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NEW YORK ACADEMY OF SCIENCES
SCIENTIFIC SURVEY
OF
Porto Rico and the Virgin Islands
VOLUME II—Parts
The Geology of the Fajardo District, Porto Rico
Howard A. Meyerhoff
Introduction, Tertiary and Quarternary Geology and
Pliysiography —//. A. Meyerhoff
Cretaceous Geology—//. A.
Meyerhoff and L
F. Smith
NEW YOEK
Published by the Academy
1931
NEW YORK ACADEMY OF SCIENCES
SCIENTIFIC SURVEY
OF
Porto Rico and the Virgin Islands
VOLUME
II
Parts
The Geology of the Fajardo District, Porto Rico —
Howard A. Meyerhoff
Introduction, Tertiary and Quarternary (ieoiogy and
Physiography —H. A, Meyerhoff
Cretaceous Geology—H, A.
Meyerhoff and
I.
F. Smith
NRW YORK
:
PlIRI.ISHKP Ry THK A('AI)F:MY
1
THE GEOLOGY OF THE FAJARDO DISTRICT
PORTO RICO
By Howard A. Meyerhoff
INCLUDING A SECTION ON CRETACEOUS GEOLOGY
PREPARED WITH THE COLLABORATION OF
ISABEL
F. SMITH
CONTENTS
Page
Introduction
202
Geological field work in the Fajardo District
202
Acknowledgments
204
Geography
206
Location and boundaries
206
Settlement and communication
206
Physical geography
208
Rainfall and run-off
213
Geology
216
Cretaceous geology
218
The age of the ''Older Series''
219
The structure of the Cretaceous
229
The structure of the western border
:
233
Structural problems of the Caguas Lowland and Luquillo Moun-
tains
241
The structure of the east coast
254
The transverse folds of the northern border
258
Summary
261
The stratigraphy of the ''Older Series''
263
Problems in nomenclature
263
The stratigraphic sequence
266
The Cretaceous formations
267
The Guzman formation
267
The Hato Puerco tuffs
272
The Guaynabo formation
275
The La Muda hmestone
276
The Rio conglomerate
278
The Luquillo formation
279
The Trujillo Alto hmestone
283
The Figuera formation
284
The Fajardo shales
285
The Juan Ascencio member
288
The San Diego formation
289
Conclusions
291
(201)
202
SOJENTIFJC SURVEY OF PORTO RICO
Page
Upper Cretaceous and post-Cretaceous volcanism
292
The early volcanic rocks
295
Surface types
295
Intrusives
300
The granitoid intrusives
306
Mineralization and contact metamorphism
314
Conclusions
319
Erosional features of the Cretaceous rocks
320
The Tertiary coastal plain
325
Distribution of the coastal plain in Porto Rico
325
The Quebradillas limestone
327
Structural and stratigraphic relations of the Quebradillas formation
.
.
330
The fauna of the Quebradillas limestone
333
Quaternary sedimentation
338
The San Juan formation
339
Recent deposition in the coastal section
344
The playas
345
The lagoonal sediments
345
The littoral deposits
347
Fluvial aggradation in the interior
347
Physiographic development of the Fajardo district
349
Tertiary erosion
349
The Quaternary cycle
353
Bibliography
357
INTRODUCTIOiV
Geologkul Field Work in the Eajaudo District
The Fajardo
district, named from the town of Fajardo, embraces ap-
proximately 450 square miles of northeastern Porto Eico.
It constitutes
one of the seven districts into which the island was divided for the pur-
pose
of
detailed geologic
study,
to
follow the reconnaissant
survey by
C. P. Berkey and C.
ISF. Fenner in 19 14.^
Prior to the
survey inaugurated by
the
ISTew York Academy
of
Sci-
ences, no systematic geological work had been done in the northeastern
portion
of Porto
Kico,
although some
general information,
applicable
directly or indirectly to the region, was available in the works of E. T.
TTilP, P. T.
Cleve^, and other authors.
In the course of the Academy's
^Berkey,
C. P.: Geologic reconnoissnnce of Porto Rico.
Annals, New York Academy
of
Sciences,
Vol. XXVI,
1915,
pp.
1-70
;
Introduction
to
the
geology
of
Porto
Rico.
New York Academy of Sciences,
Scientific Survey of Porto Rico and
tlie Virgin
Islands,
1010,
Vol.
I,
pp.
11-29.
2 See bibliography.
3 Cleve,
P.
T.
:
On
the
geology
of
the
northeastern
West
India
Islands.
Kongl.
Svenska
Vetenskaps
Akademicus,
Handlingar,
1871,
Bandet
IX,
No,
12,
Stockholm,
pp.
1-48.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
203
comprehensive survey, however, the district has received attention from
several investigators.
In 1914
it was examined by Berkey and Fenner
in their preliminary work; in 1916
its landforms were studied by A. K.
Lobeck;^ in 1921 a survey of
its
areal geology was begun, but unfore-
seen circumstances prevented the completion of this survey ; and in 1923
the
late Professor James
F. Kemp made
a
brief examination
of
the
mineral
deposits reported
or prospected
in
the
Fajardo
region
in the
course of his study of the economic geology of Porto Eico.^
In 1924,
through
the kindness
of Professor Kemp and Dr.
N.
L.
Britton,
the
writer was
asked
to supplement
his
geological
studies
in
the
Virgin
Islands
with
an
areal
survey
of
the
contiguous
and
closely
related
Fajardo district.
This report
is the outcome.
The field survey on which the present work
is based was made by the
writer
with
the
assistance
of Mrs.
Meyerhoff during June,
July, and
August,
1925.
A
superficial
acquaintance with
the
district had
been
previously obtained in May, 1924, when a hasty examination of its main
geological features was undertaken for the purpose
of
establishing the
general geologic relationships between Porto Rico and the islands to the
east.
Additional studies were made in the district in August and Sep-
tember,
1926,
to augment
the meager
paleontological
collection
from
the coastal plain strata, to determine the limits of the dioritic intrusion
in the vicinity of Rio Blanco, and to
fill in other gaips in the collections
and observations accumulated during the summer of 1925.
The data gathered in the course of
field work cannot be regarded
as
altogether
satisfactory, and
the present report on the
areal geology
of
the
district
reflects the limitations
of the material from which
it has
been prepared.
Many problems which arose in the progress of the work
were not solved, because their solution would have entailed several addi-
tional weeks of close field examination, and in most cases there was no
assurance
of
successful
results even with
further
study.
The
inacces-
sibility of parts of the district constituted one important source of
diffi-
culty.
Only four
trails thread
the Luquillo
ISTational
Forest and
the
Luquillo Mountains
belt, and two of
these, closed by uncut vegetation,
proved impassable.
In consequence, traverse lines through and around
the mountains were spaced somewhat
irregularly; and
to add
to
this
handicap the heavy covering of vegetation and the complete absence of
rocks in
situ even on the
steep mountain
slopes thwarted every
effort
* Liobeck,
A.
K.
: The physiography of Porto
Rico.
New York Academy
of
Sciences,
Scientific Survey of Porto Rico and the Virgin Islands, 1922, Vol.
I,
Pt.
4,
pp. 301-379.
5 The untimely death of Professor Kemp interrupted the preparation of a report on the
mineral resources of the island, for inclusion in Part 4 of this volume.
Tentative plans
have been made for the completion of this work.
204
SCIENTIFIC SURVEY OF PORTO RICO
to obtain
reliable structural and stratigraphie information in
this part
of the
district.
Traverses
across
the upland
section immediately
east
of the mountains were
of such
length
that in
several
cases
there was
little time for
careful study
of
local problems and
details.
Too
often
attention had to be confined to the outcrops and other features
in the
immediate
vicinity
of the caminos.^
Notwithstanding the
difficulty
of
travel through the
central part of
the
district^
a net-work
of traverse
lines was made across
it^ at no point more than five miles apart, and at
an average spacing of three miles.
The entire district was examined on
foot, for no feasible method of using horses on one-way trips over poor
and uncertain trails could be devised.
Motor transportation was utilized
but one day
for reconnaissance and was employed
otherwise
only
for
transportation to and from the immediate area in which the day's work
was
done.
For
most
purposes
the
bus
or
'^guagua''
service
proved
adequate.
In the conduct of the field survey Eio Piedras was selected as central
headquarters.
Transportation
facilities from
this town
to
Canovanas
on the east and to Caguas and Juncos on the south
(see geologic map)
proved so excellent that approximately two-fifths of the district could be
covered without change of base.
The Luquillo Mountains were studied
from the government forestry cabin at the northern edge of the Luquillo
National Forest, about three and one-half miles south of Mameyes, where
temporary headquarters were maintained
for
a week.
Fajardo
served
as a base for the work done in the northeastern and eastern parts of the
district, and a few days were spent at Naguabo during the study of the
southeastern
portion
of
the
Luquillo
Mountains.
Temporary lodging
was obtained in Barrio
''' Guzman Arriba at the western edge of the Na-
tional Forest, and at Florida estate in the valley of Kio Blanco in Barrio
Eio Blanco Abajo.
Acknowledgments
Invaluable assistance was received from many sources in the prosecu-
tion
of the
field work.
Inasmuch
as
accurate
areal geology
is
condi-
tioned by the adequacy
of the maps which guide
it, the
author's
first
acknowledgment may appropriately be made to the Military Intelligence
Division
of the War Department, which
generously supplied
copies
of
8 The word "camino"
is employed for a country road or
trail
; "carretera"
is used to
designate a "hard"
or paved road.
Less than twenty per
cent,
of the Fajardo
district
is immediately accessible to the carreteras.
"^ The word "barrio," which
will recur on subsequent pages
of the
report,
is roughly
equivalent to "township."
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
205
the reconnaissance contour maps
of Porto
Kico, made by Major Arm-
strong in 1908.
These maps were of exceptional service in planning the
work
of the summer and in geologic mapping.
The
writer
feels
espe-
cial
indebtedness
to Mr. W. P. Kramer and
to
other members
of
the
Forest Service at Eio Piedras for innumerable kindnesses, among them
the use of the cabin south of Mameyes as a base of operations during the
field examination of the Luquillo Mountains ; the assignment of a forest
guard as guide and helper during the Luquillo Mountains work ; and the
care
of mail,
supplies, and specimens during the
writer^s extended
ab-
sences from Eio
Piedras.
To
Sefior Jorge Bird Leon
of Fajardo
the
writer owes a debt of thanks for the use of his launch, Bluebird, in the
survey of the islands off the east coast of the Fajardo district.
Indebted-
ness
is
also acknowledged
to
Commissioner
of
Agriculture,
Carlos
E.
Chardon, and
to Mr.
0. W.
Barrett,
formerly
of
the Department
of
Agriculture,
for their kind attention
to the
expedition''s
affairs in San
Juan and for helpful suggestions with regard to the field work ; to Seiior
Edmundo Martinez of Eio Piedras, whose help solved many of the prac-
tical
difficulties encountered in the early stages of the work; and to the
forest
guard,
Bartolo
Peraza, whose
guidance and
thoughtful
service
made possible what work was done in the Luquillo Mountains.
A tribute
is also due the inhabitants of the
district
for their unfailing courtesy,
spontaneous interest, and friendly hospitality.
The writer feels a debt
of gratitude that can never be repaid
to the
late Professor James F. Kemp for helpful notes and suggestions which
guided both the field and laboratory study.
And finally, in the field and
the laboratory. Dr. N. L. Britton has proved an unfailing source of
in-
spiration and help, and to him is largely due whatever merit the present
report may have.
In the laboratory studies and in the preparation of the report, figures,
and maps, the writer owes much to the help and criticism given by his
former
colleague.
Professor
Isabel
F.
Smith, who has
collaborated
in
the preparation of the
section dealing with the Cretaceous geology;
to
the suggestions and criticisms given in the petrographic studies by Pro-
fessor E.
J. Colony of Columbia University; to Miss Anne E. Burgess,
who has given substantial aid in the preparation of the manuscript; and
to Mrs. Meyerhof!, whose assistance has been utilized in every phase of
the work.
The
writer
wishes
to
take
this opportunity
to
express
his
personal appreciation and that
of the
ISTew York Academy
of Sciences
for the aid given toward the prosecution of the laboratory work by Smith
College through
its president, William Allan N"eilson.
206
SCIENTIFIC SURVEY OF PORTO RICO
GEOGEAPHY
Location ajs^d Boundaeies
The Fajardo
district
includes
the 450
square miles
of northeastern
Porto Eico which
lie between
longitudes
65°34:' and
QQ^'Q'
west, and
latitudes 18°15' and 18°28' north.
In the prosecution of the field work
it was found expedient
to extend
detailed
investigations
as
far
south
as 18° 13' into the Humacao
district, and as far west as 66° 8' into the
San Juan
district.
The geologic map accompanying the
report covers
the
entire
area
studied.
The Atlantic coast forms a natural boundary on the north, and Fajardo
Eoads and Vieques Passage limit the district on the east.
On the south
a natural boundary is supplied by a lowland which extends from Caguas
to the east coast at Playa de Naguabo, a distance of twenty-four miles.
The northern wall of the lowland is a steep, youthfully dissected escarp-
ment that trends slightly south of east and crosses the southern boundary
of the district as originally defined ; but all of the scarp is included within
the somewhat larger area described
in this
report.
On the west there
is no strong topographic feature delimiting the district, but the military
road from Eio Piedras to Caguas, following a succession of low passes,
crosses the boundary at La Muda, and
at
all points
lies within
a
rela-
tively short distance
of
it.
Settlement and Communication
Except for the area included within the Luquillo National Forest, the
Fajardo district is one of the most populous in Porto Eico.
Towns and
villages are numerous, the more important
of which include
Santurce,
a community incorporated in the
city of San Juan, with
a population
estimated at 40,000; Caguas^ with 12,200 inhabitants; Fajardo, 6,600;
Eio Piedras, 5,900; Juncos,^ 4,300; Naguabo," 3,700;
Carolina, 3,150;
Gurabo,
2,550;
Eio
Grande,
2,000; Aguas
Buenas,^
2,000;
Luquillo,
1,250;
Canovanas,
1,100;
Ceiba,
850.
Trujillo
Alto,
Loiza
(viejo),
Mameyes, Eio Blanco, and El Eio are moderate-sized villages within, or
at the edge
of, the
district, and smaller ones, including the
"colonias'^
or
settlements on
the
larger
sugar
estates,
are common.
The
rural
population, which
is widely disseminated throughout most parts of the
district except in the mountains, approximates 150,000, making a
total
population of nearly 250,000, or roughly 500 inhabitants per square mile.
8 These towns
lie outside the original boundaries
of the district but were included
in
the broader limits of the survey.
All figures are from the 1920 census.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
207
The distribution of towns and of the rural population
is uneven, how-
ever, and responds rather
directly to the
agricultural pursuits followed
in the various portions
of the
district.
Agriculture in turn
is directly
related
to
soils, topography, and
rainfall.
The
suitability
of much
of
the country of low relief for large-scale sugar production and the suita-
bility
of
the lowland extending from Caguas
to Naguabo
for
tobacco
and sugar are responsible for the concentration of towns and villages in
these lower sections around the borders of the
district.
The density of
population here has naturally led to the construction of excellent carre-
teras connecting all the principal settlements with one another and with
San Juan
(see geologic map).
The old Spanish military road thus ex-
tends from San Juan through Eio Piedras across the low pass to Caguas,
connecting at La Muda with another road from the capital by way of
Guaynabo.
The Porto Kico Eailway, Light, and Power Company main-
tains a freight railroad to Caguas via Trujillo Alto and the entrenched
gorge of Kio Grande de Loiza.
Automobile highways radiate outward
from Caguas
to Aguas Buenas,
Cayey, San Lorenzo, and eastward
to
Gurabo and Juncos.
Juncos
is connected with Torres, Rio Blanco, and
Naguabo on the
east, and with Humacao on the southeast by excellent
roads.
The coastal section on the north and east
sides of the
district
is served by a carretera from Rio Piedras to Humacao via Carolina, Cano-
vanas, Rio Grande, Luquillo, Fajardo, Ceiba, and Xaguabo.
Cane rail-
roads parallel many of the carreteras in the lowlands and
all but girdle
the district.
The interior of the Fajardo
district presents a strong contrast to the
marginal
sections.
In the west
it
is
a maturely
dissected region,
the
relief and ruggedness of which increase southward and eastward.
There
is
little
flat
land;
and
the moderate
rainfall,
thin
vegetation,
heavy
lateritic soils, and steep slopes make large farms unprofitable.
It is thus
a country
of small
farms,
in which centers
of population
are
lacking.
There
is
but
one
village,
Trujillo
Alto,
with
539
inhabitants.
It
is
connected with Rio Piedras by a
carretera, which was being extended
southward at the time the field survey was made.
In the balance of the
western interior there are only two carreteras, one under construction from
Canovanas
to Juncos along Rio Canovanas
at the western edge
of the
Luquillo Mountains; the
other, a short blind road from Rio Piedras to
Barrio Cupey.
Caminos serve the rest of the area, most of them thread-
ing
the
country from
north
to
south, and
linking
the
towns
on
the
northern
carretera
with
those
in
the
Caguas-Xaguabo
lowland.
The
greater part of the eastern
interior
is a region of very high
relief, em-
208
SVIENTIFIC SUKVI^JV OF PORTO EI€0
l)raeiiig
the
JjiiquiUo
Momitaius
and
tlieir
deeply
dissectod
foothills.
The population is
tliin, and
settlers are excluded froni
tlie twenty square
luiies of tlie nioniitain slopes and summits whicli are iiitduded
in the
lui-
tioiial
forest.
Travel routes are therefore few and are
so
infrequently
utilizeil
thiit one of them was found impossible to follow, eveji with
tlie
ex|)ert guidance of a forest guard, and others w^-re reported as ini[)a8sahle.
A view from the Cauovntias-.Tuneos earretera, showing the
peak's dctacliment from
tlic
central
range,
m-liicli
forms
the
southern
skyUue.
PjT YSliCilL G KOGHAPJIT
Oonaideralion will bo given to the genesis of the physiogra|jhic features
of
tlio Fajardo district
iu^
hiter sections of the rejiort
; hut an introduc-
tory description of tlie topographic sutalivisions and their characteristics
is germane to the discussiou^
o(' the areal geology that follows.
Jn elevation the area studied ranges from sea-level
to
3fi00
!eet.
In
the Luqidllo Mountains ahoul se\'en eminences
rise well above tlOOO feet,
including
East
Peak,, West Peak, El Yuncpie
(Fig.
1), El Toro, and
others whicli
luive no
naities.
El Yuirque
(.'M96
feet)
is popularly
as-
sumed to be the highest, hut the more remote
.hjl Toro, lying in
tlie west-
ern part of the mountain area,
is about 40 feet higher.
The m,ountains
dwarf
all the otlier topographic elements
(l*^ig. 3) ami are
tlie more con-
s|hcuous
because
they
arc
located
within
short
distances
of
the
coast-
line.
The backbone of
th,e range lies hut three to
five miles north of the
ME¥KR1I()FF, GEOLOGY OF THE FAJARDO DIHTRICT
%m
Caguas-X^agiiabo
lowland.
Kast Peak
is
situated
eig-fit
miles
west
of
Vieques Sound, and Kl Yunquo lies but
six miles soutli of the Atlantic
coast.
In
eojisequenee,
tlic Luquillo
Moiuitains
constitute
one
of
the
strongest
relief
features
in
Porto
Ilieo,
surpassed
only
l)y
tlie
steep
ascent of
tlie
(^)rdillera Central from the south coast
in the south cen-
tral part
oi' the island near Ponce.
*!»*?%:/
•ast. and
tlH! low playns and
la
)izn,
extend
InlatKl
almost
to
Tlie
Pajardo
district
as
a whole may
he
diwided
into
four
nnijor
sectious:
(1) A coastal section, wddch includes a coastal plain and playas form-
ini»* an
irregular horder around
the northern
and
eastern
sides
of
the
district.
This
is a belt of flat or rolling country (Fig, 2), fringed along
sections
ot
the
coast by lagoons and marshes
that
are separated
from
the
ocean
by low
bars.
The
latter
generally
snpp(u-t
larg'<s
gro\-es
of
coconut palms, which form the
basis
of
a
flourisbiiig
inrliistry
in
this
part of the island.
The l)elt Taries in widtli from little UHrrc tlian
a strip
of beach
along
parts
of
the
eastern and
northeasterji
coast,
to
a
>^one
seven
or
eltdit
mibi'S
wide.
Alluvial
or
lagoonal
deposits
veruM-r
the
playas and coastal plain tlrroughont much
of
the
secti<)n, making
it
region of considerable
fertility that
sei-
the setting
for
a
vigorous
210
8CIEWTIFW SURVEY OF PORTO RICO
sugar
industry.
It
is
lierc,
too,
that most
of
tlie
coastal
towns
are
located.
(2)
All interior lowland, extending from a
poiiit four miles west
of
Caguas, the town from which
it
is named, eastward to Naguabo.
This
section includes the rest of the towns within, and marginal
to, the
dis-
trict.
The lowland
is flanked on the north by a strong, youthfully dis-
^^Wy<^\i
Fig. :k—The Cugtim biwlnnd
The
sllshtly
rlissected allUTlal floor may be
sotjn extending soutliward
into the
'
of Kto Tuniho.
The summits
of the
Sierra de Cayey merge
i
the
hackgi'ound
;
tlic
high bench which
flanks
the mountains
iiant
of
the
"upper,"
or
Saint
John,
peneplane;
tbe
hflls
oi
t'Jevatlon
of the Cngiiiina peneplane.
,'ith
the
clouds
to
for
right
lis
sected escarpment, 1000
to 2000
feet high.
Although breached
at the
water-gap
of Bio Grande
de Loiza northeast
of Caguas, and low and
broken for a short distance west of that point, the linear escarpment
is
the most persistent and impressive
topograpliic feature in the
district.
On the south the Caguas lowland is hemmed in by deeply dissected
hills,
which have been opened
out broadly
along
the
courses
of
the
larger
streams
(Fig.
3).
The
depression
hegin^s
on
the
west
not
far
from^
Aguas Buenas
in two slender prongs.
Eastward they
un;ite
to form
a
basinal depression approxim,ately
five miles in width, expanding
locally
where hroad
valleys
join
it from
the
south.
Some
of
these
tributary
valleys, like that of Bio Valencianos, extend as fl^at-floored alluvial plains
for distances of three to four miles into the mounting elevations
of the
MEYER1I0FF, (}imi.(M}Y OF THE FAJAIUjO Dhn'RICT
211
Sierra
fie C'ayey.
Tlie rock
floor of most of the IowIhikI
is deeply buried
beiieiitli
alliB'iirrn,
tliroiigfi which. horcTer, a
fern' low, rounded
roclc
liilli*
arid ridges protrude.
The elevation o! the alluvial surface varies within
luirrow
liirdfs,
the maximum
being
approximately
-lOO
feet
ami
the
avHriige 2oO
hn-t
nhosxi
sea^^leveL''
As
Ihe
firevaleiit
direfliun
at
tb.-
irud0w\ruh
i«
pli'^-h!!v
Tiorrli
of
(^f)-f.
tb^
loudfind. hdng on
iho
|<.a
ci,if.
of
lln;
Lncpiillu "Mountains^
in oompanitively dry ami
hot.
Tlie
rainfall
is
sufTieient,
htiwf\'er,
lo favor
the growth
of sngar
ciuie
Ihrooginnit
its
length; and
in
the
vicinity
of
C'agiin.s,
(rnndMy.
iTrnl
Jnneos
tobacco
i<
raised
in, considerable Cfuantity.
{:]) A western
hill section, whit-h extends from tbe wesfern border
of
the
district
to
ils
center,
increasing
in height and
relief
J'roin
north
to
•J The above (IfKerliitinii
diff.TS from
LolM'<:k'!<
fop.
eit.
pp.
.•]18^:',1!))
iii
r«>anrdiii!;-
wluil:
he
ciilfc the flood plahis of Urn
<iraiMlf»
«le Lojza
iuid
its
t ribiiOirios as
liic
c^siHitirtl
rto-
irieiit
ill
the lowland,
awl
tin;
n.ek
hlUs and vMmv
as
tho
sulKirdinale
hihI
niflderOnl
foatnrcrt.
T±w. vnvM
lillls
ftii
tfic- soufhern side of
tl»!
lawPiiirt aro spurs from
ll)»'
Sicrrn,
iiH
iH.riiial
tlocdplalii
imO^'rlHl
d.-|...HtwJ
1
iIm-
i'd«
212
SCIENTIFIC SURVEY OF PORTO RICO
south and from west
to
east
(Fig.
4).
On the
east the
linear north-
south valley of Bio Canovanas may be taken as
a convenient boundary,
and on the south the section terminates in the strong ridge which forms
the northern wall of the Caguas lowland, and which constitutes the ex-
tremely asymmetrical drainage divide between the short streams flowing-
south into the lowland, and
the much longer streams flowing north
to
the Atlantic.
The hill section averages about 1000 to 1500 feet in eleva-
tion
; but a portion of its surface on the north and west ranges from 200
to 500 feet, whereas the marginal ridge on the south attains a height in
excess of 2000
feet.
The depth of dissection varies with the
elevation
and
is generally well under 1000
feet.
Despite the moderate relief, the
topography as a whole
is subdued
;
cliffs and rock-walled valleys are few,
and most
of
the rounded
hills
have
a
thick
covering
of
residual
soil
which, because of excessive cultivation and inadequate protection,
is fast
being depleted by rainwash.
A few of the valleys have been aggraded,
and what alluvium has accumulated
is now undergoing dissection.
The
diversified
agricultural
possibilities
of
this region have
led
to
the
de-
velopment of small holdings, and an evenly distributed rural population
occupies almost every portion of
it.
(4) The Luquillo Mountains section, which occupies the eastern half
of the interior, and which may be sub-divided into a narrow foothill belt
and the mountain area.
High, rugged foothills flank the mountains on
tlie
east,
north, and
west, forming elongated
spurs that protrude
out-
ward from the mountain mass, separated one from the other by profound
gorges.
On the east and north they rise sharply above the coastal
sec-
tion
(Fig. 2), and on the west their height
is accentuated by the deep
valley of Eio Canovanas.
At 1600
feet many of the spurs
flatten, but
farther inland they
rise again
in steep
slopes
to the
highest
summits,
which exceed 3000 feet in elevation.
The mountains have been so deeply
scoured by stream erosion that descents of 2000 feet or more in a single
mile
are not uncommon.
Their
flanks
are
precipitous,
yet
cliffs and
crags are few in number, and what few there are prove practically
in-
accessible.
In the foot-hill belt the slopes are mantled by a thin cover-
ing of residual
soil that offers scant attraction
to the farmer.
In con-
sequence, the settlement of this section
is thin, and the agricultural pur-
suits of the inhabitants are limited to the raising of coffee and bananas
and to charcoal-burning.
Settlement and private agricultural develop-
ment are not permitted
in the Luquillo National
Forest, and as
a
re-
sult of this regulation and the lack of such restrictions in other sections,
it is the only large tract of upland tropical wilderness left in Porto Eico.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
213
Eainfall and Eun-off
The distribution of rainfall in the Fajardo district depends upon the
elevation and relative positions of the various topographic elements with
respect
to the trade winds and mountain
barriers.
The
heiglit
of
the
Luquillo Mountains and their location at the northeastern corner of the
island are factors which give them heavy precipitation.
Their summits
are often blanketed in clouds, and in every season rainfall
is abundant.
The highest points receive over 125 inches
of rain annually, and even
the flanks of the range get 90 to 125 inches.
The driest parts of the dis-
trict, on the other hand, are the lower sections along the east coast and
the western part
of the Caguas lowland, which
receive from 60
to
80
inches.
The precipitation in the balance of the district ranges from 70
to 90 inches annually.
The rainfall
is
distributed somewhat unevenly through the
year,
ex-
cept in
tlie Luquillo Mountains.
Precipitation
is comparatively heavy
from May until November, and
is
relatively low from December
nntil
April.
The distribution of rainfall thus follows the usual law for coun-
tries situated near the northern limits of the trade wind belt, where there
is a rainy season when the sun
is directly overhead and a comparatively
dry season when the sun
lies farthest
to the south.
Like most moun-
tain masses, the Luquillo Mountains are not subject to the strict opera-
tion
of
tlie law, and in general the laws of precipitation
are found to
undergo greater irregularity in
a small oceanic
island
like Porto Rico
than on continental masses in the
tropics.
Rain may
fall during any
month of the year, and local droughts have been known to occur during
the normal rainy season.
The abundant rainfall in the Fajardo district is productive of a heavy,
rapid run-oif and of numerous streams.
Stream-flow fluctuates not only
with the
seasons but between storms, and
in
parts
of
the
district the
fluctuation in water volume not infrequently amounts to several hundred
per cent, within literally a matter of minutes.
During a heavy shower
which lasted ninety minutes the writer has watched
a stream with
ai:
estimated flow of 1250 second-feet rise until
it was discharging at a rate
of not less than 20,000 second-feet.
Most of the increase in volume was
effected during the
first half-hour of the storm, and thereafter the bal-
ance between surface run-off and stream flow was established and main-
tained. Two hours
after precipitation ceased the volume had decreased
to 10,000
second-feet, and within twenty-four
hours, approximately
to
214
SCIENTIFIC SURVEY OF PORTO RICO
2000.
The storm in question was not nnusiial nor were there any local
peculiarities which made the behaviour of this stream abnormal.
Eun-off
is exceedingly rapid primarily because of the scarcity of per-
manent vegetation and the prevalciice of impervious clay
soils.
I^kcept
for
the forest growth
in the
Jjuquillo National
Forest and on some
ol'
the mountain
flanks
Avherc
coffee and bananas
are
raised,
the
wootly
^egetation
has
been removed
to
clear
the
land
for
cultivation
or
to
furnish charcoal, the only available
fuel.
There
is,
in consequence, no
restraining
factor
to
retard the precipitous
passage
of rain-water over
the
surface, and with
it a heavy burden of sediment, derived from
tlie
clay
soils, which are torn away notwithstanding a natural tenacity that
holds them to the steepest slopes.^*^
In the forested Luquillo Mountains,
on the otlier liand, the streams undergo much smaller volumetric changes,
even during abnormal
rains
or prolonged dry
weatlier, because
of
the
slower
drainage.
Little
soil
is
removed,
and
tlic
mountain
streams,
tliough they become cloudy, carry a relatively small amount of sediment,
even with peak
loads
of water.
Vegetation
is undoubtedly the principal factor
in controUijig the rate
of run-off, but the kinds of
soils have an important influence.
Imper-
vious clay
soils, which shed water
rapidly, predominate
in
the
district
and, where they are present, run-off
is rapid and variable in the extreme.
Ciround-water
is
scarce
or non-existent, and
tlie majority
oC
tlic small
tributary streams are intermittent.
In
tlie more restricted
areas
wliich
are covered by
soils of
disintegration
or alluvium, water-absorption
is
great, ground-water
is
abundant,
and more
of
the
local
streams
arc
perennial.
In all parts of the district the drainage reflects the elements
of control exerted by the types of vegetation and
soil.
Most of the drainage of the Fajardo district
is indigenous.
The only
drainage
system
of importance
that
extends
far beyond
the
district's
limits
is the Eio Grande de Loiza system, which heads in the Sierra de
Cayey, only seven miles from the Caribbean and six miles from Viecjues
Passage.
It drains an area lu^arly 500 square miles in extent, about half
of which
lies in the west central and
soutli western parts of
tlie Fajardo
^" HorUoy
has
attribiitfd
the
tenacity
of
the
soils
in
part
to
the
absence
of
*iuart/>
j?rains
in
them
(Berkey,
C.
P.,
Geological
reconnoissance
of
Porto
Rico,
op.
cit.,
pp.
:;4-'>i"i).
His
statement may
be
further
generalized.
Tliose
roclcs
which
cojitain
no
qwart'A yield tenacious residual
soils provided the minerals in tliem have been thoroughly
decomposed to
a
clay-sized residue.
Those
rocl^s. such as
diorite. which disintegrate
be-
fore
they undergo decomposition,
exhibit
no
tenacity,
whether
they
contain
quartz
or
not.
Such
soils are
readily
removed by rain-wash, and the
rocks from which they
de-
velop are rapidly reduced to lowlands.
The principle appears to be one of disintegration
versus
decomposition
:
the
presence
or
al)sence
of
quartz
is
incidental.
Further
cou-
' sideration
wiU
l>e
given
to
the
problems
of
rock
weathering
in
a
later
section
of
tin;
report.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
215
district.
Its tributaries drain the northern flanks of the Sierra de Cayey,
the western four-fifths of the Caguas lowland, practically all of the
hill
section, the western edge of the Luquillo Mountains, and a small part of
the northern coastal section.
Its more important tributaries within the
Fajardo district are KioGurabo, which drains the central part of the Caguas
lowland, rising within six miles of the Caribbean and Vieques Passage
;
Fig.
5.
The drainage
of eastern Porto Rico
The Hmits of the Fajardo
district and
of the Rio Grande de Loiza drainage system
are shown.
The position of the Caguas lowland
Is indicated by
the
large west-flowing
tributary
(Rio Gurabo)
near the southern
district boundary.
and Rio Canovanas, which heads in the ridge bounding the Caguas low-
land on the north, flows north along the arbitrary boundary between the
hill section and the Luquillo Mountains, and joins the Rio Grande de
Loiza at the southern margin of the coastal section.
In the eastern half
216
SCIENTIFIC SURVEY OF PORTO RICO
of the district there
is no dominant system, but the streams are disposed
more or
less
radially around the Luquillo Mountains, which
act
as
a
water-shed.
Here the streams are
relatively
short, and except for Eio
Fajardo they flow in direct courses to the coast.
The principal ones in-
clude
rios Herrera, Grande, Mameyes, and Sabana, which flow north-
ward into the Atlantic ; Eio Fajardo, which enters Vieques Sound on the
east ; and Eio Blanco, which empties into the Caribbean on the southeast.
The disposition of the drainage can be best visualized by reference to the
accompanying map
(Fig.
5).
The genesis of the drainage pattern will
be discussed later as a phase of the geologic development of the
district.
GEOLOGY
The salient facts of Porto Eico's geological development have been so
often stressed in the reports of the Scientific Survey that repetition here
seems unnecessary.
The history of the district
is revealed by three dis-
tinct rock groups and by the erosional forms into which they have been
carved.
Approximately
four-fifths
of
the
Fajardo
district
is
underlaid
by
rocks of the "Older Series.'^
They are largely igneous in derivation and
may for the moment be subdivided into three types: modified and un-
modified
pyroclastics, with which
a few normal
sedimentary
rocks
of
organic
origin
are
associated;
contemporaneous
extrusive and
shallow
intrusive volcanics; and somewhat younger plutonic intrusives, ranging
from irregular apophyses to the large batholith of the Humacao district,
which outcrops along parts of the southern border of the region described
in this report.
The evidence thus far adduced indicates that most, and
probably
all, of the ''Older Series"
is referable to the Upper Cretaceous.
The youngest
plutonic
intrusives
invaded
the
older
rocks
during
the
intense orogenic
disturbances which
closed
the
period, and which im-
parted to the group as a whole
its present structural complexity.
Since
the epoch of folding, these materials have constituted
a nuclear moun-
tainous core, which deep erosion and marginal sedimentation have pro-
foundly modified.
In northeastern Porto Eico early Tertiary erosion appears to have per-
sisted somewhat longer than
it did in the more westerly portion of the
tiorthern
coast,
for
only
the
youngest member
of
the
mid-Tertiary
series of limestones, the Quebradillas formation,
is represented.
It ex-
tends from Catafio, south of the city of San Juan at the western border
of the district, eastward to Loiza.
There
is good reason to believe that
MEYERHOFF, aEOLOGY OF THE FAJARDO DISTRICT
217
it continues still farther east in the broadening submarine platform which
lies offshore; but on the mainland the Tertiary outcrops completely dis-
appear, and from Luquillo to Cape San Juan Cretaceous rocks commonly
crowd to the water^s edge.
Even where best developed, the Quebradillas
limestone has been reduced to a minor topographic role in the course of
post-Tertiary erosion.
Eiddled and nearly baseleveled by subterranean
solution,
it
finds
expression
in
a few
scattered
erosional
pillars and
clusters, which
stand
like mute monuments
testifying
to
its
ancient
extent and importance.
The bases of many of these residuals lie buried
in alluvium and lagoonal
deposits, which have accumulated
in
conse-
quence of recent fluctuations in baselevel,
all but obliterating the Ter-
tiary deposits.
Opinion varies, but it
is probable that the Quebradillas limestone "was
deposited in early Miocene time.
Pronounced uplift and some warping
followed closely in the wake of mid-Tertiary sedimentation, and a long
erosional history, renewed and prolonged twice by Pliocene and Pleisto-
cene uplifts of considerable magnitude, followed.
While erosional
sur-
faces were being formed and pushed inland during the
first two of the
three late Cenozoic cycles which have
left their impress on Porto Pico's
landforms, there was no chance for regional sedimentation to take place,
and
scant opportunity
for
the
preservation
of the
local
deposits
that
undoubtedly were formed.
Since the start of the third, or Quaternary,
cycle in early Pleistocene time, on the other hand, the island has exhibited
relative
tectonic
stability.
Glacial
and
postglacial
variations
in
the
strandline have displayed no definite tendencies.
Sediments formed
in
the normal course of the erosion cycle have been spared from destruction
and in themselves offer the investigator some puzzling problems.
Along the northern coast of the island lies the oldest of these Quater-
nary deposits, a calcarenyte of eolian origin, named from its spectacular
outcrops in the city of San Juan.
Its occurrences are restricted to a few
prominent points along shore in the western half of the Fajardo** district
;
but like the Tertiary coastal plain above which
it rises,
it seems to con-
tinue eastward in a narrow, drowned ridge on the submarine platform,
to reappear in the islands of Cordilleras Reefs
at the northwestern cor-
ner
of Vieques
Sound.
The submerged
linear
ridge, with which
the
outcrops on the mainland are aligned, suggests that the dunes of the San
Juan formation were raised by the trade winds
as they swept the
cal-
careous sands of an off-shore bar, or barrier beach.
Partial submergence
of the dune
ridge,
increasing
in magnitude eastward,
aided materially
218
SCIENTIFIC SURVEY OF PORTO RICO
in its consolidation and consequent preservation but is snatching with its
left hand what it gave with its right.
Partial submergence has brought
the formation within the sphere of action of the waves, which have bat-
tered
it until but few stranded and doomed remnants
are
left exposed
apon the present shoreline.
Another chain of consequences has resulted from partial submergence.
Lagoonal and estuarine deposits have accumulated along large
sections
of the north coast and in the small estuaries in
all parts of the coastal
margins.
Fluvial aggradation has proceeded rapidly, and the alluvium
has mingled with the brackish water
deposits,
usually replacing them
completely
by
overlap
in
the
neighborhood
of
the
larger
stieams.
Fluvial aggradation has
also taken place
far inland, where
its relation
to recent changes in level
is problematical, and where other causal fac-
tors must be sought
to unravel the complicated
interplay
of
erosional
and depositional processes.
This
record
has
been
summarized
from
the
facts
revealed
in
the
Fajardo district.
Save for a few minor omissions
it
is the record of all
Porto Eico and the Virgin
Islands.
It presents nothing that
is new,
yet, considered in detail,
it provides much that is different.
It is hoped
that the details that follow will help solve some of the problems which
have arisen in the labors of the Scientific Survey and will cast new light
on
others.
Chronologic presentation, which seems the
best method
of
dealing with
the data
collected,
requires
their arrangement under
the
following topics
:
1.
Cretaceous geology
2. The Tertiary
coastal
plain
3. Quaternary sedimentation
4.
Erosional landforms
Cretaceous Geology
by
H. A. Meyerhoff and I. F. Smith
The ^^Older Series" of rocks in Porto Eico and in the Virgin Islands
has left every investigator associated with the
Scientific Survey a herit-
age
of
problems.
Many
contributions
have
been made
toward
their
solution, but few, if any, of the larger issues have been definitely settled.
In
fact,
the
claim
that each
investigation
has
raised more
questions
than
it has answered
is hardly an
extravagant
one.
The
age
of the
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
219
"Older
Series/^ the character and
significance
of the
structures
of the
formations^ and
the
stratigraphic
succession
are
still
open
questions.
The petrography
of the rock types has heen presented
in
considerable
detail^
but
the
broader
field
of
petrogenesis
has not
been
fully
dealt
with.
These four
subjects
constitute
the major problems
of the
Cre-
taceous geology of Porto Eico^ and in the presentation of the results of
the Fajardo survey they will serve as logical headings under which the
numerous minor questions may be grouped.
An historical resume of the
status
of each
of the subjects
will furnish
a desirable background and
perspective
in
the ensuing consideration
of
the
Cretaceous
geology
of
the district.
THE AGE OF THE
'"OLDER
SERIES'"
The Cretaceous age of the rocks which form the cores of the Virgin
Islands and
of Porto Eico was established by P. T.
Cleve
as
early
as
1871.^^
Cleve's conclusion was based upon the evidence of a small fauna,
most of which seems to have come from the Virgin Islands; and so far
as
it involved Porto Eico, his opinion appears to have been inferential.
He expressed grave doubt concerning the identification of two Paleozoic
corals
of
the genus
Favosites
reported from
the Virgin
Islands,
and
which probably were specimens of rudistids ; and although he leaves open
the possibility of an earlier age for some of the associated volcanic rocks,
he
is
positive
that
the
oldest
fossiliferous
formations
are
Cretaceous.
Practically
all European
geologists have concurred
in
Cleve's opinion
that they are to be correlated with the Upper Cretaceous strata of the
Alps.
With
the
authenticity
of the
correlation
well
established,
it
is
puzzling
to
find
the mountainous
oldland
of
Porto
Eico mapped
as
"undivided
Paleozoic"
by American
geologists
as
recently
as
1912,^^
notwithstanding the statement of E. T. Hill, which
is cited in explana-
tion, that the oldest rocks on the island are Cretaceous and
early Ter-
tiary.
The conviction that the history of the Antillean region dates far back
into
geologic
time
is
deep-rooted, and
attempts
are
constantly being
made to decipher the earliest events.
Schuchert has sketched phases of
late Paleozoic history in the Caribbean area and has hazarded an opinion
1^ Cleve,
P.
T.
: On the geology
of the northeastern West India
Islands,
op.
cit.,
pp.
1-48.
12 Willis, Bailey
: Index to the stratigraphy of North America.
U.
S.
Geological
Sur-
vey,
Prof. Paper 71, 1912,
p.
348.
220
SCIENTIFIC SURVEY OF PORTO RICO
concerning a few pre-Cambrian developments.^^
More
recently Matley
has differentiated Jamaican rocks which he
believes were formed long
before the Mesozoic and early Tertiary strata with which they occur.^*
Convinced that similar rocks should be found in the older mountain axes
of all the Greater Antilles;, he has reviewed the evidence in favor of the
presence
of
pre-Mesozoic
rocks
in
the
other
islands
and,
concerning
Porto Eico and the Virgin Islands, states:
".
.
.
the inability of the
A-merican geologists to find pre-Cretaceous rocks in Porto Eico and the
(American)
Virgin Islands may be due
to the inadequacy of the
field
work
.
.
.
fuller surveys of Porto Eico may result in showing that
its
^Older
Series'
possibly
contains
pre-Cretaceous
rocks
of
the same
general age as the similar rocks of the other Antilles/' ^^
In support of
this thesis K. W. Earle's opinion regarding the rocks of the British Virgin
Islands
is
cited, and K. W. Earle himself
contributes
a
discussion
of
Matley's paper/^ in which he postulates that too much importance has
been attached to the presence of Cretaceous
fossils in the limestone
at
Coki Point, Saint Thomas, as a basis for dating all the associated rocks
;
and
that, judging from comparative studies
of Cretaceous and Eocene
rocks from the Lesser
Antilles,
the
unfossiliferous sediments and
vol-
canics of the British Virgin Islands are probably older,—perhaps as old
as the Archaean.
The senior author has examined the ''Older Series'' in many parts of
Porto Eico and in all the larger islands and cays of the American and
British Virgin Islands.
He has failed to find a single item of evidence
in support of Matley's and Earle's views and has
collected data which
seem to disprove them.
A preliminary summary
of these data
is sub-
mitted
herewith, but
elaboration must be
postponed
until
the Virgin
Islands and other Porto Eican studies are completed
:
(1) The resolution of the complex rocks of northeastern Porto Eico
into a conformable series of formations, into which all the igneous rocks
are
clearly intrusive, precludes the
possibility
of a basal complex.
