AD-A137 665 SEAFLOOR ENVIRONMENTS NORTH ST CROIX MARGIN AND VIRGIN I/ ISLANDS TROUGH PAR..(U) NAVAL OCEAN RESEARCH AND DEVELOPMENT ACTIVITY NSTL STATION MS.. UNCLASSIFIED W d BURTON ET AL. DEC 82 F/6 8/10 NL moonmlillnossu ommo||Bmnom|om s!mhhEEEEEohEE Emhhmhhhhhhhml EEEEEEIEE|iiEE!U _ _3 * 2 .2 L- 40 111L I ~ .a 1.8 MICROCOPY RESOLUTION TEST CHART NATIONAL BURIALI OF SIANDARD, 1% A Seafloor Environments North St. Croix Margin and Virgin Islands Trough CPart 1 Introdudon WJ. Burton* Part 2 get Geology an Geophyscs FA. Bowles J. Egloff, Jr. Part 3 Geotechnical Investgtions R.H. Bennett D.N. Lambert 6.F. erll+ F.L. Nastav Part 4 Enitneeng Significance WJ. Burton* PRuar hy i 'm CooPrMb with 1Wd FauiKs Elhod CmummiTI ", D.C.D T ad iiI ELECT N d FEB81984 NOWil OmsM ad Abm cd IAMOMMu Su Seafloor Seosciences DMson kceen Science and Technology lboratori Novel Ocean Rmarc and Develepment ctt NSTL, Mississippi 39520 i s os o Igbmi imd FILE COY S4 2 8 0' VAN g( inuud uqn at =so amu u nu bowS mi huhi &md *Aibjus m a 6 1"S for od* wub I u po S f m ft uS .5 ft womkuui Seafloor Environments North St. Croix Margin and Virgin Islands Trough Part I Introduction WiJ. Burton* Part 2 fleology and Geophysics FA. Bowles 1. Egloff, Jr. - Part 3 -Geotechnical Investigations R.N. Bennett O.N. Lambert G.F. Merrill F.L. Nastav Part 4 Engineering Significance WiJ. Burton* W142 (N) Usmuhr 1812 Prepared by and in Cooperation with- Chesapeake Division* Naval Filities Enoneering Command Washingon, D.C. and IAtlantic oceanographic and Meteorological Laboratories" National Oceanic and Atmospheric Administration Miami. FL. Seafloor Geosciences Division Ocean Science and Technology Laboratories Naval Ocean Research end Development Activity ISIL, Mississippi 39529 A~v rwuf Fik bc sa kWSUW WON li Abstract The Virgin Islands Trough (VIT) Is a deep (4500 meters (a)), elongate basin bounded on its north and south sides by steep fault escarpments. The south escarpment forms the north submarine slope of St. Croix Island and has an average gradient of 18-23', although local gra- dients vary between 5-477. The siope is cut by two sub- marine canyons that*terminate at aid-slope. In the case of the larger canyon, erosional debris is carried from the canyon mouth to the basin floor via a system of gullies. The basin floor is a relatively smooth, gently sloping, turbidite plain that has a sediment thickness in excess of 1.5 seconds (1500 ). lost of this sediment is thought to derive from the Virgin Islands Shelf to the north. Westward transport of sediment in the basin Is blocked by a topographic high located at 17*54'U, 64'48'W causing the sea floor to be 300 a higher on the east side of the high. This area of the basin (east of the high) is also considerably narrower than the re- mainder of the basin because of a major slump and/or tectonic uplift. Sediment thickness on the north slope of the basin ranges between 0-0.15 sec (0-150 a) but cannot be re- solved seismically on the south slope (St. Croix side) where only a thin veneer of sediment exists (a few meters or less). In some areas a hard crust exists just beneath the sediment surface. Visual observations indi- cate that rock outcrops are generally infrequent. Pelagic deposition, dovnslope creep, slumping, and tur- bidity currents are all active mechanisms that trans- port sediment dovn the north slope of St. Croix to the basin floor. The sediments in the VIT are calcareous oozes and tex- turally fall within the sandy-clay silt range. Sand and gravel are also a major component of the slope sedi- ments, and the highest concentrations occur off Salt liver and Cane Bay. Large blocks of coral and rock de- bris are widely dispersed on the slope. Righ carbonate content of the sediments, differing depositional envi- ronments, and bioturbation appear to be the main fac- tors contributing to high variability in geotechnical properties, sediment type, and sediment texture. Steep gradient, sediment instability, and seismic risk, in . .. Abstract (continued) addition to vide ranging variability of geotechnical parameters. sake the north slope of St. Croix a poten- tially hazardous environment for engineering applica- tions. Accession For r;TIS GAIes DTIC TAB /K6~ Unannounced J ( U ) Justificatio -eef By Distribution/ Availability Codes Avail and/or Dist Special POW Preface This Investigation was conducted by the Kval Ocean Research and Development Activity (NOIDA), Day St. Louis, Kiss., at the request of the Chesapeake Divi- sion, Naval Facilities ingineering Comud (NAVFAC), Washington, D.C. Data collection and laboratory analy- ses were performed by NORDA cooperatively with the National Oceanic and Atmospheric Administration (NOAA), Mimi, Florida. Responsibility for the final report was retained by NORDA and NAVFAC. The purpose of the investigation was to develop a better knowledge of the geological, geophysical, and geotechnical characteristicpj of the sea floor on the north submarine slope of 4t. Croix Island, V.I., and in the VIT to supplement long-range planning of Navy ac- tivities in this area. This investigation. represents * major improvement in the current knowledge of the seafloor environment in the VIT region. In particular, it is the first investi- gation involving the engineering aspects of the sedi- ments on the north slope of St. Croix and in the trough. This investigation should be of interest to engineers involved in seafloor operations and to the general marine science comunity. - Ii FPO-1-82 (26) I SEAFLOOR ENVIRONMENTS * NORTH ST. CROIX MARGIN AND VIRGIN ISLANDS TROUGH * December 1982 Prepared for Director, Research, Development, Test and Evaluation Office of the Chief of Naval Operations Washington, D.C. S SD S /4 .. .. . .. . .. . .. . .. . .. . Abstract This investigation was conducted by the Naval Ocean Research and Development Activity (NORDA), Bay St. Louis, Miss., at the request of the Chesapeake Divi- sion, Naval Facilities Engineering Command (NAVFAC), Washington, D.C. Data collection and laboratory analy- ses were performed by NORDA cooperatively with the National Oceanic and Atmospheric Administration (NOAA), Miami, Florida. Responsibility for the final report was retained by NORDA and NAVFAC. The purpose of the investigation was to develop a better knowledge of the geological, geophysical, and geotechnical characteristics of the sea floor on the north submarine slope of St. Croix Island, V.I., and in the VIT to supplement long-range planning of Navy ac- tivities in this area. This investigation represents a major improvement in the current knowledge of the seafloor environment in the VIT region. In particular, it is the first investi- gation involving the engineering aspects of the sedi- ments on the north slope of St. Croix and in the trough. This investigation should be of interest to engineers involved in seafloor operations and to the general marine science community. ii '' ;. Preface The Virgin Islands Trough (VIT) is a deep (4500 meters (m)), elongate basin bounded on its north and south sides by steep fault escarpments. The south escarpment forms the north submarine slope of St. Croix Island and has an average gradient of 18-23, although local gra- * dients vary between 5-470 . The slope is cut by two sub- marine canyons that- terminate at mid-slope. In the case of the larger canyon, erosional debris is carried from the canyon mouth to the basin floor via a system of gullies. * The basin floor is a relatively smooth, gently sloping, turbidite plain that has a sediment thickness in excess of 1.5 seconds (1500 m). Most of this sediment is thought to derive from the Virgin Islands Shelf to the north. Westward transport of sediment in the basin is blocked by a topographic high located at 17°54'N, * 64*48'W causing the sea floor to be 300 m higher on the east side of the high. This area of the basin (east of the high) is also considerably narrower than the re- mainder of the basin because of a major slump and/or tectonic uplift. * Sediment thickness on the north slope of the basin ranges between 0-0.15 sec (0-150 m) but cannot be re- solved seismically on the south slope (St. Croix side) where only a thin veneer of sediment exists (a few meters or less). In some areas a hard crust exists just beneath the sediment surface. Visual observations indi- * cate that rock outcrops are generally infrequent. Pelagic deposition, downslope creep, slumping, and tur- bidity currents are all active mechanisms that trans- port sediment down the north slope of St. Croix to the basin floor. * The sediments in the VIT are calcareous oozes and tex- turally fall within the sandy-clay silt range. Sand and gravel are also a major component of the slope sedi- ments, and the highest concentrations occur off Salt River and Cane Bay. Large blocks of coral and rock de- bris are widely dispersed on the slope. High carbonate , Dcontent of the sediments, differing depositional envi- ronments, and bioturbation appear to be the main fac- tors contributing to high variability in geotechnical properties, sediment type, and sediment texture. Steep gradient, sediment instability, and seismic risk, in Aiii I1II . . Preface (continued) addition to wide ranging variability of geotechnical parameters, make the north slope of St. Croix a poten- tially hazardous environment for engineering applica- tions. IV : , .. . . . ;,1: .:, " '.'* "' r"'' ,- " - ... . . . v. :: , " e tf " ' . . r J Executive Summary The seafloor investigations comprising the subject of this report were performed under the sponsorship of the Director, Research, Development, Test and Evaluation (OP-098). The Chesapeake Division, Naval Facilities * | Engineering Command (CHESNAVFACENGCOM) was assigned the responsibility for accomplishing the tasking. CHESNAVFACENGCOM engaged the assistance of the Naval Ocean Research and Development Activity (NORDA) and the National Oceanic and Atmospheric Administration (NOAA) who had expertise in acquisition and analysis of sea- floor data. Better knowledge of the geological, geophysical, and geotechnical characteristics of the sea floor forming the north margin of St. Croix and the Virgin Islands Trough is essential to long-range planning of Navy ac- * tivities for the area. Such information is necessary for the development of adequate design of foundations for underwater structures, predicting requirements for anchoring systems, specifying requirements for protec- tion