If
the rock section be traced westward,
it
is found that the volcanic and
sedimentary formations of central and western Porto Eico are parts of
the same rock
series; that Cretaceous
fossils occur in the sedimentary
13 Schuchert, Charles
: Geological history of the AntiUean region.
Bulletin, Geological
Society of America.
1929,
Vol. XL,
pp. 337-360.
" Matley,
C. A.
: The basal complex of Jamaica, with
special reference to
the Kings-
ton
district.
With petrographical notes by Frank Higham.
Quarterly Journal
of the
Geological
Society.
1929,
Vol. LXXXV,
pp.
440-492.
IS Ibid., pp. 472-473.
"Ibid.,
pp. 490-491.
MEYERHOFF, QEOLOOY OF THE FAJARDO DISTRICT
221
members in every part of the series, and that the intrusives maintain the
same relationships throughout the island.
(2) The
stratified
rocks of
Saint Thomas
are
lithologically
similar
to those of the Fajardo district and are perhaps stratigraphically identi-
cal.
The Cretaceous
fossils at Coki Point may, therefore, be regarded
as a reliable index of the age of the whole rock section.
(3)
The
Saint Thomas
succession
can
be
traced
eastward
in
the
islands of Pillsbury Sound and The Farrows to Saint John and western
Tortola.
On both the
latter the same
lithologic
succession appears
to
be repeated in inverse order as the result of simple folding.
(4)
Metamorphic characteristics are present in all the stratified rocks
from Saint Thomas
to
Saint John and
Tortola, but they
increase
in
intensity eastward in the direction of the batholithic intrusion
(or in-
trusions)
of eastern Tortola, Beef Island, and Virgin Gorda.
(5) The most
intense metamorphism
is
found
in
the
schists
and
gneisses on the islands south of Sir Francis Drake Channel.
On Cooper
Island the intrusive contact between sediments and
diorite
is exposed,
and the conversion of the former to gneiss
is clearly the direct outcome
of the intrusion.
In the islands to the east and west the metamorphic
effects diminish, and the rocks involved again possess lithologic charac-
teristics that are typical
of the members
of the Tipper Cretaceous
sec-
tion.
There is no basis for ascribing to them a greater age.
Eecent investigations have thus done little to advance the question of
the
geologic
age
of
the
"Older
Series"
beyond
the
conclusion which
Cleve
reached
in
1871.
Early
in
the
history
of
the
present
survey
Berkey wrote
: ''There is no good reason
.
.
.
why the whole of the
'Older
Series' could not have been accumulated
in
a
single
geological
period.
The fossil content of the upper members of this series indicates
that this period was the Cretaceous.
.
.
.
Whether or not the older
members
date back
to
an immediately preceding
time
cannot
yet
be
definitely
stated, but whatever there
is,
is
clearly
so
closely
associated
with the Cretaceous beds that they can all be treated as a single historical
unit.'' ^'
D. E. Semmes ^^ endeavored to fix the age of the "Older Series'' more
precisely on the basis of foraminiferal faunules which he discovered in
four of the formations of the San Juan district.
Two of the collections
1^ Berkey, C. P.
: Geological reconnoissance of Porto Rico,
op.
cit., p. 58.
18 Semmes, D. R.
: The geology of the San Juan district, Porto Rico.
New York Acad-
emy of Sciences,
Scientific Survey
of Porto Rico and the Virgin
Islands,
1919,
Vol.
I,
Pt.
1, pp. 72-75.
222
SCIENTIFIC SURVEY OF PORTO RICO
came from shaly strata, and of the species identified he has listed several
long-ranged
forms,
which
in
assemblage
have
an
Upper
Cretaceous
aspect.
The other two collections, which were found in beds of limestone,
contain
species
that
are
identical
or
closely
related,
but
both
these
faunules differ strikingly from the shaly facies.
Although only one form
could be
specifically identified, Semmes believed the faunal assemblage
from the limestones to be Eocene in age.
In suggesting that the older
formations
in
the
series
are
referable
to
the
Lower
Cretaceous,
or
Comanchean, Semmes was undoubtedly influenced by Hodge's studies
in
the Coamo-Guayama district immediately to the south.
Semmes's claim
that
the foraminiferal limestones
in
the San Juan
district
are Eocene must
be
rejected
on
structural and
stratigraphic
grounds.
One of them, the La Muda limestone in the eastern part of the
district, passes stratigraphically beneath the foraminiferal shales which
outcrop southwest of Bayamon, and which Semmes on more satisfactory
micropaleontologic evidence placed in the Upper Cretaceous.
The shales
occupy the center of a northwestward pitching syncline^ which extends
from Toa Alta to Aguas Buenas, and the La Muda limestone
is exposed
upon
its northeastern limb.
The
Corozal limestone, the second
of the
supposed Eocene formations, outcrops on the southwestern limb
of the
same fold.
If this interpretation of the structure
is correct, the Corozal
limestone occupies the same
stratigraphic
position with
respect
to
the
shales as the La Muda limestone, and the two calcareous members may
be
equivalent.
The exact
structural and
stratigraphic
position
of
the
fossiliferous Unibon
shales, which Semmes
places
in
the Upper
Cre-
taceous, has not been determined, but they appear to be somewhat older
than
the
other
three
fossil-bearing horizons.
The
limestones
thus
lie
between beds that have been referred to the Upper Cretaceous, and the
faunal distinctions which Semmes recognized are ecologic and are with-
out stratigraphic significance.
Further complications
develop when
the foraminiferal
strata
of the
San Juan district are traced into the Coamo-Guayama
district.
All of
them pass directly or indirectly by way of the Fajardo district, into the
two stratigraphic divisions which Hodge has called the Eio de
la Plata
and the Barranquitas-Cayey series, and both of which he has referred to
the Lower
Cretaceous,
or
Comanchean.^^
The
same
formations
have
thus been placed in the Lower Cretaceous, the Upper Cretaceous, and the
i» Hodge,
E.
T.
:
Geology
of
the Coamo-Guayama
district.
New
York Academy
of
Sciences.
Scientific Survey of Porto Rico and the Virgin Islands, 1920,
Vol.
I,
Pt.
2,
pp.
130-137, 192-193.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
^23
Eocene.
Hodge^s claim for the Lower Cretaceous age of the Eio de la
Plata and the Barranquitas-Cayey series rests primarily upon the identi-
fication of two corals^ which he refers to the species Cladophyllia furci-
fera.
This form is an index fossil of the Edwards limestone, one of the
formations in the Fredericksburg series
of the Texas Comanchean, and
to
the
writer's knowledge
it
has
not
been
reported
outside
the
Gulf
Comanchean
area by any
other
author.
The genus
Cladophyllia, on
the other hand, ranges from the Lower Cretaceous to the Holocene and
has
a
wide
geographic
distribution.
It
seems
unwise,
therefore,
to
accept without some
reservation an
identification based on two imper-
fectly preserved specimens,
or, granting that they belong to the species
CladopJiyllia furcifera,
to assume
that
the form has
the same
strati-
graphic
value
that
it
possesses
in
Texas.
The
other
evidence which
Hodge advances
in support
of the Comanchean
age
of the Eio
de
la
Plata and Barranquitas-Cayey series
is equally inconclusive and cannot
be claimed to provide cumulative proof of his contention.
The micro-
fauna listed from the formations is unquestionably Cretaceous, but not a
species in
it
is
restricted
.to the Lower Cretaceous.
The
small, poorly
preserved flora found in one of the members of the Barranquitas-Cayey
series was examined by
E. W. Berry and
F. H. Knowlton, who
pro-
nounced
it Mesozoic ; but they were unable to state that it indicates the
Comanchean
age
of
the
associated
rocks.
The
unconformity
which
Hodge postulates at the top of the Barranquitas-Cayey series, separating
it from the overlying Sierra de Cayey conglomerates,
is indicated
pri-
marily by the abrupt change
in
lithology; but changes
of
this
kind,
which occur in several parts
of the rock
section, can be more satisfac-
torily explained as the result of renewed outbursts of explosive volcanic ac-
tivity.
Certainly they do not indicate any fundamental change in local
conditions, or a profound erosional break; and no special one of the sev-
eral that occur can be assigned the degree of importance which the strati-
graphic
hiatus
between
the
Comanchean
and
the
Upper
Cretaceous
normally exhibits.
The Eio de la Plata and Barranquitas-Cayey series have not been
re-
ported
in the Ponce and Lares
districts, and
they have not yet been
differentiated in the Arecibo
district.
In
fact, none of the workers
in
the western half of the island has found any reason to suspect the exist-
ence of these or other rocks of possible Lower Cretaceous age.
Accord-
ing
to Hodge's
interpretation
of the
structure,
the monoclinal dip
of
the
strata exposes
progressively younger formations
to
the
south and
224
SCIENTIFIC SURVEY OF PORTO RICO
west, whereas the oldest formations extend northwestward into the San
Juan
district, where
they end near
its
western boundary
against
the
dioritic and porphyritie intrusives in the
vicinity of Morovis.
If
this
interpretation
be
correct,
the
failure
of
the
surveys
in
western
Porto
Rico to reveal evidence of Comanchean rocks
is explained.
On the other
hand, MitchelFs structural studies near the eastern limits of the Ponce
district show that
all the formations are repeated by folding, and that
the dip
is not monoclinal.
The results
of
his work imply
that
strata
of the supposed Comanchean series should be present in the region west
of the San Juan and Coamo-Guayama
districts, yet the
facts
collected
indicate that all the rocks are Upper Cretaceous.
To the north in the
San Juan district, where the strata of the Rio de
la Plata and Barran-
quitas-Cayey
series
cover
a
large
area, Semmes
has pronounced them
Upper Cretaceous and Eocene.
The concensus of opinion
is thus against
the existence of rocks older than Upper Cretaceous in any part of Porto
Rico; but, as this review has endeavored
to show, the evidence used to
support both sides of the question
is indecisive or circumstantial.
ISTew
data must be sought if a final conclusion is to be reached.
Interest
in
the
possibility
that
Mesozoic
formations
antedating
the
Upper Cretaceous may be present in the island has lately received added
impetus as a result of the discovery of two ammonite molds in the ashy
shales exposed at kilometer
86.5 on the San Juan-Ponce carretera be-
tween Aibonito and Coamo.
The fact that ammonites are
characteris-
tically associated with the Jurassic has naturally led to a query concern-
ing the age of the fossils found, and as ammonites are exceptionally good
stratigraphic guides, considerable importance must be attached to their
identification.
Every physical factor
is against the Jurassic age of the
specimens.
They were found near the
top
of
the Barranquitas-Cayey
series, which Hodge considers to be the highest member
of the Coman-
chean series.
The pyroclastic character of the sediments in which they
were embedded
invites
a contrast with the only known Jurassic
strata
in the West Indies; namely, the
Jurassic limestones
of western Cuba.
The
latter
lie
unconformably
beneath
Cretaceous
strata
and,
in
the
limited section exposed, indicate an origin completely dissociated from
contemporaneous
volcanic
activity.
Physical
characteristics
are
of
limited
value, however, for both structural and
lithologic relationships
may be expected to undergo radical changes in the thousand miles that
separate Porto Rico and Pinar del Rio province in western Cuba.
The
specific identity of the fossils will, therefore, constitute the only accepta-
ble testimony concerning the age of the beds
in which they are found.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
225
Dr.
IST. L. Britton has placed the specimens in the hands
of the senior
author, but unfortunately attempts
to identify them have not yet been
successful.
One consists of a flattened and unornamented external mold
;
the other
is a cast of the exterior of a
shell, in which some features of
surface ornamentation were preserved, while the diagnostic sutures were
completely destroyed
(Fig. 6).
It
is expected that a definite statement
regarding their biologic affiliations and stratigraphic significance can be
made in a later report on the Arecibo district, into which the ammonite-
Fig.
6.
Ammonites from the
''Older Series"
These two unidentified specimens, found
in the ashy shales at kilometer 86.5 on the
San Juan-Ponce carretera, are the only ammonites which have thus far been found in
Porto Rico.
Reproduced from drawings by Miss Helen
Z. Hunt.
bearing
strata
extend.
For
the
present they must remain another of
the potential but unevaluated clues to the age of the more ancient rocks
in Porto Eico.
The age of the youngest rocks in the "Older Series^'
is as much an un-
solved riddle as the age of the oldest.
In 1899 ^^ E. T. Hill suggested
that early Eocene strata occur, but his suggestion was based more upon
the
stratigraphic
possibility of
their
existence than upon tangible
evi-
dence.
Cleve, too, made a similar suggestion.
Of the investigators as-
sociated with the
Scientific Survey, only Semmes and Hodge have ad-
vanced testimony purporting to prove the presence of Eocene rocks
in
Porto Eico.
As
stated above, Semmes opinion was based upon an
in-
conclusive microfauna and upon insufficient structural data.
His Eocene
formations
lie
stratigraphically below
shales
which
he
placed
in
the
Upper
Cretaceous, and both his Upper
Cretaceous and Eocene
are
in
part equivalent to the Eio de
la Plata and Barranquitas-Cayey
series,
which Hodge refers to the Lower Cretaceous.
]^o support can, therefore,
be given to Semmes''s claim.
Hodge, on the other hand, furnishes the only substantial evidence avail-
able for the presence of early Tertiary rocks in the "Older
Series.^'
In
2« Hill,
R.
T.
:
Mineral
resources
of Porto
Rico.
U.
S.
Geological
Survey,
Annual
Report,
1899,
Vol. XX.
Pt.
6,
p.
771.
226
SCIENTIFIC SURVEY OF PORTO RIGO
his
^^Eio Jueyes
Series'^ he
reports the
occurrence
of one specimen
of
Venericardia alticosta/^ a typical Eocene form in the Gulf
Coast.
He
stresses the familiar fact that the stratigraphic position of guide fossils
may change in widely separated geographic localities^ but he believes that
the presence of the radiolarian form Poro discus concentricus, as well as
an undetermined species of the genus Ampliistegina, conclusively estab-
lishes the early Tertiary age of the Eio Jueyes
Series and the younger
formations in the Coamo-Guayama
district.
But Hodge
questions the
identification of Porodiscus
concentricus and has assigned an incorrect
index value to the genus Amphistegina, which ranges from the late Paleo-
zoic
to the present.
In
the
last
analysis,
therefore, the Eocene age
of
the upper half of the "Older Series^^ represented in the Coamo-Guayama
district, approximately 9000 feet
of material according to Hodge^s
esti-
mates,
depends
upon
the
specific
determination
of
one
specimen
of
Venericardia
alticosta, and upon
its
position
in
the
geologic
column.
The rocks assigned to the Eocene pass westward into the Ponce district,
where G.
J. Mitchell found specimens
of the
conclusively Upper
Cre-
taceous form, Radiolites, in limestone high in the
series.^^
The age of
this aberrant pelecypod
is so firmly fixed that its presence in the fauna
is
unquestionably
of
greater
significance
than
that
of
any
or
all
of
the
species
listed
by
Hodge;
but
again
a
final
conclusion
must
be
withheld because,
if
Mitchell's interpretation
of the
geologic
structure
of southern Porto Eico, rather than Hodge's, should prove correct,
it
is
still
possible
that
basal
Tertiary
strata
are
infolded
into
the Upper
Cretaceous
sediments.
In
the
stratigraphic
sequence
proposed
by
Maury^^ the assignment
of
several members
of the "Older
Series"
to
the Middle and Upper Eocene appears to be based solely on the conclu-
sions
reached by Hodge and
Semmes,
for no new
evidence
requiring
consideration has been advanced to support the sequence suggested.^*
Theoretical considerations also militate to some extent against the ac-
ceptance
of
Hodge's
conclusions
concerning
the
occurrence
of
rocks
younger than the Cretaceous.
His estimates give the "Older Series" of
the Coamo-Guayama
district a
total thickness
of not
less than
20,000
feet.
Of this amount he places approximately 9000 feet in the Eocene,
21 Hodge, E. T.
: op. cit, pp. 193-197.
22 Mitchell,
G.
J.
: The geology
of the Ponce
district, Porto
Rico.
New
Yorls: Acad-
emy
of
Sciences,
Scientific Survey of Porto mco and the Virgin
Islands,
1922,
Vol.
I,
Pt.
3, pp. 256-257. 288.
23 Maury,
C.
J.
: Porto Rican and Dominican stratigraphy.
Science, N.
S., 1929, Vol.
LXX,
p. 609.
^
.
24 See
Meyerhoff,
H.
A.:
The
pre-Oligocene
stratigraphy
of
Porto
Rico.
Science,
N.
S., 1930, Vol. LXXI, pp. 322-328.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
227
and probably more, for the top of the section
is not exposed within the
limits of his district; 5000
feet or more he assigns to the Comanehean,
and the remaining 6000
feet go, apparently by default, into the Upper
Cretaceous.
It
is
difficult
to
believe
that
less
than
one-third
of
the
section belongs in the system which has proved to be the most important
of the older systems in the Greater Antilles.
The assignment of 9000
feet of material
to
the Eocene
likewise
creates
a problem
difficult
of
solution ; for it must be assumed that folding, injection, and local meta-
morphism, as well as the erosion of several thousand feet of rock, took
place in the relatively short interval between late Eocene time and the
Middle
Oligocene, when
coastal-plain deposition began.
The
scattered
early Tertiary rocks
in other
parts
of
the Caribbean
region,
although
deformed, have not undergone the complicated history which this view
requires, whereas the Cretaceous rocks invariably have.
Although
it
is clear that the contradictory data available do not war-
rant a categorical dismissal of the case for the Eocene, the bulk of the
evidence
is against
its
existence
in
Porto
Eico.
The
possibility
that
Lower Cretaceous, and even Jurassic rocks, may be represented
is, on the
other hand, entitled to more
serious consideration.
Comanchean
sedi-
ments
attain
importance
around
the
entire
Caribbean
border,
and
Jurassic
strata have been
differentiated
in
Cuba.
Comanchean
rocks
have been described in Haiti, but
it
is admittedly on stratigraphic and
structural grounds that their differentiation
is based; paleontologic evi-
dence confirming their age has not been found. ^^
Until Mesozoic
dias-
trophism in the Antillean region
is better understood, and some reason
for
the
absence
of
the Lower
Cretaceous
is
known,
the
presence
of
materials belonging to this epoch may always be suspected and proof of
their existence sought.
It was confidently expected that the calcareous deposits of the Fajardo
district would furnish paleontological data that might aid
in a partial
solution of the age problem.
Two limestone members, repeated by fold-
ing,
outcrop prominently
in
the
western
quarter
of
the
district
and
along its southern margin.
Calcareous tuffs and shales are conspicuous
elsewhere,
especially surrounding
the
massive
pyroclastic and
igneous
rocks which underlie the hills of the interior.
C. A. Eeeds mentions the
two limestones near La Muda and Trujillo Alto among the fossiliferous
formations
of
the
"Older
Series/'
and,
although
the
senior
author's
25 Woodring, Brown,
and Burbank
:
Geology
of
the
Republic
of
Haiti.
Republic
of
Haiti, Department of Public Works, Port au Prince,
1924,
pp.
85-93.
228
SCIENTIFIC SURVEY OF PORTO RICO
search for megascopic forms in
all of their outcrops was unsuccessful,
Dr. N.
L. Britton has recently
sent two
sets
of specimens taken from
the quarry
at the type
locality
of
the La Muda
limestone, and
these
have been added to the Fajardo collection.
The material is recrystalHzed
and
is too fragmentary for specific identification, but
its
affiliations are
unmistakable.
All of
it belongs to a single rudistid species, probably of
the genus RadioUtes,
Its timely
discovery has
settled
the
Cretaceous
age
of
the La Muda
limestone more
satisfactorily
than
the
indirect
proofs provided by the formation's structural and stratigraphic relations.
The presence of rudistids disposes of the
possibility that Eocene rocks
are present in the Fajardo and the San Juan
districts, and,
as the La
Muda limestone forms one of the members of Hodge's Kio de la Plata
series, there is little likelihood that the ammonites in the overlying Bar-
ranquitas-Cayey series are Jurassic specimens.
ISTo other megascopic fossils have been found, but both the La Muda
and Trujillo Alto limestones and some of the other stratified rocks con-
tain a microfauna.
The La Muda limestone
is invariably the most fos-
siliferous horizon and contains the greatest A^ariety of forms.
Specimens
from the quarries near La Muda and Guaynabo and along the railroad
between Eio Piedras and
Trujillo Alto
are
full
of minute
fossils, but
most of them are poorly preserved.
Many are broken or show strong
signs of mechanical wear.
Study of the forms was perforce based upon
the cross-sections exposed in
slides, and as a result
little
satisfaction
is
felt with the attempts made to identify the specimens
specifically.
Of
the few forms concerning the identity of which the writers are certain,
all
are
long-lived
species which
possess no
diagnostic
value.
A
few
species
of
Foraminifera
have been
provisionally
identified,
but
either
they are the same as those reported by Semmes, Hodge, and other investi-
gators, or they are equally worthless in determining the precise age of the
sediments in which they were buried.
Eadiolaria proved to be numerous
in some localities, the only common species being that recorded by Hodge
as Porodiscus concentricus Ehrenberg
(?).
Diatoms are sparingly pres-
ent, and fragments of algae and other unclassified organisms were noted.
The
character
of
the
assemblage
is
such
that no
general
conclusions
concerning its stratigraphic significance are valuable enough to be given
serious consideration.
It
is important that searches
for megascopic
fossils be continued in
the Fajardo district.
Unless the structure of central and eastern Porto
Rico has been
seriously misinterpreted,
the
oldest rocks
of the
^^Older
MEYERHOFF, OEOLOGY OF THE FAJARDO DISTRICT
229
Series^' are exposed
in the northeastern part of the
island.
If Lower
Cretaceous rocks are to be found, the Fajardo district
is, therefore^ the
most promising area for study.
How much more can be done with the
microfauna as a means of determining the age of the rocks is problemat-
ical.
The specimens dissolve long before the somewhat silicified rocks in
which they are embedded disintegrate or decompose, and
it
is common
to find only the space they occupied left in a rock which to the eye shows
few
of
the marks
of
weathering.
It
is
possible
that
Eadiolaria
and
diatoms may be washed from the deeply weathered calcareous tuffs, but
it
is doubtful whether Foraminifera can be
obtained by
this method.
Their identification in thin-section
is
difficult;
yet, in view of the vast
progress made in the
field of micropaleontology within the past decade,
it seems reasonable to believe that exact classification and correlation of
the Porto Kican Mesozoic deposits can ultimately be made on the basis
of the microscopic organic remains,
if megascopic fossils
fail.
With the
latter much more can be done than has been done ; but the sedimentary
formations must be combed
for
better specimens and
for more
repre-
sentative
faunal
groups.
Thus
far
fossil
collecting
has
been
unsys-
tematic;
identification and
interpretation
of
the
specimens have
been
undertaken without adequate information about
the
stratigraphic
suc-
cession, on
the
general assumption
that
the
structure
is
too complex
and the lithologic units too variable for the determination of a definite
order
of
superposition.
If
the
present
report
can make but
a
small
contribution that will aid in fixing the age of the ^^Older Series,^^ perhaps
it can make a start toward the solution of the structural and stratigraphic
problems
in the
central and eastern
parts
of
the
island.
The
results
offered in the following pages
are incomplete and imperfect; but
suffi-
cient data
are
at hand
to
correct some
of
the assumptions that have
hampered part of the work done in this half of Porto Rico; to indicate
the general character of the structure
as well
as some of the
details of
its expression
; and to sketch in a portion of the stratigraphic succession.
The structure will be examined
first.
THE STRUCTURE OF THE CRETACEOUS
The
problem
of
interpreting
the
geologic
structure
of
the
"Older
Series'^
is inherently
difficult of solution because
of the
initial irregu-
larities in the accumulation
of the materials, and because
of the intru-
sion
and
erogenic
deformation
they
have
suffered
since.
From
the
nature and distribution of the rocks
it must be inferred that accumula-
230
SCIENTIFIC SURVEY OF PORTO RICO
tion took place with reference
to a
series
of
volcanic
vents,
a few
of
which lay within the limits of the present island.
Berkey described one
vent
at
the
site
of the
Jacaguas,
or
Guayabal,
reservoir
in
the
south
central part of the island, and he noted a few others ;^^ but in the several
district
surveys
little
significance
seems
to
have
been
attached
to
this feature, either as a factor modifying the present structure, or as an
influence
in
the
initial
distribution
of the
associated fragmental and
sedimentary rocks.
The pyroclastic character of nearly all the Cretaceous
system
in
Porto
Kico
reveals
the
dominant
role
played
by
explosive
volcanic activity during the entire period of
its formation.
The
alter-
nation of massive and
stratified rocks adds
a
picture
of
intermittent
subsidence, aifecting a low platform above which the scattered volcanoes
or volcanic clusters
rose.
Violent eruption built
its surface above
sea-
level; in the intervening cycles of quiescence gradual depression carried
it slightly below.
N"ow and then new vents were opened through the
older rocks; and cones, both large and small, were heaped up on a sur-
face of simple sediments.
Aggradation was rapid, and
so
little oppor-
tunity was given for subaerial erosion of the forms produced that normal
hydroclastic sediments are extremely scarce.
The reconstruction of an orderly sequence from complex elements of
this nature
is
difficult enough
in
a
region where
the
rocks have
re-
mained undisturbed, but in Porto Rico eruptive
activity was succeeded
by
intense
folding and
plutonic
intrusion.
To
the
diverse
primary
structures of the surface-materials were added involved secondary struc-
tures, and the irregular distribution which characterized the rocks at the
time they were formed was
further
obscured by
the
dioritic magmas
which invaded the section.
As a result
it
is quite impossible to ascribe
proportionate values
to the
processes which
created
the
irregular
pri-
mary
structures,
the igneous
activity which
later modified them, and
the diastrophic movements that folded, faulted, and warped them.
The
task
is made
still more hopeless by the massive
character
of many
of
the explosive products, in which structural trends cannot be deciphered
;
by
the
deep
weathering
which
has
locally
obliterated
the
structure,
reduced the number of outcrops, and given diverse types of rocks much
the same appearance; and by the inaccessibility of many
critical
areas.
Yet some understanding of the diastrophic structures must be acquired
before any of the earlier history can be restored.
20 Berkey,
C.
P.
:
Geological
reconnoissance
of Poito
Rico,
op.
cit.,
p.
37
;
Introduc-
tion to the geology of Porto Rico, op.
cit.,
p. 20.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
231
Practically
every
criterion upon which
the
geologist
ordinarily
ex-
pects
to
rely
in unraveling the structure
of
a region has
failed when
applied
to the Cretaceous system
of Porto Kico.
In most sedimentary
areas
a
carefully plotted structure
section normal
to
the
strike
of the
strata
will
reveal the major
features
of deformation; and
several
sec-
tions^
if not too widely spaced^ will furnish a basis for sound generali-
zation.
Yet, it may be questioned whether the painstaking cross-sections
which Berkey, Semmes,
Mitchell, and Hubbard have published can be
pieced together to form a clear and adequate conception of the island's
geologic structure.
The sections which they have drawn contain no com-
mon stratigraphic units which can be used in interdistrict correlation;
in fact, correlation of folds and rocks within the separate
districts has
seldom proved
possible.
Persistent and
characteristic key horizons by
which the structures may be traced have not been isolated.
Prominent
limestones peter out rapidly along the strike or undergo radical lateral
changes;
the innumerable
stratified and massive
pyroclastic
types
are
so nearly alike and individually exhibit such lithologic inconstancy, that
their
use
in
correlation
is
precluded.
It
is
not,
therefore,
the mere
scarcity
or
inadequacy
of
structural
information
which makes
inter-
pretation
difficult,
but
the
dearth
of
firmly
established
stratigraphic
data as
well.
To some extent these two interdependent problems must
be considered together in the present section.
A
definitive treatment
of
the
structure
of
the
Cretaceous
strata
in
Porto Eico
is obviously out of the question at this time, but a general
statement
of
its outstanding
characteristics may be
formulated.
The
"Older
Series'^ has been folded along an
axis which has
a
prevailing
northwesterly
trend.
Whether
the
folding
is
to
be
referred
to
one
orogeny or
to two
is not yet known,
for some
of
its
features may be
attributed either to torsional compression in a single period of deforma-
tion,
or
to
compression from
different
directions
during two
distinct
periods.
Hubbard
calls attention to the presence
of broad north-south
flexures, which seem to be superimposed upon the dominant system of
folds in the northwestern part of the island.^^
The feature he describes
is equally pronounced
in the neighboring Arecibo
district and
is
also
expressed in the crenate character of the folds along the northern mar-
gin of Cretaceous outcrops in the Fajardo
district
(see geologic map).
Hubbard
is inclined to regard these secondary flexures as evidence of a
separate disturbance of moderate intensity, and the fact that post-Greta-
s' Hubbard, Bela
: The geology of the Lares district, Porto Rico.
New York Academy
of Sciences, Scientific Survey of Porto Rico and the Virgin Islands, 1923, Vol.
II,
p.
32.
232
SCIENTIFIC SURVEY OF PORTO RICO
ceous and post-Eocene epochs of folding have been differentiated in other
islands of the West Indies entitles his opinion to careful study.
Until
the age and subdivisions of the "Older
Series'^ are established^ and the
details of the structural features are known^ there is little chance that a
problem
as broad
as
this can be
satisfactorily
solved.
Attention may
more appropriately be focused on the effects of deformation, rather than
on its causes.
A first impression of the structure,
if obtained in one of the areas of
thinly
stratified
rocks, may
give
the observer an
exaggerated
idea
of
intricacy;
for in many such
localities the folds
are
small, compressed,
and
sometimes
distorted.
Many
of them
plunge
rapidly;
some
are
overturned;
a few pass
into minor
thrust
faults.
A more
deliberate
analysis, however,
reveals
that
these
small
structures
are
merely
sub-
ordinate parts of much larger folds, which, though less obvious because
of
their
great
size,
constitute
the
controlling
structural
elements.
Semmes ^^
and Hubbard ^^
have
both
described
the
anticlinorial
and
synclinorial character
of the dominant folds and have shown that the
smaller but more
arresting
features
of deformation
are
ordinarily
re-
stricted to the comparatively incompetent shaly sediments.
In the un-
stratified
pyroclastic
rocks
the
structures
are
practically
undetermi-
nable.
These materials appear to have been relatively competent under
pressure, and fracturing
is a more common feature than folding.
Dikes
fill many of the fractures, and their abundance in the massive rocks of
the Fajardo district contrasts noticeably with their scarcity in the areas
of thinly
stratified sediments.
Faults are more often indicated than seen.
Crush zones and slicken-
sides, abrupt changes of strike,
off-sets in topographic forms have been
noted in nearly every part of the island.
Although the amount of dif-
ferential movement along some of the fractures has been thought
to be
large, the
faults which can be
studied invariably show
small
displace-
ments.
This fact suggests that displacement in the aggregate was per-
haps moderate,
or even great, but
that
the movements
associated
with
individual
fractures were
small.
Both
thrust and normal
faults have
been identified, and
all appear to have developed during the diastrophic
and plutonic disturbances which preceded Oligocene time.
In general,
the thrust faults are simply related to asymmetric and overturned folds
;
the normal faults, on the other hand, exhibit no systematic relation to
28 Semmes, D.
R.
: op.
cit.,
p. 98.
2» Hubbard, Bela
:
op.
cit.,
p.
31.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
233
the orogenic structures^ and thus far have not been resolved into coherent
systems.
It
is
likely that many of the normal
faults can be
classified
among the displacements that ordinarily accompany and follow the
in-
trusion and solidification of subjacent magmatic bodies.
The geologic structure
of
the Fajardo
district
illustrates
all
of
the
features which
characterize
the
Cretaceous
rocks
in
the
rest
of Porto
Hi CO.
It has been studied in some
detail because
of
its importance
in
determining
the
stratigraphic
sequence, and
the
results
are
presented
in full in the hope that they may serve as a basis for a definitive treat-
ment of the structure of the entire island.
A number of the conclusions
reached
are
provisional.
The
scarcity
of good
outcrops and
the
inac-
cessibility
of
critical
areas,
especially
in the Luquillo Mountains, have
left
a
few
alternative
hypotheses
for
local
problems
on
the
authors^
hands; but in most
cases where doubt
exists concerning the true char-
acter
of
the
structure,
it has proved
possible
to
limit
the
alternatives
quite
narrowly.
In
extending
outcrops
across
some
sections
of
the
geologic map, however,
it has been necessary
to make
definite assump-
tions with regard to the underlying structure and to ignore other possi-
bilities.
Later field studies may make some revision of the map neces-
sary,
therefore,
and,
as
far
as
possible,
the
areas and problems which
need more thorough investigation will be designated.
The structure of the western border
According to Semmes and Hodge, a cross-section through Porto Rico
from San Juan
to
Santa
Isabel
would
reveal
a huge
single
arch,
or
anticlinorium,
on
the
flanks
of which
the
stratified
formations
have
undergone folding on a smaller and more complicated
scale.
The
axis
of the arch and the eroded edges of the sediments upon
its sides would
be found
to
strike
east-southeast,
passing
diagonally
into
the Fajardo
and Humacao districts.
If their interpretation be
correct, the Fajardo
district should
lie upon the northern limb
of the arch, and, except
for
local
reversals,
northerly
dips
should
prevail.
Semmes
believed
that
the structure
is further complicated by an unconformity, which
is indi-
cated by the abnormal strike
of the Aguas Buenas limestones and the
associated beds, at right angles to the predominant northwesterly struc-
tural
trend
in
the
San Juan
district.^^
His
interpretation
received
further
support from
the
work
of Hodge, who
included
these
strata
in the Rio de la Plata
series, which he believed
lies unconformably be-
»« Semmes,
D.
R.
:
op.
cit.,
pp.
64-65,
05-96
;
cf.
Berkey,
C.
P.
:
Introduction
to
the
j^eology
of Porto
Rico,
op.
cit.,
p.
17.
234
SCIENTIFIC SURVEY OF PORTO RICO
neath the Upper Cretaceous sediments of the Coamo-Guayama
district.
Semmes suggested that the unconformable contact between the older and
younger strata might be exposed in the valley of Eio Grande de Loiza,
whither the Aguas Buenas formations appeared to extend by projection.
These opinions and suggestions were utilized as a starting point in the
present study, and
in an
effort to
verify them four full-length north-
south
sections and one
partial
section were
plotted
across
the
district
between the western border and the Carolina-Gurabo camino.
Eeliable
stratigraphic markers,
or key horizons, were needed
to
fol-
low the changes in dip and strike ; and for this purpose the several lime-
stones reported by Berkey and Semmes promised the
greatest help,
in
spite of the fact that the different formational names given to each of
the limestone
exposures implied
that none
of the beds
is
repeated by
folding.
The
massive
pyroclastics
could
not
be
differentiated;
and
the ashy shales prominent in the vicinity of Eio Piedras proved too much
like those near Bayamon and the Guaynabo filtering station
to be used
as a dependable stratigraphic guide.
In this region the following lime-
stones had been differentiated and named:
(1)
the Trujillo Alto lime-
stone, a dense, bluish, microfossiliferous formation, which outcrops along
the carretera about one and one-half miles north of Trujillo Alto;
(2)
the La Muda limestone, concerning the identity of which there
is some
doubt and confusion ;^^
(3)
the Aguas Buenas
limestones, comprising
two calcareous beds that form conspicuous exposures southwest of Aguas
Buenas, and which Semmes thought rest unconformably below the other
two formations.
31 In the vicinity
of La Muda there are two limestone formations
; a grayish white,
cavernous
limestone outcropping about one
mile north
of
the
village, and the
bluish,
tuffaceous Trujillo Alto formation, which outcrops on the carretera one mile southeast
of the settlement.
Berljey's statements regarding the character and location of the La
Muda are not
sufficiently
precise
to malie
its
identity
certain.
The adjective
"cavern-
ous," which he uses in describing
it, can only apply to the formation north of the
vil-
lage
; but in this event his statement that
It
lies above a conglomerate bed
is in error.
The conglomerate rests upon
it.
The
Trujillo Alto limestone, on the other hand,
lies
above the conglomerate, but 2000-2500
feet
of
stratified
calcareous
tufCs
separate the
two.
It is safer to assume that the descriptive adjective which he used is more decisive
than an
interpretation
concerning
its
stratigraphic
relation
to an adjacent
formation,
for Berkey clearly had in mind a sedimentary rock which differs in physical character-
istics from the Trujillo Alto.
The limestone at the southern edge of Guaynabo, which
Semmes called the "La Muda,"
is the same as that north of La Muda, but in
project-
ing
it
southeastward
Semmes
extended
it
into
the
Trujillo
Alto
limestone
south
of the
village,
and,
like Berkey, seems
to have been unaware
of
the
presence
of two
distinct
beds.
The
confusion which
exists may
be
settled by
fixing
the
formational
name upon the white or light gray, cavernous strata one mile north
of La Muda and
at
the
southern
outskirts
of Guaynabo.
This
usage
of
the name
will
be
employed
throughout the present report.
MBYEEHOFF, GEOLOGY OF THE FAJARDO DI8TMICT
235
The field analysis of
ilie structure in the western, part of the Fajarclo
district and in the eastern part of the San
Jiiaii district established the
persistence of the La Muda and Triijillo Alto limestones, as well as their
repetition in the section in consequence of folding, by developing the fol-
lowing
facts:
(1) In the Trnjillo Alto region there are two distinct limestones.
The
slate-blue, tnffaccous beds to whicli Berkey gave
tlie name
are exposed
FlQ.
1
A
qnarrtetl
oaterop
along
the
spite of the development
of large
tiirul ridge for seTeral miles.
-The La Muda
limestone
iilJroad between
Rio
Pledras and
Trujillo
Alto.
In
mverns
witliiii
it,
It forms a
relatively strong
stnic^
along the carretera one and one-half miles north oC the settlement.
The
second
is buried beneath the alluvium deposited by Kio
(iJrande dc Loiza
at the nortliern edge of Trujillo Alto; but, less than a mile west-nortliwest
of the town,
its outcrops rise above the valley floor and continue west-
ward
for three miles, acquiring speetaeidar prominence along the tracks
of the Porto liico Eailway, Light, and Power Company
(Pig.
1),
The
formation
is white or
ligiit gray in
color, and
its massive beds
are ex-
ceediiiglv cavernous.
Jt
irieasiires approximately BOO
feet
in
thickness.
The two limestones
are
parallel and
strike
almost
due
east-west,
but
wcsim-ard, near the Bio Piedras-Caguas
carretera,
tlie
strike swings
to
the northwest.
The
dip
of
both
formations
is
30
degrees
north
and
clearly
establislies
their
relative
stratigraphical
positions:
the
bluisli
limestcme occupies
a
position above the gray formation, from which
it
236
SCIENTIFIC SURVEY OF PORTO RICO
is separated by 2000-2500 feet of calcareous tuffs and by a bed of agglom-
erate^
or
tuffaceous
conglomerate^ which
rests
directly upon
the
gray
limestone.
(2)
In the neighborhood of La Muda two limestones are also present.
The more northerly
is a massive^ white or light gray, cavernous forma-
tion^ which can be traced northwestward into the San Juan district for
more than three miles, until it is lost in the deep lateritic soils southeast
of Bayamon.
The more southerly
is a dense, bluish limestone, which
is
thinner and somewhat tuffaceous.
Both beds strike almost due northwest
and dip about 50 degrees southwest.
The strata separating them consist
essentially of calcareous tuffs, but immediately above the lower formation is
a ridge-making layer
of agglomerate
or tuffaceous conglomerate.
The
outcrops of this entire rock series are somewhat discontinuous, for they
are cut by small, irregular porphyritic intrusions and
closely associated
breccias.
(3)
The Aguas Buenas limestones consist likewise of an older forma-
tion of white to light gray limestone, in which some of the most spec-
tacular caves of the interior have been dissolved, and of a younger slate-
blue horizon, from which the former is separated by calcareous tuffs and a
basal agglomerate, or tuffaceous conglomerate.
The
strike
of
all these
beds south of Aguas Buenas is northeast, and the dip is 20 degrees or more
northwest.
Unsafe as correlation on the basis of physical characteristics may be, the
repetition of four distinctive formations in the same order of superposition
leaves little question concerning the exact equivalence of the white lime-
stone, the conglomerate, the stratified calcareous tuffs, and the blue tuffa-
ceous limestones near Trujillo Alto, La Muda, and Aguas Buenas.
And
the opposing dips of the beds in these three localities indicate with equal
clarity the simple anticlinal and synclinal nature of the structure
(Fig.
8)
.
In the sections at La Muda and Aguas Buenas, moreover, the sequence
can be extended upward by including an unusual member
of
strongly
banded material which lies above the blue limestone.
Semmes describes
this rock as a chert, in which crumpling has been caused by solifluction
and has been accentuated by the subsequent infiltration of hematite.^^
A
microscopic section of the rock was referred by the writers to K. J. Colony,
who has identified the chert- or quartz-like mineral as laumontite, and
who suggests that the formation was derived from a calcareous tuff by
alteration.
The rock occupies an analogous position on both limbs of the
32 Semmes, D.
R.
:
op.
cit.,
pp.
65-66.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
237
pitching syncline which
lies between La Muda
and Aguas Buenas, but
if
it
is present
in the
Trujillo Alto
section^
it was
overlooked.
No
effort was made
to
locate
it,
however,
for
its
significance
as
a
stratigraphic
guide was
not
appreciated when the
field work was
in prog-
ress.
Even
if
it prove to be missing, the white
cavernous
limestone,
the agglomerate,
the
cal-
careous
tuffs, and the bluish limestone
consti-
tute a
sufficiently
distinctive
lithologic
succes-
sion to force some revision of the prevailing idea
that
the
stratified
materials
are
lithologically
too inconstant to be employed as horizon mark-
ers, and
to
establish the simply folded charac-
ter of the structure across the district.
The
behavior
of
the
folds
is
interesting.
From north to south along the western edge of
the Fajardo district the structures comprise an
anticline,
a
syncline,
and
an
anticline.
The
crest of the last lies near the boundary between
the
San Juan and Coamo-Guayama
districts,
and
its
axis
strikes
slightly
north
of
east,
whereas
that
of
the northern
anticline
strikes
southeast.
The two meet a short distance east
of
the
valley
of
Eio
Grande
de
Loiza,
about
three miles south of Trujillo Alto, where they
merge
into
a
single
dome-like
structure
that
can be
traced
eastward
for more than twenty
miles to Cape San Juan.
Between the two con-
verging arches lies a sharply plunging syncline.
If traced westward into the San Juan
district,
the
trough
is
found
to
flatten,
and
its
axis
swings from a westerly to a northwesterly direc-
tion,
carrying
the
fold
diagonally
across
the
district
toward
Toa
Alta.
On
the
southern
limb the calcareous strata strike southwestward
from the caves near Aguas Buenas and appear
to bend sharply to the northwest a mile or two
west
of
Cidra.
The
two
limestone
members
have not been traced beyond Cidra, but one of
them
is reported
to continue
at least as far as
aq
•;:
^
-2
^.
,s ^
c
I
ej
g
4*
"^^
(y
O
S
I. ^^$o
238
SCIENTIFIC 8URVEY OF PORTO RICO
Comerio.
The syncline thus exhibits a rectangular ground plan, which
is sufficiently unique to indicate the need of more careful field study in
the elbow extending from the Aguas Buenas caves to Cidra and thence
to
Comerio.
The area
lies
outside the
limits
of the Fajardo
district,
and the senior author was able to spend but two inconclusive days de-
ciphering the structure.
It appeared to be more complex in
its details
than the general statements made by Semmes, Hodge, and the present
writers indicate, but the conclusions which have
just been stated were
fully supported by the data obtained.
Efforts to trace the lithologic succession
of the western border
east-
ward from Trujillo Alto were attended by difficulties.
The domed struc-
ture in the hill section has caused the removal of the younger stratified
series from all parts of the interior, and although the sediments continue
along the northern fiank of the anticline, the members undergo a decided
change toward the
east.
Within
a short distance
of Trujillo Alto the
white cavernous limestone ends by overlap against the underlying massive
pyroclastic rocks.
The bluish limestone outcrops on the carretera about
one mile south of Canovanas, but
it
is more thinly
stratified and shaly.
Somewhat
similar beds
are found
in
the
valley
of Eio Sabana
about
one and one-third miles southeast of Luquillo
(Fig. 9), but their corre-
lation with the bluish limestone can only be made
tentatively, because
deep weathering and a large intrusive in the intervening area made it im-
possible to prove the continuity of the horizon in the field.
It has not been
found east or south of the Eio Sabana locality.
The calcareous tuffs, on
the other hand, persist along the entire length of the structure, but with a
noticeable reduction
in the calcium carbonate
content.