and stabilization of cables, etc. The paucity of published data relevant to the aforementioned sea floor prompted an initiative to obtain the required data and to present it in a format most useful to the practi- tioner involved in seafloor engineering. Limited resources of time and money determined the scope of the investigations. Considerable economies of * Dthese resources were realized by using "ships of oppor- tunity" that were performing assignments in the general vicinity of the island of St. Croix and the VIT. During the investigations, precision bathymetric pro- filing, seismic reflection profiling, dredging, coring, ; Dand photographic operations were conducted. Sediment samples were obtained from 40 stations chosen to pro- vide representative samples from the shelf, slope, and trough in the general area of interest. Seven dredge stations and seven bottom camera stations were occu- pied. Photographic information of the bottom was great- * ly increased by the acquisition of video-tapes and still photographs taken during three submersible dives (ALVIN). The geotechnical data have been analyzed only to the extent of establishing the range and average values of * basic properties of the soils (such as shear strengths, Atterberg limits, etc.). The samples have been archived V Executive Summary, (continued) so that additional information can be generated later if required. Detailed investigations of the geotechnical character- istics of the sediments, the active geological proc- esses, and the morphological setting are important considerations for virtually any offshore engineering activity of the north margin of St. Croix. The margin is characterized by three major morphological features: a relatively narrow shallow water carbonate shelf; a steep slope (approximately 230 with steeper local slopes of nearly 50*); and a deep-sea basin which is the eastern portion of the VIT. The trough, or basin, is asymmetrical and U-shaped in cross-sectional form, with a relatively flat floor that deepens from about 3700 m on the east to nearly 4500 m on the west. The floor of the trough is interrupted at 17054'N, 64047.5'W by a topographic high that acts as a barrier to westward sediment transport. Consequently, the sea floor to the east of this feature is about 300 m shoaler than to the west. Slumping or tectonic dis- turbance (or both) has altered the normal configuration of the trough, causing it to be much narrower east of the topographic high. The south wall of the trough (northern margin of St. Croix) is cut by two submarine canyons (Christiansted Canyon and Salt River Canyon) that terminate at mid- slope. Christiansted Canyon, the larger of the two, is the principal conduit for transporting erosional debris to the basin floor. This material is distributed from mid-slope to the basin floor via a system of gullies that begin where the canyon ends. Major gullying does not occur elsewhere on the south slope. Seismic reflection profiles show that the floor of the VIT is a turbidite plain with a sediment thickness of at least 1.5 sec (1500 m). Maximum sediment thickness is about 0.1-0.15 sec (100-150 m) on the north wall of the trough, but cannot be resolved on the steeper south wall (less than 25 m thick). Extensive bottom photographs show that the south wall of the trough is sediment covered and that rock out- crops are infrequent. Evidence of significant bottom currents was not evident. Attempts to recover sediment samples indicated that the slope sediments are probably vi C''A Executive Summary, (continued) less than one meter in thickness and, in some areas, are underlain by a thin (10-15 cm) crust of probable carbonate material. Shallow water reef material and carbonate debris occur along the shelf, and a thin veneer of carbonate ooze covers the slope. Sediment recovered from piston and gravity cores in the trough (basin) reveal the presence of clayey silts and turbidite deposits composed of carbonate material from shallow water environments and carbonate debris and tests from micdle and deep water environments (abun- dances of pteropods and foraminifera). The character of these sediments indicates transport of shallow water carbonate material offshore to deeper water environ- ments of the slope and trough. Initial geotechnical analyses of the upper one meter of sediment reveal average shear strengths of cores rang- ing between 0.4-1.4 psi (3.1-10 kPa) for the slope and 0.8-5.8 psi (5.8-28.3 kPa) for the trough. Individual values within cores range as high as 10.0 psi (69.1 kPa) at sediment depths greater than one meter. Average sensitivities for the slope sediment cores range be- tween 3.2-6.1 and 4.4-12.1 for the trough (upper 1 m). High sensitivities (8-12) indicate the presence of metastable deposits and considerable strength loss upon disturbance or remolding. Average water contents (per- cent dry weight) range between 57-77% (101% for grab sample) on the slope and 52%-64% for the trough. The proportions of sand, silt, and clay sized fractions and the nature of the particles (i.e. water filled cavities of carbonate tests), as well as bioturbation in some deposits, strongly affect the variability in the water contents and porosities. Grain densities (solid par- ticles) are fairly uniform 170.4-171.7 pcf (2.73-2.75 Mg/m 3 ) and wet unit weights average between 97.4-106.8 pcf (1.56-1.69 Mg/m 3 [92.4 pcf or 1.48 Mg/m 3 grab sam- ple]) for slope and trough sediments. Average poros- ities among cores range between 61-68% for slope sedi- ments and 59-65% in the trough. Whole core averages (depths >1 m) of the geotechnical properties are slightly different. The steep northern slope of St. Croix and the seismici- *ty of the Virgin Island region constitute a potentially vii 4 .. Executive Summary, (continued) unstable engineering environment, requiring that spe- cial attention be given to the location and design of seafloor installations to prevent sliding. Also to be considered is the presence of rugged local terrain, active dowuslope movement of sediment, sediment cracks, bioturbation, and currents. The floor of the trough provides a more stable environ- ment because of much lower gradients. This virtue is offset, however, by the fact that the sediments in the trough, as well as on the slope, exist in a metastable state. The calcareous nature of the sediments is also a complicating factor because the dynamic behavior of this sediment type is poorly understood. Evidence sug- gests that calcareous oozes are susceptible to failure under repeated loadings. Thus, large factors of safety are advisable when choosing bearing capacities for the design of structural footings. The variability in local morphology, sediment proc- esses, and sediment properties demonstrate the need to obtain detailed data in specific areas of engineering interest along the northern margin of St. Croix. In addition, a comprehensive determination of surface and bottom obstructions (man-made) should precede any planned engineering activities. Viii Contents ABSTRACT ............................................ ii PREFACE ............................................ i i EXECUTIVE SUMMARY ..................................... v TABLE OF CONTENTS .................................... ix NOMENCLATURE ......................................... x LIST OF FIGURES .................................... xiii LIST OF TABLES ...................................... xvi ACKNOWLEDGMENTS AND CREDITS ....................... xvii PART 1. INTRODUCTION ................................ 1.1 PART 2. GEOLOGICAL AND GEOPHYSICAL OBSERVATIONS .... 2.1 PART 3. GEOTECHNICAL INVESTIGATIONS ................ 3.1 PART 4. ENGINEERING SIGNIFICANCE ................... 4.1 PART 5. CONCLUSIONS AND RECOMMENDATIONS ............ 5.1 PART 6. GLOSSARY .................................... 6.1 1 I I I ix W , Nomenclature number O degrees (latitude, longitude, slope or temperature) minutes (latitude or longitude) (foot or feet) min minutes (time) sec seconds (time) inch(es) inch(es) ft foot (feet) 'Um micrometer(s) micron(s) mm millimeter(s) cm centimeter(s) cc cubic centimeter m meter(s) km kilometer(s) lb(lbs) pound (pounds) ton 2000 pounds psi pounds per square inch TSF tons per square foot g/cm2 grams per square centimeter kPa kilopascals PVC polyvinyl chloride plastic IPS iron pipe size ID inside diameter OD outside diameter CaCO 3 calcium carbonate fps feet per second in/min inches per minute mm/min millimeters per minute g/cc grams per cubic centimeter Mg/m3 megagrams per cubic meter pcf pounds per cubic foot x - - - . . 4.-i Nomenclature, (continued) "C degrees celsius % percent ppt parts per thousand P load pressure LL liquid limit PL plastic limit PI plasticity index LI liquidity index w water content a1 maximum principle total stress 02 confining stres 03 confining stress oc cell pressure al-0 3 deviatoric stress ac effective cell pressure * *d effective compression stress (- l - 03) 5 effective normal stress n a maximum principle effective stress 0 minimum principle effective stress 03 rF shearing stress at failure u excess pore water pressure linear strain (axial strain or shear displacement) * bp back pressure O angle of internal friction OCU total angle of internal friction for consolidated-undrained tests 1CD effective angle of internal friction for consolidated drained tests effective angle of internal friction for Oconsolidated undrained tests wet unit weight A pore pressure coefficient xi Nomenlature, (confinued) cc compression index Cu coefficient of consolidation C s recompression swell index > greater than < less than - approximately xli 4L. Figures Part 1 Figure 1.1. Bathymetric map showing the regional geologic setting of the Virgin Islands * Trough area ............................... 1.4 Part 2 Figure 2.1. Ship's survey lines along which seismic reflection and 3.5 kHz profiles were collected ................................. 2.4 Figure 2.2. Bathymetric map based on survey lines shown in Figure 2.1 ....................... 2.6 * Figure 2.3. Generalized diagram showing relationship of widebeam and narrowbeam echosounders with a sloping seafloor of 300 ............. 2.9 Figure 2.4. North-south topographic profiles across the Virgin Islands Trough ................. 2.10 Figure 2.5. Sesimic reflection profile showing south wall of the Virgin Islands ................ 