The nature
of
these changes suggests the existence of a volcanic center near the central
and eastern parts of the
district, and this hypothesis
is strengthened by
the profusion of porphyritic intrusions and andesitic flows in many sec-
tions of the interior, culminating in the almost continuous outcrops
of
igneous
rocks north
of Juncos and eastward along the borders
of the
Caguas-Kaguabo lowland.
ISTear the south-central boundary of the Fajardo district both limestone
members reappear, but their structure almost defies interpretation because
of the complicated series
of andesitic and dioritic intrusions associated
with them.
Fettke has described them under the name
''Collores lime-
stone",^^ but he appears
to have misinterpreted
their
relationship and
33 Fettke,
C.
R.
: The geology of the Humacao
district, Porto
Rico.
New
Yorli Acad-
emy of Sciences,
Scientific Survey
of Porto Rico and the Virgin
Islands, 1924,
Vol.
II,
pp. 149-151.
Magnetite deposits of eastern Porto Rico.
Amer. Insti. Min. and Met. Eng.,
Trans., 1924, Vol. LXX, pp. 1024-1042.
MBYEltnOFF, GMOLOGY OF THE FAJARDO DI8TMIGT
339
lias in coiisequciiee
failed
to
{liU'ereiitiate between
tliem.
The two
cal-
careous formations outcrop in association with the magnetite deposits east
of Juncos.
Notwit1istaiidin<( the
intense
contact nietamorphism which
the beds have
HiiffertKl
h3eally.
botli have
i-etalned
their
identity.
The
lower bed is an
iiiifossilifcroiis grayisfi white limestone composed
of me-
240
SCIENTIFIC SURVEY OF PORTO RICO
dium to coarse, interlocking calcite crystals.
Fettke lias traced it along its
southeasterly strike for a distance of six miles, throughout which it main-
tains an estimated thickness of 250 feet and a persistent dip of 30 to 40
degrees to the northeast.
The second bed
is exposed a
little
less than a
mile northeast of the first, approximately one mile south of the village of
Torres.
Circumscribed by the alluvium
of the Caguas lowland on the
northwest and of the Kio Anton Euiz valley on the southeast, and ob-
scured by the porphyritic intrusives on the southwest and northeast,
it
has been traced but
a
short
distance.
Its
strike
parallels
the grayish
white
limestone,
hut
its
dip
has
not
been
satisfactorily
determined.
Fettke has given
it as 52 degrees southeast, but he measured the angle,
not on the limestone, but on an associated ledge of andesite, the relation
of which to the calcareous bed
is not clear.
On the basis of this uncer-
tain determination Fettke postulated a synclinal structure between the
two limestones and concluded that they are one and the same formation.
This conclusion disregards the fact that the two beds are lithologically
different.
The formation near Torres is dark gray in color and
is much
finer and less even in texture than the light gray limestone. The latter is
comparatively pure ; the former
is tuffaceous, and the microscope reveals
fragments of plagioclase, quartz, and ferromagnesian minerals within
it.
Eecrystalization and the destruction of micropaleontologic remains render
direct comparison of the dark gray limestone with the Trujillo Alto in
other parts of the
district of little value; but both the megascopic and
microscopic features
differentiate
it
at once from the
light gray
lime-
stone.
If both beds are assumed to possess the northeasterly dip observed
in the light gray bed, their relative stratigraphic positions are exactly the
same as those of the La Muda and Trujillo Alto limestones in
all other
localities where the two are present in the section.
In view of the facts
that the two formations are lithologically different, and that
it is possi-
ble Fettke's dip measurement was made on the contact plane of an in-
trusive body, the authors believe that the two formations are distinct, and
that they must be correlated with the La Muda and Trujillo Alto lime-
stones.
More careful study of these two beds in the Juncos-Torres section
is necessary, however, before a final conclusion
is justified.
And, should
the correlation postulated prove correct, field evidence should be sought to
verify the synclinal structure which must
exist between these outcrops
and the upland to the north.
Whether the La Muda and Trujillo Alto limestones extend
into the
central and western parts of Porto Eico,
it
is impossible to say until a
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
241
careful restudy
is undertaken.
They appear
to
be important
horizon
markers, but an effort to correlate them with the limestones described in
the districts to the west must be based on many more field observations
than are available at the present time.
It is hoped that some contributions
may be made to this question when the Cretaceous of the Arecibo district
is differentiated.
Structural problems of the Caguas Lowland and Luquillo Mountains
In the central and southern parts of the Fajardo district the geologi-
cal structure
is not easily deciphered.
In the interior the rocks consist
of porphyries and
andesites, and of massive
tuffs and agglomerates,
in
which the only observable structures
are
joints and
intrusive
contacts.
These features have not been resolved into systems that can be used to
interpret the broader structural relations; but the northward dip of the
calcareous tuffs and limestones near the northern limits of the Cretace-
ous outcrops, the apparent
repetition
of these
strata
in the magnetite
belt east of Juncos, and the low southerly dip of the rocks near Guzman
Abajo suggest that the dominant structure
is
anticlinal.
The conver-
gence of the two lower anticlines from the west appears to have given
the arch great height and breadth near the point
of junction,
east
of
which the structure forks and declines in elevation; one broad, pitching
anticline stretches toward Cape San Juan, and the other, a much nar-
rower
fold, extends eastward between
the
central
range and
southern
foothills of the Luquillo Mountains to the east coast.
The evidence of
bifurcation in the structure
is not entirely conclusive, and the field data
on which the interpretation is based require further exposition.
Eather
closely linked with the problem
is the structural character
of
the Lu-
quillo Mountains and of the Caguas-ISTaguabo lowland; and the
discus-
sion that follows will center largely on the origin of these two arrestinix
topographic forms.
The Luquillo Mountains,
in
spite
of the clustered and unsystematic
arrangement
of the prominent peaks, may
be
resolved
into
three
ele-
ments, two of which exhibit definite east-west elongation:
(1)
a central
range, with El Toro marking its western end, and East Peak its eastern
extremity;
(2)
the southern foothills, which are rather irregularly
dis-
tributed
yet roughly
parallel
the
eastern
portion
of
the main
range
;
and
(3)
the El Yunque
outlier, an isolated eminence which stands out
appreciably beyond the northern flank of the central ridge, but
is tied
to
it by a narrow divide that sags nearly 1000
feet below the adjacent
242
SCIENTIFIC SURVEY OF PORTO RICO
mountain summits
(Fig.
1).
The trend
of the
central range and the
southern foothills, as well as that of some of the ravines and valleys asso-
ciated with them,
coincides
closely with the
strikes
of the underlying
rocks,
so
far as could be determined from
scarce and widely
scattered
structural observations.
The raison d'etre of the mountains, which tower 1500
feet and more
above the
fringing upland remnants, and 3000
feet above
the nearby
lowlands
and
coastal
areas,
is
not
immediately
apparent.
They
are
located near
the
northeastern
corner
of
the
island where
rainfall
is
abundant, and where, in the course
of
late
Cenozoic erosional
history,
reduction with respect to the Atlantic base-level should ordinarily have
proceeded rapidly.
During the Tertiary they were situated between the
two major river systems of the region, which comprised Eio Grande de
Loiza and
its
tributaries, and the stream system draining the lowland
that now
forms
Vieques
Sound
and
Fajardo
Roads
(Fig.
5)
;
but
their seeming remoteness from these main drainage lines was more than
offset by
their proximity
to
the
Atlantic.
There
is,
moreover,
mucli
more reason to believe that the Luquillo Mountains served as a partial
cause of the location of the Tertiary drainage, than that they constitute a
result.
The fact that the headwaters of both the Eio Grande de Loiza
and
the
Vieques
Sound
systems
surround
and
isolate
the
mountain
area
is a convincing indication that the latter has persisted in spite of
the growth of extensive river systems on every side, and in spite of its
nearness
to the
sea.
Some special cause must be sought, therefore,
to
explain its prominence.
Lobeck dismissed
the hypothesis
that comparatively
recent warping
may account for the abnormal elevation and position
of the mountain
mass, on convincing physiographic evidence;^* but
it
is not
clear that
he gave due consideration to the possibility of recent faulting and
tilt-
ing.
Although
his arguments
against warping apply
equally
well
to
such a theory,
so strong a
case in
its favor can be constructed on the
basis of the topographic features that it
is at least entitled to discussion.
Superficially
the upland
comprising
the
western
hill
section
and
the
Luquillo
Mountains
possesses many
of
the
characteristics
of
a
tilted
fault block.
Except for local conformance to the underlying structure,
the
streams
that
drain
it
flow northward
as
if
consequent upon
the
back-slope of a tilted block; and the upland increases in elevation from
north
to south ending
at the high,
linear
east-west escarpment which
84 Lobeck, A. K.
:
op.
cit., pp. 311-315.
MEYERMOFF, GEOLOaY OF THE FAJAEDO B18TBICT
343
overlooks the Caguas-Kagiiabo lowland
(Fig.
10).
This escarpmoot hm
e\x»rj a.speet ol a fault searp.
Altliough for
!«ojiie miles east of Guarabo
it eoiiicides witli the trend of the structural and topograpliie elements in
the
higiilaiid,
elsewhere
it
truiieates the upland topography
at
a low
angle, in the manner of a normal fault scarp.
It
is oversteepened and
Fm.
10.
The northern wall
of the Caguas lowland
Over 2000
feet high wear Joncos,
this scarp
perisists mith
scareelj' a break from
tlie
western part
of the
rllstiict
to
Ktt)s:uabo.
The
alhiYiiil
fans which
luive been
built up
t Its base bury the floor of
tlie basin, but here and there rock
hills rise abO¥e them
like
3'ouug, and at
its base coalescing alluvial fans spread far into the low-
land, overlapping the floodplains of the large streams which parallel
it
(Fig.
10).
The scarp
persists
in
spite of
radical changes
in
the
I'ock
types that compose
it.
Tuffs, andcsites and andesite porphyries, grani-
toid
iiitrusiveSj
asliy and
calcareous
shales
are found
along
successive
portions of its imposing slopes: jet, iiotwith,standing local modifications
which
reflect
the
lithologic
changes,
it
continues
almost
unbroken.
Thoughts
of the Quaternary and
Late Tertiary
rift-faulting which has
furrowed the Antillean region, and which has fashioned
several
of
the
lowhiiids within and marginal to Haiti and Santo Domingo, involxc the
idea of comparati'voly recent faulting here.
Certainly the youth of the
escarpment and the
recency of
alluviation
at
its
l)ase
give
substantial
support to the view.
244
SCIENTIFIC SURVEY OF PORTO RICO
South of the Caguas lowland the Sierra de Cayey upland rises in some-
what
the same way
as
the
Luquillo and western
hill
sections
of
the
Fajardo district (Fig. 3).
In a distance of eight or nine miles elevations
increase from 300 to 3000 feet, and high altitudes continue as far as the
southwestern
coast.
Between Yabucao and
Patillas,
cliffs
rise 1200
to
1600 feet above the waters of the Caribbean.
Shore forms that charac-
terize other sections of the Porto Kican coastline are missing, and their
absence
indicates
that
this
part
of
the
coast was formed by
faulting
late in Quaternary time.^^
The inference that the escarpment north of
the Caguas lowland
is
of similar origin and of similar date
is
all but
irresistible.
This explanation of the Luquillo Mountains and the Caguas lowland,
how^ever, must disregard several features which point to a much longer
and more involved history.
As Lobeck has shown/^ remnants
of
ero-
sional
surfaces
surround
the
higher
portions
of
the mountain
range,
against which they end with topographic discordance.
Correlative
ero-
sional forms
fringe
the
Sierra
de Cayey upland
at
corresponding
or
slightly higher elevations
; but in both uplands the low gradients
of the
erosional terraces
are unrelated
to the more rapid
rise
in elevation
in
the associated highland areas.
The
rise
is not uniform but
is made in
three
distinct
steps, each
of the three
erosional
levels present forming
one of the treads, while comparatively steep scarps form the
risers.
If
the Luquillo and
Cayey uplands
are
tilted
blocks,
tilting must
have
occurred so long ago that the blocks have undergone the same erosional
history as the rest of the island.
In this case their elevation and relief
are to be explained, not as the outcome
of diastrophic processes, but
as
the result of their inherent resistance to the processes of denudation.
Equally
serious
objections
to the hypothesis
of
recent
block-faultiu'^-
develop when some
of
the
drainage
features
are
studied.
If
eastern
Porto
Eico
consists
of two
fault
blocks
tilted
toward
the
north,
the
drainage from the
southerly
block would
follow
the linear
depression
along the fault plane between them, especially as the latter provides an
unobstructed route eastward to Vieques Passage.
The Caguas-IsTaguabo
lowland, however,
is not occupied by an east-flowing mainstream.
On
the contrary, the greater part of the lowland drainage
is tributary
to
Kio Grande de Loiza, which cuts through the western
hill
section,
fol-
35 MeyerhofF, H.
A.
: Tertiary physiographic development
of Porto
Rico and
the
Vir-
gin Islands.
Bulletin, Geological Society of America, 1927,
Vol. XXXVIII,
pp. 572-575.
8« Lobeck, A. K.
: op.
cit.,
p. 314.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
245
lowing a long^ tortuous course to the Atlantic.
Scattered wind-gaps
in
the escarpment forming the northern boundary of the lowland show that
other streams^ which have since been captured by Rio Grande de Loiza,
initially had a simifer relationship to the upland^ and that the behavior
of the latter is normal.
The measure of independence which the streams
display toward the topographic features
is a common phenomenon
if
it
is related to a long erosional history of two
or more
cycles^ duration;
it
is entirely incompatible with the development of consequent drainage
in young block mountains.
The superposition of Rio Grande de Loiza
across the highland and the features of entrenchment within
its course
indicate clearly their genetic association with an early
cycle
of erosion
and
their lack
of connection with
a hypothetical Quaternary
displace-
ment.
A more speculative^ yet interesting^ line of evidence against the fault
hypothesis
is furnished by
the
distribution
of
granitoid
intrusives
in
the Fajardo
district.
If
it be assumed that stocks and apophyses that
invade overlying rocks from a subjacent magmatic reservoir ordinarily
taper upward^ there should be a rough relationship between the depth of
erosion and the
areal extent of their outcrops.
If the Caguas lowland
and Luquillo upland were formed from a
simple^, peneplaned mass by
displacement and tilting, the intrusives now exposed on their respective
surfaces, in spite of the differences in elevation, would have been eroded
to
tlie same relative depths.
Yet the numerous granitoid intrusives in
the upland
are invariably small
in
areal
extent, whereas
those
in
the
lowland are characteristically large, a feature which suggests that deeper
erosion has taken place
in
the
latter.
This general
line
of reasoning
loses
its force, however^ when
it
is recalled that the source of the intru-
sives was the Ilumacao batholith
to the
south, and that
their
relative
sizes are probably due as much to their proximity to the batholith as to
the depth
of dissection.
On the other hand, three of the intrusives
of
the lowland area extend into the upland: an apophysis of moderate ex-
tent is present between the La Muda-Caguas carretera and Aguas Buenas
;
a small stock invades the margin of the upland near El Rio; and an
intrusive tongue
passes from lowland
to mountains along
the
falls
of
Rio Blanco,
^o one of these cases could be exhaustively studied in
tlie
field, and to some extent the boundaries given them on the map are only
approximate.
All of them are alike, however, in beginning with broad
bases in the lowland, and ending in much more
restricted exposures in
the highland.
In
the
Rio Blanco
intrusive,
tapering can
actually be
246
SCIENTIFIC SURVEY OF PORTO RICO
detected from bottom to top of the steep mountain
slopes.
Evidence
of
this kind has merely an incremental value;
it
is in harmony with an
erosional origin of the Luquillo upland and Caguas lowland, and seem-
ingly contradictory to a diastrophic interpretation.
It is unfortunate that a problem as elementary as this cannot be settled
by direct observation, but the rocks of the lowland are exposed
at the
base of the northern escarpment at only one point—at the divide between
the Kio Grande de Loiza drainage and the Kio Blanco drainage.
Here
the escarpment and lowland are both composed of a porphyritic intrusive.
Although evidence of a structural or lithologic break is lacking, the char-
acter of the exposures forbids an unqualified
assertion.
Elsewhere the
continuity of rock outcrops from scarp to lowland
is interrupted by the
alluvial outwash from the upland; but east of Gurabo the scattered rock
"islands'^
in the
alluvium, and the rock exposures along the
southern
margin of the depression afford little basis for postulating a normal fault
of large displacement between them.
On the contrary, in the magnetite
belt east of Juncos a simple synclinal relation appears to exist between
the rocks in the southern part of the lowland and those in the scarp on the
north.
West of Gurabo most of the basin is underlaid by quartz diorite.
The tuffs to the north and south of the intrusive are much alike, and com-
parisons between them have little significance.
N'ear Aguas Buenas the
lowland seems to end with the diorite intrusive, and the entire country
rises to an elevation of 700 or 800 feet ; yet even here an east-west belt as
wide as the Caguas basin, and continuous with
it, forms a valley pene-
plane bounded both north and south by higher
flat-topped ridges and
uplands.
The ridge on the north is continuous with the escarpment that
bounds the Caguas lowland farther east and appears in every way to be
analogous to it; but in the Aguas Buenas region the numerous and un-
broken rock exposures reveal no structural break between the ridge and
the floor of the valley peneplane.
Alluring as a theory of block-faulting may be to explain the juxtaposi-
tion of the Luquillo upland and the Caguas-Naguabo lowland,
it clearly
does not fit many of the facts.
Suggested by the youth of the scarp be-
tween the highland and lowland, by the truncation of physiographic and
structural elements, and by the alluviation of the Caguas basin, the hy-
pothesis cannot account
for the
presence
of
the same
erosional
levels
around the
flanks of the mountains and the margins
of the basin
as
occur in all other parts of Porto Eico ; for the occurrence of the same, or
of stratigraphically related, rocks in the lowland and on the scarp; for
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
247
the absence
of convincing evidence
of
faulting where
the
base
of the
scarp is exposed; or for the superposition of streams across the northern
upland^ notwithstanding the existence of an unobstructed outlet to the
east coast.
Lobeck suggests that the contrast in relief along the southern margin
of the Fajardo district is the outcome of differential rock resistance.^'^ He
considers the Caguas lowland to be the result of rapid excavation of the
underlying granitoid rocks ; and, though puzzled by the diversity of rock
types in the Luquillo range, he concludes a brief descriptive passage with
the statement
: "It is quite possible that a better knowledge of the rocks
involved would show that the higher areas are actually made up of more
resistant types /^
In field and laboratory the authors have given LobecFs suggestion very
careful
consideration, but
the
evidence
assembled
fails
to
support
his
opinion.
Quartz
diorite underlies
only a part of the Caguas lowland,
and where
it is present, it has undoubtedly played an important part in
facilitating denudation.
In the vicinity of Caguas, where the depression
is broadest, its limits are almost precisely determined by the extent of the
quartz diorite.
East of Gurabo, however, outcrops of other rock types are
common, and at the divide between Eio Gurabo and Quebrada ^^
Peiia
Pobre the granitoid intrusive is absent, yet the lowland persists.
In the
entire ISTaguabo section of the lowland outcrops of quartz diorite are scat-
tered,
and, notwithstanding
the
ease with which
the streams working
headward from the east coast have excavated them, they appear to have
been
subsidiary,
rather than dominant,
factors
in
the
location
of
the
drainage
lines.
At the
falls
of Rio Blanco
the
non-resistance
of
the
diorite has unquestionably determined the position of the river; but the
fact that the coarse-grained intrusive passes indiscriminately from valley
to mountain mass without a structural break
is of greater significance,
and in following it Eio Blanco drops a thousand feet or more in a broken
series of falls down the sheer mountain wall of the central range.
The
most prominent valley in this portion of the Luquillo Mountains
is not
that of Eio Blanco, which has barely notched the highland ; it is the east-
west valley of the much smaller stream which flows between the central
range and the southern
foothills
(Fig.
11).
It must be admitted that wherever rocks of a non-granitoid character
occur within the lowland, they tend to rise to elevations somewhat higher
37 Lobeck, A. K.
: op.
cit,
pp.
314, 318-319.
3« The word "quebrada" may be translated "creek" or "brook."
248
SVIEWTIFIG SURFEY OP PORTO R1€0
than its
iioo,r, making prominent; knobs,
ritlges, and spurs that protrude
through
the
<illiiviuni
or form
local
st^'eam
divides.
West
of
Cagiias,
where the quartz diorjte ends,
tlie entire eoujitiy rises 600
feet or more:
}'et even liere there
is a topographie lowland hetwxjen high ridges.
From
the persistence of this linear depression along the southern boundary of
•he
!'";!.iardo
iiislriir
hi
^|iM(^
of
rh;i\]i[><
n\
rock
l>iH-.
il
>[ni>t
\>v
rs.si-
Tills deep
east-
worketl
hcfKlvvard
Klo Blanco, cascades
(h
plmlogrnph was
taken,
eluded that the non-resistant intrusives did not: eonstitute a primal cause
in localizing denndoiion, much as tliey may have ai;eelcrai:e(l
its progress.
A somewhat similar cf»nclnsion most
l)e drawn coni'eriiing the
role of
rock resistance in the oj'igin of the Ijucjuillo '^^lonntains and of the ridge
that extends westward from them,
foTjning
the
scarp on
the northern
sid(! of the lowland.
Field traverses merely confirmed Lobeck's observa-
tion
that
atnmst
every
litliologie
variety
found
elsewhere among
the
(Cretaceous
roe-ks
of the
district
outcrops
in
the
raoimtajn
region
(cf.
geologi(: map).
JIf a few individual beds, like the conglomerate north of
La Miida, be excepted, none of the varieties present exhibits notew-orthy
resistance in other parts of northeastern Porto Eieo.
The sediments on
the southern flank of the central range display a greater degree of indura-
tion than those near the north and east coasts, hut this cliaracteristic
is
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
249
attributable to the rapidity of dissection and to the consequent freshness
of the exposures, rather than to an initial difference in composition.
The
marked tendency of the stratified tuffs and ashy shales to form subsequent
valleys in every part of the island stresses the unlikelihood of abnormal
elements of resistance in the sediments of the Luquillo Mountains.
In
the extensive areas of high elevation and relief where sediments are not
present, moreover, superior lithologic resistance cannot be entertained as
a means of interpreting the topography. The hypothesis must be rejected
also as an explanation of the ridge that bounds the Caguas lowland, for
this, too, is composed of many lithologic types.
Analysis
indicates,
therefore,
that
the Luquillo Mountains and
the
Caguas lowland have not been formed by recent diastrophic movements of
any kind, because they have gone through the same phases
of a long
erosional development as the rest of Porto Rico; that they did not origi-
nate in consequence of geographic isolation, for they seem to have occupied
a vulnerable position with respect to streams and ocean since early Mio-
cene time ; that they have not been etched into relief because of the differ-
ential resistance of their constituent rocks, for the same, or similar, for-
mations are present alike in mountains and lowland, as well as in every
other part of the Fajardo district.
It is true that lithologic factors have
proved important in the development
of portions
of the Caguas
basin,
especially near the town of Caguas, but they had at best a small part in
determining
its initial location.
There
is available, however, another explanation
of the origin of the
Luquillo Mountains and the Caguas lowland, and although
its applica-
bility cannot be fully demonstrated at the present time, it
is supported by
all the data thus far assembled.
The relative resistance of rocks to pro-
cesses of erosion may depend upon their lithologic characteristics, or, on
the other hand, it may be more directly related to their structures.
Mass
resistance, as contrasted with lithologic resistance, may, for example, be
imparted to rock formations by the development of compressional struc-
tures.
In nearly every region of folded mountains, especially if the fold-
ing
is close, the synclinal troughs more effectively withstand denudation
than any other kind of orogenic structure.
In pitching folds the ends of
both the anticlines and the synclines seem to possess the quality of mass
resistance, as the canoe-shaped forms of the central Appalachian region
so strikingly show.
Anticlinal crests and the summits of domes are, in
contrast, easily excavated, and not infrequently they serve to localize the
formation of subsequent depressions.
250
SCIENTIFIC SURVEY OF PORTO RICO
There is some reason to believe that an explanation of this kind applies
to the major topographic features in the southeastern and southern parts
of the Fajardo district.
It is unfortunate that concrete structural observa-
tions in these two sections were few and scattered ; yet the evidence they
supply is suggestive, if not conclusive.
It may be summarized briefly
:
(1)
The steep southern flank of the central range
is composed largely
of ashy and tuffaceous shales which dip at a moderate angle toward the
north.
In spite of their somewhat darker
color, which
is due in great
part to the fact that they have not undergone deep residual weathering,
these stratified rocks are tentatively correlated with the beds found near
the
coast
at Luquillo and other
localities, where
also they
dip north.
Repetition of the strata cannot be interpreted as the outcome of faulting in
this case, and from the facts which follow, it appears definitely to be the
result of folding.
(2)
The presence of folds within the mountains
is further indicated
by the determinable structures at their eastern and western ends.
J^ear
Guzman Abajo at the northwest end of the central range the
crest of a
large anticline has been identified.
Along the east coast from Fajardo
to l^aguabo two large folds involving several thousand feet of sediments
and flows have been found, and they have been traced for several miles
westward into the foothills of the Luquillo range.
From the folds which
have been differentiated, from the repetition of strata in the mountains,
and from the distribution of the outcrops of other rock types, the struc-
ture from the north
coast to the southern front of the central range
is
believed
to
consist
of a
synclinal trough, an
anticline, and
a syncline
(Fig.
12).
The latter
is an eastward plunging structure, which seems
to undergo partial
closure
at East Peak, but which opens broadly
to-
ward the
east coast.
(3) The central range and southern foothills
of the Luquillo Moun-
tains and the Caguas lowland are all elongated in an east-west direction.
Although they parallel the general
structural
trend
of
the underlying
rocks,
the
occasional independence
of
strike
and
topography
is
note-
worthy.
The
cases
of independence can
be
readily understood
if
the
topographic elements
coincide with the
axes
of
the
folds
rather than
with the strikes of individual formations ; and in the few localities which
could be studied carefully, this relationship was found to exist.
(4) The elevated valley peneplane which lies west of the Caguas low-
land beyond
the outcrops
of
the
non-resistant
granitoid
intrusive
has
already been mentioned.
Semmes and Hodge have determined the anti-
clinal character of the underlying structure, and the field studies of the
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
251
present survey have made
it possible to extend
the
anticlinal axis far into the Cagnas basin.
Further study may show that
it
is continuous
with the
anticline between the
central range
and southern
foothills, thereby extending the
structure the full length
of the Fajardo
dis-
trict; but at present there
is some doubt con-
cerning
its
continuity
between
Gurabo
and
El Rio.
In any case
its presence in the west-
ern
portion
of
the
depression
strongly
sug-
gests that the Caguas
section
of the lowland
was
initially an anticlinal
valley, the excava-
tion
of which has been
locally
facilitated by
the
dioritic
intrusives.
The
valley between
the
central
range
and
southern
foothills
is
also anticlinal, and its discontinuity and nar-
rowness
as compared with the lowland to the
west
reflect
the
greater
compression
of
the
structure and the scarcity of intrusives.
The
structure of the Naguabo section of the low-
land
was
not
determined
carefully,
but
its
development
appears
to
depend
more
upon
its proximity to the
coast, and upon the free
play
of
the
lithologic
characteristics
of
the
rock formations than upon simple
structural
features.
Its
connection
with
the
Caguas
basin
is
due
to
the
accidents
of
headward
erosion.
(5)
The
origin
of
the El Yunque
outlier
can merely be conjectured.
It
lies somewhat
north
of the main
range, almost
isolated by
a
ramifying
system
of
stream -heads
which
have cut deep gorges on nearly every side.
A
slender
divide
ties
it
to
the
central
ridge,
but
the
divide
lies
a thousand
feet
or more
below
the
mountain
crests.
Although
the
principal
drainage
lines
on
the
northern
slopes
of the mountains trend northward
to-
ward
the Atlantic
coast,
the
rapid
extension
of
tributaries
in
an
east-west
direction
be-
m^
^
<M
w
'-;
5^.
a>
r^
'
^:
H
«i
^
!>»
s
^ ^
"^
OS
s
^
\o
.,
252
SCIENTIFIC SURVEY OF PORTO RICO
tween El Yunque and the central range
is suggestive of a zone of weak-
ness, which the variety of rocks
across the divide proves
is not
strictly
related to the lithology.
On the geologic map this belt
is shown occupy-
ing the limb of a fold, where relative lithologic resistance would be the
principal
element
affecting
erosion,
whereas
theoretical
consideration
would lead one to expect that the east-west gorges are being opened out
along the
axis
of an
anticline.
More thorough
field
study may show
the map to be in
error, notwithstanding the weak support given
it by
the physiography and the distribution of outcrops, which imply that El
Fig.
13.
Sketch
profile
of El Yunque
As
seen
from
Vieques
Sound.
The
recurrent
scarps
facing
southwest
suggest
a
northeastern dip in the rocks that form this side of the mountain, but the structure at
the summit
is
uncertain.
The
flat-crested spur approximating 1600
feet
in
elevation
is conspicuous.
Yunque
forms
the
crest
of
the
anticlinal
structure.
The
scattered
readings that were obtained on the northern slope of the mountain are
persistently to the north.
An unusual
profile view of the northeastern
spur
of El Yunque was
sketched from the mail launch
that
plies
be-
tween Fajardo and
Isabel Segunda.
The
sketch, which
is reproduced
in Figure 13, suggests from the recurrence of scarps fronting southwest
that this side of the mountain
is a partly stripped dip
slope, composed
of northeastward dipping strata.
If
this
interpretation be
correct. El
Yunque forms the end of an anticline which pitches east-northeastward,
and which, because of
its closure, has given mass resistance to the peak.
The interpretation gains some support from the fact that Cape San Juan,
which
is structurally aligned with the northeastern spur of El Yunque,
is anticlinal in character.
The synclines which flank the cape, moreover,
pitch east-northeastward and necessitate a similar pitch in the anticline
between them.
Meager
as
the
observations
presented may
be,
they warrant
several
definite conclusions and permit a few inferences regarding the structural
character of the Luquillo Mountains and the Caguas lowland.
Both are
MEYBRHOFF, GEOLOGY OF THE FAJARDO DISTRICT
253
composed
of folded
strata, which include most of
the formations that
appear in all other parts of the Fajardo district.
These formations have
no
peculiar
lithologic
properties
affecting
their
ability
to
resist or
to
undergo erosion in the mountain and lowland belts, and the features of
the topography exhibit a degree of independence of the rock types that
makes a lithologic explanation of their origin untenable.
To some ex-
tent, however, dioritic intrusives have accelerated erosion, but only where
denudation was localized by other factors.
In general, the
relief features are intimately
related to the orogenic
structures, but the relation is with the axes of the folds rather than with
the
formational
outcrops
associated
with
them.
An
anticlinal
valley
separates the southern foothills from the central range, and the presence
of an anticline, which pitches at both ends, may be responsible both for
the
isolation
of El Yunque and
for
its prominence.
Between
the two
anticlines
lies the pinched
synclinal structure which forms the
central
range.
Closure upon
the
west determines
its
limits
at El
Toro,
the
highest peak
in
eastern Porto Eico;
partial
closure on
the
east may
also account
for East Peak, and the compression
of
the
fold between
the two points of closure has made
it possible for the central range as a
whole
to
maintain
an
average
elevation
approaching
3000
feet.
The
Caguas basin
offers
difficulties in
interpretation because
the
intrusives
have obscured and to some extent distorted the original structures of the
older rocks.
In the west, however,
its anticlinal character provides rea-
son to believe that its excavation was due initially
to the structure.
If
this interpretation of the structure be correct, supported as
it is by a few
measured
dips, by the distribution of the fragmental rocks, and by the
physiographic
features,
the Luquillo upland and Caguas lowland have
been carved from two anticlines and a syncline,
all with east-west axes
that pitch toward
the east
(Fig.
12).
Limiting them on the south
is
the syncline of the magnetite
belt, which has already been mentioned;
on the north lies an undulating synclinal trough, which will receive fur-
ther study in a later section.
It may justly be claimed that the
relief in
southeastern Porto
Eico
has not yet been explained, even
if the
structural
interpretations
pre-
sented
be
correct.
Along
the western border
of
the
district
pitching
folds are
also present;
yet, granting the excavation
of the
anticline
at
the southwestern corner of the district into
a valley peneplane flanked
by
somewhat
higher
synclinal
uplands
to
the
north
and
south,
the
topographic
effects are by no means comparable with those in the
east.
254
SCIENTIFIC SURVEY OF PORTO RICO
Some
factor
other than simple
folding must
be found
to
explain
the
contrasts in relief which occur in the Luquillo Mountains region.
The additional factor
is provided by the relative amounts of compres-
sion in the folds of the two localities.
Along the western border of the
district the folds are broad and open^ and the processes
of erosion have
not encountered the mass resistance
of tightly pinched
structures.
In
the Luquillo Mountains the synclinal fold of the central range
is nar-
row and
compressed, and the
anticline
to
the south
is almost
equally
so.
The mountains are wholly restricted to these two compressed folds
and
to- the sharply creased end
of
the plunging
anticline which seems
to compose El
Yunque.
In
every
direction from
the mountains
the
major folds shallow and broaden; the inherently low erosional resistance
of the rocks comes into
play; and the
elevation
subsides
at the
point
where the change in the character of the structure first appears.
A study
of the east coast will make the significance of these statements
clear.
The structure of the east coast
Confirmation
of
the
inferences which have
been drawn
concerning
the structure of the Luquillo Mountains and the Caguas-Kaguabo low-
land might logically be expected from the natural structure section ex-
posed along the coastline at the eastern border of the
district.
Again,
however,
realization
falls
short
of
expectation, and
it
can merely
be
claimed that the rock section along the east coast partially confirms and
nowhere
contradicts
the
interpretations
which have
been
advanced
to
explain the structures of the
interior.
Minor folding and faulting ob-
scure
the larger features;
deep weathering makes
field and
laboratory
identification of rock-types precarious; the appearance of thick volcanic
sills and flows, not present in the western part of the district and not so
conspicuous in the central part, makes stratigraphic determinations
less
certain.
More serious than these partly surmountable
difficulties which
are of a positive character, are the wide gaps in the
cross-section that
have been caused by deep dissection, submergence, filling of the estuaries,
and
aggradation
of
the
valleys.
The
unconsolidated
deposits
of
the
broad
playas, which
extend
for
moderate
distances
inland
along
the
stream
courses, bury many
parts
of the
rock
section—usually
critical
parts, because the larger aggraded valleys were excavated along the axes
of folds and upon the less resistant stratified rocks, in which structural
data are easily gathered.
In consequence, most
of the outcrops in the
rocky divides between the playas consist of the more
resistant, massive
volcanics, which supply few
structural
details
that
are
serviceable
in
MEYEMHOFF, GEOLOGY OF THE FAJARDO DISTMIOT
25.1
identifying the major folds of
tiie region.
The result
is an incomplete
set of facts, especially for the area sontli of Fajardo; and the conclusions
presented
in
the
following
paragraphs
and
ntilized
in
drawing
the
geologic map are
provisional.
Some
revision
will be necessary when a
tliorough investigation of the stru(;turo has heen made.
From Playa de Fajardo to Cape San Juan <)utcrops are almost unbroken
along the shoreline, and they are also very numerous inland.
They dis-
play
a bewildering succession
of small
folds, many
of which
are over-
id fan
soiitli
of
Fajarilo,
turned from, the south.
Buft ashy shales, like those at Bio Piedras, domi-
nate in the vicinity of Fajardo, and they continue southwcstward along
Eio Fajardo
into
the
Liiquillo
foothills and
n,orthwestward
along
the
r<uite
of the Fajardo
Railroad
to
tlie Atlantic
coast.
Korth-northeast-
w'ard toward the (,-ape and southw'ard toward Ceiba, however, beds of
tu;ft
increase in number and in prominence, ami in both these directions the
tuffaceous strata ultimately replace the
shales.
The strikes of the sedi-
ments are as variable as the points of the compass, but a prevailing east-
northeasterly
trend
that
coincides
with
the
direction
of
the
Fajardo
valley
is
apparent.
The
dips
are even
less
constant than
the
strikes.
The numerous
small,
sharp
overturned
folds,
some
of which
involi'e
thrust faults
of minor displacemerd;, show
the
extraordinary extent
to
which
distortion
of the
rocks has proceeded.
In
the
rocky knob
that
256
SCIENTIFIC SURVEY OF PORTO RICO
protrudes above the alluvium a half-mile south of Fajardo, one
of the
quarries
along
the
carretera
has
uncovered
an
overthrust which
de-
veloped
at the crest of a slightly overturned
anticline
(Fig. 14).
The
succession of anticlines and synclines exposed in the wave-cut cliffs north
of Playa de Fajardo
is the most
striking
structural
display which the
island contains.
Many of the small folds plunge sharply, a feature which
is well exhibited in the flexures and contortions that the strata undergo
when exposed on horizontal
surfaces,
as along the Fajardo
valley road
about one kilometer southwest of Fajardo.
The prominence and obviousness of the structural details in the north-
eastern corner
of the
district, however,
obscure
the broader and more
significant features, for the aggregate of many strike and dip readings
fails to disclose the major structural elements of which the minor defor-
mation
is
part.
The
larger
structures have been determined
only by
plotting the distribution of the sedimentary formations.
The buff shales
which outcrop in the valley of Eio Fajardo give place to calcareous tuffs
on the south
; and to the north and northwest they partly encircle a simi-
lar
series
of
tuffaceous
sediments.
The
reappearance
of the
shales
in
scattered outcrops
along the
contact between
the
Cretaceous and Ter-
tiary
rocks
shows
that they were
originally continuous with
the
ashy
shales
at Eio Piedras; and wherever they are exposed
at the northern
edge
of
the
Cretaceous
oldland
they
lie
stratigraphically
above
the
calcareous
tuffs.
Their
position
between
the
tuffaceous
strata
in
the
valley of Rio Fajardo seems, therefore, to be a reliable indication of the
synclinal character of the main structure.
The
tuffs in the
vicinity of
Cape San Juan, on the other hand, are lithologically distinct from the
calcareous beds which
lie beneath the
ashy
shales.
They
are non-cal-
careous and
contain
a
greater
admixture
of
pyrogenic
material, and
their
location
at the
northeastern
extremity
of the
pitching
anticline
which has been traced from El Yunque to the Cape San Juan peninsula
affords evidence
of their younger
age.
This
anticline
is the northern-
most
structure which
emerges
at
the
east
coast,
and
it
provides
one
clearly defined boundary for the syncline of the Fajardo valley.
The
village of Ceiba, four miles
south of Fajardo,
is situated upon
tuffaceous
deposits which include a large proportion
of
calcareous and
shaly members.
Weathering and deformation have so completely
oblit-
erated the structural features of the rocks that in the score, or more, of
outcrops studied not a
single
reliable structural determination was
ob-
tained ; in most exposures it was impossible to identify the bedding, even
though
petrographic
analysis
has shown
the
rocks
to
be
sedimentary
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
257
in character.
Here
agaiii^ the lithologic succession must be
utilized in
forming a
tentative
conclusion regarding
the
structure.
Between
the
tuffs of the Ceiba district and the ashy shales of the Fajardo valley is a
band^ one kilometer wide, of volcanic rocks, which range from microfelsite
and typical andesite to a highly altered andesitic derivative.
Their fine
textures and flowage lines make their effusive origin certain.
They were
traced westward more than four miles to Kio Fajardo and throughout this
distance they parallel the ashy
shales and
stratified
tuffs
to
the north
and the calcareous tuffs to the south.
About half a mile south of Ceiba
a similar band occurs across a zone with approximately the same width.
It was not traced westward, and in showing
its extent on
the geologic
map accompanying this report the writers have made some use of Fettke's
brief notes concerning it.^^
On the map of the Humacao district
it ap-
pears as a lens-shaped body which loses
its identity in the extensive out-
crops of porphyritic andesite surrounding
it.
The work
of the present
survey failed to disclose the presence of the porphyry north of the felsitic
flows but discovered that, like those north of Ceiba, they come into direct
contact with the
tuffs.
It seems highly probable from the limited
evi-
dence obtained that the two bands of extrusive material north and south
of Ceiba are identical, and that they outcrop on the opposite limbs
of
an anticline.
The structural alignment of the Ceiba spur with the anti-
cline between the central range and the southern foothills of the Luquillo
Mountains supports this opinion without proving
it.
Interpretations of the structure south of the Ceiba anticline so nearly
approach guess-work that none will be offered.
Andesite porphyry pre-
dominates among the deeply weathered rocks, but a short distance north
of Playa de Naguabo an area of felsite resembling the flow material near
Ceiba, and a small patch of calcareous shale suggest duplication
of the
formations by folding
or by faulting.
The
distribution and
limits
of
the felsite and shale in the
vicinity of Playa de N'aguabo could not be
determined
in
the
field, and
it
is
impossible
to form any
satisfactory
opinion
about
them.
Fettke,
whose
studies
overlapped
those
of
the
Fajardo survey
at
this
point, did not differentiate
the
sediments, and
his map shows the felsite as an ill-defined lens within the porphyry.
He
makes no attempt to interpret the structure.
The most puzzling feature
in the Naguabo region is the huge mass of andesite porphyry which cuts
the adjacent stratified rocks like a wedge, roughly paralleling their struc-
ture and stretching westward to the Caguas lowland, perhaps as far as
»» Fettke,
C.
R.
:
Geology
of the Humacao
district,
Porto
Rico,
op.
cit.,
p.
148 and
map.
258
SCIENTIFIC SURVEY OF PORTO RICO
the large plug-like mass at the head-waters of Eio Canovanas.
In many
places
the porphyry maintains
a
sill-like
relation
to
the
surrounding
strata, but in others it cuts irregularly through them.
Some of the prob-
lems
of
its
origin and
structure
will
be reserved
for consideration
in
an ensuing
section, but
its presence
in the southeastern corner
of
the
district creates a structural chaos that defies analysis.
Its intrusion has
isolated sedimentary areas, which have been further affected by the
in-
vasion of the Humacao batholith.
In view of the extent of volcanism,
it may be doubted
if any of the sedimentary remnants
in this area has
retained a wholly normal
relationship with
the more
regularly
folded
rocks
to the north.
The character of the structure along the east coast suggests a tentative
explanation
of the lowland
east
of
Porto
Eico,
which,
as
a
result
of
submergence, now
forms
Vieques
Sound.
The
sharp
folds
of
the
Luquillo Mountains plunge eastward;
the two
anticlines
flanking
the
central range become narrower and -lower ;
the
syncline between them
broadens, and within its trough non-resistant sedimentary rocks appear.
I^otwithstanding the development of sharp drag folds in the more thinly
bedded
strata,
the
general
structure
flattens,
and
in
consequence
the
erosionally weak rocks are spread over a large area.
Culebra, composed
in part of horizontal tuffs, appears to
lie in the center of this synclinal
trough,^^ which, because
of the
coincidence
of
relatively
flat
structure
and of non-resistant rocks, has remained an area
of low elevation and
relief throughout Cenozoic time.
A second period of gentle folding or
warping may have modified
it during the
diastrophic
disturbances
of
the Miocene, for a remnant of the Tertiary coastal plain has been pre-
served within
it
in
spite
of
its
long
erosional
history.^^
It
is
clear,
however, that the Yieques Sound lowland begins where the close folding
ends.
ISI'ot only
is
this statement
true
of
its
eastern end
against
the
Luquillo Mountains, but
also
of
its
southern margin
against Vieques,
where the Cretaceous sediments and volcanics again possess high
struc-
tures.
The transverse folds of the northern border
In the preceding
analysis the
structure
of the
Fajardo
district has
been studied in cross-section
at
its eastern and western boundaries and
*« Meyerhoff, H.
A.
: The
physiography
of
the
Virgin
Islands,
Culebra and
Vieques.
New York Academy of Sciences,
Scientific Survey of Porto Rico and the Virgin
Islands,
1927, Vol. IV, Ft.
2, pp. 204-205, including footnote 38,
p. 205.
«Idem,
pp.
195.
198-200.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
259
across
its
center.
The repeated references
to plunging folds have
in-
dicated that there are additional complications
in longitudinal
section,
aome of which may be appreciated by a brief examination of the struc-
tures along the northern edge of the Cretaceous outcrops.
The carretera from Kio Piedras to Fajardo follows the contact between
the
Tertiary
strata and
the
Cretaceous
rocks
rather
closely
until
the
former
disappear.
Eock
cuts
are
numerous
and,
in
spite
of
deep
weathering, the predominating stratified tuifs and shales have retained
their structural
features
sufficiently well
to supply ample data for
in-
terpretation.
From Bayamon to Rio Piedras the rocks strike southeast,
and
their
dips
are
steep
to the northeast.