2.13 Figure 2.6. Seismic reflection profile showing the south wall (A' - G') and north-south crossings of Virgin Islands Trough ........ 2.13 Figure 2.7. Seismic reflection profile showing the south wall of the Virgin Islands Trough and multiple crossings of Christiansted Canyon (C) and Salt River Canyon .......... 2.14 I Figure 2.8. Seismic reflection profile showing east- west profile down the axis of the Virgin Islands Trough ............................ 2.14 Figure 2.9. Seismic reflection profile showing north- south crossings of Virgin Islands Trough .................................... 2.15 Figure 2.10. Ship's track showing the location of seismic profiles in Figures 2.4-2.8 ....... 2.16 xiii Figures, (continued) Figure 2.11. Selective bottom photographs shoving the floors of the: A. Virgin Islands Trough ............................. 2.19 B. South Wall ....................................... 2.21 C. South Wall ....................................... 2.22 D. South Wall ....................................... 2.24 E. South Wall ....................................... 2.25 F. South Wall ....................................... 2.26 G. South Wall ....................................... 2.27 H. South Wall ....................................... 2.28 Part 3 Figure 3.1. Bathymetric map of north slope, St. Croix Island and adjacent Virgin Islands Trough .................................... 3.4 Figure 3.2. Hydroplastic corer ........................ 3.10 Figure 3.3. Operation of the free-fall method of coring using the modified "Ewing-type" large diameter piston corer with a hydro- plastic corer used as the trip weight ..... 3.10 Figure 3.4. Drawing of the core retainers and cutters used with the large diameter piston corer ..................................... 3.10 Figure 3.5. Modified piston used with the large diameter piston corer ..................... 3.11 Figure 3.6. Drawing of the Shipek grab sampler ........ 3.11 Figure 3.7. Torvane device and sensitive vane adapter ................................... 3.12 Figure 3.8. Miniature vane shear apparatus ............ 3.12 Figure 3.9. Typical arrangement for inserting mini- vane into short core section .............. 3.12 Figure 3.10. Sediment sample shown in split ring soil sample retainer for use in the direct shear tests ............................... 3.13 xiv Figures, (continued) I Figure 3.11. Example of shear configuration at failure in the direct shear test .................. 3.13 Figure 3.12. Typical configuration of triaxial test apparatus and cylindrical soil sample ..... 3.14 Figure 3.13. Typical example of principal stresses on cylindrical soil sample in triaxial test ...................................... 3.14 Figure 3.14. Schematic of single piston helium pycnometer ................................ 3.15 Figure 3.15. Textural classification of sediments from the northern margin of St. Croix using Link's (1966) method ...................... 3.17 Figure 3.16. X-radiograph of a section of Core 22 showing one complete turbidity sequence and the lower portion of another .......... 3.18 Figure 3.17. Plot of porosity (%) versus depth in core * for all samples ........................... 3.19 Figure 3.18. Plot of void ratio versus depth in core for all samples ........................... 3.19 Figure 3.19. Plot of wet unit weight (pcf) versus * depth in core for all samples ............. 3.19 Figure 3.20. Plot of natural shear strength (psi) versus depth in core for all samples ...... 3.19 Figure 3.21. Plot of remolded shear strength (psi) * versus depth in core for all samples ...... 3.20 Figure 3.22. Plot of sensitivity versus depth in core for all samples ........................... 3.20 Figure 3.23. A. Stress-strain plot from consolidated- * drained direct shear tests--Core 22. B. Plot showing the drained angle of internal friction for Core 22 ...................... 3.21 Figure 3.24. A. Stress-strain plot from consolidated- drained direct shear tests--Core 35. B. * Plot showing the drained angle of internal friction for Core 35 ...................... 3.22 xv * Figres, (continued) Figure 3.25. Plots of triaxial shear tests results for Core 35............................. 3.23 Figure 3.26. Plot of liquidity index versus depth In core .. .. .................... ...... 3 .24 Figure 3.27. Plasticity chart for all samples tested...3.24 Xvi I m Tables TABLES Table 3.1. List of core locations ...................... 3.5 * Table 3.2. Geotechnical properties ..................... 3.8 Table 3.3. Reported limits of reproducibility for selected geotechnical tests and analyses .... 3.16 Table 3.4. Calcium carbonate analyses .................. 3.24 APPENDIX A 6.7 Tables: A-6.7.1. Size classification ........................ 3.30 A-6.7.2. Soil properties ............................ 3.41 A-6.7.3. Strength measurements ...................... 3.56 1 A-6.7.4. Atterberg limits ........................... 3.66 6.8 Plots: A-6.8.1. Soil properties ............................ 3.73 A-6.8.2. Strength measurements ...................... 3.89 Ii I xvii : ____ ___ ___ ____ ___ ___ ___ ____ ___ __ ____ ___ ___ __ The authors express their appreciation for the assistance of the following scientists and technical assistants during field and shorebased activities: Drs. William Harrison, Gideon Almagor, and Terry Nelsen; Messrs. Evan Forde, John Burns, J. Egloff, Roy Burke, Sam Bush, Steve Madosik and Thomas Tuma; Ms. Gail Romero, Sharon Madosik, and Karen Sullivan. Special thanks are due the officers and crew of the U.S. Navy Ships LYNCH and BARTLETT and the NOAA ship RESEARCHER. The support given by NOA's Atlantic Oceanographic and Meteorological Laboratories during the field activities is gratefully acknowledged. Encouragement given by Drs. H. Eppert, L. Maynard and T. Holcombe is appreciated. Special thanks are due Dr. Dennis Hubbard, West Indies Laboratory, for access to the information and data col- lected during DSRV ALVIN dives off St. Croix. Support for this project was given by the Naval Facilities En- gineering Command, Chesapeake Division, and was spon- sored by the Director, Research, Development, Test and Evaluation, Office of the Chief of Naval Operations. xviii PART I INTRODUCTION W.J. Burton PART I * INTRODUCTION W.J. Burton PART 1. INTRODUCTION A limited investigation has been per- outer shelf and slope off the north side formed to determine the geological and of St. Croix and the neighboring VIT geotechnical characteristics of a se- where precision bathymetric mapping, lected geographical region that incor- seismic reflection profiling, dredging, porates a portion of the northern margin coring, and camera operations were of the Island of St. Croix, U.S. Virgin conducted. Water depths ranged from tens Islands, and the adjacent Virgin Island of feet (ft) on the shelf to greater Trough (VIT). The gathering and analyses than 13,000 ft (4000 meters (m)) in the of the data have taken place intermit- trough. The overall region investigated tently over an extended period of time, is considered to be favorably situated beginning in 1980. The effort has been from the standpoint of using the deep performed jointly by the Chesapeake Di- portion of the VIT in combination with vision, Naval Facilities Engineering shore mounted equipment positioned on Command (CHESNAVFACENGCOM); the Naval St. Croix. Ocean Research and Development Activity (NORDA); and the National Oceanic and 1.3 Scope Atmospheric Administration (NOAA). This report summarizes the results obtained The extent of this investigation was from the observations and analyses of limited by time and funding consider- the data and provides insight into the ations. Economy of both these resources placing of underwater structures and was realized by using "ships of oppor- equipment on or beneath the sea floor. tunity," that is, vessels that were Such applications are of interest to a working in the vicinity of the VIT and wide-range of activities within both the could incorporate the tasks of this pro- Navy and NOAA. ject into their schedules. 1.1 Purpose of Investigation Bathymetric, seismic reflection, dredg- ing, and bottom photography operatior- This project was an interdisciplinary were conducted to determine the morro- effort integrating geology, geophysics, logy, primary sediment patterns, akd and geotechniques to provide fundamental active sediment processes of the north data for offshore engineering applica- margin of St. Croix and VIT (Part 2). tions on a carbonate island margin. More specifically, this investigation is con- Piston core, hydroplastic giavity core, cerned with the engineering properties and Shipek grab samples were recovered of the surficial sediments, the morpho- for geotechnical and sedimentological logical characteristics, and the proc- study (Part 3). esses active on the margin. Selected data were reported for the up- 1.2 Geographic Area per 18 ft (5.5 m) of surficial sedi- ments; however, the greatest emphasis in Figure 1.1 depicts the geological envi- this report is on the upper 3 ft (1 m). ronments of interest, including the 1.3 4DIM A ,ANK-UIM iMrlq ~B 30' 650 30' 7 7 30 ST~~S CROXOIXCOI 30' Figure 1. 1 Bathymetric map showing the r'egional geologic setting of the *Virgin Islands Trough area. 1.4 PART 2 K GEOLOGY AND GEOPHYSICS F.A. Bowles J. Egloff, Jr. * 2.1 PART 2. GEOLOGICAL AND GEOPHYSICAL OBSERVATIONS The following sections discuss the re- the geology and geophysics of St. Croix sults of the Joint geological and geo- Island and adjacent sea floor. Signifi- * physical investigations undertaken by cantly, nearly all the references up to NORDA and NOAA in the Virgin Islands the time of their report deal primarily Trough and on the north margin of St. with aspects of island geology, the Croix. In addition, they provide a broad regional features of Caribbean geological setting for the geotechnical geology, and specifically with the sea investigations and engineering interpre- floor west of the island where the St. tations discussed in Parts 3 and 4. Croix Tracking Range is located. Thus, outside of the Range area, little direct 2.1 Previous Investigations information exists for most of the sea floor adjacent to St. Croix. St. Croix Island represents an exposed portion of the east/west-trending St. Frassetto and Northrop [2.7] published Croix Ridge (Fig. 1.1). Immediately to the results of an early bathymetric the north, the deep, elongate VIT survey of the VIT between approximately separates St. Croix from the