Between Eio
Piedras and
Carolina the strike changes to due east-west, and the dips become lower
but remain persistently northward.
At Carolina the sediments swing
to the northeast, and from this point to Cape San Juan northeasterly
strikes prevail.
The exceptions are few but noteworthy.
At the valley
of Rio Grande de Loiza, at the town of Rio Grande, and again a short
distance southeast
of Luquillo the
strikes
veer
in a
direction
at right
angles to the general trend exhibited elsewhere, and the
stratified rocks
swing inland.
In these three localities structural observations were diffi-
cult to
obtain, because igneous
intrusions, uncommon elsewhere in the
section, cut the strata.
In the Rio Grande-Mameyes region porphyritic
and granitoid masses have completely engulfed the sediments except for
one or two small patches that accidentally escaped destruction.
Throughout most of the section from Carolina to Cape San Juan the
dips are very low—locally even horizontal or gently undulating. In many
places
at
the extreme
northern
edge
of
the
Cretaceous
outcrops,
the
strikes turn from northeast to due north, and,
if the strike readings be
plotted on a map, the impression
is inescapable that the massive pyro-
clastic rocks of the interior pass into the Atlantic and in their passage
suffer
conversion
from
massive
to
stratified
types.
A
more
careful
evaluation of the facts must alter this alluring hypothesis.
There
is no
other evidence that the unstratified
tuffs and agglomerates grade later-
ally into sedimentary equivalents to this extent.
On the contrary, part
of the same sedimentary succession that
is present in the neighborhood
of Rio Piedras and Trujillo Alto persists without interruption to Cape
San Juan, in
spite
of
its apparent tendency to move out to
sea.
Ex-
planation
of the seemingly contradictory
facts must be sought
in the
structural character of the three belts in which the rock section swings
back into the interior.
260
SCIENTIFIC SURVEY OF PORTO RICO
Once again the critical features are not entirely clear in the field, and
interpretation
must
supplement
direct
observation.
The
rocks
that
strike northeast and north have
gentle
dips and
are
relatively
free
of
minor deformation and the
effects of igneous intrusion; but those that
strike southeast have high dips, which are locally complicated by faulting
and
contortion.
The
intrusives
associated with them
appear
to have
been localized by the intensity of deformation at these points, and they
have
the unfortunate
effect
of
confusing
or
obscuring
the
significant
orogenic structures.
The igneous bodies near Carolina and Luquillo are
comparatively small, but the granitoid mass near Eio Grande
is of con-
siderable
size, and
its invasion destroyed a large and important area of
sediments.
In each of the three localities, however, enough of the sedi-
mentary
succession
remains
to
supply
some
structural
data,
on
the
basis
of which an explanation
of the larger
orogenic
features may be
postulated:
(1)
The major
structure along
the northern edge
of
the
Cretaceous
exposures
comprises
an
east-west
syncline,
which
extends
from a point two miles east of Eio Piedras to a point about three miles
east
of
Luquillo.
The
contact between
the
Tertiary
and
the
^"^Older
Series^' roughly coincides with the axis of the fold.
(2)
Strongly asym-
metric
transverse
folds
in
the
trough
cause
undulations
which
sink
gradually
toward
the
west and
drop
sharply
toward
the
east.
(3)
Diastrophic
conditions
on
the
steep
limbs
of
the
cross-folds
favored
magmatic penetration of the
stratified rocks.
The second and third of
these features are illustrated in the accompanying generalized structure
section
(Fig. 15).
The geologic map shows the peculiar areal distribu-
tion of rock types which has prompted the interpretation outlined above.
Transverse folding of
a
less pronounced character
affects every por-
tion
of northeastern Porto
Eico,
as may be judged from
the number
of plunging folds which have been described.
The writers are not pre-
pared
to read any
final meaning
into
these
cross-structures,
for
their
significance can be gauged only
after
a
careful
structural
analysis
of
the entire Porto Eico-Yirgin Islands region has been made.
The
inti-
mate
relation existing between
intrusives and the asymmetric
folds
in
the northern part of the Fajardo
district, however, implies that
all the
deformation occurred
in
a single period
of
orogenesis, and that
it
in-
volved strong torsional
stress, which was
relieved by movement along
transverse
lines.
Folding may
have
occurred
in
two
distinct
pre-
Oligocene
cycles,
as Hubbard
suggests,
but
this
supposition
receives
little
unqualified
support from
the
structural
features
in
the
north-
eastern part of the island.
SI
rw
•z
w
NW
n
wo
u
5^
MEYEBHOFF, GEOLOGY OF THE FAJARDO DISTRICT
261
Summary
Despite
the many
problems which
the
foregoing
discussion
of
the
structure has left unsettled, it is clear that the formations of the Fajardo
district are systematically folded.
The main system of folds extends in
an east-west direction across the district, but it is complicated by several
well
defined
flexures which
cross
it from north
to
south.
Subsidiary
systems of the drag fold type are found among the incompetent rocks,
but they are genetically related to the larger structures with which they
are associated.
The major folds
are few
in number, and
their
distribution may
be
briefly reviewed.
Two anticlines enter the corners of the
district from
the west and merge into a single large structure in the hill section, a short
distance west of Eio Grande de Loiza, thereby crowding out the syncline
between them.
The enlarged anticline
rises into
a dome-like structure
on
the
northern
flanks
of
the
Luquillo
Mountains and
then
pitches
moderately toward Cape San Juan. A narrower
anticline
appears on
the south near the eastern end of the Caguas lowland, passing eastward
between
the
central
range
and
southern
foothills
and
plunging
into
Vieques
Passage.
Between
these
two
folds
lies
a
tightly
compressed
syncline, the rocks of which acquired through folding a mass resistance
that has enabled them
to form
the high
central
range
of
the moun-
tains.
On the north the
district
is enclosed by an asymmetrically un-
dulating syncline, the northern limb of which
is buried beneath Ceno-
zoic deposits or
lies upon the submarine platform beyond the limits of
observation.
The southern border of the
district
is complicated by the
dioritic
outliers
of the Humacao
batholith and by
older
intrusives
of
andesite porphyry.
From Gurabo eastward the structure is anticlinal in
character, but
in
the Naguabo
lowland
the
vestiges
of
structure
still
remaining appear to be synclinal
or monoclinal.
The
folds
of the
in-
terior flatten eastward ; the two anticlines narrow and disappear, whereas
the
syncline
of
the
central range broadens
at
their
expense, forming
the basinal expanse that underlies Vieques Sound.
Within the syncline
minor folding and thrust faulting have developed
to a marked degree
in the incompetent
shales, but the small
folds show convincingly
their
genetic relation
to the major structure.
The regularity
of the folded
structures
is surprising in view
of the
heterogeneous character
of
the
rock
section.
There
is some
basis
for
believing
that
the
variations
in
the
widths
of
the
folds
are
related
closely to the relative massiveness and competence of the rocks involved
262
SCIENTIFIC SURVEY OF PORTO RICO
within them; and that the characteristic features of cross-folding which
all
of them
display were
in some
instances,
at
least,
localized by
the
presence of competent intrusives or volcanic masses, which rose through
the rocks of the normal stratigraphic section.
Further analysis will un-
doubtedly reveal a very intimate connection between the competence of
the
lithologic elements and the form
of
th^
orogenic
structures.
The
work of unraveling the
structure has
also brought out two
additional
features, the significance of which
is of more immediate importance in
the
present
study;
namely,
the
strong
influence
which
the
structure
has exerted upon the development of
the topographic forms; and the
possibility
of
differentiating
the
confused
and
variable
rocks
of
the
region into an orderly stratigraphic series.
The first of these points has
already been discussed; the second may appropriately be considered in
the following section.
THE STRATIGRAPHY OF THE ""OLDER
SERIES'^
Problems in nomenclature
The
development
of
stratigraphic nomenclature
in most
regions
is
inevitably haphazard because
of the piecemeal
differentiation
of
litho-
logic units on which
it
is generally based.
The law of priority cannot
always be
rigidly
applied,
for
stratigraphic
subdivisions
acquire
final
validity only when the rock
section
of a given
locality
is
fully under-
stood, and when the individual units and the groups of which they are
part have been traced to their farthest geographic
limits.
The names
assigned to well defined formations are likely to prove permanent unless
there has been inadvertent duplication, but the names given
to forma-
tional series usually need revision as knowledge of the stratigraphic sec-
tion grows.
This preface to an examination of the Cretaceous stratigraphy of the
Fajardo district outlines the situation which must be met in delineating
the lithologic sequence.
The problems may best be understood from an
historical resume.
In 1899 E. T. Hill briefly described the limestones
which outcrop prominently
in
parts
of
the
interior, and which form
the crests of several of the more conspicuous ranges,^^ referring to them
as the mountain limestones, to distinguish them from the younger Ter-
tiary strata of the coastal plain.
Hill did not regard his designation of
these rocks as a formational name, nor did he apply it to a specific strati-
*2 HiU,
R.
T.
:
Mineral
resources
of
Porto
Rico.
U.
S.
Geological
Survey,
Annual
Report, 1899, Vol. XX, Pt.
6, pp. 771-772.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
263
graphic
unit;*^
yet Hodge
definitely
assigned
the name
to
the
shaly
limestones in the vicinity of Barranquitas and included them as one of
the members in his Barranquitas-Cayey
series.**
Berkey,
too, has sug-
gested that the name be retained in the
stratigraphic nomenclature.*^
Such a course seems decidedly unwise, because the name has no definite
geographic connotation in Porto Eico, hence a type locality must arbi-
trarily be chosen, and because the application
of the term to a
specific
part of the Cretaceous section was not Hill's intention.
What
is more,
the name
is pre-occupied.
In this report, therefore, no attempt will be
made to fix the term "mountain limestones'' upon any of the calcareous
strata which occur in the district, notwithstanding the fact that the shaly
limestones of the Barranquitas-Cayey series are probably present.
As
an
outcome
of
his
geological
reconnaissance
Berkey
assigned
names
to
several
well
defined and
conspicuous
stratigraphic
horizons
located
in
different
parts
of
the
island.
Those which
lie within
the
limits
of
the
Fajardo
district
include
the La Muda
limestone
and
Trujillo Alto limestone, and the Fajardo
shales.
Berkey differentiated
these beds before their stratigraphic
relationships were known, and he
foresaw the
possibility that any of them might prove to be equivalent
to the formations elsewhere in Porto Eico, for which names were simul-
taneously proposed.
The only possibility of this kind which the present
survey has developed has not been proved; but a
careful study
of the
pitching syncline which extends from Aguas Buenas to Toa Alta may
show that the Corozal and La Muda limestones are
identical beds
ex-
posed on opposite limbs of the fold.
Semmes added few names to the short
list proposed by Berkey.
He
applied the term "Aguas Buenas limestones^' to the two calcareous beds
that outcrop
in the southeastern corner
of the San Juan
district; but
the
two
formations
are
apparently
equivalent
to
the La Muda
and
Trujillo Alto limestones, in spite of their separation from the latter by
an interposed porphyritic intrusion, and in
spite of the discordance in
their direction of
strike, a feature which Semmes and Berkey thought
indicated their unconformable position beneath the rocks
to the north
and northeast.*^
The name "Aguas Buenas" must, therefore, be placed
in synonymy.
3 Hill, R. T.
: personal communication.
** Hodge, E. T.
:
op.
cit.,
p. 135.
« Berkey,
C. P.
: Introduction to the geology of Porto
Rico,
op.
cit.,
pp.
17-18.
46 Ante,
pp. 231-232.
264
SCIENTIFIC SURVEY OF PORTO RICO
The "Juan Ascencio Chert Beds" near Aguas Buenas were also named
by Semmes.
As stated in the preceding section/'^ the material in these
strata
is laumontite rather than some form of
silica^ and^ according to
E. J. Colony, the rock was probably derived from beds of calcareous tuff
by alteration, perhaps of the hydrothermal type, like that postulated by
Hodge for two comparable rocks found in the south-central part of the
island. ^^
Notwithstanding
the
fact
that
the Juan
Ascencio
beds
did
not originate as Semmes believed, the name
is
still a useful appellation
for the banded strata to which he gave
it, and it will be retained in the
present report.
So little is known about the lateral and vertical extent of
the beds, however, that
it seems safer to treat them as one of the mem-
bers within the
associated and genetically
related
calcareous
tuffs and
ash beds, rather than as a distinct formational unit, at least until more
information concerning them is obtained.
The name "Collores limestone'^ was given by Fettke to the two bands
of limestone that outcrop in the magnetite belt east of Juncos.^^
He be-
lieved
that both bands
are
parts
of
the same
formation
exposed on
opposite limbs of a synclinal fold, but the two are lithologically
differ-
ent and bear more
striking resemblances
respectively
to the La Muda
and Trujillo Alto limestones than they do to each other.
The indeter-
minate character of the structure in the magnetite belt iftakes
it neces-
sary to attach greater significance to the physical differences in the two
beds, and in the present report they will be regarded as separate forma-
tions.
Provisional
correlation
with
the La Muda
and
Trujillo
Alto
limestone
is made on
the grounds
that
the
lithologic
similarities
are
definite, and that these two formations are the only persistent calcareous
beds
in the
eastern part
of the
island.
The names "La Muda" and
"Trujillo Alto"
will be used for the
Collores limestones,
therefore,
al-
though the isolation
of this narrow
belt
of sediments makes complete
verification of the writers' opinion difficult, if not impossible.
In his report on the geology of the Coamo-Guayama
district, Hodge
has divided the Cretaceous rocks into seven more or less arbitrary "series,"
in each of which he has endeavored to group genetically related materials.
The tendency to
restrict the word "series" to the complete sequence of
sediments accumulated in a single geological epoch makes its employment
for the
indefinite
lithologic
divisions
in Porto Eico inappropriate,
be-
cause
the
latter
reflect
fluctuations
in
the
vigor
of
volcanic
activity
*'Ante.
p.
234.
*8 Hodge, E. T.
: op. clt, pp. 208-209.
*» Ante, pp. 236-238.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
265
rather than
progressive
changes
in
life and environment
during
well
marked
divisions
of
geologic
time.
In
a
rock
succession
character-
ized by rapid alternation of hydroclastic, pyroclastic, and pyrogenic
ele-
ments^ moreover^ any grouping is likely to have severely limited applica-
tion, and
it
is
significant in
this connection that neither Mitchell nor
Fettke saw fit to extend the ^^series'^ of the Coamo-Guayama district into
the
contiguous
areas
to
the west and
east.
The
validity
of Hodges^
stratigraphic succession may also be questioned on more serious grounds.
He has assumed that successively younger beds are exposed from north to
south across the Coamo-Guayama district; but Mitchell's studies in the
adjacent Ponce district show that the structure of southern Porto Kico
involves a series of folds, in which the strata are repeated several times.
Hodge has given
little information concerning the
structure, and
it
is
difficult to reconcile the few statements he has made with the superposition
of beds he has assumed in dealing with the stratigraphy.
Until the nature
of the structure is determined with some degree of finality,
it seems un-
safe, therefore, to accept the succession which Hodge has proposed, for
it
is probable that some of its parts are equivalent.
Of the seven lithologic divisions differentiated in the Coamo-Guayama
district, the two oldest, at least, extend into the Fajardo
district.
The
basal
division,
the Eio
de
la
Plata,
consists
chiefly
of
massive
tuffs,
whereas
the overlying Barranquitas-Cayey
series
is composed predomi-
nantly of sediments.
Both are mixed lithologic series rather than simple
formational units, however, and, as Hodge indicates, the contact between
them is intergraditional and rather vague in character.
To employ these
indefinite group names for any of the
clearly defined formational sub-
divisions which can be recognized in the Fajardo district would inevita-
bly create confusion in what
is otherwise an orderly and
differentiable
sequence.
In the present work, therefore, Hodge's names for the strati-
graphic divisions of the Coamo-Guayama district which extend into the
Fajardo
district
will not be accorded
recognition,
because
there
is no
point in utilizing broad group terms based on questioned structural in-
terpretations, when definite formational names can be applied.
Eecently
C.
J. Maury has proposed the following stratigraphic
suc-
cession for the ^'^Older Series'' in Porto Eico :^°
Upper Eocene
:
Eio Descalabrados and La Muda deposits
Middle Eocene
:
Eio Jueyes beds
«o Maury, C. J.: Porto Rican and Dominican stratigraphy.
Science, N.
S., 1929, Vol,
LXX,
p. 609.
See also, Science, N.
S., Vol. LXXII, 1930,
p.
254.
266
SCIENTIFIC SURVEY OF PORTO RICO
Upper
Cretaceous:
San German
deposits, Ensenada
shale,
Fajardo
and Cape San Juan limestones
Lower Cretaceous
( ?)
: Limestone south of Cidra
For the most part the sequence suggested utilizes formational names
that are already in use for strata in the several districts, although it erro-
neously refers to the Fajardo shales as limestones.
It introduces one new
term for a lithologic unit in the Fajardo district ; namely, the "Cape San
Juan limestones/^
This designation cannot be accepted for the rocks of
the Cape San Juan region because, in the
first place, they are not lime-
stones, and, in the second, the name "San Juan" is already in use for the
Quaternary dune sands of the north
coast.^^
Based upon the work of
Berkey, Semmes, Hodge, Mitchell, and Yaughan, the stratigraphic suc-
cession proposed by Maury
is vitiated by the lack of correlation among
their several studies.
The limestone south of Cidra
is equivalent to the
La
Muda,
and
both
lie
stratigraphically
below
the
Fajardo
shales.
Mitchell has shown that Hodge's Eio Jueyes and Rio Descalabrados series
are not simply superposed members of the rock succession, but that they
comprise the same
closely folded formations.
His
discovery
of
Creta-
ceous fossils in the limestone strata, moreover, indicates the need of cau-
tion in assigning them to the Middle and Upper Eocene.
The stratigraphic sequence
The writers approach the task of offering a geologic column for north-
eastern Porto Eico with some hesitation.
Although the earlier studies of
the Scientific Survey imply that this area contains the most ancient mem-
bers of the "Older Series," the rock system has no base in the true sense
of that word.
The younger Cretaceous formations, moreover, have been
eroded; hence the succession presented
is but a
slice from the middle.
Its exact place in the entire system cannot be determined until the whole
of the "Older
Series" in Porto Rico has been resolved into an orderly
sequence of superposed or graditional and interfingering units.
The only
defense that can be offered for publishing a new table of formations at
this time rests upon the fact that the data obtained within the limited area
of the Fajardo district make
it possible to place part of the Cretaceous
system in order.
The chaotic condition which exists as a result of the
contradictions and lack of correlation among the earlier studies is evident
« Meyerhoff, H.
A.
: The
pre-Oligocene stratigraphy
of Porto
Rico.
Science,
N.
S.,
1930,
Vol.
LXXI.
pp.
322-323.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
367
from Maury's attempt to construct a much needed stratigraphic section
from them.
As the foregoing discussion has shown, Berkey gave names
to several lithologic units whose relative positions were unknown ; Semmes
and
Fettke
duplicated
his work
to
some
extent
but
did
not
fill
in
the gaps which Berkey left for the more detailed district surveys ; Hodge
presented
a
stratigraphic column
composed
of
generalized
units,
but
based
it upon too simple a view of the geologic
structure;
his
strati-
graphic conclusions are contradicted both by Mitchell and by Semmes.
The determination
of the structural relationships
of the rocks along
the western border of the Fajardo district has made
it possible not only
to fix the positions of the formations which Berkey segregated, but also to
differentiate
the
associated
rocks
into
definite
formations.
The
entire
section has been correlated throughout the Fajardo district and has been
traced into the immediately adjoining areas, and some of the discrepancies
in earlier reports have been
settled.
Obviously the section given in the
accompanying table and discussed
in
the following pages will need
re-
vision and correction when correlation can be extended still farther west,
for some of the strata may prove equivalent to formations in other parts
of the island which have already been named.
It
is not likely that the
succession will apply in every part of Porto Eico.
In faet, the geologic
sections at opposite ends of the Fajardo district are not identical, as the
restored
section
(Fig.
16)
shows, and
it
is
to be expected that
lateral
changes of as great magnitude take place westward.
The sequence pro-
posed, therefore, does not purport to be regional in application, for
its
utility has not been tested outside the northeastern portion of the island.
THE CRETACEOUS FORMATIONS
The Guzman formation
The oldest rocks exposed in the Fajardo district have been named from
their occurrence in the western part of Barrio Guzman Abajo, where they
outcrop along Eio Canovanas about nine kilometers south of the village
of Canovanas.
They embrace an imperfectly understood series of slate-
blue calcareous shales and tuffaceous limestones, massive
tuffs, volcanic
breccias, and andesite flows.
Their differentiation as a formation distinct
from the overlying Hato Puerco tuffs is based upon their stratification and
upon the presence of calcareous members, for these features indicate cir-
cumstances of genesis that contrast with those under which the superim-
posed tuffs were formed.
The Hato Puerco tuffs rest upon the calcareous
268
SCIENTIFIC SURVEY OF PORTO RICO
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MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
269
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270
SCIENTIFIC SURVEY OF PORTO RICO
shales of the Guzman formation directly, and evidence of a disconformity
is absent; but the change from conditions favoring the accumulation of
pyrogenic,
pyroclastic, and
hydroclastic materials
in
shallow
vi^ater,
to
those favoring extremely rapid subaerial deposition of ill-assorted explo-
sive products may well have involved a temporary break in deposition,
even
if erosion did not intervene.
The stratigraphic position of the Guzman formation has been deter-
mined from its situation at the crest of the pitching anticline which under-
lies the northern slopes and foothills of the Luquillo Mountains.
In con-
sequence of its structural position the strata composing it have low dips,
which undulate gently in the exposures along the carretera that follows
the valley of Eio Canovanas.
Comparatively
little
of the formation
is
exposed, for both to the north and to the south it passes beneath the over-
lying tuffs, which are cut off southward within a short distance by an ex-
tensive intrusion of andesite porphyry.
The areal extent of the Guzman
formation is unknown.
The limits shown on the geologic map can lay no
claim to accuracy, for the ruggedness of the country, the absence of roads
and the scarcity of trails proved insurmountable obstacles in the
efforts
to trace
it toward the east.
It has been projected as far as El Yunque
partly on the basis of the geologic structure, the details of which are none
too certain, and partly on the strength of a brecciated limestone which
outcrops high on the northern flank of El Yunque.
There fragments of
gray, coarsely crystalline limestone are embedded in an andesitic
sill or
flow.
Although the limestone bears little or no determinable relationship
to the darker calcareous sediments in the Eio Canovanas valley,
it has no
demonstrable affinity with the two higher limestone members in the strati-
graphic section,^^ both of which appear to be absent in the northeastern
part of the district. A correlation made on as flimsy evidence as this, how-
52 In making
this statement
the
authors
disregard
a resemblance which
appears
in
microscopic
sections among the limestone fragments of the El Yunque
breccia, one of
the zones in the La Muda limestone, and a bed of limestone in the Barranquitas-Cayey
series
(cf. Hodge,
op.
cit.,
pp. 134-135).
These three
roclss are composed of
rolled and
abraded
particles, many
of them
consisting
of small
fossils, cemented
in
a
recrystal-
lized calcium carbonate matrix.
This mutual resemblance may
prove more important
than the writers believe, but the feature appears to be
local
in
its development and
is
not one that can be used
safely
in
regional
correlation.
The
other
zones
in
the La
Muda
limestone,
for
instance,
are
genetically
different, and on
the
basis
of
physical
characteristics exhibited in thin
sections might easily be thought to belong
to a
differ-
ent formation.
The coincidental occurrence
of limestone and volcanic
brecciation, and
the structural position
of the breccia on El Yunque are features which
relate
this and
the
associated beds with
the Guzman
formation, and which appear
to
be
of
greater
significance than an accidental resemblance to other strata.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
271
ever,
is worth little, and the Guzman formation needs more careful field
study before it can be accurately mapped and adequately defined ; or before
the possibility of its correlation with younger beds in the section can be
entirely eliminated.
In Barrio Guzman Abajo the lithologic characteristics of the formation
are distinctive.
Massive andesitic breccias predominate and are character-
ized under the microscope by pronounced flow structure in a cryptocrystal-
line matrix, which is but rarely interrupted by phenocrysts of plagioclase
feldspar.
The included fragments
are
invariably
volcanic.
Alteration
processes have involved the development of chlorite, epidote, and zoisite,
and the rocks are considerably indurated by introduced quartz.
Second-
ary calcite is so abundant in the fracture planes and pore spaces that in
the field some of the thinner layers were mistakenly called limestone brec-
cia.
Clearly the secondary calcite was derived from the intimately asso-
ciated tuffaceous limestones and calcareous shales.
The latter vary from
medium-bedded
to
thinly
stratified blue-gray
rocks,
all of which have
undergone extensive recrystallization and induration.
In relative abund-
ance and importance they are second only to the breccias.
Non-calcareous
tuffs occur, but in Barrio Guzman Abajo they comprise but a minor por-
tion of the
section, whereas andesite flows, containing large amygdules
rimmed with chlorite and filled with calcite, occupy a somewhat more con-
spicuous place.
In the type locality the Guzman formation has escaped
extensive intrusion, but it
is penetrated by a small number of porphyry
dikes.
The outcrops along Kio Canovanas do not constitute an ideal type sec-
tion, because the base of the formation is not revealed and only the upper
300 to 400 feet are exposed.
Whether a more satisfactory section
is to be
found elsewhere in Porto Eico
is unknown.
It
is possible that the
for-
mation outcrops near the border of the San Juan and Coamo-Guayama
districts along the axis of the anticline which brings the Kio de la Plata
"series" to the surface; but Hodge^s account of the geology of this area
contains no mention of rocks that can be correlated with
it.
Unless the
genetic conditions which gave
it
its
distinctive
characteristics changed
laterally at the time of origin, it can be easily recognized from its strati-
fied calcareous members and
its
brecciation.
Apparently the
anticline
near the headwaters of Eio de la Plata has not been eroded deeply enough
to bring it to the surface, and there is no information available on which
its occurrence in the western half of the island can be postulated.
272
SCIENTIFIC SURVEY ^F PORTO RICO
The Hato Puerco tuffs
Approximately half of the rocks of the Cretaceous section in the Fajardo
district consist of massive tuffs, in which there is practically no vestige of
structure, except in one thin member of bedded ash exposed in the valley
of Eio Grande de Loiza, two miles due south of Trujillo Alto.
In the
northwestern part of the district, however, the upper portion of the for-
mation contains an increasing number of crudely stratified layers, which
grade vertically into the overlying Guaynabo formation ; and the fact that
the latter thins eastward and finally disappears south of Trujillo Bajo,
whereas
the massive
tuffs
thicken,
indicates the
probability
of
lateral
gradation as well.
Southwestward the massive
tuffs can be traced into
the Eio de la Plata series of the Coamo-Guayama district, but they form
only part of the
latter, in which Hodge has
also included some of the
overlying stratified rocks.
In the accompanying table
it has been sug-
gested that the massive tuffs of the Fajardo district may be equivalent to
those which Berkey described and named from their occurrence in the
Sierra de Cayey between Guayama and Cayey,^^ but this conjecture can
be substantiated only by additional field work of a structural character.
Until the rocks of the two localities are proven to be identical,
it seems
necessary to propose a provisional formation name which will include the
unstratified
tuffs
which
lie
between
the Guzman
formation
and
the
Guaynabo formation in the western part of the Fajardo district and in the
northeastern corner
of
the Coamo-Guayama
district; and between
the
Guzman and Luquillo formations nearer the eastern end of the island.
The tuffs outcrop throughout such a large part of the Fajardo
district
that many satisfactory type localities are available.
Barrio Hato Puerco,
situated on Eio Canovanillas midway between Barrio Guzman Abajo and
Trujillo Bajo, has been chosen because here the formation attains what is
believed to be its maximum development, and because its base and top are
exposed or can be very closely approximated.
The Guaynabo formation is
either thin or absent, and the Hato Puerco tuffs give way abruptly to the
overlying sedimentary series.
The estimate of thickness which has been
given is at best a rough one, based on the width of outcrop and the dips of
the subjacent and superjacent strata, with a conservative allowance for
duplication by faulting.
Even in those few places where the dips of the
^ Berkey,
C. P.
: Geologic reconnoissance of Porto Rico, op.
cit.,
pp. 25 and 61.
Unfortunately the name
"Sierra de Cayey" was
later used by Hodge
for a thick
conglomerate which appears not
to have been embraced
in the formation
differentiated
by Berkey.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
273
associated rocks are steep, the Hato Puerco tuffs outcrop in a band con-
siderably more than one mile wide; hence, the maximum figure of 5000
feet ascribed to the formation seems neither too
little nor too much,
al-
though the great areal development which the tuffs attain gives an im-
pression of a much greater thickness.
The competence of the formation under lateral compression
is every-
where apparent, for major fractures are far more abundant than in any
other lithologic unit in northeastern Porto Eico.
Some fractures have
been filled by dikes; others show from associated slickensides the
effects
of displacement, although the simple character of the faults implies that
the amount of movement was not great in any single case.
The rocks comprising the formation have been called
tuffs, and
cer-
tainly the monotony of the deep, reddish residual soils or of the scattered
gray-green outcrops gives an impression
of
the
lithologic homogeneity
which a
single
descriptive name
implies.
But the impression
is com-
pletely dispelled by the microscope.
Perhaps four-fifths of the formation
consist of tuff, which ranges in texture from ashy to agglomeratic.
The
large fragments in the coarser varieties are invariably volcanic in char-
acter but were obviously derived from several distinct sources.
They in-
clude glass, felsites, and porphyries of several kinds, but the majority of
the pieces consist of andesite porphyries
of different textures and com-
positions.
In many parts of the formation the pieces are angular, but in
others definite rounding may be noted, and some outcrops were described
in the field as conglomeratic.
In the medium and fine-grained horizons
the determinable fragments usually consist of crystal grains,
of which
altered plagioclase feldspars are the commonest.
Among the components
that are conspicuously absent from the assemblage of explosive materials
are bombs, lapilli, and other ejectamenta that result from the subaerial
consolidation of molten matter.
The few glass fragments identified appear
to have been secondarily derived.
It
is quite possible that a more thor-
ough search and a more careful selection of specimens will reveal some
primary fragments ; for in a formation as prevailingly pyroclastic as the
Hato Puerco tuffs, a considerable amount of this kind of material
is to
be expected.
Its scarcity, or absence, raises a question whether the for-
mation represents an old volcano, or whether the whole was formed by
the accumulation of fragments transported fluvially from a nearby high-
land composed of diverse volcanic rocks.
The angularity of so much of
the material casts some doubt on the second possibility, but in itself does
not constitute conclusive evidence in favor of the
first.
In any case, the
274
SCIENTIFIC SURVEY OF PORTO RICO
included fragments offer a means of surmising some features of the pre-
Hato Puercan history of the region.
Although the earlier development
need not have been geologically long^, it was exclusively volcanic, and the
extrusive activity was essentially of the same type as that which occurred
in Guzman and Hato Puerco times.
An epoch or period antedating vol-
canism cannot be envisioned from any of the second-hand evidence sup-
plied by the rocks represented in the Hato Puerco tuffs.
The remaining fifth of the formation comprises volcanic flows.
Per-
haps some
sills are present,
for not a
little of the volcanic material
is
massive and porphyritic ; some porphyry intrudes the tuffs as dikes, but
much of
it cannot be oriented with respect to
structural
lines, and
its
original form is unknown.
The proportion of volcanic rocks mentioned
above
is not claimed to be mathematically
accurate,
for the amount of
flow material
present
varies
greatly from
place
to place and
in some
localities makes up
a
larger
percentage
of
the
whole.
Furthermore,
alteration and weathering have so changed the appearance of the volcanics
that without careful
microscopic
analysis
it
is
difficult
to
distinguish
them from the
tuffs; and
a
final opinion regarding the
pyroclastic or
pyrogenic origin of a small number
of specimens has not been reached
even by microscopic study.
The greater number of the flows consists of
amygdaloidal or vesicular andesites that exhibit a moderate compositional
range,
chiefly in a subsilicic
direction,
for several augite andesites and
a few basalts have been
identified.
Alteration has been
extensive and
was dominated
as a rule by the process
of
chloritization, some
of the
specimens having been changed almost completely to that mineral.
In
a moderate number of the rocks flowage
lines are apparent not only in
thin-section but also
in outcrops, and when
visible in the exposures
of
the formation, they furnish one of the few clues to its structure.
The great thickness of the formation and
its occurrence in the broad
anticlinal structures which underlie so large a part of the Fajardo
dis-
trict combine to make the outcrops of the Hato Puerco tuffs very exten-
sive.
Massive
tuffs underlie nearly
all
of the
hill
section
east
of Rio
Grande de Loiza, and from this central area they follow the crests of the
anticlines to the northeast toward Cape San Juan, to the east into the
Luquillo Mountains,
to
the west-southwest beyond Aguas Buenas
into
the Coamo-Guayama
district,
to the northwest as far as Barrio Cupey.
In the last direction the steep pitch of the structure and the gradation
of the tuffs into the Guaynabo formation cause them to disappear beneath
younger formations within a relatively short distance.
They also occur
in the hills south of the Caguas basin.
Despite a moderate amount of
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
275
induration, the formation
is deeply weathered, partly in consequence of
long exposure, partly as a result of the alteration which took place dur-
ing the later igneous history of the island.
The brick-red
soils which
its rocks yield
are more
characteristic than
its gray-green outcrops
in
the large areas over which
it is exposed ; but in the fresh rock surfaces,
which may be found along the streams and in a few road cuts,
its un-
relieved massiveness
is
its most
diagnostic
property;
for
in no
other
formation
in the
district
is evidence
of
resorting and
stratification
so
completely absent.
The Guaynabo formation
The name "Guaynabo^^
is proposed
for the
succession
of sediments
which lie below the La Muda limestone and above the Hato Puerco tuffs
in the northwestern part of the Fajardo district.
The town of Guaynabo
lies near the southwestern edge of the broad formational outcrop, which
extends beyond the
limits of the
district to Bayamon, where the
sedi-
ments lose their identity in the deep residual soils.
The formation thins
rapidly southward and somewhat more gradually eastward, where a slen-
der wedge of
it may be
identified
as far as the camino from Carolina
to Gurabo.
Along the carretera south from Canovanas, four miles far-
ther east,
it was not found.
The thinning and disappearance
of the Guaynabo formation may be
interpreted in one of two ways
: either it rests disconformably, or uncon-
formably, upon the Hato Puerco
tuffs, against which
it ends eastward
by
overlap;
or,
it grades
laterally
into the massive
tuffs by
replacing
overlap.
The problem of interformational relationship was not appre-
ciated when the field studies were made, and data on which a choice be-
tween the alternatives might be based were not collected.
Coarse water-
worked
pyroclastic
sediments
are
numerous
enough
throughout
the
Guaynabo
section
to
suggest
a
close
genetic
relation
with
the
Hato
Puerco, but not necessarily a contemporaneous origin.
The abundance
of
stratified ash, ashy shales, and bedded tuffs suggests that the north-
western corner of the district lay on the periphery of an eruptive center,
where the explosive products were
smaller, save during short sporadic
outbursts, and where the fragments were sorted and rounded in shallow
marine waters as they
fell.
The volcanic center appears to have been
situated toward
the
southeast and may have been located
in
the
area
of andesite porphyry between Barrio Guzman Abajo and Juncos.
Many
of the features of the Guaynabo formation foster the view that its deposi-
tion was modified and
controlled by
its proximity
to
a
large
volcanic
276
SCIENTIFIC SURVEY OF PORTO RICO
cluster, but the view must seek
its support from too many untested as-
sumptions.
Until the
field relationships
of the Hato Puerco
tuffs are
studied more
critically,
there
is
little
to
be gained by
discussing the
hypothesis at greater length.
Lithologically
the Guaynabo
formation
is more
heterogeneous than
any
of
the
other sedimentary units in the
district.
In
the exposures
along the carretera from San Juan to Caguas by way of Guaynabo, and
along the old military road between Eio Piedras and La Muda, no one
rock type can be said to dominate.
Beds of ash, tuff, and shale, ranging
from buff to dark gray and green in color, increase in relative abundance
northward, but in the
central and southern
outcrops,
beds
of
coarser
material
are numerous.
Interstratified with
the
tuffaceous
sandstone,
ashy shales, and agglomerates are some calcareous shales and shaly lime-
stones.
A more discriminating field analysis may resolve the formation
into
several members,
possibly
into
separate
formations;
for notwith-
standing the marked
vertical and lateral changes that occur, there are
zones
in
which
one
sedimentary
type
is
predominant.
The
authors^
collections and
field notes are not adequate to the task of subdivision,
however, and
it will not be attempted.
The stratification of the mate-
rials, their position beneath the La Muda limestone and above the un-
stratified Hato Puerco
tuffs, and their gradational or overlap
relation-
ship
to
the
latter imply
sufficient kinship among the many
lithologic
types for inclusion in a single formation in the present report.
The La Muda limestone
The name ^'La Muda'^ was proposed by Berkey ^* for "a rather heavy
development
of limestone in the
vicinity of La
Muda.''^
He described
the rock as having a coarse fragmental structure almost completely ob-
scured by healing, and he noted also the cavernous character of its out-
crops.
Berkey was not aware of the presence of two limestone formations
in the vicinity of La Muda, but his brief description applies more fully
to the calcareous bed situated one mile north of the village than to the
shaly, or tuffaceous, strata situated to the south.
The latter have been
correlated with the Trujillo Alto limestone, and the name "La Muda^^
will therefore be used for the more northerly of the two calcareous beds.
Semmes likewise restricted the use of the term to the light gray crystal-
line limestone which he found a short distance south of Guaynabo and
traced northwestward toward Bayamon, where it becomes undifferentiable
in the deep residual soils; but in the type locality he failed to discover
" Berkey, C. P.
: op.
cit.,
p. 22.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
277
the
corresponding limestone and erroneously
applied the name
to
the
higher Trujillo Alto.
As the structural studies presented in the preceding
section have shown^ the La Muda limestone
is to be correlated with the
cavernous
limestone
which
outcrops
prominently
along
the
track
of
the Porto Eico Eailway, Light, and Power Company between Rio Piedras
and Trujillo Alto
(Fig.
7)
; with the cavernous member of the Aguas
Buenas
limestones, which were
differentiated by Semmes;^^ and with
one of the two limestones south of Cidra.
On the basis of physical char-
acteristics, the authors have also suggested its equivalence with the more
southerly of the two limestones in the magnetite belt east of Juncos.
The La Muda formation
is exposed in relatively narrow bands on the
limbs of the several folds which furrow the western part of the Fajardo
district.
It measures from 275 to 300 feet in thickness at several widely
scattered
localities, and the constancy of these figures implies that the
irregularities which
have been
observed within
the
strata,
as
well
as
the discontinuity noted in some of
its outcrops, are secondary features
that have developed since its deposition.
In the outcrops near La Muda
and south of Aguas Buenas, as well as in those east of Juncos, it is severed
by intrusives.
In these and other localities the extensive solution of the
calcium carbonate has led to slumping in the overlying pyroclastic mate-
rial, which now
fills many of the older caves.
Obscured by weathering,
features of this kind may too easily be interpreted as evidence of lateral
variation ; but the purity of the formation, which in hand specimen and
in thin-section reveals no pyroclastic content, makes such an interpreta-
tion untenable.
On the other hand,
the limestone disappears
at some
point east of Eio Grande de Loiza, and projection suggests that
it ends
either unconformably or by replacing overlap against the Hato Puerco
tuffs.
Its lateral passage into massive fragmental rocks, as well as the
presence of tuffaceous beds beneath it and of a thick pyroclastic conglom-
erate immediately above
it, makes
its
purity
all the more remarkable.
It seems to have been formed during an exceptionally long
lull
in the
volcanic activity of the time.
It
is a highly fossiliferous formation, in which the megascopic forms
have been less commonly preserved than the smaller types, for the latter,
with fragments of larger fossils, make up the chief constituents of many
of the beds.
In one of the zones the fossils have been worn and rounded,
and under the microscope the rock looks not unlike the much younger
San Juan formation; but the
greater part
of
the formation
contains
algse, Foraminifera, Eadiolaria, and other organic types, some of which
"Ante,
pp. 234-236.
278
SCIENTIFIC SURVEY OF PORTO RICO
have been broken but not rounded.
The organic remains
are
set in a
calcareous matrix which has undergone thorough
recrystallization, and
locally the fossils, too, have been destroyed by recrystallization.
In the
outcrops
east of Juncos extensive metamorphism and replacement have
taken place where the limestone comes in contact with intrusives, and
here the rock consists of an aggregate of calcite, garnet, chlorite, quartz,
and pyroxene, with occasional crystals or crystalline masses of magnetite
and hematite. ^^
One of the most striking traits of the La Muda limestone
is
its sus-
ceptibility to
solution.
In
its outcrops along the railroad between Eio
Piedras and Trujillo Alto large and extensive caverns have been opened
up within it, and the caves which occur in it south and southwest of Aguas
Buenas
are
well known among
the
caverns
of
Porto
Eico.^''
Partly
filled caverns
are
also present at the type
locality.
The recurrence
of
this
striking
erosional
habit, combined with the formation's compara-
tively constant lithologic
characteristics, make the La Muda limestone
an ideal key horizon in the western part of the Fajardo district and in
the adjacent portions
of the San Juan and Coamo-Guayama
districts.
It has no counterpart in the eastern part of the Fajardo region, where
presumably the horizon
is represented by some
of the higher members
of the Hato Puerco tuffs, unless the fragments in the flow breccias on El
Yunque belong to this formation instead of the older Guzman.
The Rio conglomerate
In all the localities where the La Muda limestone has been identified,
except
in Barrio
Collores
in the
eastern
part
of
the Caguas
lowland,
it
is succeeded by relatively massive beds of conglomerate.
Berkey was
under the impression that the conglomerate lies stratigraphically beneath
the limestone at the type locality, but his interpretation
of the relative
positions of the two formations
is at odds with their physiographic ex-
pression.
The resistant conglomerate stands as a prominent linear ridge
which
crosses the Eio Piedras-Caguas highway
at
the northern
limits
of La Muda, presenting a steep scarp toward the northeast and a com-
paratively
gentle
slope toward
the
southwest.
The
scarp
is
erosional
in character and cuts across the beds, whereas the gentler southwestern
slope
appears
to be
the
dip-slope.
The La Muda
limestone
emerges
from beneath the erosional
cliff on the northeastern
side
of
the ridge.
w Cf. Fettke, C. R.
: Magnetite deposits of eastern Porto Rico, op.
cit.,
p. 1028.
57 Dinwiddle, W.
: The great
caves
of Puerto
Rico.
Harper's Weekly,
March,
1899,
Vol. XLIII,
p. 293.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
279
The southerly dip of the two members can be seen and measured in their
exposures
along
the
San Juan-La Muda
carretera,
immediately
south
of the village of Guaynabo, where the limestone
is succeeded by a well
exposed
section
of the
conglomerate, which
outcrops
continuously
for
nearly two kilometers along the road.
In this area
the formation ap-
pears to be very thick, and
it
is perhaps of greater thickness than the
500 feet which have been assigned to
it in the table; but the impression
obtained may err because of the
fact that the
strike
of
the beds cuts
the road at a low angle, and the dips are gentle and undulating.
The
conglomerate is also associated with the La Muda limestone near Aguas
Buenas and along the railroad west of Trujillo Alto, but in Barrio Col-
lores intrusion and metamorphism have occurred
so extensively
that
a
corresponding
horizon
was
not
differentiated.
Its
persistence
in
the
other
localities where
the La Muda
is found
entitles
it
to
recognition
as a separate member of the stratigraphic section.
Some
difficulty has
been experienced in selecting a formational name for
it, because
its nar-
row outcrops form
linear ridges that have not been conducive
to
the
development
of settlements upon them.
The name "La Muda^^ would
be most appropriate, but
it cannot be used; hence Barrio Eio, a small
community
situated
about one
mile
southeast
of La Muda,
has been
chosen
as
the type
locality.
The rocks of which the formation
is composed are coarse in texture,
but not uniformly so.
Some of the beds consist very largely of boulders,
usually of andesite porphyry, embedded in a tuffaceous matrix; in other
strata the
tuffaceous matrix
is the dominant feature, and the boulders
are small and scattered; a few layers are almost wholly tuffaceous.
The
Rio conglomerate
is stratified throughout, although some of the individ-
ual beds are very thick ; and its marine origin is indicated by the minute
fragments of fossils
it contains.
In
its stratification and in the round-
ing of the
constituent fragments
it can be readily
distinguished from
the coarser horizons of the Hato Puerco
tuffs, and
it
is to be differen-
tiated from
the
coarse-textured
strata
in the Guaynabo formation by
reason of
its greater thickness and the persistence
of
its conglomeratic
characteristics.
Like the Guaynabo and La Muda formations,
it passes
westward
into
the Hato Puerco
tuffs,
or ends unconformably
against
them.
The LuqvMlo formation
The Eio conglomerate is succeeded by a thick section of stratified tuffs
and tuffaceous shales which are characteristically calcareous.
They con-
880
SCiEWTIFiC SURVEY OF PORTO RICO
stitute a formation which
is one
of the most persistent
in^ northeastern
Porto Bico, and its oiiterops cover large areas in many localities.
In the
western part of
the
Fajarclo
district both
the
upper and
lower
limits
of the tuffaceons beds are clearly defined, and here they form an
easily
distinguishable
unit
in
the
stratigraphic
section.
Along
the
Trujillo
Alto carretera the formation
is exposed from top
to bottom,
bnt deep
weathering mahes a careful study and analysiB of
its characteristics im-
t the
type locaUty
resistance, and in many places has been carved into a subsequent lowland
or valley by local streams.
Because of the weathering and erosion
it has
undergone in most inland
regions,
it has been named, not from any of
the areas in the western part of the Fajardo
district, but from the town
of Luquillo, where it forms low but prominent dills both along tlie shore-
line and along the banks of Bio Sabana near
its mouth
(Fig. 17).
In
this region the Rio conglomerate, the La Mnda limestone and the Guay-
nabo
formation
are
absent, and
the
stratified
Luquillo
tuffs
rest
di-
rectly upon the massive Hato Pucrco
tuffs.
Because
of the depth
of
weathering
it
is difficult
tx) determine the exact position of
tlic contact
between the two formations, but the change from the massive non-cal-
careons pyroclastics to the stratified calcareous beds can be detected within
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
281
a short distance by the slight yet significant changes in the topography,
soils, and outcrops.
Luquillo and
its environs do not furnish an ideal
type section, because the upper part of the formation has been destroyed
by fluvial and marine erosion.
On the other hand, the freshness of
all
but the basal outcrops and the clarity of the geologic structure make
it
possible to learn more about the strata than can be learned elsewhere;
and the prominence which these calcareous tuffs attain along the north-
ern margin
of Cretaceous rocks
furnishes another reason
for
selecting
a formational name from a town within this belt.
From the type locality the Luquillo formation extends in a continuous
band southeastward almost to Fajardo and thence southward along the
east
coast.
A
synclinal prong
carries
it far
into
the Luquillo Moun-
tains, but elsewhere the bedded
tuffs
occupy a
position
in the eastern
foothill
belt.
They
are
also
present near
the southern margin
of
the
district, but here the outcrops are discontinuous, and their textural and
compositional features are profoundly modified by contact metamorphism.
Except
in
the
central
range
of
the
Luquillo Mountains
and
in
the
vicinity of La Muda, where synclinal structures carry the beds into the
district, the Luquillo formation serves
as a jagged frame for the older
deposits which have been described
above.
It
encloses the underlying
Cretaceous
rocks
so
completely,
that
the
younger
formations
in
the
"Older
Series^^ have
succeeded
in retaining a
place within the
district
borders in very few localities.
In the west the evenness of the outcrop
bands indicates a constant thickness, which has been estimated with a
fair
degree
of
accuracy
at 2500
feet.
In
the
east,
too,
its
thickness
seems
to undergo
little,
if any, change, notwithstanding
the tendency
of all the underlying members
to thin and disappear
in this
direction.
The relation of the Luquillo sediments to the Hato Puerco tuffs suggests
that
its deposition occurred when
the
rapid accumulation
of
the
con-
tinental pyroclastic rocks ceased, and that it was formed by the sorting
and redistribution of the finer materials in the highest portions of that
massive
series
of
explosive
products.
The
relative
quiescence
which
existed throughout Luquillo time
is
attested by the abundance
of
cal-
careous material in the strata.
The calcium carbonate, although some-
what irregularly distributed, increases in the higher beds and culminates
in the Trujillo Alto limestone, which forms the top.
The calcium car-
bonate content also increases laterally to the west, away from the Hato
Puercan mass, which seems to have served as the source for the
clastic
material in the Luquillo formation.
^82
SCIENTIFIC SURVEY OF PORTO RICO
Petrographic study reveals a moderate range in texture and composi-
tion among the members of the formation.
Some of the beds are mas-
sive and coarsely tuffaceous, but a much larger proportion of the strata
exhibits an
arenaceous
texture,
and
in
thickness
the
individual
beds
range from a fraction of an inch to one or two feet.
In both the coarse
and medium-grained layers, the individual particles consist almost wholly
of rounded volcanic fragments, chiefly of andesite and andesite porphyry,
with minor amounts of glassy lava and pumice.
Some
of the beds
in
the middle of the formation and many of those toward the top are shaly
in
texture and
tuffaceous,
ashy, and
calcareous
in
composition.
The
original calcium carbonate content of the individual beds varies in quan-
tity in inverse proportion to the grain-size.
Commonly
it
is present in
the form of microscopic organic remains,
especially Foraminifera.
In
the shales it also consists of finely comminuted shell fragments, in which
all semblance to specific forms of
life has been
lost.
Secondary
calcite
has been introduced into both the calcareous and non-calcareous layers;
but,
as the latter contain a large amount of interstitial pore space, the
secondary calcite
is usually more conspicuous in them.
The abundance
of marine life at the time these tuffs were deposited
is further indicated
by the dark gray
or
slate-blue
colors
of the
fine-grained beds;
in
the
coarser beds, as might be expected, organic material was less often pre-
served, and these rocks have the characteristic greenish hue that
is im-
parted to them by the volcanic fragments.
Calcareous tuffs have been described from many parts of Porto Eico,
and at present it is difficult to correlate those reported in localities out-
side the Fajardo district with the Luquillo formation.
In the present
survey the formation has been traced for a short distance into the San
Juan district, where
it forms a broad band paralleling the outcrops
of
the La Muda limestone
as far as Bayamon.
It has
also been followed
across the southeastern corner of the San Juan
district into the north-
ern part of the Coamo-Guayama district, where
it forms part of Hodge's
Barranquitas-Cayey
series, and perhaps
a small
portion
of
the Rio de
la Plata series.
It seems probable that the Luquillo formation
is partly
or wholly equivalent to the Barranquitas shaly limestones of Berkey, and
if future
field work should prove these two formations to be correlates,
it will be necessary to discard the name "Luquillo^^ for the prior name
^^Barranquitas,^' which was
given by Berkey
in
1915.
Correlation
of
the
Luquillo with
the
calcareous
tuffs
described
in
the
districts
still
farther west
is not yet possible.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
283
The Trujillo Alto limestone
Berkey gave this name to the dense bluish limestone which outcrops
on the carretera about one mile north of the settlement of Trujillo Alto.
He did not know that the formation is present also a short distance south
of La Muda, where
its outcrops are nearer the village than those of the
La Muda limestone.
Because
of the
geographic
relation
of these two
limestones to the localities from which their names were
derived, there
is some danger of confusing them, but their physical characteristics are
so entirely distinct that there
is little risk of mixing them once they are
differentiated.
The Trujillo Alto limestone
is invariably dark in color,
the La Muda light ; the former is visibly tuffaceous, the latter chemically
and mineralogically pure; the former displays few effects of the solvent
action of ground water, whereas the La Muda limestone is perforated with
caves and solution channels wherever its beds come to the surface.
The Trujillo Alto limestone grades downward into the calcareous tuffs
and shales of the Luquillo formation, and in most places
its lower beds
are so extremely tuffaceous that it is impossible definitely to establish its
base.
The
limestone
is
clearly
a
gradational
phase
of
the
Luquillo
formation,
representing the end point
of the sedimentary
cycle which
started with
the
deposition
of
the
Eio
conglomerate.
Because
of
its
relation to the underlying
rocks, the
Trujillo Alto may be considered
merely as a member of the Luquillo formation ; but the formational status
given it in the present report seems preferable in view of
its distinctive
lithologic characteristics and
its geographic distribution, features which
make it one of the most satisfactory stratigraphic horizons in the eastern
two-thirds of the Fajardo
district.
From the type
locality
it may be
traced westward toward Eio Piedras, although
its identity
is lost in the
deep residual
soils near the town.
Eastward
it extends at least as far
as Canovanas, and
it
is probable that the calcareous beds exposed along
Rio Sabana about one mile south of Luquillo are to be correlated with it
(Fig.
9).
It has not been found along any portion of the east
coast,
in spite of the fact that the calcareous tuffs and shales of the Luquillo
formation occupy a conspicuous place in this area.
In the southern part
of the
district it reappears as a short band truncated by igneous intru-
sives
not
far from
the
village
of
Torres
(the
CoUores
limestone
of
Fettke).
Its exposures on both flanks of the plunging syncline
at La
Muda have been followed for
short
distances
into the San Juan
dis-
trict and into the northeastern corner of the Coamo-Guayama
district,
where
it may form one of the members of the Barranquitas shaly lime-
284
SCIENTIFIC SURVEY OF PORTO RICO
stones
of Berkey
(Barranquitas-Cayey
series
of Hodge).
Its
correla-
tion with the latter, however, was not fully established.
The Trujillo Alto limestone seems to have attained
its maximum de-
velopment in the type section north of Trujillo Alto, where
its exposures
measure not more than 200
feet
in
thickness.
In
other
parts of the
district the gradation of
its lower layers into the underlying calcareous
shales and
tuffs reduces the thickness of the limestone beds.
Eastward
along the north
coast the thinning
of the horizon
is
appreciable, and
beyond Eio Sabana, as has been pointed out,
it no longer acquires sepa-
rate
lithologic
expression.
In
spite
of
slight
variations,
the
physical
characteristics
of
the
formation
are
dependably
constant.
Normally,
the beds are microfossiliferous, and Foraminifera,
algae, and Eadiolaria
have been
identiJfied within
it.
For the most part,
it
is a fine-grained,
somewhat recrystallized lime-mud, or calcilutyte, in which the evenness
of texture is occasionally broken by crystal fragments, including feldspar,
augite,
and
other mineral
grains
from
the
associated
volcanic
rocks.
JN'ear Torres the fossils have been obliterated by contact metamorphism,
and a
suite
of metamorphic
minerals
has
been
developed.
In
other
parts
of
the
district induration and
partial
recrystallization have
oc-
curred but have
not
obscured
the primary
features which
the
strata
The Figuera formation
Along the east coast in the vicinity of Ceiba a series of volcanic flows
is
intimately
associated
with
the
calcareous
tuffs
and
shales
of
the
Luquillo formation.
From the distribution of the flows and their posi-
tion with respect to the Fajardo
shales, they appear to be intercalated
with the higher strata of the Luquillo formation.
They may, in fact, form
the top, although they
are
succeeded by more tuffaceous
layers, which
seem to pass by insensible gradations into the basal portion of the Fajardo
formation.
The
flows
may,
therefore,
be
regarded
either
as
a
local
volcanic member of the Luquillo beds,
or, if the same line of reasoning
be followed as was used
for the Trujillo Alto limestone,
as a
distinct
formation.
In the
latter case the tuffaceous beds above them must be
grouped with
the
Fajardo
shales.
The
second
alternative
is
adopted
in this report, but the disappearance of the
Trujillo Alto limestone in
this region makes
it impossible to determine whether the limestone and
the flows occupy wholly analogous positions in the stratigraphic section.
The geographic extent of the extrusive rocks is unknown.
Two parallel
bands occur near Ceiba, one of them beginning at the east coast at Punta
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
285
Figuera, from which the formation has been named; the other extend-
ing from Puerto Medio Mundo into the
interior
for an undetermined
distance.
The more northerly of the two bands has been traced inland
for a distance
of four miles, but
it has not been followed west
of the
valley of Rio Fajardo.
The more southerly band has not been differen-
tiated from
the
associated
andesite
porphyries
west
of
the
Fajardo-
Humacao carretera, which
it crosses one and a half miles south of Ceiba.
A
third
occurrence
of
the
volcanic
flows
was
found
near
Playa
de
JSTaguabo, but its limits could not be determined.
The material comprising this formation has suffered intense alteration
and deep weathering, yet in many of its greenish outcrops flow structure
is clearly visible.
A mile and a half north of Ceiba an isolated boulder
resembling pitchstone or obsidian was discovered in association with the
flows.
Under
the
microscope
the
rock was found
to
be
microcrypto-
crystalline, spherulitic, and amygdaloidal, with cavity fillings of hematite
and a colorless mineral which has been tentatively identified as quartz.
Although unlike the andesitic material that constitutes most of the Fig-
uera formation,
it appears to be genetically related.
The more common
andesite
is
fine-textured but entirely
holocrystalline.
Some
of
the
in-
dividual flows are amygdaloidal, but many of them lack vesicular cavities
;
and the few scattered phenocrysts, usually of orthoclase feldspar, indicate
a
relatively narrow compositional range, varying between andesite and
trachyte.
Alteration
has
introduced
large
quantities
of
epidote
and
chlorite, and subordinate amounts of quartz and kaolin, into the rocks,
and
a
few
specimens
contain
serpentine.
Weathering
has
converted
the ferro-magnesian minerals partly or wholly to limonite, which colors
many of the more deeply weathered exposures brown.
In a few respects
the Figuera flows resemble the extrusive rocks identified at several points
within and marginal to the Caguas basin, and in a more detailed survey
the
possibility
of
their equivalence should be given
careful
study.
In
the western part of the district the Juan Ascencio member of the Fajardo
formation occupies a stratigraphic position which
is so nearly analogous
to that of the Figuera flows that the two may prove to be related upon
more thorough
investigation.
At
present
the
scant
evidence
favoring
the correlations suggested is not sufficient for the expression of a definite
opinion.
The Fajardo shales
The town of Fajardo
is almost completely surrounded by low hills of
buff-colored
ashy
shales, which form
the
valley
walls
of Rio Fajardo
2S6
SCIENTIFIC SURVEY OF PORTO RICO
for several miles above
its mouth.
The name "Fajardo'' which Berkey
gave these sediments is therefore doubly appropriate ; and in spite of the
complexity of the structure, which makes accurate stratigraphic analysis
difficult,
a
better
type
section
could
not
have
been
chosen.
In
the
Fajardo region the base of the formation has been
arbitrarily extended
downward to the top of the Figuera flows, notwithstanding the fact that,
as thus defined,
it includes a moderate amount of calcareous tuf! which
might with equal propriety be placed in the Luquillo formation.
In the
localities west of Rio Sabana, the designation of the Trujillo Alto lime-
stone as the lower limit of the Fajardo shales raises the same problem.
The writers believe that it is more satisfactory to use well defined horizons
for the
base,
at the expense
of including material
genetically
related
to
an
underlying
formation,
than
to
try
to
discover
a
non-existent
boundary
in
the
interstratified and intergraded
tuffs and
shales
that
mark the passage into the homogeneous strata in the middle and upper
parts
of the formation.
The
top
is
exposed
in
the
Cape
San Juan
peninsula north
of Fajardo, where
the
shales
lie conformably beneath
the San Diego formation and to some extent intergrade with the
tuffs
that belong to the latter.
From the Fajardo valley the ashy shales appear to extend west-south-
westward
into
the
Luquillo Mountains,
where
they make up
part
of
the
crest and southern front of the
central range.
Other
occurrences
of the Fajardo formation are restricted to the sags in the synclinal struc-
ture which forms
the
northern
limit
of
the
Cretaceous
exposures
in
the
district.
Outcrops are found along the Eio Piedras-Fajardo carre-
tera between the valley of Rio Grande de Loiza and
Canovanas; they
occur again in the section between Rio Piedras and Carolina, and in the
environs of Rio Piedras the
tilted beds of the formation outcrop
con-
spicuously.
The
Fajardo
shales have
been
differentiated
in
the
San
Juan district, notably in the large area of stratified ash included between
Bayamon, Toa Alta, and La Muda.
Probably some
of the ashy
sedi-
ments in
other parts
of the San Juan
district can be
correlated with
the Fajardo shales as soon as the geologic structure
is more fully known,
and
their
identification
in
the Coamo-Guayama
district
is
also
to
be
expected.
In
fact,
it
is possible
that these beds continue
to the west
end of the island, where, as Berkey has suggested, the Mayagiiez shales
look suspiciously like those which occur at Fajardo.
The Fajardo formation has proved extremely incompetent under the
compression of post-Cretaceous deformation.
In the northeastern part
of the district
it
is closely folded and thrust-faulted
to such an extent
MEYERHOFF, OEOLOQY OF THE FAJARDO DIBTMIGT
287
that
accurate measurement
of
its
tliiekness
was
impossible,
«iii(l
the
figures given in the table
(750-1000 feet)
are
little more than a rough
approximation.
In
other areas the upper beds have been removed by
erosion, and
the
present
thickness
is
considerably
less
even
than
the
miiiimnm
figirre given.
If the basal strata be left ont of consideration,
the physical characteristics oC the formation are relatively uniform.
The
beds
are
thinly
stratified, and
tlie
closely
s])a('ed
Joints normal
to
the
bedding
planes
give them
a
bloeky
fnieture
(Fig.
18).
Yew
of
the
h'm.
is.
The ftijardo ^haUb
A qiiarrj face at the type locality, one klloinetei
\.>:,?'W>^*"
iith of Fajardo.
layers exceed four inches in thickness, and most of them average about
two.
The rocks are exceedingly fine-grained, and under the microscope
the individual particles which can be isolated consist ebielly of anguhir
fragments of volcanic glass and, to a less extent, of mineral fragments.
The minute grains which compose the groundmass exhibit the polariza-
tion of quartz, and, if tlie mineral
is correctly identified, its presence and
abundance
ai-e indeed
puzzling, unless
it be assumed
that
it has been
introduced
into
the
rocks
by
processes
of
alteration—an
assumption
which could not
be
verified.
The buff
color, which
is such
a
striking
feature of the formiition in
its deeply weathered exposures in the coastal
areas,
is due to the presence of interstitial hematite.
Weathered speci-
mens have an unusually low specific gravity, a feature which, in micro-
288
SCIENTIFIC SURVEY OF PORTO RICO
scopic
sections, may be traced
to the abundance
of
cavities,
formerly
occupied by minute Foraminifera that have been dissolved in the course
of rock decomposition.
In the Luquillo Mountains
the
strata which have
been
referred
to
this horizon do not resemble superficially the beds in the coastal region,
save in their shaly structure.
The former are
dense,
slate-blue
rocks,
which look more like the calcareous shales of the Guaynabo and Luquillo
formations than they do like the buff shales in the vicinity of Fajardo
and Eio Piedras.
The differences exhibited appear to be due largely, if
not wholly,
to the freshness
of the outcrops on the steep
flanks of the
mountains, and not to differences in the initial chemical or mineralogic
composition.
The mountain sediments lack the cavities but contain the
Foraminifera and so have a higher calcium carbonate content than the
beds at the type locality.
The organic material has not been oxidized,
and
its presence prevents
the
iron oxide from imparting
color
to the
rocks.
In other more fundamental respects, however, they are identical
with the buff shales at lower elevations.
Berkey, too, has pointed out
the evident genetic relationship between the weathered buff shales and
the unweathered bluish
shales ;^^ but in view of the need to verify the
correlation of the coastal strata with those in the Luquillo Mountains
and
other
elevated
portions
of the
interior,
it
is important
that
the
identity of the two types of material be more fully confirmed by addi-
tional studies.
At present the possibility that the beds in the mountains
which have been referred to the Fajardo shales may prove to be a more
shaly phase of the Luquillo formation, has not been wholly eliminated.
The Juan Ascencio member
Closely associated with the calcareous tuffs that lie above the Trujillo
Alto limestone in the western part of the district is a series of deposits
of distinctive lithologic character, but of uncertain derivation.
Semmes
called them the "Juan Ascencio chert beds,'^ from Barrio Juan Ascencio
at the eastern edge of the San Juan district, not far from Aguas Buenas.
It has already been pointed out that these rocks are not cherts, but that
they consist largely of the mineral laumontite, with hematite filling the
interstitial spaces and giving the rock a reddish
color and a strongly
banded appearance.
E.
J. Colony suggests that they may have been
formed by hydrothermal
action upon calcareous
tuffs, and Hodge has
»® Berkey, C. P.
: Geological reconnolssance of Porto Rico, op.
cit.,
p.
19.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
289
postulated a somewhat similar origin for petrographically related rocks
on the southern slopes of the island.
The Juan Ascencio beds bear some physical resemblance to a series of
flows which outcrop near the village of Torres and again in the escarp-
ment on the north side of the Caguas basin a
little more than a mile
from Juncos.
Correlation of the latter deposits with the Juan Ascencio
member cannot be made from the data
collected
in the
course of the
field survey, but the possibility
is sufficiently strong to warrant further
investigation.
The
fact
that
the
Figuera
flows
occupy an
analogous
stratigraphic position along the east coast suggests the idea that they, too,
may
be
related
genetically
to
the
processes
that formed
the
banded,
laumontitic rocks
along the western
border, but
this
conjecture com-
pletely lacks substantiation.
Because of the striking lithologic charac-
ter of the Juan Ascencio beds,
as
well
as
their
clearly defined
strati-
graphic
position and
their
persistence in the La Muda-Aguas Buenas
region,
it seems desirable to accord them recognition as a basal member
in the Fajardo
shales.
Future study may show that the horizon
rep-
resents
a phase of widespread volcanic
activity that assumed
different
modes of expression in various parts of the island.
The San Diego formation
This name
has been
applied
to
the
series
of
stratified
tuffs which
form much of the Cape San Juan peninsula and the tied island that
now
constitutes Punta
San
Diego
(Fig.
19).^^
The
validity
of
the
formation depends wholly upon the accuracy with which
the
authors
have interpreted the structure of the Cape San Juan area.
Along the
rugged shoreline on the eastern side of the peninsula where exposures are
best, the Fajardo shales appear to pass beneath the tuffaceous deposits;
but critical sections of the shoreline were found difiicult of access, and a
re-examination of the region is desirable to prove with greater certainty
that an unobserved
reversal
of the
dip
has not brought
the
Luquillo
formation back to the
surface.
If the writers'
interpretation
is
right,
the San Diego formation occurs in no other part of the
district, for
its
high stratigraphic position has led to its removal by erosion.
Pyroclastic
beds
which
are
tentatively
referred
to
it
outcrop
extensively
in
the
pitching syncline which stretches from Aguas Buenas to Toa Alta.
Xear
the center of this structure a complete
section
is undoubtedly present,
and had time permitted,
it
is probable that a better type locality could
*» Meyerhoff, H. A.
: The pre-Oligocene stratigraphy of Porto Rico, op. clt,
p. 323.
290
SCIENTIFIC SUMA'EY OF FORTO RICO
have been discovered in this part of the San Jueb district
It may be
questioned,
however,
whether
the
deeply
weathered
exposures
in
the
interior are as suitable for study
as those in the imposing
cliffs
of
the
northeastern coast.
The principal disadvantages which
the
hitter area
possesses as a type region
lie in the fact that the top of the formation
does not occur, and the upper beds are considerably modified and con-
fused by the
dioritic intrnsions which
cut them.
If
these
disabilities
are overlooked, the exposures are unrivaled and they reveal more of the
Fig.
1ft.-
Piintn
»>!«« Diego
The
type
locnUty
of
the
San
Diego
formation.
Tlie
tuffs
and
sill:
appear
to
be
"resistant,
for
they
underlie Cape
San
Juao
and
the
nor
Culebra, where,
too, thej are expo.sed to the vigorous attaek
of
tlie
Atlai
stratigraphic
details than
those
of any
other
locality
in
northeastern
Porto Bico.
The conformable relation of the San Diego formation with the under-
lying Fajardo
shales, and the intcrgradational character of the contact
are features which have already been mentioned.
The
strata are com-
posed of thin
to medium bedded
tuffs with some
interstratified shales
and
intercalated
sills.
A few of the fragmental
layers
are
calcareous,
but the greater part of the calcium carbonate
is secondary, and there
is
much
less of
it than there
is in the I/uquillo formation.
The greenish
andesite fragments, which give many of the beds a coarse sandy texture,
impart
a
characteristic
color
to
the
entire
formation.
These
physical
and chemical features, the associated
sills, and the absence of horizons
corresponding
to
the
Trujillo
Alto
limestone
and
the
Figuera
flows
are facts which give the writers grounds for believing that the San Diego
formation
is the youngest
stratigraphic unit in
the
district, and
not
the Luquillo formation resurrected on the limb of an unidentified
fold.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
2&1
Conclusions
In the foregoing description of the Cretaceous system^ the persistence
of the several formations has been especially emphasized, but a broader
view of the rock section reveals the marked lateral changes that occur.
From the western border of the Fajardo
district to the east coast the
change
is little less than a transformation; for along the former, strati-
fied materials comprise nearly three-quarters of the total section; along
the latter,
three-fi.fths, or more, consist of unstratified pyroclastic rocks
and volcanic flows
(Fig.
16).
The
vertical change that occurs in the
10,000 to 12,000 feet of material is of a similar nature.
At the bottom
the unstratified
tuffs of the Hato Puerco formation are found
resting
upon limestones and volcanic breccias.
Except for the interpolation of
an
occasional
flow
or
a
sill,
the
coarse fragmentals
continue upward
with scarcely a break for 5000 feet in the east, and for at least 3000 feet
in the west and northwest.
When eruptive activity and rapid accumula-
tion ceased, marine conditions followed, and the upper part of the Hato
Puerco tuffs was reworked into the sediments of the overlying Luquillo
formation.
For the development of the highest formations, the Fajardo
shales and the San Diego
tuffs, new accessions of pyroclastic materials
from remoter sources must be postulated.
The
full
significance
of
the
lateral and
vertical
changes
cannot be
grasped
until more
information
about the
distribution
of
the
several
formations
in
adjacent
areas
is
obtained.
The Fajardo
district
obvi-
ously does not embrace the whole of the picture which the stratigraphic
section suggests, but provisional
inferences
are
irresistible.
The Hato
Puerco formation appears to have been a huge volcanic cone or cluster,
the
profile
of which was
materially lowered
by
the
contemporaneous
effects
of marginal wave
action; and on
its
northwestern
margin,
at
least, strata composed of sorted and water-worn fragments either inter-
finger with the massave
tuffs
of the volcano or end against
its
flanks.
Eruption was
intermittent, and
in
the
quiescent
intervals
calcareous
strata were formed.
The episode ends with complete submergence and
renewed volcanic activity, which this time emanated from a more
dis-
tant center.
The reconstruction of the Hato Puerco tuffs into a volcanic cone or
cluster may,
it must be admitted, involve more imagination than
fact.
The disappearance of the Guaynabo, La Muda, and Kio formations, a
part of the sedimentary section involving 1500 to 1800 feet of material,
takes place within the space of ten miles, and in the same distance the
292
SCIENTIFIC SURVEY OF PORTO RICO
Hato Puerco tuffs seem to thicken a corresponding or greater amount.
If
these
estimates
are
at
all accurate, replacement
or
overlap
of 200
feet per mile
is required.
A slope of this declivity
is, of course, too low
for a normal volcanic cone composed
of fragmental materials;
but, on
the other hand,
it
is too high for a graded marine platform, unless
a
rapid rate and an abnormal amount of subsidence are postulated.
The
available
data
are
thus
not wholly consonant
with
the
interpretation
offered,
or with any
other
reasonable
hypothesis;
and
only
a
better
acquaintance with all the facts can lead to a sound and acceptable inter-
pretation.
UPPER CRETACEOUS AND POST-CRETACEOUS VOLCANISM
In the preceding pages repeated mention has been made
of volcanic
activity and
of igneous
intrusion.
In
fact,
it has been
seen
that the
accumulation of the thick stratigraphic section was controlled through-
out Upper Cretaceous time by eruptions which varied in intensity and
duration, and which produced
effects that were closely dependent upon
the remoteness of the active vents.
During the early part of the period
which
is represented by the fragmental and organic
deposits,
explosive
activity took place at close range, but later
it was more
distant.
The
change
is
recorded
in
the
lithologic
contrasts between
the
lower and
upper parts
of the
stratigraphic
section, which
likewise mirrors
every
minor igneous and diastrophic pulsation that occurred.
Volcanic activity
started before the oldest sediments in the Fajardo district were deposited,
and
it did not entirely
cease until the youngest Cretaceous
rocks had
been formed.
Volcanism ended, however, some time before the
Oligo-
cene and Miocene marls and limestones
of the
coastal plain were
laid
down, stopping, apparently, with the erogenic disturbances that preceded
Tertiary deposition.
Whether this happened in the Cretaceous or in the
early Tertiary
is unknown;
for,
like the stratigraphic history, the be-
ginning and end of igneous activity fade into the obscurity which
veils
so many of the pre-Oligocene events.
Notwithstanding the imperfection of the record, more has been learned
about the volcanism than about any
other phase
of Mesozoic
history.
Events centered around the activity of a dioritic magma; and pyroclas-
tic
materials,
flows, and
intrusives approach
this
composition
as
the
norm,
though
differentiation
gave
rise
to many
local
compositional
varieties.
A few
distinct tendencies are apparent: Among the micro-
crystalline and porphyritic rocks rhyolitic types are scarce, whereas basalts
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
293
are more
abundant.
Among
the
granitoid
rocks, on
the
other hand,
granites are commoner than gabbroic differentiates.
A study of the forms
which the igneous material assumed when it came to rest within the older
rocks or upon the surface, indicates that few important structural types
are missing.
Among the surface forms, volcanoes were indubitably pres-
ent during Cretaceous time.
Throughout the 200-miles' length of the
Porto Eico-Virgin Islands platform, the recurrence of thick sections of
unmodified tuff and agglomerate implies the
existence
of
several large
and vigorous eruptive centers.
Dissected by fluvial agencies, planed or
modified marginally by marine
erosion, and folded, none
of the cones
has retained its initial shape, and the few that can still be identified must
be isolated by inferences based upon dike systems and plug-like masses
that suggest the positions of ancient vents.
Only rarely does a feature
like the replacing overlap which has been discussed in connection with the
stratigraphy make partial reconstruction of a specific form possible.
Sills
are
so
often
intercalated
with
the
strata
of
the
sedimentary
formations
that the presence
of
flows
has
generally been
overlooked.
Berkey wrote
: "On the whole, evidence of lava flows on a large scale are
wanting.
This kind
of product seems
to have been
very much more
rare than
the fragmental
type." ^^
Semmes
spoke
of
the
scarcity
of
flows in the San Juan district,^^ and Mitchell ^^ stated flatly that "all of
the igneous rocks in the Ponce district have an intrusive relation to the
Cretaceous
sediments.^^
But
Mitchell
apparently
missed
one
of
the
largest single lava flows which the senior author has seen in Porto Eico
a
striking amygdaloidal
basalt that outcrops continuously
for
several
miles along the road connecting Adjuntas with the Yauco-Lares
carre-
tera, not far from the northern boundary of the Ponce district.
Hodge
mentioned but a small number of flows in the Coamo-Guayama district
;
Hubbard described only one in the Lares district; and Fettke was un-
willing to commit himself as to the intrusive or extrusive origin of the
felsitic rocks in the southeastern corner of the island.^^
It is possible that the outpouring of lava was accidentally concentrated
in the Fajardo district, but in view of the abundance of flows there, the
writers must take issue with Berkey's general statement.
Lava was ex-
truded in every part of the district, and it is present in most of the bedded
formations of clastic origin.
The authors have, in fact, found
it neces-
^ Berkey, C. P.
: Geological reconnoissance of Porto Rico, op.
cit., p. 26.
« Semmes, D. R.
: Geology of the San Juan
district,
op.
cit.,
p.
76.
«a Mitchell, G.
J.
: Geology of the Ponce
district,
op.
cit.,
p. 261.
«« Fettlie,
C. R.
: Geology of the Humacao
district, op.
cit., pp. 142 and 147.
294
SCIENTIFIC SURVEY OF PORTO RICO
sary to differentiate a series of fine-textured andesites that outcrop near
the east coast as a separate formation, under the name "Figuera
fiows.'^
The extrusive origin of the latter is scarcely less certain than that of the
other felsitic rocks which comprise parts of the Hato Puerco, Luquillo,
and Fajardo formations; for in nearly
all
of them their amygdaloidal
character, strong megascopic flowage lines, columnar jointing, and rela-
tion to the superjacent and subjacent strata furnish
a body of evidence
concerning
their
eruptive
character
that
leaves no room
for
question.
The importance of this kind of product
is
still further emphasized by
the common, if not abundant, occurrence of flow fragments in the tuffs
and agglomerates.
The majority of the extrusive sheets may have been
comparatively local in extent, yet in the aggregate they form a large frac-
tion of the rock section.
Even
in the Fajardo
district, however, the
effusive
rocks do not
ac-
quire the importance exhibited by the intrusives.
Sills invade the sedi-
ments in every part of the island, some hovering on the lit-par-lit injec-
tion type, others attaining large proportions, a few shading into
lacco-
liths.
Dikes are probably the most numerous of the intrusive forms and
display a remarkable petrographic range.
The eroded granitoid bodies
of deeper origin are more scattered than the shallow intrusives and are
exposed in irregular apophyses and stocks, and in a few batholithic masses.
So many petrographic varieties are present among the igneous rocks
of the Fajardo
district, that a
systematic treatment
of
all
the
species
which have been differentiated
in
the preparation
of
this report could
easily furnish material for another of equal length.
Systematic studies
have already been made of the igneous rocks in the adjoining San Juan
and Humacao
districts by Semmes and Fettke, and their petrographic
descriptions and general conclusions apply
equally well
to the igneous
geology
of the Fajardo
district,—the more because many
of
the rocks
which they classify and describe extend into the
district without under-
going any detectable change.
A similar treatment
of the same or
like
products would be in large part repetitious.
On the other hand, the igneous phenomena have an historical
signifi-
cance which may appropriately receive consideration, for they require cor-
relation with the stratigraphic details which the Fajardo district reveals.
To this end they may be grouped,
first, into those features which were
more or less contemporaneous with Upper Cretaceous sedimentation and,
second, into those which were concomitant upon the
orogenic
disturb-
ances that later caused deformation in all the older rocks.
The members
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
295
of these groups
differ slightly in composition, and markedly in texture
and form.
The contemporaneous types invariably possess finer textures,
and they may be less silicic than the phanerocrystalline intrusives of later
origin.
The
latter invade
the
stratified
rocks
as
irregular
apophyses,
stocks, and batholiths.
The former, on the
contrary, assumed simpler
shapes which were more intimately related to the original sedimentary
structure
:
sills are abundant ; the dikes are simple, tending to
fill single,
well defined fractures; other intrusives
are, with few exceptions, small,
compact masses, which have upon and within them the marks of later de-
formation and magmatic alteration.
Gradational varieties that link the
two groups are not present, and in the
field and laboratory
it has been
found consistently possible to classify the igneous products on this two-
fold basis.
On the geologic map which accompanies this report the con-
temporaneous intrusives, which in some localities are so intimately asso-
ciated with the sediments as to be inseparable from them, are accorded
a degree of prominence and distinctiveness that is not encountered in the
field, whereas the deeper intrusives which invaded the stratigraphic sec-
tion at a later date are as sharply set
off from the surrounding rocks as
the red color employed to represent them on the map implies.
A brief
examination
of the igneous
rocks will make
their
similarities and
dif-
ferences more apparent.
The early volcanic rochs
The igneous rocks which were formed during the period of Upper Cre-
taceous sedimentation may be subdivided into surface types and intrusives.
Obviously the intrusives in this group were later in origin than the rocks
they invaded, but they were formed at shallow depths while higher mem-
bers
of the
stratigraphic
section were being
deposited, and before
the
orogenic movements
occurred.
Except
for
their unfailing
porphyritic
habit and
their usual
lack
of flowage
lines they
differ
little from the
closely related flows.
(1)
Surface types,
Pyroclastic rocks cover about three-quarters of that
portion of the Fajardo district which is underlain by the ^^Older Series.'^
A large quantity of the fragmental material was ejected, or washed, into
shallow marine waters, where it underwent sufficient resorting and modi-
fication to assume the stratified structures, as well as the textures, of nor-
mal sedimentary
rocks.
So completely
have
the
explosive
features
of
many of the finer pyroclastics been modified by wave action that they are
usually described under sedimentary names, and their ultimate volcanic
396
SCIENTIFIC SURVEY OF PORTO RICO
origin is indicated at most by a qualifying adjective.
The ashy shales of
the Fajardo formation^ the tnffaceons sandstones and calcareous tuffs of
the Guaynabo and Luquillo formations^ the pyroelastic conglomerate
of
the Eio formation are among the examples which the Fajardo district con-
tains.
Of the
several stratigraphic units that have been
differentiated,
only the Hato Puerco t^ffs seem to be subaerial in origin.
Even in this
formation, the diversity exhibited among the constituent particles and the
rounding which the fragments locally display suggest that water action
played some part in assembling the materials.
The salient features and
problems of the pyroelastic rocks have already been briefly discussed, how-
ever, and attention may more appropriately be turned to the flows which
are intercalated with them.
Extrusive sheets have been found in association with most of the clastic
formations which have been differentiated within the boundaries of the
district.
In the Guzman formation amygdaloidal andesite flows are inter-
bedded with the tuffs and limestones, and on the northern
flank
of El
Yunque, at an elevation of 2400 feet, one of the flows has brecciated a bed
of limestone.
The recrystallized and partly absorbed fragments
of
the
limestone have weathered more readily than the andesite
in which they
are embedded, giving the outcrop a deeply pitted surface, in which the
white or light gray calcareous remnants afford a striking contrast with
the weathered greenish exposure of the volcanic matrix.
It is not known
to what extent flows characterize other portions of the Guzman formation,
for along the northern flanks of the Luquillo Mountains, where
it
is be-
lieved to acquire
its maximum extent,
its outcrops could not be visited.
Until
this area
is
studied,
a more
exact statement concerning
its
con-
stituent members cannot be made.
In the overlying Hato Puerco tuffs the percentage of flow material
is
extremely variable, but in every section which was examined
flows were
found to compose a moderate proportion of the whole.
Practically all of
them are andesitic in character; most are amygdaloidal, and flowage lines
are commonly exhibited.
In the southwestern corner
of
tlie
district
a
typical extrusive belonging to this horizon is exposed about one kilometer
from the Caguas-Aguas Buenas carretera along the
trail to the caves in
the La Muda
(or Aguas Buenas)
limestone.
The rock
is a vitrophyric
flow breccia with the composition of a dellenite.
The small phenocrysts
of feldspar, altered biotite, and magnetite are embedded in a groundmass
MEYERHOFF, QEOLOOY OF THE FAJARDO DISTRICT
297
composed largely of glass.
Fragments of an older glass^ which exhibits
strong flow structure^ are also present^ but megascopic and microscopic
flowage lines are equally prominent in the younger glassy matrix.
Vein-
lets of
sericite and epidote penetrate the rock^ and alteration has mate-
rially affected the phenocrysts.
The biotite has been
all but destroyed;
the magnetite has undergone partial alteration to leucoxene ; and second-
ary quartz has been introduced into every cranny in the rock.
Across the western hill section on the caminos from Trujillo Alto to
Gurabo and from Trujillo Bajo to Juncos flows are very common.
Most
of them are so extensively altered that scarcely an original mineral
re-
mains; but vestiges
of plagioclase phenocrysts can usually be
detected,
and the prominent amygdules and flowage lines have not been much dis-
guised.
Perhaps as many as 1000 feet of these amydaloidal andesites are
present in the central portion of the district.
Grouped tentatively with the
Hato Puerco formation on the geological map are two related rocks which
are somewhat more
silicic in composition than the majority of volcanic
products in the district.
One outcrops at the summit and on the higher
slopes of the escarpment that forms the northern wall of the Caguas low-
land near Juncos ; the other occurs in the two conical hills that rise con-
spicuously above the lowland
floor not far from the
village
of El Eio.
Both are composed in great part of secondary hematite and quartz,
in-
cluding jasper and vein quartz, and the distribution of these minerals has
given the rocks a strongly banded appearance.
In the exposures north of
Juncos the nature
of the banding
is
suggestive
of
flowage, and
petro-
graphic analysis of the specimens confirms the impression of their effusive
origin by revealing
a pronounced
vesicularity and a large quantity
of
glass.
Professor
E.
J.
Colony
suggests
that
the
rock was
initially
a
scoriaceous
lava
of
felsitic
or
rhyolitic
composition.
The
rocks
near
El Eio consist in part of similar material but also comprise associated frag-
mental rocks that have been identified as rhyolite tuffs.
In some respects
the specimens from both localities bear an arresting resemblance to the
Juan Ascencio member of the Fajardo shales, and it is possible that addi-
tional field study will show the latter to be a correlate of the Caguas low-
land deposits.
The authors have not found sufficient evidence to justify
the correlation.
It would require the presence along the escarpment north
of the lowland of a deep synclinal fold in which a considerable portion of
the stratigraphic section must occur.
As has already been
stated,^* the
64 Ante,
pp. 236-238.
SCIENTIFIC Sl'Jti'KY OF PORTO RICO
f^piA.".,:'
A
tilted
estriisive
sheet
exposed
on
the
carretera
about
three
kilometers
soutli
of
Canovanas.
MEYERHOFF, GEOLOGY OF TEE FAJARDO DISTRICT
299
structure of this belt probably is synclinal, and if the CoUores limestones
have been correctly correlated with the La Muda and Trujillo Alto forma-
tions, the basal portion of the Fajardo shales in which the Juan Ascencio
member occurs may also be present.
But in this case the entire section
should be repeated in the upland, and in the
field studies in this region
it was not found.
Its absence can too readily be attributed to the porphy-
ritic intrusives which penetrate the formations along the
trails, and the
missing strata may be present in less accessible parts of the upland.
Until
they are discovered, opinion regarding the correlation suggested is reserved.
In the comparatively small area underlaid by the Guaynabo formation
in the northwestern corner of the
district, the majority of the igneous
rocks that came under observation are sills.
In a few cases the possibility
of an extrusive origin was noted in the field, but none of the specimens in
the
collection which has been studied microscopically shows convincing
signs of having cooled at the surface.
The Luquillo formation, however,
contains one of the two most striking flows in the district, and perhaps
others which were not differentiated are present.
The flow in question
(Fig.
20)
is
a
tilted
sheet about 30
feet
thick,
intercalated with
the
bedded tuffs and ashy shales that outcrop on the carretera nearly three
kilometers south of Canovanas.
Like the sediments with which
it
is as-
sociated,
it dips 25
degrees north-northwest, and
its
regular columnar
joints are inclined an equal amount from their original vertical position.
A similar tilted flow in the lower portion of the Fajardo shales has been
used as a bridge abutment where the carretera crosses Kio Canovanillas
between Carolina and Canovanas.
The rocks composing both these extru-
sives are amygdaloidal and aphanitic.
Marked flowage
lines have been
preserved by the oriented lath-shaped feldspars, which are set in a glassy
matrix.
The
amygdules,
as
a
rule,
exhibit
reaction
rims
of
chlorite
which envelop a calcite
filling, but in the specimens from the Fajardo
formation a zeolite, probably heulandite,
fills some of the cavities.
Both
rocks possess too fine a texture for accurate classification, but the extinc-
tion angles of the small feldspar laths suggest a composition intermediate
between basalt and andesite.
The nature of the volcanic material in the
Figuera
formation
has
already
been
described.®^
In
the
middle
and
upper part of the Fajardo shales and in the San Diego tuffs no flows were
found.^'
65 Ante,
pp.
282-283.
«" Vaughan
(Journal, Washington Academy of
Sciences, Vol. XIII, 1923,
p. 307) men-
tions a
folded
basalt
flow
at Cape San
Juan,
but
the
writers'
field
observations and
microscopic studies indicate a probable
sill origin for the andesitic rocks of this locality.
300
SCIENTIFIC SURVEY OF PORTO RICO
From this summary statement of the areal and geologic distribution of
volcanic flows in the Fajardo district, it
is clear that their importance in
the Porto Eican section must be re-evaluated, for locally they comprise
as much as 1200 to 1500 feet of the total geologic column.
Effusion took
place sporadically during most of IJpper Cretaceous time, but two par-
ticularly vigorous cycles are noteworthy
: one in Hato Puerco time, another
during Figuera time.
If the puzzling rocks of the Juan Ascencio member
and of the central and eastern Caguas lowland were not formed correla-
tively with the Figuera flows, a third cycle must be recognized.
Although
the eruptive rocks are typically andesitic, local differentiation gave rise to
the
less
silicic varieties near Canovanas and to the more
silicic variety
north of Juncos.
(2)
Inirusives.