Virgin 69*50'W and 65*50'W. Their bathymetry Islands shelf and the islands of map compares well with the regional Vieques, St. Thomas, and St. John. bathymetry shown by Garrison et al. Evidence strongly indicates that the VIT [2.8] which was also used as the base is a graben structure formed by post- map for Figure 1.1. Miocene extensional forces [2.1, 2, 3, 4]. At the 3000 m isobath the VIT is Both maps show the VIT as an elongate, approximately 55 nautical miles (nm) flat-floor feature bounded by a steep wide, although the actual floor is southern escarpment (up to 430, [2.7]) better defined by the 4000 m isobath. which forms the north island-slope of The VIT consists of two basins, the St. Croix. Aside from showing a large smaller of which is at the extreme west first-order indentation in the isobaths end of the trough and is separated from off Christiansted Harbor, neither ba- the larger basin by a narrow sill having thymetry map reveals any fine-scale de- a water depth of about 3600 m. The east tail. and west extremities of the trough lead into the Anegada and Jungfern passages, A later survey by Shepard [2.9], how- respectively, which provide the deepest ever, does provide more detailed bathym- link between waters of the Atlantic etry of the north slope of St. Croix Ocean and Caribbean Sea [2.5]. between about 64°39'W and 64*55'W. This data also reveals a steep escarpment, Holcombe et al. [2.6] have presented a but suggests over-steepening in some geological study of the Fredericksted places and gentler slopes in others. Plateau area (Fig. 2.1), in which they Although this is probably the case, it provide detailed bathymetry, seismic is noteworthy that the detail displayed reflection profiles, bottom photography, in Shepard's bathymetry does not appear and sediment engineering information, to be warranted by the number of survey They also assembled all available data lines. His contours do, however, plainly published prior to 1975 and, where ap- reveal the existence of a large canyon- propriate, have synthesized it into a like feature directly off Christiansted detailed and comprehensive summary of 2.3'M A 15 65° 45" 30 I " i 15 VIEQUES 4__ - ~ ~ ~ ~ ~ ~ 4 + - I - - - ; . --- ..- - .- I F --t . , I ,1l:1 /! / 7- L:_ CANS BAY\ .0b sBCK I,. '-CHRISTIANSTED CANYON HAMS BLFF 45 FREERICKSTED ST. CROIX PLATEAU 30 Figure 2. 1. Ship's survey lines along which seismic reflection and 3. 5 kHz profiles were collected. Stippled area delineates "shadow zone" where reception of trans- ponder located on east end of island was shielded by Buck Island. Dashed arc shows maximum extent of signal reception for this transponder. Harbor. Interestingly, the canyon ter- Holcombe et al. [2.6] note that the flat minates at a depth of 2600 m above the character of the floor of the VIT is base of the escarpment. A smaller, simi- indicative of a turbidite plain and sug- larly discontinuous "canyon" is also gest that much of the sediment contribu- indicated off Salt River. Bottom Current tion to the plain may originate on the studies in the upper portions of Lhese island of St. Croix and on the crest of canyons [2.10] show that the currents the St. Croix Ridge. In this regard, it are generally slow (rarely exceeding is significant that little information 0.33 feet per second (fps) (10 cm/sec)) exists in the literature concerning the but exhibit high-frequency alterations nature and source of the sediments in of direction and occasional periods of the VIT. Hubbard et al. [2.11] have re- relatively strong down-canyon flow. cently investigated the quantity and distribution of shallow-water sediments In general, the south escarpment inter- and biological debris to the VIT off St. sects the floor of the VIT with a sharp, Croix. Details of their investigation angular contact, indicating the absence are presented in Section 2.4.5. of a depositional fan [2.91. Some echo- grains, however, reveal a hummocky topog- In addition to the investigations men- raphy at the base of the slope, suggest- tioned above, nearshore, shallow-water ing possible slumping of material. hydrographic surveys around St. Croix 2.4 pi have been, and are continuing to be, 2.3.2 Data Collection conducted by NOAA's National Ocean Sur- vey To explore and evaluate the seafloor conditions and nature of the sediments, 2.2 Area of Investigation the following operations were accom- plished. The extent of the investigation area is shown by the ship's track chart present- * Installation of an acoustical, short ed in Figure 2.1. Initial plans called baseline positioning, and tracking for detailed surveying to be done at two system. locations; one off the general area of * 3.5 kilohertz (kHz) bathymetric pro- Salt River and the other north of the filing. east end of the island. These areas are e Seismic reflection profiling. evident in Figure 2.1 by the greater den- e Collection of sediment samples. sity of track lines (0.5-1.0 nm line * Collection of rock samples. spacing) relative to the remaining areas * Collection of bottom photographs. where much wider reconnaissance survey * Acquisition of additional bottom pho- lines were run (2.0-4.0 nm). Because the tographs taken by the submersible survey coverage west of about 64*55'W ALVIN. was broad, the data collection was in- * Laboratory testing of physical and complete (no seismic reflection, no bot- engineering properties of sediment tom photographs, three cores), and as samples (Part 3). the area was of secondary importance, this report deals almost exclusively The location of camera stations, and with the sea floor and sediments north ALVIN dive sites are given in Section of the island. 