Igneous
intrusions
are
more numerous
than
flows
among the rocks of Cretaceous age, and, as might be expected, the older
and
lower
formations
have
been more
extensively
invaded
than
the
younger.
If
it be assumed that the greater number of intrusives in the
older rocks is the outcome of continuous igneous activity throughout the
depositional period represented by the stratigraphic section, wide textural
variation in the intrusions might be expected in view of the differences in
the thickness of the overburden as successive invasions occurred.
To an
extremely limited extent this assumption
is substantiated by a study of
the dike
systems which
penetrate
the
formations,
especially
the Hato
Puerco tuffs.
Textural variation is almost wholly restricted to the dikes,
however, for in other kinds of igneous bodies the textures are monoto-
nously felsophyric.
The cryptocrystalline matrix which characterizes most
of the intrusives implies that they cooled at relatively shallow depths, and
that their formation followed close upon the deposition of the rocks which
they invade.
A more critical analysis also shows that the large number
of
intrusives
penetrating
the Hato Puerco
tuffs
is
due
rather
to
the
greater vigor of volcanism in Hato Puerco time than to the cumulative
effect of igneous activity over a long geological period.
The time factor
was of slight importance only in the development of the younger dikes,
through which the magma was supplied
to higher stratigraphic
levels.
Description of a few of the intrusive types, chosen to illustrate their rela-
tionships
to the surrounding rocks and
their compositional range,
will
serve to show their historical significance, as well as a few of the problems
they present.
On the geologic map all are represented by a single symbol,
and only those which attain moderate proportions, or which appear to have
especial significance, are shown.
AlEYERBOFF, O'EOLOOT OV THE EAJABBO DISTRICT
301
Dikes occur in
all the Cretaceous rocks, but thej attain
tlieir best de-
velopment
ill
tlie massive and
relatively competent
tnll's
of
tlie Hato
Puerco formation.
Kio Grande
tie Loiza and Bio Canovanas have cut
excellent natural sections across this formation
in the center of the dis-
trict, and in thr* steep valley walls and in the beds of
tlieso two streams
tlie scattered dikes form coiispieiious outcrops
(Fig. 31).
]iIost of tlnaii
consist of altered
andesite
porpliyry,
to which
tlie white, or light gray
plagioclase phenocTysts, set in a dull green mieroscrystalline groundmass,
impart a
niottlcd
a.-^pcct.
A few
dikt'S are
iiuyrc
sili(:ie
liian the
ainli'.aitiu
norin."^
At
kilooieter
21. <> along
the
triiek
ol"
ilie
I^orto Hieo
llailway.
LishL atnl Power i'ompauy, a
l>mid oC rock witli diked ike
charaer.erisiii-,-^
cuts across IJio (Iraiiile de Loixa, forming
ii jmrrow zone nf
rai>ids.
It has
been
cbis-^ilicd as a dike, altbuiigh
its .mdac-rs wirb
tlie surrouuding
tults
are burimj beueath^ n thick
soil rovnr nud
...uld
not
b.- studied.
liiii<,ii.:iy
oiioiigb, Ihe
inas'-
is roiniHiseil of two rlo>.dv r.dah'd igm'oas
rm-k:^. one iu^
trnsive
luio
ihe other.
Tiie older
is
a
bo-tonih:.
iiironspuaiosly iH-rphy-
' In
111
302
SCIENTIFIC SURVEY OF PORTO RICO
clase.
The younger rock, which
is more distinctly porphyritic, paraUels
the older and has sent narrow stringers into the fractures that had formed
within it.
The plagioclase feldspars of the later intrusion are higher in
soda than those of a normal andesite, and except for accessory magnetite,
biotite is the only determinable ferro-magnesian mineral
it contains.
It
has been
classified as a porphyritic trachy-andesite.
Dacite porphyries
have also been encountered in a small number of dikes in the western third
of the district.
One comparatively unweathered occurrence was noted at
kilometer 15.5 along the railroad in the valley of Eio Grande de Loiza,
where the intrusive cuts a series of indurated shales.
Quartz and andesine
compose both phenocrysts and groundmass, and
biotite
constitutes
the
ferro-magnesian component.
Other kinds of chemical and mineralogic variations among the dike
rocks are more common than those responsible for the differentiation of
the trachy-andesites and dacites. One type is to be found in the relatively
common hornblende andesite porphyries, such as occur in the dike which
outcrops on the carretera eight and one-half kilometers south of Cano-
vanas.
In this rock moderate-sized phenocrysts of plagioclase, ranging
from andesine
to
bytownite,
occur with huge phenocrysts
of greenish
black hornblende in a matrix of aphanitic andesite.
In most porphyries
of
similar
character
the
hornblende
phenocrysts
are
relatively
incon-
spicuous, and in several intrusives they are difficult to distinguish mega-
scopically from augite, which
is a common essential mineral.
Among
the older intrusive dikes a graded series of rocks ranging from bostonite
to augite andesite porphyry could easily be arranged, but beyond these
extremes
lithologic representatives are
rare.
Fragments
of basalt and
diabase, including olivine-bearing
varieties,
are
locally
so abundant
in
the
tuffs and agglomerates
as
to compose
the major part
of some
of
the
minor
pyroclastic
members.
These,
together
with
the
scattered
basaltic flows and sills in various parts of the district, make
it necessary
to postulate the existence of feeder dikes of similar composition, which
escaped detection in the
field.
The single body of olivine basalt which
was found in the district about four kilometers north of Gurabo on the
Trujillo Alto
trail,
does not seem
to have a
dike-like
relation
to
the
surrounding tuffs, and
it may upon further study prove to be a flow.
In the foregoing
discussion no emphasis has been placed upon the
textural features of the dike rocks, other than to stress their general por-
phyritic habit.
As phenocrysts have
little value in indicating the
rela-
tive position in which the magma ultimately crystallized, attention must
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
303
turn to the groundmasses.
With no important exceptions the latter are
uniformly cryptocrystalline, and a few contain small quantities of glass.
None of the specimens is truly phanerocrystalline.
It must be concluded,
therefore, that nearly
all the older dikes consolidated near the surface,
and that they do not represent an unbroken series which links the earliest
manifestations of volcanic activity with the deep-seated intrusions that
followed the Cretaceous.
On the contrary, the facts suggest a distinct
hiatus between Cretaceous and post-Cretaceous volcanism.
There
is
little need to enlarge upon Berkey's general
description
of
the
occurrence
of
sills :^^ "The
smaller
intrusives
are
chiefly
andesite
porphyry in composition and have everywhere penetrated the shale and
ash
beds.
The commonest occurrence
is in the form
of small
sills or
sheets conformable
to
the bedding structure and varying in
thickness
from only a few inches to many feet.
These
sills are so perfect in form,
have so little disturbed or modified the adjacent beds, and are so similar
in general composition and appearance, after weathering, to the associ-
ated sedimentary beds that it is quite impossible to determine in all cases
how much
intrusive and how much
original sedimentary
rock are
in-
volved.
The only thing noticed
as
a
rule
is
the uniformity
of
petro-
graphic structure that seems to be characteristic of the intrusive as com-
pared with the associated beds.
The simplest occurrences of sills of this
kind, which
at the same time show
their igneous
intrusive
character,
were
seen
near
Fajardo,
near
Eio
Piedras
and
in
the
vicinity
of
Comerio.^^
The
two
examples
mentioned
by
Berkey
as
occurring
within
the
limits of the Fajardo district are typical of many other similar intrusions
in the strata of the Guaynabo, Luquillo, Fajardo, and San Diego forma-
tions. Locally it proved possible to isolate the sills by the perfection of the
spheroidal weathering affecting them, but where
this method
of
differ-
entiation
failed,
identification
was
possible
only
by means
of
petro-
graphic
analysis,
save
in the
rare unweathered
exposures along
a few
streams and along parts of the northeast coast.
The uniformity in tex-
ture and in composition which the sills exhibit is certainly a more arrest-
ing feature than their elusive occurrence in the field.
Nearly all of them
consist
of
andesine
or
labradorite
phenocrysts
in
greenish
cryptocrys-
talline matrix,
in which hornblende
or,
less commonly, augite may be
identified.
In a few sills the extinction angles of the plagioclase indicate
^ Berkey,
C. P.
: Geological reconnoissance of Porto Rico,
op.
cit.,
p.
26.
304
SCIENTIFIC SURVEY OF PORTO RICO
a composition that approaches bytownite^ but basaltic
varieties
are
ex-
tremely
scarce.
Their
texture^
their
thinness, and
their conformance
with the sedimentary structures are characteristics which at once
indi-
cate their contemporaneity in origin with the formation in which they
are found and set them apart from the phanerocrystalline
sills of much
later origin.
Other kinds of porphyritic intrusives present in the district are
difficult
to
classify, and only the larger and more important bodies have been
represented on the map.
Many consist of plug-like masses with circu-
lar, oval, elongate, or irregular ground-plans.
They usually appear sud-
denly in the midst of a section of sediments, their outcrops covering, in
some cases, but a few acres ; in others, from one to two square kilometers.
The
dikes
associated with a few
of them suggest that they mark the
sites of subsidiary volcanic cones, and at least one that came
to atten-
tion near La Muda has the aspect of a volcanic plug.
Marginally some
of the intrusives have incorporated such large quantities of country rock
that several of them could scarcely be distinguished in thin-section from
the pyroclastic
rocks, but in the
field their intrusive
contacts with the
surrounding formations make their origins clear.
The
nature
of
the
larger
intrusive
bodies
of
andesite
porphyry
is
indeed
perplexing.
Semmes mapped
a
large
mass
north
of
Aguas
Buenas, and it has been traced for some distance into the Fajardo
dis-
trict, although
its boundaries could not be determined with any degree
of precision.
It
is composed of andesite and andesite porphyry and
is
cut locally by
irregular
dikes
or apophyses
of
diorite, some
of which
appear to be
direct offshoots of the granitoid intrusives of the Caguas
basin.
The areal distribution of the porphyry and its location at the end
of the pitching syncline that crosses the western border
of the
district,
suggest a sill-like origin, yet wherever its contacts with the surrounding
sediments were
observed, they have been found
to
cut
the
structures.
Its ground-plan and structural relations are much like those of a stock,
yet
its comparatively fine texture, grading from
felsitic at the margins
to felsophyric nearer the center, casts serious doubt upon such
a mode
of origin.
It is possible that the mass developed as a subsurface feature
in
association with
one
of
the younger
volcanic
vents
in
the
interior
of
the
island,
but
the
erosion
of
the
higher members
of
the
strati-
graphic
series has destroyed whatever evidence of concomitant eruptive
activity there may have been to support such a postulate.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
305
The large porphyritic intrusion near Mameyes
is associated with sedi-
ments that have relatively low dips^ and
it occupies a stratigraphic and
structural situation almost identical with that of the Aguas Buenas por-
phyry.
Its development in the trough of the undulating syncline that
underlies
this region again suggests
a
sill-like
relationship
to the sur-
rounding
rocks^ but such an assumption
is precluded
in view
of
the
nature of its intrusive contacts.
Were
it not for the unmistakable signs
of
deformation
that
are
revealed when
this
and
the Aguas
Buenas
porphyry are studied under the microscope^
it might be postulated that
both
entered
the
zones
of minimum
pressure
in
the
troughs
of
the
synclines
as folding occurred, much
as
incoiiipetent
clays
are
squeezed
into the axial portions of folds whenever strong compression takes place.
Fettke has already called attention to the large areas of andesite por-
phyry
in
the
northeastern
part
of
the Humacao
district and
in
the
adjacent portion of the Fajardo district/^ but he has
left their origin
unexplained.
As
the
geologic map
accompanying
the
present
report
shows, these intrusives display a tendency to
parallel the structures
of
the
sedimentary
rocks,
yet
in many
places
they
cut
indiscriminately
across them.
Minor petrographic changes in the rocks imply that the
extensive area which Fettke and the present writers have mapped
as
a
continuous body of porphyritic andesite along the southern edge
of the
Luquillo Mountains may prove to be a more complex series of intrusives.
At the western limits of the mountains the igneous rocks occur at the
end of the pitching syncline that forms the central range, and from the
meager data obtained in the
field
it has been inferred that they do not
penetrate the overlying sediments of the Luquillo formation, which
are
present in the structure.
This assumption is based partly on the form of
the intrusive and partly on the
associated dike system, both
of which
indicate that the area was the locus
of
explosive
activity during Hato
Puerco time.
The inference was not supported by direct field observation
of the contact between the porphyry and the sediments, and
if the intru-
sive
is continuous and contemporaneous with the one in the eastern part
of the Caguas lowland and in the Naguabo region, the hypothesis
is in-
correct, for the latter
is associated with the younger rocks in the
strati-
graphic
series.
If two
distinct
intrusives
are
present,
as
the
geologic
map
implies, the
older was formed during Hato Puerco
time and
in-
volved the development of a plug-like volcanic core of moderate
size
at
the western end of the Luquillo Mountains.
The younger involved the
69 Fettke,
C. R.
: Geology of the Humacao
district,
op.
cit.,
pp.
136,
144-147.
306
SCIENTIFIC SURVEY OF PORTO RICO
injection of thick sheet-like masses of andesite porphyry into the strata
of the Luquillo formation.
By postulating two cycles of intrusion the
contrast between the forms of the intrusive in the upland and of that
along the southern border of the district can be explained; but no physi-
cal evidence was disclosed in the field to justify the recognition of two
distinct periods of invasion at different times, and microscopic analyses
of the specimens from the two localities are non-committal.
The
essential petrographic features of the
irregular and unclassified
intrusives have been described by Semmes and Fettke.
Most of them are
normal
andesite
porphyries with andesine
or
labradorite forming
the
phenocrysts.
Several of the smaller masses show a slightly higher con-
tent of silica and have crystallized as dacite and trachy-andesite differen-
tiates.
In the larger masses an opposite tendency
is
locally
exhibited.
The plagioclase shades toward bytownite, and augite replaces hornblende.
ISTone of the rocks
quite
reaches
the
composition
of
diabase, although
stray diabasic fragments among the coarse pyroclastic rocks show that
differentiation
proceeded
to
this
extreme
in
a few
cases.
In
all
the
intrusives
of
this group,
irrespective
of
size and composition,
the
fel-
sophyric texture shows no sign of gradation into the phanerocrystalline
textures of the younger granitoid intrusives.
The gap between the two
types
is broad, even though they occur
in bodies
that
are comparable
in form and
size.
The granitoid intrusives
The coarsely crystalline igneous intrusives have a wide distribution in
the Fajardo district, and it is probable that there are even more outcrops
than the geologic map shows.
The majority of those which lie north of
the Caguas-Naguabo lowland
are
small,
yet,
irrespective
of
their
size,
they
are
composed
of
rocks
with
phanerocrystalline
textures
and
mineralogic
characteristics
that
set them
clearly
apart from
the
por-
phyritic intrusives.
In thin-section the latter exhibit the
effects
of ex-
treme
alteration and
of mechanical
strain and chemical healing.
The
granitoid
igneous
rocks,
on
the
other
hand,
are
relatively
unaltered,
and, although a few specimens possess a faintly gneissoid structure, sug-
gestive of crystallization under compression, none
exhibits the
features
of strain, shattering, and healing, which develop
if pressure
is applied
after crystallization has taken place.
The relation between some of the
smaller granitoid intrusives and the folded structures of the surround-
ing
sediments
also
indicates
that
the movement
of
the magma
was
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
30?
locally governed by the structures the rocks assumed when folded, and
that invasion took
place
as the period of
orogensis was drawing
to
a
close.
The
textural hiatus between
the porphyries and the
granitoid
rocks thus
reflects the gap in time that separated the earlier from the
later intrusions, as well as the vastly different physical conditions under
which the two invasions occurred.
In both epochs of volcanism the source of the magma was evidently the
same, for each porphyritic variety can be matched by a corresponding
granitoid variety.
Yet,
if the aggregate of coarsely crystalline rocks be
compared with the aggregate of the finely crystalline types, the former are
found to be somewhat more silicic.
Subsilicic species occur in the Fajardo
district, but they are not common.
One of them Fettke designated by the
name "Juncos gabbro,^'
'^^ from
its exposures along the
carretera from
Naguabo to Juncos, a little more than a mile from the latter town, where
its contact with the younger diorite that cuts it may be studied.
Accord-
ing to Fettke the rock is composed of labradorite and augite, with horn-
blende forming reaction rims around the latter.
The writers^ specimens
were undoubtedly taken from a point nearer the contact, for hornblende
comprises a very abundant constituent throughout the rock, and the py-
roxene consists of colorless diopside.
In
its fine-grained, inequigranular
texture it resembles a gabbro boulder which was picked up in the bed of
Eio Blanco a short distance below the falls.
Mineralogically, however, the
fragment represents a normal gabbro, and the abundance of boulders like
it in the stream bed indicates an outcrop of unknown extent somewhere
along the upper reaches of the river.
At the time the field work was done
there were no trails in this section, and
it was impossible to ascend the
water course beyond the
falls, so nothing further was learned about the
source of the gabbroic fragments.
Although none of the other rocks in the district was classified as a gab-
bro, several of the
diorites exhibit mixed characteristics.
A few carry
augite as a primary constituent.
Specimens from one locality about three
kilometers west of Caguas were found to contain no other ferro-magnesian
mineral.
They were taken near the contact with the country rock, and to
what extent the presence of augite is due to endomorphic reactions
is un-
certain.
The abnormal composition of the rock as a whole suggests modi-
fication of this kind,
for both andesine and orthoclase
are
constituent
feldspars.
The intrusive exposed along the carretera between Rio Grande
'• Fettke, C. R.
: Geology of the Humacao district, op.
cit.,
p.
153.
i^C/f'JNTIFIC Sl'ltVMY OF PORTO RICO
.^^^The
},'U, mane
(]vnii
ite
falls
of
:
>ive
intrt
Uio
BliUH
JSivo
in
i
io show
.ta
pretM
s
the
:
di'oii
whieli
the
river
Is
front of
the central
range.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
309
and Mameyes
is another
in which augite plays a primary
role, but the
calcic character
of
its plagioclase indicates that the composition
of the
entire rock
is normal.
The
diorites composing part
of
this
intrusion
and one
of the
smaller apophyses
that penetrate
the San Diego
tuffs
near the Cape San Juan lighthouse exhibit the closest approach to gabbro
which was observed.
The relations of both to the surrounding strata in-
dicate an origin contemporaneous with the other
dioritic masses in the
district, and their compositional deviation from the norm
is apparently
the result of simple differentiation.
In the diorites elsewhere in the northeastern part of the island the feld-
spars range from andesine to bytownite, but generally have the com])osi-
tion
of
labradorite;
hornblende
is
invariably
present,
and
magnetite,
titanite, and apatite are the usual accessory minerals.
In the majority
of the smaller intrusives and in the western part of the stock underlying
the Caguas basin no other minerals are present; but in the San Lorenzo
batholith, in the Eio Blanco stock
(Fig. 22), and in a few other dioritic
masses, quartz becomes so abundant that the rocks must be classified as
quartz diorites.
Andesine
is commoner than the more calcic plagioclase,
and in a few places biotite appears in small quantities among the ferro-
magnesian constituents.
As Fettke's excellent account of the San Lorenzo
batholith and the associated intrusives shows/^ careful sampling will re-
veal in each of the large granitoid intrusives the presence of hornblende
granites,
grano-diorites,
quartz
diorites,
normal
diorites,
and
augite
diorites,
so
perfectly
intergraded
as
a
result
of
relatively
slight
geo-
graphic differentiation that sharp delineation between the species
is not
only impossible but pointless.
These graded variations in single masses
provide an ideal demonstration of the unity which must exist among the
small granitoid
intrusions
in the
district,
irrespective
of
their present
accidental
isolation and
divergence
in
composition.
Only
the Juneos
gabbro exhibits a distinctly older origin, and a few granitic rocks, which
are described below, appear to have been formed at a later date.
Two silicic differentiates, probably products of relatively late magmatic
processes, have been noted in the district.
In the valley of Kio Grande de
Loiza, at kilometer 23.3 along the railroad, an exceedingly coarse-grained,
almost pegmatitic, granite dike cuts through the surrounding tuffs, form-
ing a band fifteen or twenty yards wide on both sides of the river.
Slrreds
of bleached biotite are distributed among the large crystals of quartz and
'^1 Fettke,
C. R.
: Idem,
pp.
153-159.
310
SCIENTIFIC SURVEY OF PORTO RICO
kaolinized feldspar, the rock forming at once the nearest approach to nor-
mal granite and
to pegmatite which the
district
affords.
A
stock-like
body of graphic granite
is situated at the southern edge of the area in-
cluded in the present survey, about two kilometers southwest of Torres,
Mixed with the graphically intergrown quartz and pink
orthoclase
are
minor amounts of albite and scattered shreds of biotite.
The rock as a
whole is easily differentiable from the quartz-diorite with which it is asso-
ciated, but the character of the contact could not be determined.
Fettke
regards this granite as a phase of the analogous Yabucoa granite, which
at Playa de Yabucoa is intrusive into the diorite/^
There is every reason
to believe that the relation holds in the occurrence near Torres, and that
the dike in the valley of Kio Grande de Loiza is an essentially contempo-
raneous injection.
About five kilometers east of Caguas on the carretera to Gurabo a road-
cut reveals a small intrusive of quartz monzonite situated between outcrops
of diorite and tuff.
Fettke has recorded similar patches of quartz mon-
zonite in the magnetite belt east of Juncos, but they were not differentiated
by the authors ; two other small areas, respectively three and one-half and
seven kilometers west of Naguabo, are shown on Fettke's map, but a re-
study of the material collected from them has indicated that the rocks
must be classified as quartz diorite.
Although the quartz monzonite near
Caguas is finer grained than the large body of diorite in the Caguas basin,
mineralogically
it falls within the range of variation which the dioritic
intrusion displays, and the authors were inclined to regard
it as a mar-
ginal phase of the main mass, in
spite of the prevalence of
less
silicic
differentiates in situations of this kind.
Fettke, however, found quartz
monzonite cutting the Yabucoa granite along the coast two miles south-
east of Patillas, and, on the assumption that all occurrences of monzonite
were formed at the same time, he believes the intrusives along the south-
ern border of the Fajardo
district to be younger than any of the other
kinds of granitoid rocks.
The writers do not have sufficient information
to support or to refute his hypothesis, but some of their conclusions differ
from his and at least indicate the need for supplementary studies of the
quartz monzonites in the Caguas-Naguabo lowland.
72 Fettke,
C. R.
: Idem,
pp.
159-161.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
311
If Fettke^s chronology of post-Cretaceous igneous activity be applicable
in the Fajardo, as well as in the Humacao,
district, the four stages of
granitoid intrusion which he distinguished may be recognized.
Gabbroic
differentiates of limited size and extent were intruded early in the cycle
and were succeeded by the widespread invasion of dioritic magmas, the
later products of which were decidedly
granitic.
Perhaps as an
after-
math, small masses
of quartz monzonite were formed
in,
or near,
the
areas affected by the earlier invasions.
The center of igneous activity was
obviously in the Humacao region, where the San Lorenzo batholith, the
largest batholithic mass in Porto Eico, appears to have played a parental
role to the smaller intrusives to the north.
Even the older gabbros near
Juncos and on the southern slopes of the Luquillo Mountains are closely
associated, and the younger granites either cut through the batholith or
outcrop
in
the
peripheral
areas.
The more remote
intrusives
are
in-
variably dioritic, and their deployment over so wide a region bespeaks the
intensity of igneous activity when
it reached the climax of
its develop-
ment.
The textural features of the granitoid intrusives require mention,
al-
though their variety makes it impossible to give them extended treatment.
All are phanerocrystalline, but the crystal size
is invariably small in the
gabbros and is usually small in the quartz monzonites.
Among the rocks
of the
dioritic group
fine textures occur along their
contacts with the
country
rock, and
in
a few
of the smaller masses
the
crystals remain
small throughout the entire extent of the exposures.
In the majority of
cases, however, the crystals become larger away from the contacts, except
in the granites, where they maintain about the same relative sizes in
all
parts of the intrusive.
Although the constituent crystals have sometimes
grown to moderate proportions, none of the rocks is very coarsely crystal-
line, with the notable exception
of the granite dike along Bio Grande
de Loiza, which
is almost pegmatitic.
In nearly
all
of the intrusive specimens which were
studied micro-
scopically the fabric
is inequigranular.
Generally the inequigranularity
is seriate, but in a few specimens it is sufficiently hiatal to warrant the ap-
plication of the term porphyritic.
The gabbro specimens found in the
stream boulders near the falls of Kio Blanco, for example, contain a few
enormous phenocrysts of augite, which have undergone more marked alter-
ation than the augite in the equigranular groundmass.
A diorite por-
phyry containing large hornblende phenocrysts in a
finer matrix
is ex-
posed in the low ridge which rises above the alluvium of the Caguas basin
312
SCIENTIFIC SURVEY OF PORTO RICO
between Caguas and Gurabo.
The rock is clearly a marginal phase of the
diorite, for
its contact with the associated tuffaceous
strata
is well
ex-
posed in another part of the ridge.
These two cases are exceptional, for
the ineqnigranularity rarely goes to these extremes.
The forms which the granitoid rocks assumed npon crystallization were
seldom influenced by the structures of the strata they invaded, but a few
exceptions to this statement may be noted.
About two and one-half kilo-
meters east-southeast of the village of Luquillo a lens-like intrusion
of
diorite
rises
above
the
deeply
weathered
tuffs,
attaining
a maximum
breadth of 200 to 300 yards and tapering gradually toward the ends in
the manner of a tilted laccolith.
Fo strata are exposed in its immediate
vicinity, and
it
is possible that the diorite ^lls a pinching fracture that
parallels the sedimentary structures ; but its unique and distinctive shape
has led the authors to classify it provisionally as a thin laccolith.
Four
kilometers south of Kio Grande a slender wedge of diorite conforms with
the structural trend of the subjacent and superjacent agglomerates and
tuffs.
It has been classified as a sill, although on the map it has not been
differentiated from the larger stock-like intrusive with which it is closely
associated.
At Cape San Juan a network
of small intrusions cuts the
San Diego formation, and a few of them conform somewhat crudely to the
bedded structures.
In the fine-grained diorite porphyry which protrudes
above the alluvium between Caguas and Gurabo, a faintly gneissoid struc-
ture appears
to parallel the bedding planes
of the
associated ashy and
tuffaceous shales.
Here the intrusive mass cuts through the sediments,
but marginal offshoots interfinger for short distances with the beds.
So far as could be determined, the other granitoid rocks cut jagged
swaths through the formations
of
the
stratigraphic
section.
The San
Lorenzo batholith truncates nearly all the formations and their structures
indiscriminately.
The irregular apophyses which radiate outward from
the
stock beneath
the Caguas
basin,
apparently
filling
the
joints
that
penetrated the surrounding country rock, illustrate clearly the magma's
intrusive course through the clastic rocks on every side.
The Rio Blanco
stock ^2
is elongated in a direction normal to the trend of the regional
'^sFettke caUed
this
intrusive
the "LuquiUo
stock,"
on
the
basis
of
erroneous
in-
formation
that
it forms
a
considerable
part
of
the
Luquillo
Mountains
(op.
cit.,
pp.
154,
158-159).
The
senior
author was
unable
to
determine
the
exact
limits
of
the
quartz-diorite
in
the
central
range, but he
at
least found that
it
is
restricted
to
the
relatively small area drained by
the headwaters
of Rio
Blanco.
Its
close
association
with
this stream and
the anomalous
part
it
plays
in
the mountain mass
render
the
name
LuquiUo
inappropriate,
and
the name
"Rio
Blanco"
is
therefore
proposed
to
replace
it.
(Cf.
Fig.
22.)
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
313
structure.
The smaller intrusives in the district are similarly related to
the older rocks which they invade^ but their movement into these rocks
seems to have been in part occasioned by the development of lines of easy
penetration during the period of folding.
The crests and steep flanks of
the asymmetric
cross-folds in the northern part of the Fajardo region
served as particularly favorable sites for invasion, although by no means
all of the intrusive masses can be definitely correlated with
specific oro-
genic structures.
Most of them have stock-like forms, but only the large
mass near Eio Grande is of sufiicient size to be classified as a stock.
It ap-
pears to be a complex intrusion, composed of several closely related, and
perhaps detached, elements, the relations and distribution of which were
not determined in the
field.
The
other
small granitoid bodies
in
the
northern and eastern part of the district penetrate the rocks as irregular
apophyses, to which the names stock and dike can scarcely be
applied.
They are usually xenolithic, a few strongly so, and they give the impres-
sion of being the summits of stocks that have just been uncovered by ero-
sional processes.
Only one normal dike was identified in the district—the
pegmatitie granite dike which cuts through the Hato Puerco tuffs along
Eio Grande de Loiza near the southern limits of the western hill section.
The foregoing account of the granitoid rocks does not pretend to add
much that
is new to a subject which has been ably ^nd more fully pre-
sented by other workers in Porto Eico, especially C. E. Fettke.
The post-
Cretaceous igneous activity in the district was, in fact, but a smaller edi-
tion
of
the
profound
invasion
that
took
place
simultaneously
in
the
Humacao district.
A statement of the conclusions which may be drawn
from the studies in both these areas at the eastern end of the island seems
pertinent
:
(1) A comparison of the older andesites and the younger diorites with
respect to composition, texture, degree of alteration, and form, shows that
the two rock groups cannot be regarded as early and late products of the
same uninterrupted cycle of volcanism.
On the contrary, they represent
two wholly distinct cycles
of igneous
activity, and their
respective
de-
velopments took place under
different
sets
of conditions, although
the
magma was unquestionably derived from the same source.
(2) The dioritic invasions appear to have been started by the orogenic
movements which followed the accumulation of the stratigraphic section.
Judging from the relations between intrusions and folds, the dim traces
of
gneissoid
structures, and
the lack
of
strain
effects,
shattering, and
314
SCIENTIFIC SURVEY OF PORTO RICO
alteration in the
diorites, the magma made
its way into the overlying
rocks toward the close of the period of diastrophism.
(3)
It seems likely that intrusion took place at moderate, rather than at
great depths below the surface, for the chilling effects which the diorites
exhibit near their contacts, and the fine textures of the early gabbroic and
later monzonitic
intrusives,
are
facts which
indicate
that
the molten
masses penetrated
relatively
cool
rocks.
It
is unnecessary
to imagine,
therefore, that the stratigraphic section was very much thicker than it
is
now, and the character of the folds furnishes ample reasons for believing
that there was not a heavy overburden that has since been removed by
erosion.
(4) The magmatic invasions were
cyclic
in
character, involving the
evolution of more silicic types as time went on.
Chronologic differentia-
tion is pronounced, whereas geographic differentiation is relatively slight.
(5)
If the geographic distribution of the granitoid rocks may be used
as a criterion, intrusion was as extensive in the Fajardo district as in the
Humacao district.
The molten material did not rise to the same eleva-
tion, however, and subsequent erosion has not been deep enough to expose
the igneous rocks which probably underlie the entire area.
To what ex-
tent post-Cretaceous volcanism may have been responsible for the struc-
tural anomalies in the stratigraphic section is unknown.
(6) The scarcity of veins of magmatic origin
is a feature which must
sooner or later come to the attention of the investigator who
is studying
the igneous geology of eastern Porto Eico; and a brief enquiry into the
disposition of the mineralizers during the closing stages of volcanism may
appropriately conclude the present section.
Mineralization and contact metamorphism
In an
island as large as Porto Eico, where volcanism dominated
its
early geological history, the scarcity of pegmatite dikes and the practical
absence of veins are surprising features, especially when the
silicic char-
acter of the
later magmatic
differentiates
is considered.
In
all
of the
Fajardo district one pegmatitic dike was found, and a single specimen of
vein material was discovered in a boulder along the trail between El Yun-
que and El Toro, uiider circumstances that made location of the vein
it-
self impossible.
Assumptions that the mineralizers escaped without leav-
ing a sign of their departure, or that pegmatites and veins were formed at
higher levels and have been lost through erosion, are entirely gratuitous.
It
is logical, therefore, to look for the effects of mineralization in other
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
315
situations, and the search need not go far.
In every member of the strati-
graphic section^ in
all the andesitic intrusives, and even in some of the
earlier granitoid intrusives, the subtle effects of mineralization are evident.
Scarcely a rock in the "Older
Series'^ has completely escaped alteration,
and in a few, little of the original material remains.
About thirty minerals
of secondary origin have been identified among the Cretaceous specimens,
and twenty-five or more
of them were
characteristically introduced by
late magmatic activity.
The processes which produced the most widespread effects were silifica-
tion, chloritization, and epidotization.
Secondary silica
is present in the
volcanic rocks and sediments in the form of quartz ; loss commonly, opal
and jasper may be detected.
Rarely the quartz has replaced older con-
stituents, but in the majority of instances it infiltrated through the inter-
granular spaces, where it serves as an important cementing agent.
In a
few specimens
it forms veinlets that ramify through fracture and crush
zones, but
this type
of occurrence was observed
at very few
localities.
The comparative scarcity of quartz as a primary mineral in the rocks of
Upper Cretaceous age makes its presence more notable and
its magmatic
origin more certain.
Silica,
it
is true, may be derived from feldspathic
rocks by normal weathering, and
it may be deposited in the
interstices
of fragmental rocks by ground water ; but petrographic study of the speci-
mens shows that such a theory of origin
is inadequate to explain its dis-
tribution in the sedimentary and andesitic rocks of northeastern Porto
Eico.
Chlorite is an equally common alteration mineral, and, although usually
a product of replacement,
it was formed under a large variety of condi-
tions.
It fills the vesicular cavities in some of the andesitic and basaltic
flows ; in others it forms reaction rims around the calcite which occurs in
many of the amygdules in the effusive rocks.
In most of the porphyries
and coarse tuffs the process of chloritization involved
selective
replace-
ment of ferro-magnesian minerals, but in some of the cryptocrystalline
and stratified rocks partial or complete replacement appears to have gone
on without much regard for mineral or chemical composition.
Although
quantitatively less abundant, epidote
is scarcely less common than chlo-
rite, being present in a majority of the Cretaceous specimens which have
been studied under the microscope.
It
is found both as an
interstitial
mineral and as a replacement product, and
it also occurs in veinlets and
in irregular aggregates of anhedral grains.
Sericite and zoisite were like-
316
SCIENTIFIC SURVEY OF PORTO RICO
wise noted in a large number of specimens, but the remaining secondary
silicates are only sparingly present and are seldom important.
Locally mineralization involved the introduction of sulphides, includ-
ing pyrite, pyrrhotite, and chalcopyrite; and of the iron oxides, magnetite
and
hematite.
About
three
miles
northwest
of
Florida
estate
in
the
southeastern portion
of the Luquillo Mountains,
the country rock has
been exploited for copper.'^*
The old mine, located high above the anti-
clinal valley that separates the central range from the southern foothills,
has not been re-opened since the hurricane of 1898, and the trails are so
completely overgrown that the senior author was unable to reach it, or to
find guides willing to undertake the several days' labor of opening a trail.
The geology of the deposit
is therefore unknown, but, judging from
in-
formation supplied by
Sr. David Koble,
it
is
like that of most of the
copper prospects in Porto Rico, which usually occur in veinlets that pene-
trate fracture zones in shattered andesite or andesite porphyry.
In the
area drained by Eio Blanco andesite porphyry
is abundant, and
it seems
safe to guess that an intrusive of this character has cut the folded sedi-
ments which make up most of the central range, and that ore formation
has been localized within
it.
According to Sr. Noble, the principal min-
eral
is chalcopyrite, the higher grade material assaying six to eight per
cent, copper, in addition to small values in gold.
It is unlikely that ore
of this quality continues either laterally or vertically for any distance, but
even though the chances that th^ Eio Blanco deposit will prove richer and
more extensive than the other copper prospects in Porto Rico are not very
great,
it deserves a careful field study to evaluate
its possibilities.
A second and less important showing of copper was found 31.1
kilo-
meters east of Rio Piedras on the carretera near Mameyes, where mala-
chite conspicuously stains the weathered surface of an andesite porphyry.
The primary sulphide, probably chalcopyrite, was present in such small
quantities in the minute veins in which it was formed that
it has under-
gone complete conversion to the copper carbonate.
Other small sulphide
deposits occur within the limits of the district, for the senior author has
seen specimens of copper ore that had been collected from three unvisited
localities, but in none of them was the tenor of the ore high enough to
have encouraged
serious
prospecting.
Sulphide
replacement
has
also
taken place to a moderate extent in some of the sediments, but in the
"^^ Colony,
R.
J., and Meyerhoff, H, A.
: The copper prospect at Barrio
Pasto,
Porto
Rico.
Econ.
Geol., Vol. XXIII, 1928, pp. 515-516.
On
tlie location map in this
article
(Fig. 1)
the mine Is incorrectly shown to the east of Rio Blanco, instead of to the west.
Cf.
also, Low, Bela
: The mineral
deposits
of Porto
Rico.
Engineering and Mining
Journal, 1929.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
317
cases studied the minerals are disseminated through the rocks in amounts
too small to warrant economic exploitation.
With the possible exception of the Eio Blanco copper deposit the fea-
tures
of mineralization which have thus
far been discussed
are widely
distributed and affect large masses of rock of diverse origins and kinds.
Very few of the lithologic elements^ the formation of which antedated the
granitoid intrusives, completely escaped alteration, and even some of the
latter were mildly
affected.
Mineralization
is
so
general
that
it can
rarely be attributed to any specific intrusive mass, and in many places it
is by no means clear whether part of it occurred during the volcanic ac-
tivity of Upper Cretaceous time, or whether all of it occurred as an after-
math of the later period of dioritic invasion.
The changes which
all the
sedimentary,
pyroclastic,
and
pyrogenic
rocks
have
suffered
are
dis-
tinctly metamorphie in character; and,
if the word
^^^regional" may be
used in its literal sense, they may appropriately be designated as regional,
to
set them apart from the
local and more intense changes which are
found in the rocks that come in direct contact with the larger
dioritic
intrusives of the district.
Comparable
in many particulars with
the
regional
alteration which
has taken
place,
the
local,
or
contact, metamorphie phenomena
differ
principally in degree.
The magnetite
belt
east
of Juncos exhibits the
most spectacular results of
local metamorphism which the
district con-
tains.
Here, not only have large masses of magnetite and smaller quan-
tities of hematite been introduced into the La Muda and Trujillo Alto
(Collores)
limestones and the
closely
associated
calcareous
tuffs
of the
Luquillo formation, but the calcareous beds have been converted to aggre-
gates of garnet, epidote, chlorite, amphibole, pyroxene, and talc, whereas
calcite has become a relatively unimportant constituent.
Contact meta-
morphism occurred when the San Lorenzo batholith and the contempo-
raneous
dioritic
stocks and
apophyses invaded
the
region,
for
intense
alteration
is
directly
related
to
these
intrusives,
and
its
intensity
diminishes away from the contacts.
Fettke has studied the deposits of
the magnetite belt quite carefully,^^ hence a restatement of the geologic
details and of the economic importance of the iron ores
is unnecessary.
Local metamorphism occurs
elsewhere
along
the southern border
of
the Fajardo district
in association with the
dioritic intrusions.
Endo-
morphic changes
are numerous, and some
of them
have
already been
mentioned ;^^ attention may be
turned,
therefore,
to
a few
of
the exo-
^5 Fettke,
C. R.
: Magnetite deposits of eastern Porto Rico, op.
cit., pp.
1028,
ff.
'« Ante, pp. 305, 309-310.
318
SCIENTIFIC SURVEY OF PORTO RICO
morphic modifications which a number of the rocks outs de the magnetite
belt have suffered.
At Casa de
la Mina, in the valley 1000
feet below
the abandoned copper mine
in the Rio Blanco
region, an
altered
tuff
illustrates some
of the processes that undoubtedly occasioned the
intro-
duction of chalcopyrite in the vicinity of the mine.
The tuffaceous origin
of the rock is evident from its fragmental texture, but none of the initial
pyroclastic material remains.
Stout but minute crystals of diopside are
scattered profusely through
the
entire
rock,
the
other
constituents
of
which
include
a
colorless
chlorite,
orthoclase, soda
plagioclase, and an
undetermined amphibole.
Zoisite, calcite, and epidote are common, and
chalcopyrite, pyrrhotite, magnetite, and titanite form prominent, though
less abundant, constituents.
This one rock seems
to epitomize most of
the
features
of
magmatic
alteration
which
are
present
in
somewhat
milder and less varied form in the other lithologic types of the district,
especially
in, and around
the
edges
of,
the
Caguas-Naguabo
lowland,
where the dioritic intrusions are large and numerous.
In a few places
the country rock
is distinctly schistose for short distances from igneous
contacts, and in one locality the intrusive
itself has assumed a gneissoid
structure.
Structural changes
of
this kind
are, however,
less common
than they
are
in the
adjoining Humacao
district, where
similar
phe-
nomena have been studied and described at length by
Fettke."^^
In the foregoing discussion contact and regional metamorphism have
been differentiated; but, as has been pointed out, the difference is chiefly
one of degree rather than one of kind, for in the field the two types are
completely gradational.
Minerals diagnostic of one or the other cannot
be distinguished, because the whole gamut of alteration products occurs
indiscriminately in both regional and contact situations.
The fact that
the products of mineralization are so widely disseminated in the Creta-
ceous rocks leads to one possible explanation of the alteration phenomena.
The comparative
scarcity of dikes,
especially in the higher members of
the stratigraphic section, has already been mentioned, and the abundance
of thin sills in the shales and stratified tuffs has also been noted.
From
the nature of the shallow intrusions, as well as from the lithologic char-
acteristics of the rocks themselves, the several formations of the
strati-
graphic section appear to have been relatively impervious to the magma
which essayed to
rise through them, and their unconsolidated condition
seems
to have made them too incompetent to fracture.
The consolida-
tion which the rocks now exhibit was acquired primarily as a result of
the
induration which
the magmatic emanations
effected.
There
were
77 Fettke,
C.
R.
: Geology
of the Humacao
district,
op.
cit.,
pp.
170-176.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
319
thus no easy channels through which the mineralizers could make a ready
escape, and in consequence they permeated the entire rock section.
Their
action was
often
highly
selective,
for
strongly
altered
types
occur
in
proximity to rocks that suffered mildly or scarcely at all, a feature strik-
ingly illustrated by the silicated Juan Ascencio member of the Fajardo
formation, which lies between beds that have been but slightly modified.'^^
Usually
selective alteration was fostered by the chemical composition of
the
individual
strata
and
their
relative
susceptibility
to
mineralogic
changes, but in some cases it was clearly due to contemporaneous activity
of a hydrothermal
character.
The enforced dissemination of the mineralizers had, beyond question,
a deleterious effect on metallic ore concentration, except in a few of the
limestones,
in which replacement could readily
occur.
The
local
con-
centration of copper sulphides in the andesites
is natural, for the latter
were the only rocks
in the
section competent enough
to fracture;
but,
engulfed in incompetent pyroclastics and sediments, the shallow igneous
intrusives were compelled to yield to compression by flexure more often
than by fracture, and crush zones are few and extremely limited in ex-
tent.
In the
other rocks sulphides
are commonly present, but in such
low concentrations that the prospects for economic development are not
promising.
Conclusions
The preceding sketch
of
the igneous
features
of northeastern Porto
Eico has aimed merely
to add areal
details
to
a phase
of the
island's
geological history, the main features of which have been known for some
time.
It departs from the conclusions advanced in
the
earlier reports
of the
Scientific
Survey in but one
respect;
namely,
in
insisting
that
the sharp textural differences between the surface flows and shallow
in-
trusives, on the one hand, and the later dioritic intrusions, on the other,
indicate not only the contrasted conditions of depth and pressure which
existed when the two cycles of volcanism occurred, but also the probability
of a long lapse in time between the earlier and later periods of igneous
activity.
Early
volcanism
was
explosive
in
nature
and
was
causally
related to the development of the stratigraphic section.
In Hato Puerco
time a volcano appears to have been built up subaerially in the
south-
ern or eastern part of the
district, although
its geographic position
is
unknown.
Several subsidiary vents were opened on
its flanks, and flows
and dikes gave strength
to the fragmental materials that composed
it,
TSAnte,
pp. 286-287.
320
SCIENTIFIC SURVEY OF PORTO RICO
while
marginally
the fragments were
resorted by an
overlapping
sea.
Activity waned and finally ceased, but more remote centers remained, or
became,
active.
Encroaching
seas
planed and
buried
the
old volcano
beneath sediments
of
pyroclastic and
biogenic
origins, although
a few
submarine flows and
sills, and local cinder cones were mingled with the
sedimentary
deposits.
Later,
after several thousand
feet
of sediments
had
accumulated,
volcanism was
revived,
probably
as
a
result
of
the
orogenie disturbances that compressed and folded the rock section.
Ee-
surgent magmas, which became increasingly
silicic in composition, rose
to various levels, forming the granitoid series of rocks, and concomitant
mineralization produced
telling
effects upon
the
entire mass
of
older
rocks.
In chemical
compositions
the
older and younger
igneous
rocks
are
essentially alike, although the latter are somewhat more
silicic than the
former.
Among the felsitic types differentiation produced both basaltic
and
trachytic,
or
rhyolitic,
varieties, but the
differentiates are comple-
mentary, the average or norm approximating the composition
of ande-
site.
Among the granitoid rocks time played a more important part than
space
in
effecting
differentiation,
for the
first
intrusives
of
this
cycle
were gabbroic, and the
later,
if not the
last, were granitic.
Again the
aggregate is more or less complementary, the whole approaching the com-
position
of
quartz-diorite.
The
pyroclastic
rocks,
too,
are dominantly
andesitic, for silicic and subsilicic variants occupy -but little place in the
stratigraphic
section.
It was
this assemblage
of
andesite-diorite mate-
rials that formed the mountainous oldland around which the subsequent
Cenozoic events have centered.
EROSIONAL FEATURES OF THE CRETACEOUS ROCKS
The erosional history of the "Older
Series'^ will be treated in a later
section on
the physiography
of
the
district, but the
various
lithologic
types react to the processes of weathering and erosion in ways that are
sufficiently characteristic to merit brief exposition.
In a humid tropical
region decomposition normally predominates over disintegration, but in
the mountainous parts of Porto Eico the high relief and youthful topog-
raphy accelerate some of the mechanical phases of destruction, and the
resultant erosional forms which each kind of rock assumes depend upon
the balance between the chemical and mechanical forces which are
act-
ing upon
it.
Chemical processes of rock decay are affecting every type of rock which
the Fajardo
district contains, and the results are especially striking in
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
321
the low coastal sections and in the interstream areas of the moderately
elevated
uplands.
Lateritic
soils
characterize
so much
of
the
island^s
surface that^ in the intensely cultivated districts where the soils are often
bared by the plough, their red and brownish red colors are an inseparable
part of every landscape.
Lateritization has proceeded most extensively
in the areas underlaid by the andesitic rocks.
Limonite forms early in
the process, and the development of residual clay products soon follows.
In spite of their felspathic content, the patches of diorite, quartz diorite,
and granite, on the other hand, exhibit negligible decompositional effects,
and are commonly exposed in relatively fresh outcrops, even where they
have
been
eroded
to
greater
depths
than
the
surrounding
andesitic
materials.
One of the most widespread results of the hydration which accompanies
chemical weathering
is
exfohation.
Although
operative on
practically
every lithologic variety in the ''Older Series,'' it affects the relatively dense
rocks
to
a greater
degree than the
coarser and more
porous
types,
in
which
there
is more
latitude
internally
for
this kind
of katamorphic
change.
In the andesite porphyries exfoliation
is most marked, and
it
is often displayed
to best advantage
in
fresh road cuts and in stream
gorges, sometimes as much as six and eight feet below the natural sur-
face
of the land, notwithstanding the protection
afforded by the
thick
blanket of residual
soil
(Fig. 23).
In several localities in the Fajardo
district and
in
other parts
of the
island,
the
limits
of an outcrop
of
andesite porphyry can be accurately determined from the extent of the
comparatively fresh
exfoliation boulders
scattered thickly over
its
sur-
face.
Berkey called attention to
this
feature, and the resemblance
be-
tween the boulders and glacial erratics led him to postulate glaciation as
a
possible
explanation
of
their
origin.^^
In
the
cases which came
to
Berkey's
notice, the process of exfoliation has produced spectacular
re-
sults because of the extremely irregular spacing of the fractures which
governed
its operation; in other rocks, where the joints are closely and
more evenly spaced,
this mode
of weathering
is no
less
effective,
even
though
residual
boulders
do
not
endure.
Its
workings
are
revealed,
however, in the rounded edges and minute
scaling of joint blocks
dis-
played in road cuts and in quarries.
The depth beneath the surface at
which exfoliation may occur, and the equable climate of Porto Eico are
factors which indicate clearly that the process
is the
result
of normal
decomposition
rather than
of
disintegration.
Blackwelder
has
argued
79 Berkey,
C.
P.
: Geological reconnoissanee
of Porto
Rico,
op.
cit.,
p.
35.
tiCIKNTJFJC SURVEY OF PORTO Ef(;0
whieli
the prow
ledras
to
Barrio Cupey
<lc<!oiniH»S!iti.m have
pei
illustrates; the depth
to
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
323
that exfoliation
is more
characteristically
a chemical process than one
dependent on thermal
or mechanical phenomena.^^
Certainly
the
ex-
foliation which is taking place in every part of Porto Kico
is open to no
other interpretation.
Two anomalies
in the
effects of weathering come to attention rather
early in a
field study of Porto Eican geology.
In many
localities the
purer
crystalline
limestones
in
the
"Older
Series^''
exhibit
greater
re-
sistance to weathering and erosion than any other kind of rock, whereas
diorites
usually
exhibit
less.
The
limestones
of
the Fajardo
district
are thin and do not exert a strong influence on the topography, yet the
La Muda limestone forms a distinct linear ridge in a number of its inter-
stream outcrops
(Fig.
7).
In the interior of the
district the granitoid
intrusives have proved so non-resistant that stream erosion and gullying
are usually localized upon them, and the outcrops of all the larger masses
have become
the
sites
of topographic basins
or
lowlands.
In
the
low
coastal
areas, however, the
relationships
are not infrequently reversed:
The limestones have been baseleveled, but
the granitoid
intrusives
rise
above the graded levels to which the other types of rocks have been
re-
duced.
Within
the
limits
of
the
present
report
the many
details
of
weathering cannot be described, but a brief summary and explanation of
the outstanding features and anomalies seem pertinent.
Aside from the regional precipitation, humidity, and temperature, four
special,
or
local,
factors
are exerting an important influence upon the
weathering processes
at work.
These include,
(1)
the degree of mag-
matic alteration which the rocks underwent before exposure;
(2)
relative
solubility and porosity of the several lithologic types;
(3)
present rela-
tion between the land surface and the water table; (4)
relief.
(1)
All the Upper Cretaceous rocks were altered to a greater or
less
extent by the processes of mineralization which were attendant upon the
formation of the younger
dioritic
intrusives.
Except
in the few rocks
which were silicified but not otherwise altered, susceptibility to chemical
weathering appears
to have been increased
in
direct proportion
to
the
amount of alteration which took place.
Because of the similarity of the
changes that occurred within them, the andesites, the andesite porphyries,
and the massive and stratified tuffs, all exhibit analogous erosional forms,
in spite of the initial differences in their respective abilities to withstand
weathering and erosion.
Conversely the superior resistance of the grani-
«o Blackwelder,
Eliot
: Exfoliation
as a phase
of
rock weathering.
Journal
of
Geol-
ogy, 1925, Vol. XXXIII, pp. 793-806.
324
SCIENTIFIC SURVEY OF PORTO RICO
toid rocks in regions
of low
relief may be traced
to the comparatively
small amount of alteration which they suffered.
(2) The factor of relative solubility acquires expression chiefly where
limestones, shales and andesitic rocks of both pyroclastic and pyrogenic
origins are juxtaposed.
In areas of low
relief and low elevation where
the water-table is near the surface, the rate of decomposition reflects quite
accurately the proportionate solubilities of the varieties of rock present.
The
limestones
are
rapidly reduced;
the
shales,
slowly;
the
andesitic
rocks, at an intermediate rate.
The simple operation of this rule may
be modified by the porosity of the rocks, a factor which retards solution,
though not appreciably in the areas of low
relief.
(3)
In the regions
of moderate and high
relief, on the
other hand,
permeability
becomes
a
factor
of
increasing
importance.
The
water-
table
is situated at a greater depth beneath the surface, and the down-
ward movement of meteoric waters becomes too rapid for them to exert
their maximum solvent effects upon the porous rocks.
Under such
cir-
cumstances both the initial porosity and the development of solution chan-
nels and caverns retard the rate of solution in the limestone strata, and
they
are
in
consequence
relatively
resistant
elements
in many
parts
of the
interior.
Conglomeratic rocks Hke the Kio formation may,
too,
if they have not too impervious a matrix, display their superior
ability
to
resist
decomposition
in
comparison
with
the
tuffs and
porphyries
around them.
(4)
High
relief
also
accelerates mechanical weathering and
erosion
to such a marked degree that in many parts of the island they assume
leading roles in the reduction of the land surface.
There
is
little
dis-
integration in the stricter sense of the word, for most of the work
is ac-
complished
by
rainwash
and
by
landslides
on
oversteepened
slopes.
Decomposition
initiates the process by loosening the minerals
or
frag-
ments of which the rocks are composed, but thereafter the work
is carried
on by rain and gravity.
Every downpour, from
the
short showers
of
the summer season to the occasional deluges of sporadic hurricanes, pre-
cipitates a host of earth slides, involving the shifting of vast quantities
of material
in
the
youthfully
dissected
regions.
Where
the
rocks
are
andesitic, the natural tenacity of the residual clay soils slows the process
markedly, but in the areas underlaid by diorite or granite it proceeds un-
checked.
Decomposition speedily loosens the interlocked crystals, as even
a
slightly weathered
exposure
shows, and rainwash
or,
under
proper
topographic
conditions,
a
slide,
removes
the
crystal
fragments
before
they have the chance to undergo further decay.
The arkosic sands that
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
325
cover much of the floor of the Caguas basin^ as well as the comparative
freshness
of the
diorite exposures, provide eloquent testimony concern-
ing the
efficacy of these mechanical processes.
In this manner depres-
sions have been excavated in
dioritic
areas, while nearby andesitic out-
crops, seemingly more deeply weathered, have been etched in relief.
On
the other hand, in places where the
relief
is not great, whatever
their
elevation, removal of loosened crystal grains takes place at a more mod-
erate
pace.
Here
the
feldspars
of the granitoid
rocks
are undergoing
thorough decay, and weathering
is progressing at a slower rate than in
the
associated
volcanic
rocks.
The minor features
of rock weathering
are
too complicated and
too
manifold
to be covered in the present
report, and the foregoing
state-
ment does
little more than outline a few of the principles and explain
some of the paradoxes.
Though granitoid rocks may underlie the Caguas
basin and form the peaks of the Sierra de Cayey, though the La Muda
limestone may form the valley of Eio Grande de Loiza at Trujillo Alto
and make the strong linear ridge just a few miles to the west, the erosional
expression which each lithologic variety acquires
is perfectly adjusted to
the set of factors operative in the topographic situation that exists where
it comes to the surface.
JSTone of the rocks can be rated on
a scale
of
absolute resistance to katamorphic change, for their respective resistances
vary from one
type
of environment
to
another.
The
climate
governs
merely the general aspects of weathering, but the behavior of the individ-
ual rock types depends almost wholly upon the minor factors which char-
acterize
different
environmental
conditions.
Discussion
has
therefore
centered upon the
latter.
The Tertiary Coastal Plain
by
Howard A. Meyeriioff
DISTRIBUTION OF THE COASTAL PLAIN IN PORTO RICO
The Cretaceous oldland of Porto Eico
is flanked both upon the north
and south by overlapping coastal plain deposits of Middle Tertiary age.
Along the south coast these deposits form a relatively narrow and irreg-
ular strip between the Cordillera Central and the present shoreline, ex-
tending with one or two breaks
in
their continuity from Santa
Isabel
to Cabo Eojo.
Two formations have been differentiated: a basal
shale,
called the Juana Diaz formation, the occurrence of which
is limited to a
326
SCIENTIFIC SURVEY OF PORTO RICO
small area near the eastern extremity of the Tertiary outcrops; and the
Ponce formation^
essentially a limestone, in which chalky marls, semi-
crystalline and fragmental strata, calcarenytes, and reefs are interbedded,
although
the marls predominate.
Both
formations
display
a
physical
and faunal detachment which has made their correlation with the Ter-
tiary strata along the north coast one of the perplexing problems in the
Cenozoic geology of Porto Eico.
The northern Tertiary
is much more
extensive, stretching from the western extremity of the island to Loiza
in the north central part of the Fajardo district, and occupying approxi-
mately five-sixths
of the entire coastal area.
It acquires
its maximum
width of fourteen miles in the longitude of Lares, narrowing westward
to
less than eight miles in the vicinity of AguadiUa, and eastward to a
minimum of three miles at Loiza.
At the latter locality the strata make
their easternmost appearance in the
hills lying east
of Eio Grande de
Loiza.
Several lines of evidence indicate, however, that they originally
extended some five or six miles farther east to Punta Miquillo, and that
even now they continue
still farther east upon the submarine platform.
The increasing breadth of the latter, the presence of typical coastal plain
topography upon
its drowned surface north of Culebra and the Virgin
Islands,^^ and the low angle
of
tilt which carries
all the landforms of
Porto Kico downward toward the east, the lower ones to positions below
sea-level, are facts which leave little room to question the presence of the
Tertiary rocks immediately off-shore in the northeastern portion of the
Fajardo
district.
The Tertiary coastal plain not only acquires
its greatest width in the
Lares district in the northwestern part of the island, but also
its maxi-
mum thickness and
stratigraphic range.
Here
it
is
divisible
into
five
distinct formations, to which Hubbard has given the following names and
estimates of thickness :^^
5.
Quebradillas limestone, 700-785 feet
4. Los Puertos limestone,^^ 550-1000 feet
3.
Cibao limestone,^^ 250-1000 feet
2. Lares formation, 350-1275 feet
1. San Sebastian shale, maximum, 700 feet
81 Meyerhoff,
H.
A.
: The physiography
of the
Virgin
Islands,
Culebra, and
Vieques,
op.
cit.,
pp. 107-119;
194-207.
82 Hubbard,
Bela
: The
Tertiary
formations
of Porto
Rico.
Science,
1920,
Vol.
LI,
pp. 395-396.
Geology of the Lares
district,
op.
cit., pp. 38-49 and Table
3.
83 Maury
(Amer.
Jour.
Sci.,
4th
Ser.,
1919,
Vol. XLVIII,
pp. 209-215)
proposed
the
name
. "Aguadilla"
for the
strata which
lie between the Lares and Quebradillas forma-
tions, on the basis
of faunal studies.
Two very
distinct formations
are
present, how-
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
327
Where
all five formations are present the Tertiary
series averages not
less than 3000 feet in thickness, and along the valley
of Eio Camuy a
maximum
of nearly 4000
feet
is
attained.**
East and west from
this
stream the section thins ; westward, because the rate of accumulation was
less rapid, and the individual formations decrease in thickness ; eastward,
because
the
younger
stratigraphic members
progressively
overlap
the
older.*^
Thus the San Sebastian
shales pinch out near the eastern ex-
tremity of the Lares district.
The other four formations continue into,
and across, the Arecibo district, but the Lares limestones become extremely
thin and, not far from the western border of the San Juan district, they
disappear.
The Cibao formation extends but
little farther, and the Los
Puertos
limestone
gradually ends
against
the
Cretaceous
rocks
in
the
eastern part of the San Juan district.
Only the Quebradillas formation
passes
into the Fajardo
district.
THE QUEBRADILLAS LIMESTONE
The
Quebradillas limestone has
already been
described by Hubbard
and by
Semmes,*^
but
the
formation undergoes
gradational
lithologic
changes from west to east, and
its characteristics in the Fajardo district
are sufficiently different from those exhibited in the type locality to merit
brief supplementary description.
The calcareous strata of the formation
are exposed in a
relatively small proportion
of the
coastal area
in the
northeastern corner of the
island.
They are found
in widely scattered
patches that
rise
as
isolated
conical
hills
or
hill
clusters, the
bases
of
which are buried by overlapping lagoonal and fluvial deposits of recent
ever, and in none of Maury's work
is
it clear whether the term "Aguadllla" as originaUy
employed embraced one or both
of them.
The rocks
at Aquadilla are
referable
to
the
higher formation, the Los Puertos of Hubbard, and the lower, or Cibao, outcrops within
a
mile
of the
center
of
the town.
The Los
Puertos
is
faunally
allied with
the
Que-
bradillas and
is
Miocene
in
age,
whereas
the
Cibao's
fauna
is
distinctly
Oligocene.
Maury's early studies placed the "Aguadilla" formation
in the
Oligocene,
associating
it
with the Cibao rather than with the Los Puertos.
Recently, however, she has placed the
Aguadilla
in the Miocene
(Science,
1929,
Vol. LXX,
p.
609),
apparently accepting the
opinion
of Woodring.
It
is
not
clear,
however, whether
this change
is based upon
a
re-evaluation
of
the
paleontologic
evidence,
or
upon
the
restriction
of
the
term
"Aguadilla"
to the higher of the two formations.
In view of Maury's failure
to
define
the formational name at the time
it was proposed, or
to state
the basis for her recent
change,
its validity cannot be established, although
its equivalence to the Los Puertos,
as employed
in her
recent statement,
is
clear.
In the
present
report,
therefore, Hub-
bard s carefully defined names, "Cibao" and "Los Puertos,"
are retained.
8* Hubbard, Bela
: Geology
of the Lares
district,
op.
cit.,
PI.
II.
8B Ibid.
Fig. 23,
p.
75.
8«Ibid.,
pp. 46-49.
Semmes, D. R.
: Geology of the San Juan district, op.
cit.,
pp. 55-57
(in part).
1128
8lHE?iTlFIG SURVEY OF PORTO RICO
origin.
As miiv be seen from
tlie geologic^ map which aeooiiipauies this
re])ort, the foriufttioii
is exposed in
i'oiir priBcipal areas:
(1)
ill the clustered
hills lying between San Joan harbor aud Baya-
mon
{¥ig. 24)
;
(2)
in the coirical hillocks to the east of Lago San Jose:
(3)
in
tlie
scattered eminences
whicli
rise
above
the
plain
between
Lago San Jose and the Eio Piedras-C'arolioa carretcra;
(-1)
in
a comparatively
large^. maturely
dissec.'ted area on
either
side
of Bio GraiMle de Idnm, extending from the (Jarolina-Canovanas
car ret era nor tli ward to
tiie coast.
view south across San
.liiati Harbor,
sliowiiig the
slightly upraised lagoonal deposits
at Catafiy
(eeiitrr right)
ovwrlappiui; the erosioiuil
remiiHiilK of the IVrtlnry strata.
Other
isolated
t'xposures
ocenr
w-itlrin
the
distriet,
but
their
size
is
small, and most of them arc so remote from roads and
trails that they
vfere not visited in the course of the
field survey.
Ill
all
of
its occurrences
in
northeastern
Porto
Eico
the
limestone
has been recliiced by subsurface solutioUj and
tljc irregular, jagged
liills
tlnit characterize
its outcrops
are undissolved
residuals
that have
thus
far escaped destruetiou.
In the type locality at Quebradlllas the forma-
tion consists in large part of impervious marly beds which have yiehled
to solution but slowly.
Here, in consequence, the strata have been reduced
primarily by
the
usual
processes
of
suljaerial
w'eathering and
surface
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
329
erosion, and they form smooth, rounded hills and
flat upland expanses,
in
which
sink-hole
topography
is
not
especially
prominent.
These
lithologic and erosional characteristics persist throughout the Lares
dis-
trict and across the greater part of the Arecibo district, but near the east-
ern boundary of the latter the lithology undergoes minor changes, which
become more pronounced farther eastward.
The marls give place to an
increasing
percentage
of
reef
limestone,
calcarenytes, and beds
of
re-
crystallized limestone, all of them types which readily succumb to subsur-
face
solution.
Sink-holes become abundant, and
as
their numbers
in-
crease toward the east, conical hills become the outstanding topographic
form marking the presence
of the Quebradillas formation.
These
ero-
sional
pillars, called "haystack
hills'^ by Berkey, and "pepino
hills^^ by
E. T. Hill, who used the local term applied to them, formed the supports
of
caverns
in an
earlier
stage
of
the
present
erosion
cycle and have
attained their prominence by the collapse of the cave-roofs around them.
The extent to which solution has proceeded in the limestone increases
markedly eastward until, in the northeastern portion of the San Juan dis-
trict, the formation appears above the surface of the overlapping lagoonal
and fluvial deposits in bunched pepino hills.
In the Fajardo district the
groups of pepinos become more widely spaced, and in some localities the
formation
is represented by solitary erosional pillars of diminutive
size.
The varied effects of solution are striking.
All the outcrops are exceed-
ingly cavernous ; in the reef limestone some of the caverns are original or
primary, but most of them are of secondary origin, and each cave is lined
with a thick encrustation
of travertine.
Even the arrangement of the
clusters
is obviously the outcome of subsurface channeling.
The eastern-
most outcrops of the formation consist of a series of aligned pepinos, the
trend of which reflects the influence of north-south subsurface drainage
lines.
The pepinos and sink-holes on the west bank of the Rio Grande de
Loiza have a checkered pattern, in which north-south main channels and
east-west tributary channels can be distinguished.
The three detached
hills of Quebradillas limestone between Lago San Jose and K. 3.6 on the
Eio Piedras-Carolina
carretera
are
similarly
in north-south alignment,
and in other parts of the
coastal plain the plan
of the original under-
ground drainage, although more complex, can
still be deciphered from
the erosional remnants which are present.
In the Fajardo
district the
cycle
of
subsurface
erosion
reached
the penultimate
stage
of
develop-
ment,
for
the
Quebradillas formation has been reduced
to
a
perfectly
graded peneplane above which the pepinos rise as monadnocks.
Since the
330
SCIENTIFIC SURVEY OF PORTO RICO
erosional plane was formed, the recent changes in level at the coast have
caused some aggradation to
occur, and
its
surface
is buried beneath
a
veneer of alluvial and lagoonal materials.
In the Fajardo district the lithology of the Quebradillas formation
is
about as varied as the lithology of limestones can be.
Cavernous reef lime-
stones and
crystalline or semi-crystalline types predominate, and inter-
stratified with them are beds of marl and a few layers
of impure
cal-
carenyte.
Two and one-half miles northeast of Carolina an exposure of
breccia,
consisting of fragmented
reef materials
in a clay matrix, was
noted.
The
reef members
characteristically
consist
of
scattered
coral
heads and
recrystallized
coral
bases,
with
coral and
shell
fragments,
rounded sand-grains, and, more commonly, marl
filling the
spaces
be-
tween the heads.
Most of the original cavities in the reef limestones have
been filled in this way, and, from this characteristic, can generally be dis-
tinguished from the younger solution cavities.
As a rule, the reef and
crystalline beds are massive, varying in thickness from two to twelve feet,
and the predominance of these two types in the section has governed the
erosional development which the formation has undergone.
The marls
and calcarenytes, typical exposures of which may be seen in the railroad
out
about
one
kilometer
west
of
the
Canovanas
Sugar
Central,
are
habitually thin-bedded.
In
several
of the pepino groups beds
of marl
appear to have furnished local basements above which the erosional rem-
nants
rise.
In the limestone
hills between Carolina and Loiza,
for ex-
ample, drainage ditches in the bottoms of the sink-holes reveal impervious
marls, through which the underground drainage was unable to penetrate
by solution.
The haystack hills which surround the sinks on every side
consist almost wholly of massive beds of pure calcium carbonate, some of
it partly recrystallized, in which marly strata
either
are absent or
are
limited to beds of negligible thickness.
STRUCTURAL AND STRATIGRAPHIC RELATIONS OF THE QUEBRADILLAS
FORMATION
The structure of the Quebradillas formation appears to be simple, but
the character of its outcrops made it impossible to obtain an unequivocal
series of
definite
observations.
Many exposures
consist
of the massive
strata which form the pepinos, and in them irregular bedding and local
slumping around the margins of sink-holes tend to obscure the true struc-
ture.
The impressions obtained from distant views are probably more
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
331
reliable than detailed measurements of individual strata, and they indicate
that the strata of the coastal plain dip uniformly toward the north at an
angle which
is not lower than three degrees, and which does not exceed
six.
In the railroad cut one mile west of the Canovanas Sugar Central,
the thin-bedded marls and calcarenytes dip
five degrees north, and this
amount may be taken as the average for the formation in the Fajardo dis-
trict.
It
checks
almost
exactly with
the numerous measurements
the
writer has made in the Arecibo district and in portions of the San Juan
and Lares districts.
The dip of the strata is discordant with the slope of
the
surface, whether the
latter be measured
across the present surface
or across the relatively accordant tops of the clustered pepinos, which rise
to the elevation of an old erosional level that may be traced far inland in
the hills of the Cretaceous oldland immediately to the south.
Obviously
the
five degree dip
is an acquired deformational characteristic, that has
been completely disregarded in the recent erosional history of the coastal
region.
The discordance between the one degree slope of the erosional surfaces
and the five degree angle of dip affords a means of estimating the thick-
ness of the Quebradillas formation.
The angle of discordance is not more
than four, nor less than three degrees.
Between Bayamon and Canovanas
the limestone belt maintains an average width of four miles, making the
estimated thickness of the formation not less than 1100 feet and not more
than 1450 feet.
In the Lares district Hubbard gives the thickness as 700
feet at Quebradillas and 875 feet at Camuy.^^
At Arecibo it approximates
1000 feet, and in the vicinity of Manati it is somewhat in excess of 1200
feet.
From this point eastward the limestone maintains a relatively con-
stant thickness, and the computations for the Fajardo district appear to
be normal.
The rapid decrease toward the northwestern corner of the
island
is probably due to the more extensive erosion which the beds have
undergone since their exposure, and
it need not be attributed to local or
regional differences in the rate of subsidence during Quebradillas time.
In fact, several lines of evidence show that the period of its accumulation
was characterized by more marked crustal stability than obtained during
any other Tertiary age.
It may be questioned, however, whether the 1500
feet maximum thickness which the formation now possesses represents its
total thickness at the end of the period of deposition ; for its tilted strata
have already passed through one cycle of erosion and have entered upon a
87 Hubbard, Bela
: Geology of the Lares district, op.
cit., Plate
I.
332
SCIENTIFIC SURVEY OF PORTO RICO
second.
In each cycle some of the upper beds have been removed, yet pro-
jection of the dipping strata and the erosional planes to the edge of the
Porto Eican platform indicates that the loss through erosion may not have
been great, and that the formation probably did not exceed 1800 to 2000
feet when deposition ceased.
In view of the number of uncertain factors,
however,
it would be unwise to place too much confidence in this rough
estimate of original thickness.
In the Fajardo district the Quebradillas formation
rests
directly and
unconformably upon the underlying Cretaceous volcanics and sediments.
The contact
is not exposed within the boundaries of the district, because
the
erosional surface above which the pepinos
rise
is buried beneath a
veneer of lagoonal and alluvial deposits of variable thickness.
The Ter-
tiary hills between Bayamon and
Cataiio, however, are locally separated
from outcrops of Cretaceous rocks by as short a distance as 200 yards, in
which the residual soils of the ''Older Series'' have been modified by sur-
face wash to an extremely limited extent.
The Cretaceous materials of
this area consist of tuff and ash, both of which have been deeply lateritized.
In places the weathered surface appears to have undergone so little gully-
ing that
it undoubtedly represents the unmodified erosional plane upon
which the Tertiary coastal plain was originally deposited.
It is impossible
to gauge how much lateritization the rocks underlying this surface have
undergone since stripping re-exposed it, but the depth and extent of alter-
ation are greater than in the Cretaceous rocks of any other portion of the
district.
This fact appears
of
especial
significance, because
it implies
that the greater part of the weathering must have taken place before the
coastal plain was laid down, rather than in the brief lapse of time since
its removal, else the older erosional surfaces would exhibit far more pro-
found effects of residual decay.
The assumption that marine strata were
deposited upon
a
subaerially eroded land
surface which
escaped com-
pletely the usual planation by wave action appears absurd, yet this con-
clusion
is inescapable.
And fortunately
it can be proved by direct evi-
dence from other parts of northern Porto Rico.
In several stream gorges
in the San Juan, Arecibo, and Lares districts, the basal beds of the coastal
plain may be seen in direct contact with the underlying Cretaceous vol-
canics.
In every case which the writer has had opportunity to study, the
Cretaceous rocks had been deeply lateritized before
the Tertiary
strata
were deposited on them, and in several places, notably in the canyon of
Rio Arecibo and in the valley of Rio Yunes, lateritic soil zones, ten to forty
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
333
feet in thickness^ are present immediately beneath the unweathered Ter-
tiary limestones.
The profound weathering which has taken place in the
Cretaceous rocks at the margin of the coastal plain in the Fajardo district
indicates that here^ too, the subaerially eroded oldland underwent little or
no marine planation when Middle Tertiary submergence occurred.
The
basal strata of the coastal plain were laid upon a practically unmodified
surface of deeply weathered volcanic rocks.
Further proof of this interpretation
is furnished by the nature of the
contact between the Tertiary and Cretaceous rocks.
Broadly considered
the line of contact is relatively straight, but in detail it is exceedingly ir-
regular.
It
is composed
of moderately strong
salients and
re-entrants
that show the undulating character of the pre-Miocene surface, for they
are unrelated to the features of Quaternary stream dissection.
All of the
re-entrants
contain
outcrops
of
the
Quebradillas
formation,
some
of
which may be seen in the pepino hills south of San Juan harbor, south of
Lago San Jose, and between Carolina and Canovanas.
Between the
first
two of these three localities a salient protrudes about a mile northward in
the vicinity of Eio Piedras
; between the second and third, at the town of
Carolina, another extends
nearly two
miles north
of
the
adjacent
re-
entrants.
As shown on the geologic map of the district, outcrops of Que-
bradillas limestone occur within the recesses in locations that are consider-
ably south of the northernmost points reached by the Cretaceous Ipurs.
The significance of the irregularities
is more noteworthy if
it be recalled
that the surface on which the coastal plain strata rest has been tilted ^ve
degrees north, and it seems probable that the fluvially eroded surface on
which the Quebradillas limestone was accumulated had a relief of several
hundred feet, in spite of the deep weathering which had affected every part
of it.
An examination of the Cretaceous rocks and rock structures in the
spurs shows that their elevation was in no small measure due to the re-
sistance given them by their compressed synclinal structure.
THE FAUNA OF THE QUEBRADILLAS LIMESTONE
The Quebradillas limestone is the most fossiliferous of the Tertiary for-
mations
in Porto
Kico, and from
its
outcrops
in western
Porto
Eico
Maury ^^ and Hubbard ^^ have described 104 different molluscan forms.
A comparatively small number of corals has been listed from this horizon
S8 Maury,
C. J.: Tertiary Mollusca from Porto Rico.
New York Academy of Sciences,
Scientific Survey
of Porto Rico and
tlie Virgin
Islands, 1920,
Vol.
Ill,
Pt.
1.
89 Hubbard,
Bela
: The
Tertiary molluscs
of
the
Lares
district,
ibid,
1021,
Vol.
Ill,
Pt.
2.
334
SCIENTIFIC SURVEY OF PORTO RICO
in the recent work of Coryell and Ohlsen.^^
Their study was based upon
the specimens in the Eeeds and Hubbard collections, both of which con-
tain
material
taken
only from
the
northwestern
districts,
where
this
phylum is but meagerly represented.
Eastward, as has been pointed out,
the lithologic characteristics of the Quebradillas limestone change from a
preponderance of clastic layers to an abundance of reef beds ; and with the
change in the character of the strata, the complexion of the fauna changes
accordingly.
Corals increase in number and variety, and within the limits
of the Fajardo
district they equal the Mollusca in importance.
Other
groups of organisms are present in fewer numbers.
Reeds, Hubbard, and
the writer have found foraminiferal beds, but the microfauna is somewhat
more prolific in the Lares and Arecibo districts than in the northeastern
part of the island, where the remains are poorly preserved and appear to
have suffered considerably from the
effects of wave action prior to their
burial.
Echinoids are not uncommon despite the fact that only a single
specimen was found in the Fajardo
district, in the limestone hills south
of Catano.
Sharks' teeth,
also, have been taken from the strata of the
formation, but none were discovered in the course of the present survey.
There are not many exposures
of the
Quebradillas limestone
in the
Fajardo
district, and
the
opportunities
for
fossil-collecting
are
corre-
spondingly limited.
Nearly all of the outcrops are fossiliferous, it is true,
but many of the massive strata composing the scattered pepinos contain
coral heads which are so firmly crystallized in the rock, or are so large in
size, that their recovery with the equipment which the author had at his
disposal was not feasible.
Other types of organic remains are rare in the
coralline beds, many of which are
so heavily encrusted with travertine
that
it is impossible to detect the small fossils within them; but in the
semi-crystalline
beds,
the
calcarenytes, and
the
coarser-grained
marls
Foraminifera are common, and Mollusca are unfailingly present.
Their
state of preservation
is uniformly poor, and in the majority of exposures
only the internal molds of the Mollusca can be recovered.
In five locali-
ties, however, comparatively good fossils were obtained, and all but a few
of the species identified were taken from them.
Four of the localities are
situated
in the limestone
belt between Canovanas and Eio Grande
de
Loiza; the fifth is located in the hills south of Catano
(Fig. 24).^^
^ Coryell, H.
N., and
Ohlsen.
Violet
:
Fossil
corals
of
Porto
Rico, with
descriptions
also of a few Recent species,
ibid, 1929, Vol.
Ill, Pt.
3.
w Professor Horace Elmer Wood,
II, has kindly supplemented the author's
fossil
col-
lections with specimens, many of which were gathered in the Catano locality.
MEYERHOFF, QEOLOOY OF THE FAJARDO DISTRICT
335
The study
of the fauna
is incomplete.
The many samples
of marl
which were collected have been examined for Foraminifera^ and several
faunules have been isolated and are awaiting study.
The single fragment
of an echinoid has not been identified^ and some of the most poorly pre-
served
corals and Mollusca remain
unclassified.
A small
collection
of
corals was referred to Dr. H. N. Coryell, who has kindly made the follow-
ing identifications
:
Antiguastrea cellulosa (Duncan)
Porites douvillei Vaughan
Cyatliomorylia tenuis (Duncan)
Brachyphyllia sp.
Plocophyllia sp.
Antilloseris sp.
(?)
Thirty-eight molluscan forms have been differentiated, of which twenty-
six
are
Pelecypoda,
and
twelve
Gastropoda.
Many
specimens
were
referred provisionally to described species, because their identification has
been based upon internal molds or upon none too diagnostic shell frag-
ments.
The faunal
list follows:
Pelecypoda
""Leda peltella Dall ''
Area n.
sp.
^''Arca (Scapharca) guajatica Sheldon and Maury ^^
Area (Scapharca) n. sp.
'^Glycimeris portoricoensis Hubbard, n. var.
Ostrea cahohasensis
( ?)
Pilsbry and Brown
Pecten {Plagioctenium)
cercadica Maury
Pecten {Plagioctenmm) n. sp.
Pecten n. sp.
Pecten (Chlamys)
cf. waylandi Cooke
Venericardia scahricostata (Guppy)
Venericardia santodoming ensis Pilsbry and Johnson
^Phacoides {Miltha) hubhardi, n. sp. (=P. sp. indet. Hubbard) ^^
"»2 Reported
in~th^
Quebradillas
formation
of
western
Porto
Rico
by
(*)
Hubbard,
(*) Maury
(***) Hubbard and Maury.
«3 Several 'specimens
in
the Fajardo
collection
indicate
the need
of recognizing
as
a
rli«tiTipt
snecies
the form which Hubbard
described
as
Phacoides
(Miltha)
sp.
indet.
7The Tertiary moUuscs of the Lares
district,
op.
cit,
p.
112, PL XVIII,
fig.
5).
The
specific name P.
{Miltha)
huhDardi is herewith proposed.
336
SC/ENTIFIG SURVEY OF PORTO RICO
'^Phacoides
(Lucinisca)
cf. calhounensis Dall
'-'-'-Bivaricella prevaricata Guppy ^^
Codahia spinulosa (?) Dall
'-^-'-Qardium (Laevicardium)- serratum Linne
'^'^^Cardium
(Trigonocardia)
haitense cercadicum Maury
^'Cytherea herheyi Hubbard
^"^^Chione ivoodwardi {?)
(Gruppy)
^'^Antigofm tarquinia antillica Maury
Antigona tarquinia (?) Dall
^"^"^Metis trinitaria Dall
Tellina sp. undet.
Semele (?) sp. undet.
^^^^ Teredo incrassata (Gabb)
Gastropoda
Solarium sp. indet.
Turritella tornata Guppy, n. var.
^Stromhus hifrons (?) Sowerby
Orthaulax aguadillensis Maury, n. var.
( ?
)
'''Orthaulax portoricoensis Hubbard
'''Cypraea cf. spurcoides Gabb
"^''Columhella (Stromhina)
cf. portoricana Hubbard
^Alectrion gurabensis varicum Hubbard
'-'^^Cancellaria laevescens (?) Guppy
'''Conus catenatus
( ?) Sowerby
^''Conus marginatus Sowerby
Conus symmetricus semio'bsoletus (?) Maury
Especial attention has been given the molluscan fauna in the paleon-
tological work which has been done thus far because
of
its immediate
utility in the problems of correlation.
If allowance be made both for the
varietal differences which the author has distinguished and for the poor
state of preservation which has made some of the identifications uncer-
tain, twenty-one of the
thirty-eight forms
differentiated have been
re-
ported in the Quebradillas limestone in the Lares district.
The percent-
age
is extraordinarily high when the lithologic changes that occur in the
intervening distance are considered, and the exact stratigraphic equiva-
lence of the beds at both ends of the island cannot be questioned.
There
are, it is true, six species in the fauna which are found in the Los Puertos
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
337
limestone, a comparatively high percentage, considering the small famia
described from the Los Puertos strata.
Five of the six species, however,
are known to be common to both horizons, three of them being relatively
long-ranged types which are widely distributed in the Middle Tertiary
rocks of the Caribbean region.
Only one species which has not been re-
ported
above
the Los
Puertos
limestone
in
Porto
Eico
occurs
in
the
Quebradillas
formation
of
the
Fajardo
district;
namely,
Orthaulax
aguadillensis Maury .^^
This species has been
listed from the Tertiary
limestones of Vieques and Saint Croix, however, and its occurrence in the
Quebradillas fauna was to be expected.
There
is no reason, therefore, to
postulate the presence
of Los Puerto
strata
in the
district, and Hub-
bard's opinion that they end by overlap near the eastern border of the
San Juan district appears to be fully substantiated.
The correlation of the Quebradillas limestone with stratigraphic units
in other parts of the West Indies was seemingly well established, but the
recent studies of Woodring have raised several points which re-open the
question.
The partial equivalence
of the
Quebradillas with the Ponce
limestone along the south coast of Porto Eico was demonstrated by Hub-
bard, and Woodring regards
it as a correlate of the Tertiary strata on
Vieques and Saint Croix.
The affinities of the Quebradillas and Bowden
faunas were announced by Hubbard as early as 1917, although the
first
published correlation
is Maury's, which appeared in 1919.^^
Woodring,
however, considers the Quebradillas to be older than the Bowden, but the
basis for his opinion
is not very fully presented.^^
The writer is not quali-
fied to discuss the change of status proposed by Woodring, but he
is con-
vinced that the faunal facts require more careful scrutiny before
its cor-
relation with the Bowden
is to be dismissed.
A
little more than twenty
percent, of the Quebradillas fauna occurs in the Jamaican Miocene, a re-
markably high figure when the distance separating the two islands is con-
sidered.
A study of a small molluscan fauna from Long Point, Saint
Croix, completed recently by Miss Ella-Kate Wemple, has yielded a
list
in which fifty percent, are Bowden species, although a study of the fossils
M The writer does not agree with Woodring that
0.
aguadillensis Maury and
O.
por-
toricoensis Hubbard
are
the same
species.
Both
are
represented
in
the
B^ajardo
col-
lection as
distinct and
easily
differentiated
forms.
. ^
.^
«5 Maury
C
J
•
The
correlation
of
Porto
Rican
Tertiary
formations
with
other
Antillean^ and* mainland
horizons.
Amer.
Jour.
Sci.,
4th
Ser.,
1919,
Vol. XLVIII,
pp.
9AQ.91 Q
~
98 Woodring W
P
• Miocene Mollusks from Bowden, Jamaica.
Contrlb.
to the
Geol.
and Pal. of the West
Indies,
Pt.
2, Carn.
Inst,
of Wash.,
1928, No.
385,
pp. 62-63 and
table facing
p: 41.
338
SCIENTIFIC SURVEY OF PORTO RICO
from other Saint Croix localities may be expected to lower the proportion
of common forms.
A hasty classification of a collection from southern
Vieques has revealed more Bowden than Quebradillas species.
There
is
good reason to believe, as Woodring does, that the Saint Croix and Vieques
Tertiary and the Ponce and Quebradillas limestones are essentially con-
temporaneous deposits, but the cumulative evidence seems also to favor
their correlation with the Bowden, rather than with some older horizon.
The geologic age of the Quebradillas
is even less firmly fixed than
its
correlation, for
it has been variously assigned
to the Upper
Oligocene
and to the Lower and Middle Miocene.
The problem involved is too vast
in scope to be given consideration in the present report, and it need merely
be stated that the concensus of current opinion places this formation and
its correlates in the Lower Miocene.
This point of view appears to have
grown out of the not fully explained tendency to rob the Oligocene to pay
the Miocene, and among recent authors only Hubbard has argued strongly
for the Oligocene age of these transformed horizons.
The question
in-
volves fundamentally the settling of the position of the boundary between
the Oligocene and the Miocene, on the one hand, and correlation with
European Tertiary horizons, on the other.
On these moot points opinions
are undergoing too rapid evolution for dogmatism, and toward their solu-
tion the present report has nothing to contribute.
Quaternary Sedimentation
During the latter part of Tertiary time the whole of Porto Rico was
above sea-level.
Gentle arching seems to have put an end to coastal plain
accumulation; and, after the ensuing erosion
cycle had reached an ad-
vanced stage of development, vertical uplift raised the island about 800
feet and erosion started anew.
At no time was the elevation quite high
enough for differential erosion to carve the mountain and coastal elements
into forms with high relief, such as have in recent time led to extensive
fluvial aggradation in parts of the interior.
The edge of the island and of
its submarine platform
coincided, and marginal
deposition must have
been restricted to the steep slopes that are found on every side except the
east, where the late Tertiary landmass extended to the farthest limits of
the Virgin Islands.
In consequence of this emergent condition, no per-
manent deposits either of fluvial or of marine origin were formed until
the Quaternary, when, in spite of renewed uplift, fluctuations in the posi-
tion of the strandline and rapid heightening of the relief during the early
stages of dissection have been followed by the formation of the continental
MEYERHOFF, QEOLOGY OF THE FAJARDO DISTRICT
339
and marine materials which occupy large areas within the limits of the
Fajardo district
(cf. geologic map).
The variety of the Quaternary de-
posits
is surprising, for they include littoral, estuarine, lagoonal, eolian,
delta,
floodplain, and
alluvial fan
types.
The
local problems
of
their
genesis are too numerous to receive extended treatment, and their more
important
features must
be
briefly
summarized
under
the
following
headings
(1)
The San Juan formation
(2)
Recent deposition in the coastal section
(3)
Fluvial aggradation in the interior.
THE SAN JUAN FORMATION
The name "San Juan^^ was proposed by C. P. Berkey to include the
scattered
outcrops
of
consolidated
calcareous sands which
occur
along
the northern coastline of Porto Rico.^"^
As used by Berkey and later by
Semmes,^^ the formational name was applied strictly to these calcarenytes,
in which conspicuous and persistent cross-bedding provides ample evidence
of their eolian origin.
At a somewhat later date the differentiation of
deposits comparable either in age or in origin to the San Juan formation
induced Lobeck^^ and Hubbard ^^^ to expand the term "San Juan'', until
it embraced not merely the partly cemented eolian sands, but also con-
temporary and younger materials
of both wind-blown and hydroclastic
derivation.
Far from being useful, the hybrid concept which has been
thus created makes
it impossible even to discuss the individual varieties
included in the sedimentary miscellany without constant danger of con-
fusion, and it is here proposed to restrict the term to its original meaning,
for the following reasons
:
(1)
Formational names are applicable only to single lithologie units.
The
eolian and marine
deposits embraced
in the expanded San Juan
formation
are
lithologically and
genetically
distinct, and they do not
intergrade ; hence the use of the same name for all is erroneous.
(2)
All lines of evidence support the opinion advanced by Berkey ^°^
that the San Juan formation may be as old as Pleistocene, whereas most
of the deposits which
it
is proposed to add are of Recent origin.
The
employment of a single formational name for
all of these sediments in-
»^ Berkey.
C.
P.
:
Geologic
reconnoissance
of Porto
Rico,
op.
cit.,
pp.
11-12.
es Semmes, D.
R.
: Geology of the San Juan
district,
op.
cit.,
pp.
53-55.
»» Lobeek, A. K.
: Physiography
of Porto
Rico,
op.
cit.,
pp.
361-366.
100 Hubbard, Bela
: Geology of the Lares district,
op.
cit.,
pp. 98-103.
101 Berkey,
C.
P.
:
Geologic reconnoissance of Porto Rico,
op.
cit.,
p.
12.
340
SVIENTIFW SURVEY OF PORTO KICO
volves the creation of a terrain, and this policy
is not sanctioned by cur-
rent practice.
(3) The San Juan formation was
deposited,
perliaps
continuously,
along 125 miles of coastline under conditions tliat were similar tlrrongh-
ont
tlie entire
distaiice.
The other Qiial:ernary sediments
ol'
the; coastal
areas are the products of factors wliich operated locally
aiul
diseonij'mi-
ously, and
they
are,
witliont
e.xeeption,
di scon forma biy
related
to
the
Sail Juan beds.
effects
Tal«
Jmm formxitum
along
the
exposed
enfftt
at
I»ii
The San d'uaji fornniilon, sensn redridu, may
therefoi'C! be defined as
tlie
poorly
consolidated
cross-bedded
calearenyte
of
eoliaii
origin,
out-
cropping in isolated and often widely scattered bluffs and cays situated
chiefly along the noTthern coast of Porto ]?ico.
Tliert* are four outcrops
in the Fajardo
district: namely,
(1)
in the northern
part
of the
city
of San Juan,
the type
locality;
(2)
along the Condado
section
of the
shoreline immediately east of fSan Juan and in other
|)aj'ts of Saiitnrce;
(3)
at puntas Cangrejos and Maldonado, two adjacent peninsulas north
of Lago Torrecilhi; and
(-1)
at Punta
'\''acia Telega.
Iji
aJI
iliese
localities except one the characteristics
of
the formation
are
the same.
It
consists
of massive
bedSj,
in which
the
only
visible
structure
is the regular cross-bedding
(Fig.
2-">).
Tlie
latter has a con-
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
34I
stant direction of dip slightly south of west;, and the angle of dip varies
within the narrow
limits
of 22° and 30°.
The
constituent grains
are
well cemented, although the degree of consolidation differs somewhat ac-
cording
to the exposure
of
the
rock.
Where
it
fronts
the
sea and
is
drenched continually by the spray from breakers,
it
is firmly indurated
for an
inch
or more beneath the weather-beaten
surface.
Where
not
so exposed, the degree of consolidation
is decidedly
less, although
suffi-
cient to render the formation a consistent ridge- or cliff-maker.
Berkey,
Semmes,
and
Lobeck
have
described
secondary
horizontal
structures
in some
of
its outcrops west
of San Juan, notably
in the
vicinity
of
Arecibo, but in the Fajardo
district secondary
structures
of
this
type
were not observed.
As in other parts
of Porto Eico, the rock
is com-
posed primarily of rounded fragments
of
corals and
shells, but micro-
scopic
examination
reveals
other
constituents,
including
worn
and
polished foraminiferal
tests, angular quartz grains, and, less commonly,
subangular feldspar fragments and pieces of other igneous minerals.
The
calcareous fragments have been rounded, but the other particles stand out
prominently in thin-section because
of
their
angularity.
Calcium
car-
bonate forms the cementing material, binding
the contiguous
surfaces
of adjacent fragments, but only rarely entering the
interstices.
The
lithologic features
of the rock
indicate
that the materials com-
posing
it were not blown far.
Wind-blown sands that have been trans-
ported even for moderate distances show more homogeneity in grain size
and in mineral composition,
as well
as more perfect rounding, than
is
exhibited
in
the
San Juan
calcarenyte.
In
the
latter,
although
the
calcareous fragments are rounded, the degree of rounding
is not great,
whereas
the quartz
particles and
the
other mineral fragments
display
practically no
effects of rounding, for many of them
still possess sharp
edpes and corners.
The very presence of the non-calcareous constituents,
in fact, supplies additional evidence of the slight amount of transporta-
tion, for the wind, a perfect sorting agent, separates materials that have
different specific gravities, while abrasion disposes of the softer particles,
before movement has gone on for any length of time.
A study of the
surfaces of the grains, moreover, reveals that only those of the calcareous
fragments are typically pitted and frosted, whereas the surfaces
of the
quartz grains
are
clear and
glassy.
Even with moderate wind
action
frosting takes place in grains of all types.
These physical characteristics
in themselves are sufficiently suggestive, but, if considered in conjunction
with the location and distribution
of the sands, they permit no escape
from the conclusion that the San Juan formation represents
a
line
of
343
SCIENTIFIC SURVEY OF PORTO RICO
coastal dunes, the materials
of which were gathered by the prevailing
northeasterly trades from an immediately adjacent beach.
The peculiar distribution of the San Juan formation indicates its gene-
tic
association with a
shoreline
older than
the one which now bounds
the island of Porto Bico on the north.
Not alone in the Fajardo district,
but along the entire coastline to the west,
its occurrences are restricted
to the prominent
salients and headlands and
to small detached islands
which lie short distances offshore.
The
all but perfect alignment of the
San Juan outcrops offers the temptation of projecting them into a con-
tinuous series of dunes formed originally on the landward side of an off-
shore bar, which occupied a position now retained by the headlands and
by the islands of Cordilleras Eeefs.
Lobeck suggested this explanation
of
the formation's
origin
in
a
figurative way, but
left the
hypothesis
without support.^ ^^
There
is, however, adequate reason to believe that
the explanation holds ; for the linearity of the outcrops, contrasting with
the haphazard and
local occurrence of the modern dunes; the uniform
structural and physical characteristics
of the deposits, betokening iden-
tical conditions
of
genesis along 125 miles
of
coastline;
the recurrent
lowering of sea-level during the cycles of Pleistocene glaciation, a feature
favoring the development of offshore bars along the emergent coast of the
island; and the invariable association
of the San Juan dunes with the
Tertiary coastal
plain,
are
facts which provide unanimous support
for
the theory, and which at the same time are incompatible with any other
mode
of
origin.
Certainly the modern
dunes, which
are
absent from
long stretches of the coastline, even where conditions seem to favor their
development, and which fit into the irregularities of the present shoreline
without a suggestion of alignment, afford an instructive contrast.
The constant
association
of
the San Juan formation with
the Ter-
tiary coastal plain, mentioned in the preceding paragraph, merits addi-
tional comment.
The outcrops of the dune sands are co-extensive with
the
Quebradillas
formation, and eastward
in
the Fajardo
district
the
occurrences
of
the
calcarenyte
cease where
the
Quebradillas
limestone
disappears from the mainland a short distance east of Loiza.
Even on
such prominent points
as
Miquillo,
Picua, and Embarcaderos,
not
to
mention Cape San Juan,
the San Juan
deposits
are
absent,
as
is
the
Quebradillas formation along this portion of the
coast.
Two miles
or
more offshore, however, a prominent ridge makes
its appearance on the
submarine platform, and on the United States Coast and Geodetic Sur-
102 Lobeck,
A.
K.
: Physiography
of Porto
Rico,
op.
cit.,
p.
362.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
343
vej charts it may be traced east-southeastward along Cordilleras and Her-
manos reefs to Culebra.
It rises more than 100 feet above the floor of
the submarine platform, and in Cordilleras Keefs
it attains the surface
in a number of small islands, such as Icacos, Eatones, Lobos, and Diablo
cays.
The rock composing these islands consists of strongly cross-bedded,
pure white calcareous sand, the characteristics of which indicate an origin
similar
to, and contemporaneous with, the San Juan formation on the
mainland.
The nature and significance of the islands have been previ-
ously
discussed by
the
writer,^ ^^ and
the
conclusion was
reached
that
the
rocks
of which
they
consist must
be
referred
to
the
San Juan
formation.
In
acquiring
expression
above
the
surface
of
the
sea
the
consolidated dunes rise some 50 or 60 feet above a much broader base of
older material, which physiographic
evidence
indicates
is an extension
of the Quebradillas formation from the mainland.
The persistence
of
the high calcium carbonate content of the calcarenyte shows that
it was
formed at a time when the Cretaceous volcanics did not constitute a part
of the shoreline, as they do at present in Culebra and in the northeastern
corner of Porto Eico.
The age
of the San Juan formation has only been
tentatively
fixed.
From its relationship to the Tertiary coastal plain and to Eecent forma-
tions, and from
its degree of lithification Berkey concluded that
its de-
velopment must be referred to the Pleistocene.
The .writer believes that
it can be dated somewhat more
closely by using the physiographic
evi-
dence
available.
In the first place, the major physiographic features in Porto Eico and
the Virgin
Islands show that the
island elements have been
relatively
stable since Middle Miocene time save for two short periods of positive
movement, the latter of which was accompanied by slight eastward down-
warping.
The
short emergence which
initiated
the
construction
of
a
barrier beach and associated dunes can best be explained by postulating
a change in sea-level rather than a change in land-level, and the world-
wide changes in the strandline which accompanied the advance and
re-
treat of the Pleistocene ice-caps furnish an entirely adequate explanation
for the temporary emergence of San Juan time.
In
the
second
place,
the
formation
has
been
tilted.
Near
Arecibo
its base is approximately fifteen feet above sea-level, and it declines east-
ward to sea-level at San Juan, and
to a position not
less than 30
feet
below
sea-level
in
the
islands
of
Cordilleras
Eeefs.
The amount
of
108 Meyerhoff, H.
A.
:
Physiography
of the
Virgin
Islands,
Culebra, and
Vieques,
op.
elt., pp. 198-207.
344
SCIENTIFIC SURVEY OF PORTO RICO
tilting
is much smaller than
that which has
taken
place
in
the same
direction in the older^ erosional landforms, but it is enough to show that
the formation was
deposited before
the
cessation
of
the
tilting move-
ment.
The latter occurred wholly within Pleistocene time.
In the third place, the minor modifications which the San Juan de-
posits have undergone
since
slight
tilting
partly submerged them
are
all
referable
to Eecent
time.
They
are
so
definite
in
character
that,
taken
in
conjunction with
the
tilting,
they furnish an adequate
basis
for dating the formation of the calcarenyte as relatively late Pleistocene.
In the southern and western part of Santurce the partly consolidated
and deeply weathered sands which are exposed in some of the street cuts
and along the American Eailroad
are only
tentatively
referred
to
the
San Juan formation, because they exhibit departures from the diagnostic
features of the latter that entitle them to a more analytical study than
could be given them.
They are situated south of the line of San Juan
outcrops, and they
fail
to
reveal
the
characteristic
cross-bedding, even
in the many fresh excavations which the author found and examined.
Although
a
careful
microscopic
analysis
has
not
been made
of
the
samples taken, they appear to contain a much higher percentage of quartz
than the San Juan calcarenyte.
Toward the west they merge with the
marly
deposits which
are now
being formed
along
the
shores
of San
Juan harbor,
but whether
they
constitute
a
source
for
the
latter,
or
are themselves similar deposits exposed by the slight emergence of recent
geological date,
is not known.
Sediments of like nature were not found
in any other part of the district, and the extent to which they have been
modified and buried in the rapidly growing settlement of Santurce made
their relationship
to
the San Juan formation
obscure.
Until
a more
definitive
field and laboratory study
of them
is undertaken, judgment
concerning
their
age,
origin, and
stratigraphic
affiliations may
wisely
be suspended.
RECENT DEPOSITION IN THE COASTAL SECTION
In the Fajardo
district the other sediments of the coastal region are
unconsolidated materials
of recent origin.
Their accumulation was
so
intimately
related
to
the
physiographic
development
of
northeastern
Porto Eico that their distribution and characteristics can best be under-
stood
if they are considered in connection with the
factors which gov-
erned
their
deposition, and
for
this mode
of
treatment
they may
be
grouped
genetically
into
(1)
the
playas,
(2)
the
lagoonal
sediments,
and
(3)
the
littoral
deposits.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
345
(1) The playas—The term ^^playa;'^ which in literal translation means
"beach'^
or
^^shore/' has been applied
also
to the
flat triangular
areas
that extend like wedges from the shoreline into the interior.
Their dis-
tribution
is so obviously dependent on the features of fluvial dissection
that the essential elements involved in their origin require
little exposi-
tion.
They
are
best
developed
along
those
portions
of
the
district's
coastline where the Tertiary coastal plain is absent, and the extent which
each attains
is roughly related to the
size of the stream that drains
it.
The materials composing the playa
deposits
are almost wholly
alluvial
in character, and
their
relatively
flat surfaces, which
slope seaward
at
a
very low
angle,
consist
of a veneer
of
floodplain and fan materials
covering slightly older delta deposits that were built outward into estu-
aries.
The delta sediments are occasionally exposed in the marshy tracts
at the seaward margins of a few of the playas, where recent emergence
has carried them several feet above sea-level.
The dark massive clays of
which they are composed contrast sharply with the bedded sands or fine
gravels that form the playa surfaces farther inland, and the delta muds
appear to have
settled in the salt or brackish water of the bays beyond
the reach of the stream currents that deposited the coarser and higher
strata.
In other parts of Porto Kico marine shells have been discovered
in some of the playa beds, but none were found in the Fajardo district,
although the combined fluvial-estuarine conditions of sedimentation were
undoubtedly alike in every portion of the island.
The playas associated with Kio Fajardo and Eio Blanco are the largest
within the region studied, but neither
is entirely
typical, because
their
playas
are
inseparable
from
their
extensive
floodplains.
The
other
streams in eastern and northeastern Porto Kico are small and many are
intermittent, and as a result of their small volumes some have failed to
fill
completely
the
estuaries
into which
they
discharge.
Several
have
steep gradients, and the
deposits they have formed near
their mouths
are more
like alluvial fans than
deltas or
floodplains.
In
spite
of the
variety the individual playas exhibit, their formation represents the out-
come
of
a
simple
sequence,
involving normal
fluvial
dissection, mar-
ginal submergence of 30 to 40 feet, delta and floodplain construction in
the estuaries, with concomitant upstream aggradation, and, finally, slight
emergence.
The
form,
size, and
sedimentary
materials
of each playa
are dependent largely upon
the
stage
of development the
local stream
associated with it had reached prior to submergence.
(2) The lagoonal sediments—From San Juan Harbor eastward to the
mouth of Eio Grande the coastal section
is low and
flat
(Fig. 2), and
346
SCIENTIFIC SURVEY OF PORTO RICO
near the shoreline marshes and swamps are common.
Associated with
the latter in the western part of the coastal belt are a few enclosed bodies
of open water, such
as Lago
Torrecillo and Lago San Jose.
Shut
off
from the Atlantic by low barrier bars, they are being rapidly
filled by
mud and vegetation.
Like them in all other respects, San Juan Harbor
differs chiefly in maintaining an open connection with the ocean.
The
enclosed water bodies
lie
practically
at
sea-level, and the water within
them
is kept brackish by
the
inflow
of
the
tide through
the
tortuous
and narrow channels
that connect them with the
sea.
The unconsoli-
dated sediments surrounding them form a relatively broad plain, which
rises in elevation almost imperceptibly inland, and above which the few
remnants of the Quebradillas limestone protrude with sharp discordance.
The material composing the plain varies from place to place, but its varia-
tion
is systematic.
Muds, some of them black from their high organic
content, are most characteristic.
In interstream
localities they are not
infrequently calcareous, but near the streams they are usually argillaceous
and contain admixtures of river
silt and sand.
Along the northeastern
margin of San Juan Harbor where the water of the bay laps exposures
of the San Juan calcarenyte or an associated deposit of poorly
consoli-
dated sand, marls are in the process of formation.
And in several locali-
ties pockets of white quartz sand, surrounded on
all sides by the normal
deposits of clay, endow the formation with its one baffling mystery.
There can be
little question regarding the
origin
of the muds.
The
absence of stratification, their association with partly
filled lagoons, and
their position between the hills of the interior and the bars which now
form
the
coast, show beyond
dispute
that
they
are
lagoonal
deposits
which have been exposed as a result of the recent slight emergence which
eastern Porto Eico has undergone.
The
sources
of
the sediments,
ex-
cept
the
quartz
sands,
are
equally
clear.
The
lateral gradation from
clays to
silts and sands, both in the direction of the streams and in the
direction of the interior hills, as well as the slight rise in elevation in the
same
directions,
indicates
that
the
materials
were
brought
into
the
lagoons by streams and rain-wash, and except for
the
organic matter,
were
initially
alluvial
in
character.
It must
be assumed
that Lobeck
was considering only the agent of transportation which carried them
to
their
present
resting
place when he
dismissed
the
possibility
of
their
marine origin and
classified them
as alluvium ;^^* but this principle
of
classification cannot be accepted, for the deposits along the north
coast
of Porto Eico must be deemed lagoonal in origin.
It
is true that allu-
10* Lobeck,
A. K.
: Physiography of Porto Rico, op.
cit.,
p. 349.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
347
vium
is associated with them, overlapping them in places; especially on
their landward margin.
Locally,
as
in the
area
east of Kio
Herrera,
an alluvial cover may completely mask the lagoonal muds beneath, and
in mapping the deposits
it
is quite impossible to effect a clear separation
of the two types, as the geologic map attempts to show.
At
the
present time no
explanation
is
available
for
the
pockets
of
quartz
sand.^^^
Their isolated position makes
it extremely
difficult
to
regard them
as mechanical deposits, and there
is no tangible proof of
organic precipitation.
They increase in size and number in the coastal
sections to the west, and
it is hoped that the studies in the Arecibo dis-
trict will provide some solution of their mystery.
(3) The
littoral deposits—The remaining sedimentary deposits which
occur
in
the
coastal
region
are
directly
associated
with
the
coastline,
the evolution of which will be discussed as a phase of the physiographic
history.
In addition
to the
bars,
spits, and tombolos, there
are
a few
older marine deposits which were formed before the recent uplift
of
a
few feet, and which occur as peninsulas along some stretches of the shore-
line.
Among them may be mentioned
Miquillo and Picua
points,
in
the marine sands of which, three to six feet above sea-level, Kecent Mol-
lusca are embedded.
They both have spit-like forms, although the pre-
cise reason for the formation of two such prominences along
this part
of the coast
is not
clear.
Punta Embarcaderos neat Luquillo appears
to
be
a
cuspate
bar which
developed
contemporaneously.
Coral
reefs
are forming marginal limestones
off
several
portions
of
the
coast, but
their development cannot be discussed within the limits
of the present
report.
FLUVIAL AGGRADATION IN THE INTERIOR
There
is no
line
of demarcation between the playa deposits and
the
floodplains
of
several
of the
large
streams,
like Eio
Fajardo, nor
can
a sharp boundary be drawn
to
separate the lagoonal and normal
river
deposits in the northern part of the
district.
The extent of the
flood-
plains and their relation to the playas and lagoons show that their deposi-
tion has been markedly
accelerated
as
a
result
of submergence, which
necessitated
aggradation
for
long
distances upstream.
The
rivers
are
slightly entrenched
in
their
floodplains
in consequence
of
very
recent
uplift, but the amount of entrenchment
is rarely adequate to keep them
from overflowing their banks in times of abnormal rains.
105 cf . Britton, N. L.
: Botanical investigations in Porto Rico.
Jour., N. Y.
Botanical
Garden, 1922,
Vol. XXIII,
pp.
56-57.
348
SCIENTIFIC SURVEY OF PORTO RICO
The
depositional
features
found
near
the
mouths
of
the
streams,
however, present few problems, but inland several drainage systems have
undergone local aggradation which is more difficult to explain.
Terraces
are haphazardly
distributed
along
a
few
of
the
stream
courses
(Fig.
27).
Their formation was undoubtedly due to changes in water volume
and load, but why the changes should have occurred in each
case
is by
no means obvious.
The terraces are not associated with
all the drain-
age systems, hence
local
factors must be sought
to account
for them.
One of the most obvious
is stream capture, but whether piracy can
be
held responsible for every example which the Fajardo
district contains
the author does not know.
The terraces along Eio Grande de Loiza may
well have been developed when
it captured the headwaters of Rio Cano-
vanas.
The steep gradient of the latter and the coarse materials it trans-
ported from the dioritic outcrops of the Caguas lowland were undoubtedly
more than
the mainstream
could
carry
over
its
lower
gradient,
even
with the additional volume of water.
Aggradation to the point of bal-
ance would have followed, to be succeeded by degradation and terrace de-
velopment after equilibrium was achieved.
This hypothesis has the ad-
vantage of plausibility, although close scrutiny brings to light objections
which have not been disposed of.
In any event its application
is limited,
and the terraces along other streams remain unexplained.
Regional fac-
tors, such as change of level, tilting, and climatic cycles cannot account
for them, and it was not possible to determine whether piracy and tempo-
rary overload
as headward erosion tapped
easily reduced rocks furnish
a satisfactory set of causes for every case.
Of greater areal importance
are
the
alluvial
deposits
of the Caguas
basin, which bury the floor of the lowland
to moderate
depths.^^^
The
alluvium
is
of
a
two-fold
character,
consisting
in
part
of
local
flood-
plains deposited by Rio Grande de Loiza and the several large tributaries
which join
it within the areal limits of the basin, and in part of mar-
ginal alluvial fans which coalesce with each other and interfinger with
and partly overlap the floodplain deposits toward the center of the basin
(Fig.
10).
Three interrelated
factors appear
to have been
responsible
for the extensive aggradation which has occurred:
(1)
the abnormally
rapid excavation of the underlying dioritic stock, causing an unbalanced
relation between the slopes and stream gradients in the lowland areas, on
the one hand, and in the surrounding uplands, on the
other;
(2)
the
stream piracy which occurred as the limits of the lowland were extended
;
108 Ante,
pp. 208-209, 241.
MEYERIIOFF, OEOIAMIY OF THE FAJARDO I}1HTEI€T
349
(3)
the
ultimate^
coirliuerice
c»f many
large
and
soniewhat
discordant
streams in the topographic depressifm.
Given
the non-resistant diorites
and the line of structural weakness which ]oc4ilized the development of the
Cjagiias lowland, and
the operation of the
thrcic
factors enumerated
wiis
inevitable.
Perfect
adjnsl.ment^ has
not'
yet hv.m
acliieved, and, where
80 many diverse fluvial elements are concerned,
it
is natural to find that
the various
portions
of
llie hasin are
in
different stages
of adjustment.
Near
its
eastern
and
western
ends
deLTadatioii
is
already
in
progress
Flii.
zn.—DiKKwlca
iiliuftm
/cif<8
f.i
Iht
i-ii!ii,,t.>
km- ifiiiii
Klo Gurabo
has
cut
n:s
cbaniwn
about
thirty
f<H't
hito
the
mu:•oiisolicintetl
east
of
.Iiineo s.
The scarp
from
which
tlM!
c'lastl);•
mnn-Tlals
were
wastied
In
background.
(Fig. 26,
cf. also Fig. 3),
an,<l locally the alluvium, has been dissected
to
depths of 30 or 40 feet.
I^'dsevvdiere the fans are
still being built np, an,d
in the center of the basin some
flood plain
con,struction continues.
An
unestimated
amount
of
the
hieal
aggradation now
oc{;urring
can
be
tracked to the intensive cultivation of the regi(«i during
re('eiit years, but
the natural processes at work have not been greatly modified or disguised.
PlIYSlOORAI'JI.Ki DeTEJ.OPMI'INT OF TirK Fl-J.^EOO DlSTlUCT
tert:iary krosiox
The Fajardo district has undergone the same physiographic evolution
as the rest of Porto Bico; hut. because of the unusual opportunities which
the rocks and structures have provided for
diff<»reiitial
erosion, the older
350
SCIENTIFIC SURVEY OF PORTO RICO
landforms have not retained the prominence which they exhibit in other
parts of the island.
In several portions of the district the
flat crests of
structural
ridges,
the accordance
of detached remnants
of upland
sur-
faces
in interstream
localities, and
the horizontal spurs radiating
out-
ward from the higher slopes of the Luquillo Mountains, all approximating
elevations of 1400
to 1600
feet
(Figs.
3 and 4), provide unmistakable
evidence of an old erosional
level that has
all but succumbed
to more
recent dissection.
In northeastern Porto Eico scarcely enough of
it re-
mains
to constitute a clearly defined peneplane, but
if
its remnants be
projected from the flanks of the Luquillo Mountains along the ridge that
forms the northern boundary of the Caguas lowland, to the upland areas
north and south of Aguas Buenas, or across the Caguas lowland to the
northern slopes of the
Sierra de Cayey,
it
is seen to occupy a place
of
greater importance; and in the highland west
of Caguas
it forms the
highest element in every panorama. Toward the west its height increases,
and
if
its gently tilted surface be reconstructed and extended eastward
over Vieques Sound, it is found to coincide with the Saint John peneplane
of the northern Virgin
Islands. ^^^
Above
its undulating
surface
the
Luquillo Mountains rise as monadnock ranges that resisted reduction be-
cause of their structure, and to the south the Sierra de Cayey forms a
lower monadnock group.
This high erosional level was called the "Upper
Peneplane" by Lobeck,^^^ and the name "Saint John'^ has been applied
to
it by the writer.^^^
The date of origin of the Saint John peneplane has not been settled.
Lobeck advanced the opinion that
its formation antedated the develop-
ment of the surface on which the coastal plain was deposited in Middle
Tertiary time, whereas Semmes
considered the two
surfaces
to
be
the
same.
The former hypothesis cannot be entertained in view of the fact
that the erosional surface which lies beneath the coastal plain rises south-
ward at a five degree angle, and, if projected,
it would intersect the Saint
John
level and
rapidly pass
above
it.^^^
Semmes^s
interpretation, on
the other hand, seems possible
if
it be also postulated that the declivity
of the erosional plane beneath the Tertiary strata decreases inland; but
it does not account for the equally significant relationship that exists be-
tween the Saint John and the sub-coastal plain surfaces from the eastern
10''^ Meyerhoff, H.
A.
: Physiography
of the Virgin
Islands,
Culebra,
and Vieques,
op.
cit.,
pp. 100-103.
108 Lobeck, A. K.
: Physiography of Porto
Rico,
op.
cit.,
pp.
312-314.
109 Meyerhoff,
H.
A.
:
Tertiary
physiographic
development
of
Porto
Rico
and
the
Virgin Islands, op.
cit.,
pp. 560-567.
"0 Ibid.,
fig.
3,
p. 564.
MEYEEIIOFF, QEOTjOOY OF THE FAJARDO DISTRICT
351
to the western end
oi'
tlie
islainl.
The Saint
Joliii
peneplaiie
rises at a
uniform rate from 1U)() feet in
tlie Fajardo district
to 3000 feet in the
highlands betwecji Yaueo and Lares.
Discounting the
five degree slope
to the
iioriii,
tlie plane
ol; I'crtiary deposition sank, perhaps as much
as
3000 feet, in the game distance at tiie time the coastal piaiji was formed
a aag which
is now itdlected in ttie inci'easing thickness and widtli of ont-
orop of the Tertiar}' strata toward the west, bnt which
is not reflected
in
any way by tlie position and relatioiisliips of the Saint .Jolm
level.
The
riio Cnsiiana ero^minl levol
Is
pi-oiiuiient
in
tlie western MO
seetioii.
At
thlft
liRUil-
,
2CS.4 kilometers south wl San
.luati on the i-oad
to OaKti«s,
aUiivinl
terraces occwyy a
iBpiciiouR
plar;e in
tlu:
Qijal<>nm!'3' valleys.
latter, on the eojilrnry, maintains a pusition
tliat
is exai;tly parallel to a
younger and lower peneplane, which
indiscriminately hevels
the
Creta-
ceous and Tertiary rocks.
In view of
its striu'tnral accordance
witli the
younger,
late
Tertiary peneplane and
its
dfsfhute discordance with
tlie
erosional
surface
heneatli
the
Tertiary
strata,
the
(hn-xdopment
of
the
Saint dohn level must have started after the vxnmnl phiin was formed and
arched in Miocene time.
In every part of the
distrird. except the Luquilh) Mountains, the lower
and votrnger erosionallevel menliooed
in the preceding paragraph forms
a
conspieunns,
thougli
not
always
a
dominant,
plivsiographic
clement
(h'ig. 27).
In the coastal section it is found in the crests of the dissected
Cretacetms hills, and even the f)epiiios in the larger Tertiary clusters show
352
SCIENTIFIC SURVEY OF PORTO RICO
a tendency
to
seek
its
elevation.
In
the Caguas lowland
the
isolated
hills of harder rock invariably rise to
its position, which
is clearly indi-
cated by the accordant levels bordering the basin, especially on the south
(Fig.
3).
In other parts
of the
interior, on the other hand,
it forms
the floors of high interior valleys, as the anticlinal valley in which Aguas
Buenas
is
located
well
illustrates.
Like
the
Saint John peneplane
it
rises westward from an elevation of 500-600 feet along the coast front-
ing Vieques Sound and Fajardo Roads to a height in excess of 800 feet
at Aguas Buenas.
Beyond the limits of the district, where
it
is farther
removed from
the
coastline, and where
its
preservation has not been
impaired by the rapid excavation of non-resistant rocks,
its areal extent
increases, attaining maximum expression in the north-central and north-
western
interior.^^^
It has been named from Barrio Caguana
in
the
Arecibo
district, where
it
planes
the
folded
Cretaceous
rocks and the
gently dipping Tertiary strata without regard for structure or lithology.
In the Fajardo
district
its imperfection
is due not alone to features
of
differential
erosion, which
initially
limited
its
development and
ulti-
mately facilitated its destruction, but also to the small amount of uplift
which it underwent at the end of Tertiary time.
Quaternary erosion has
progressed
far
at
its expense,
yet,
in the many inland
localities where
remnants of it still remain, the well defined topographic unconformities
that separate it from the Saint John peneplane on the one hand, and the
Quaternary
levels on the
other, provide the Fajardo region with some
of its major relief features.
The scarp between the Caguana and Saint
John
erosional
planes
is
uniformly
800
feet high;
that between
the
Caguana and Recent
levels varies from 300
feet along the north
coast
and 500 feet along the east coast, to 600 feet in the southwestern corner
of the district.
The Caguana cycle appears to have been inaugurated by
800
feet of vertical uplift at the beginning of Pliocene time.
In parts
of the island
its development continues,
for Quaternary
dissection has
not yet reached its headward limits ; but in the region under considera-
tion large sectors of
its surface have been entirely destroyed, and
all of
it is undergoing rapid reduction.
The preceding sketch does not pretend to solve the many local prob-
lems connected with the two cycles
of Tertiary erosion, for space does
not
permit
exhaustive
treatment.
The
extension
of
north-flowing
streams, their superposition across the ridge that bounds the Caguas low-
land, the expansion of the Rio Grande de Loiza system, the inception of
111 Ibid.,
Fig.
4.
p. 569.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
353
the series of captures which led to the slow consolidation of all of its head-
water
tributaries within the Cagnas basin—a process which continued
into Quaternary time—are but a few of the
special features connected
with
the
Saint John and Caguana
cycles
that
deserve more thorough
analysis than has been accorded them in the present study.^^^
THE QUATER]S"ARY CYCLE
In every other part of Porto Eico the elevation which ended the Ter-
tiary was so great that Quaternary erosion
is
still in a youthful
stage
of
development.
The movement,
however,
was
differential and
slow,
lasting perhaps through
all of Pleistocene time, and even when it ended,
the Caguana peneplane was but 300
to
600
feet above
sea-level along
different sections of the Fajardo coast, and only 800 feet in the south-
western corner of the
district.
To the east the
flat synclinal structure
and
the
non-resistant
rocks
facilitated
the
speedy
excavation
of
the
Vieques Sound and Naguabo lowlands; on the south, erosion proceeded
quickly in the Caguas basin, abetted by the size of Eio Grande de Loiza,
which flows close to the western boundary.
Thus carved into relief, the
highlands within the district have undergone vigorous headward attack
from every side, though partial protection was afforded on the south by
the
character
of
the
structure.
The
Quaternary, cycle,
favored by
so
many factors, has therefore reached a much more advanced stage of de-
velopment than elsewhere in Porto Eico.
No single adjective is adequate to describe the stage of dissection which
has
been
reached.
Most
of
the
district
has
been
maturely
dissected
(Fig. 4)
;
yet,
as
is usual in a region
of comparable
size, the upland
areas remote from major drainage lines are
still struggling through the
vicissitudes of youth, while the coastal belt exhibits the features of old
age.
Locally special factors have retarded progress, as in the Luquillo
Mountains;
or again they have
accelerated
it,
as
in the
coastal
plain,
where the early stages were carried on
entirely by subsurface
solution.
Here a third cycle peneplane has been formed, with only the few isolated
clusters
of limestone monadnocks
to mar
its
perfection.
Inland non-
resistant lithologic elements, like the diorite of the Caguas lowland, have
been carved into old forms, while the country upstream and down, drained
by the same
streams,
is in a
less advanced stage
of development.
In
places erosion has produced over-steepened slopes, has caused discordant
stream junctions, or has led to stream capture.
In such situations aggra-
dation has temporarily stayed the destructional processes until the nor-
112 Ante, pp. 239-252, 346-347.
354
ISCIENTIFIG SURVEY OF PORTO RICO
mal. balance between stream gradients,
valley
slopes, and water volume
was restored.
Gradation
is rapidly being achieved, but maladjustments
are
still numerous.
In the interior of the Fajardo district Quaternary erosion and deposi-
tion are wholly fluvial in character, and they represent the usual results
of stream processes acting on a region that has undergone differential and
rather protracted uplift during the earlier part of the
cycle, and minor
vertical
oscillations during the
latter
part.
Along the
coast, however,
the diastrophic movements have yielded extremely complex results, some
of which have been described by Lobeck.^^^
A brief chronology of the
events
is needed for an understanding of the physiographic forms which
are present.
The deposition of the San Juan formation was the
first of the events
which can still be deciphered.
The deposits formed a line of dunes, per-
haps unbroken at the time of origin, except opposite the mouths of the
larger
rivers,
extending from
the
northwestern
corner
of
Porto
Eico
at least as far as Culebra.
The position which the formation occupies
with respect to the present shoreline, as well as the alignment of its scat-
tered
outcrops,
provides
reason
to
believe
that
it was formed on
the
lee side of an offshore bar along an emergent coast.
During the period
of
its
origin and
for
a short time
thereafter, however, the
island was
being
tilted downward toward
the
east and upward toward
the
west,
and
it
is
unlikely
that
a
simultaneous
vertical movement
caused
the
emergence that started bar and dune development along the entire coast-
line.
On the other hand, the variations in the strandline during the ad-
vances and retreats of the Pleistocene ice-caps are entirely adequate to
explain a temporary condition of general emergence, which could occur
in the midst of tilting.
The appearance of an offshore bar and the growth
of dunes upon
it
required but little of Pleistocene time, and with the return of the waters
to the ocean as the glaciers melted, these normally transient deposits ac-
quired a measure of permanence when the resulting submergence effected
their
partial
consolidation.
Tilting
continued,
meanwhile,
and
as
it
progressed, the broad fluvially degraded flats east of the Fajardo district
subsided beneath sea-level, and with them went the Virgin Islands plat-
form, all but the extreme Atlantic margin of which had, until this time,
occupied a position above the sea.^^*
The conversion of the eastern low-
118 Lobeck, A. K.
: The Physiography of Porto Rico,
op.
cit,
pp.
343-371.
11* The inappUcability of Daly's and Vaughan's hypothesis of low level abrasion dur-
ing maximum glaciation to the Virgin Islands-Culebra-Vieques platform
is discussed
at
length in Volume IV,
pp.
107-119, 198-207 of this
series.
MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
355
lands
into
shallow
inter-island
channels
gave
the Fajardo
district
its
natural boundary fronting Fajardo Roads and Vieques Sound, although
the present shoreline differs in many minor details from the one which
formed at the close of the Pleistocene.
The Holocene record
is comparatively clear.
Tilting ceased, and the
streams in every part of the island began the task of adjusting themselves
to the new level.
Degradation was the dominant type of activity, except
in the newly drowned areas east of Porto Eico.
Before fluvial processes
had made much headway, however,
a general submergence
of thirty to
forty feet occurred, and in the estuaries that were formed, the develop-
ment of the playas was inaugurated, while in the old Pleistocene lagoons
between the San Juan dunes and the coastal hills sedimentation started
once again.
It was during this
cycle
of development that the features
of
the
present
shoreline
took
definite
shape.
Bars
were
hastily
con-
structed
across the open mouths
of the embayments;
islands
detached
from the mainland by the subsidence were
tied to
it again by tombolos
(Fig. 28)
; Cretaceous headlands were cut back by waves wherever they
fronted the sea
(Fig. 19), while elsewhere the line of Pleistocene dunes
was vigorously attacked and reduced to smaller and more widely spaced
remnants connected by slender
bars,
like beads strung upon a
delicate
thread.
Scarcely had marine gradation
fully entered upon
a youthful
stage
of development when
its work was
interrupted by
slight
uplift
of
re-
gional extent.
The amount was not great—perhaps only five or six feet;
but it was enough to nullify the work already done.
On the eastern side
of the Cape San Juan peninsula and along other rocky sections of the
coast the old wave-cut terrace, only
a few yards wide, now supports
a
growth of vegetation and sometimes a camino, which could not otherwise
obtain access to the rugged shoreline.
Steep wave-cut
cliffs, their slopes
scarcely modified,
are succumbing
to
the
processes
of
subaerial
decay.
The narrow bars which tied the headlands have been appreciably broad-
ened and heightened, and
their
surfaces
are furrowed by beach
ridges
which may possibly mark the progressive
stages
of
uplift.
Vegetation
has taken root on nearly all the bars in the Fajardo district, but in places
the wind shows some inclination to heap the sands into dunes.
The physiographic record ends appropriately with an emergence which
has been of inestimable service to man.
Not only did it elevate the shore-
forms, but
it raised the estuarine deposits which now make the playas,
and the lagoonal muds which make the fringing lowlands of the north-
ern
coast.
In
the
lagoons and
playas
streams have
entrenched
their
SCfENTlFlC ^CRVEY OF PORTO RICO
•
Jtglifliouse on rape
now
tieail and
sliglM
:a,
iM'iU'plHne.
uit
cbaimels, providing either natural clraiuago or the pussihility of artificial
drainage, and as a result the yonng phiiiis have been converted
into fertile
agricultural lands, upon
wlii<di
a
Uirge
ftart of the s^ugar crop
is grown.
Partly in consec|iiejice of their agricultural development, these phdns have
become a nucleus for human setthmient and
ijidustry.
At
tlie heginning
of this report the distribution of population within
tlie Fajardo district
was briefly outlined.
A sketcli of
its depcsndence u,pou the physiographic
features, which, in turn, constitute but the visible expression of tlie under-
lying geology, will serve as a suitable conehision.
Settlement and progress
iu northeastern Porto
Rico have been domi-
nated by Quaternary landforrns, and, almost symbolically,
tlie inhabitants
of the interior uplands seem, as remote from the modern world as the older
landforrns on whicli they live.
Rarely does a carretera reach them.
Ao
energetic government
is extending roads
ijito many parts
of
the
inac-
cessible interior, but ninch of it has so little prospect of development that
improvement of many of the routes of travel wouhl be without economic
justification.
The steep slopes precdudi? most profitable agricultural pur-
suits, and the few occupations which cmi be follow^ed, such as coffee an,d
banana raising and charcoal burning, support a severely linrited number
of families.
The meager livelihood available
is mirrored in every phase
of the
life of the people.
In
physiographic terms they
are isolated by
Quaternary uplift and dissection, and
the influence of these factors be-
comes more striking when the density of population
is seen
to increase
in the
areas of lower
relief.
So
perfect
is
the gradation
that
one
is
MEYEh'HOFF, GEOIAM'JY OF THE FAJARDO DISTRICT
lie
cape
io
the
I Sub Diego
(cti
lieneplane stauc
.»
been
foriried,
and
in
the
larger
a
• Joined
to
the ma Inland.
The
hills
.<>
M'<ninl:ains
in
the Irackgronncl.
teiiiptcjd
to see
if the ratios between
relief ami density of settlement do
not maiiitain
a constant
value.
Where Quaternary erosion has produced
ofjeii valleys and broad lew-
landfa. settlement
is thiek. as the
i'agiias
hasiii and
the lower
valleys of
llio Grande
cle
Loiza,
Riu
Fajarrio. and
l?iu
Blaiieo show.
And where
processes
of
ag'gradation
liave
yielded
not
only 'flat
plains
but
fertile
soils, as in the eoastal seetion. the deiisit}' of
tlie population
far exceeds
the
high
avera^-e
for
the
entire
district.
Concentration
has been
still
further encouraged
in
tliose few
loi-alities
in
the
coastal
T(-^gioii where,
as in the iieigbhoriiood
ol' San Juan, the estoaries and lagoons, deepened
hv Qoaternary
sul)iiiergence, have
not been
wliolly
obliterated
l)y
suh-
seqnent
sedinnsntation.
8hipt)irig
facilities,
agricultural
development,
settlement,
and
communication
are
so
completi»]y
dominated
by
the
Quaternary
landfomis
aiid
are
so
profoundly
modified
where
older
physiographic elements persist, that a map showing in
detail the culture
and agriculture of the Fajardo district can be used as accurately and
as
reliably io
irtt^erpret the
pliysiography,
as
a physiograijhic map can be
employcxl
to
interpret even
local
aspects
of settlement and
occupation.
Tfuman adjustment to environment
is
extraordinarily
perfect.
BIBLIOCRAPHY
The
following bibliography
is
not
int'cnded
to
be
exhaustive.
Full
lists of
tfie early studies dealing
ivitli the natural
liistory of Porto Rico
appear
in some
of
the
previous
reports
of
the
Seientifie
Survey, and
358
SCIENTIFIC SURVEY OF PORTO RICO
greater emphasis has therefore been placed upon works published
since
the
Survey was
inaugurated.
Of
the
older
contributions
only
those
which have been utilized in the preparation of the present report are in-
cluded.
A survey of the literature^ old and new^ shows impressively the
steadily growing interest
in West Indian geology, and no bibliography
can hope to remain long up-to-date.
The writer wishes to acknowledge his debt to the authors of the titles
listed,
for,
without
the background
they
have
provided,
the
Fajardo
studies would have been impossible.
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1915.
Geological reeonnoissance of Porto
Rico.
Ann.
N.
Y. Acad.
Sci.,
Vol. XXVI,
pp.
1-70.
1919.
Introduction to the geology of Porto Rico.
New York Academy of
Sciences,
Scientific Survey of Porto Rico and the Virgin Islands,
Vol.
I,
Pt.
1,
pp.
11-28.
BlackWELDER, Eliot
1925.
Exfoliation
as
a
phase
of
rock
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Geol.,
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Britton, N. L.
1919.
History of the Survey.
New York Academy of Sciences,
Scientific
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I, pp.
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1923.
Botanical investigations in Porto Rico.
Journal,
N.
Y. Botanical
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Cleve, p. T.
1871.
On
the
geology
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Svenska Vetenskaps Akademicus,
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No.
12, pp.
1-48.
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1883.
Outline
of
the
geology
of
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Colony,
R.
J., and Meyerhoff,
H.
A.
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Econ. Geol., Vol.
XXIII, pp. 515-527.
Coryell, H. N., and Ohlsen, Violet
1929.
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species.
New York Academy
of
Sciences,
Scientific
Survey
of
Porto Rico and the Virgin Islands, Vol. Ill, Pt.
3, pp. 167-236.
Dinwiddie, W.
1899.
The great caves
of Puerto
Rico.
Harper's Weekly,
March,
Vol.
XLIII,
p. 293.
Earle, K. W.
1924.
The geology of the British Virgin
Islands.
Geol. Mag.,
Vol. LXI,
pp. 339-351.
Fettke, Charles R.
1924.
Magnetite deposits of eastern Porto
Rico.
Amer.
Inst. Min. and
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MEYERHOFF, GEOLOGY OF THE FAJARDO DISTRICT
359
The
geology
of
the Humacao
District,
Porto
Rico.
New
York
Academy
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Sciences,
Scientific Survey
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the
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I, Pt.
2, pp. 111-228.
Hubbard, Bela
1920.
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Science,
Vol. LI, pp. 395-
396.
1921.
The Tertiary
molluscs
of
the
Lares
District,
Porto
Rico.
New
York Academy of Sciences,
Scientific Survey of Porto Rico and
the Virgin Islands,
Vol.
Ill,
Pt.
2, pp. 79-164.
1923.
The geology
of the Lares
District, Porto
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the
Vir-
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1922.
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Low, Bela
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The mineral deposits of Porto Rico.
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209-215.
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Porto
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of
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Porto
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