2.4.1. Locations for sediment samples are given in Part 3. 2.3 Operational and Technical Considerations 2.3.3 Equipment The following subsections provide spe- cific information relating to the The following equipment was employed in research vessels, equipment, and proce- support of the survey operations: dures used. * Bathymetric Survey System: 2.3.1 Vessels --Transducer--EDO, Model 240 H, 3.5 The first bathymetric, seismic, coring, kHz, hull-mounted and dredging operations for this in- --Transceiver--ORE, Model 140 vestigation were carried out aboard the --Recorder--Raytheon LSR-1811-10, USNS LYNCH in August 1980, followed by electrostatic additional bathymetric surveys and bot- tom photography by the USNS BARTLETT in * Seismic Reflection System: November 1980. Normal survey speed dur- ing both ship operations was approxi- --Sound source--Teledyne, Model 27290, mately 8 knots. 30,000-Joule "Sparker" --Hydrostreamer--Teledyne, Model 28420 The most recent surveying and bottom --Amplifier--Teledyne, Model 300 LF sampling operations were conducted with --Recorder--Raytheon LSR-1811-10, the NOAA ship RESEARCHER in August 1981 electrostatic (discussed in Part 3.). 2.5 • . . .m.m. * Navigation--Racal Decca, Model 540 Digital Distance Measuring System: .3, --Range--Line of sight to 35 miles --Accuracy--±l m, typical --Resolution--O.l m Photography--Hydro-Products, 70 milli- 40M meter (mm) multi-exposure cameras (2) and electronic strobe lights (2) e Dredging--Benthos chainlink bag dredge 4 A4 * Coring--Ewing piston corer, hydroplas- KIIA).VI HI tic gravity corer, Shipek grab sampler 2.3.4 Data Quolity Figure 2. 2. Generalized diagram showing relationship of widebeam and narrowbeam The following factors are critical re- echosounders with sloping sea floor of 300. garding the quality of the data collect- Dashed line represents hypothetical ed. rough bottom. 2.3.4.1 Errors Resulting from Beamwidth of 4000 m the area of sea floor insoni- The geometrical relationship between the fied is about 4600 m. Thus, at this sea floor and spherical wave front of depth a feature would have to be approx- the sound pulse emitted by wide-beam imately 5 km wide to be resolved by the echosounders can create errors in the echosounder. At shallower depths the measurement of water depth and feature radius of curvature of the echosounder's shape (resolution). In the latter case, wave front (i.e., the insonified area) the role of side echoes in obscuring the decreases with the result that resolu- true shape of sea floor features has tion increases. For example, resolution long been appreciated [2.12, 13, 14]. at a depth of 2000 m is about 2500 m Any sharp projection above the sea floor (Fig. 2.2). Clearly, feature resolution gives a hyperbolic echo trace resulting is improved by reducing the wavefront in distortion of seafloor topography by radius of curvature. This can be accom- generally increasing the apparent width plished by towing a transducer close to of a feature, although its actual height the bottom or by limiting the half-angle is preserved. The critical factors in- of the transducer through beam forming volved are the feature's radius of curv- techniques. Figure 2.2 compares the ature relative to the radius of curva- width of a narrow-beam echosounder ture of the echosounder's wave front. An (2-2/30) with that of the wide-beam echosounder will, with reasonable accu- (600) showing that the narrow-beam is racy, resolve a feature if the radius of capable of much finer feature resolu- curvature of the wave front is much tion. larger than that of the feature. How- ever, distortion results when the re- More importantly, however, from the verse is true. standpoint of bathymetry, are the inac- curacies in recorded bottom depth. Ac- In general, wide-beam echosounders (300 cording to Krause [2.13] a transducer half-angle) preserve features whose will receive an echo from any part of wavelength (or width) is greater than the sea floor that is tangent to the the water depth in which they operate. spherical wave front, provided the echo As shown in Figure 2.2, at a water depth takes place within the half-angle of the 2.6 bA transducer; outside the half-angle the trace is less than the actual seafloor sound energy emitted and, therefore, the gradient. Krause [2.13] notes, however, return echos are too weak for detection that for slopes up to 150 the difference purposes. is negligible and that even up to 30* the departure in gradient is not very On a sloping sea floor the echo does not great (about 3°). originate from directly beneath the ship (as in the case of a flat sea floor) but 2.3.4.2 Resolution of Sediment Cover from a point where the slope is tangent to the curvature of the wave front. This Seismic reflection profilers and echo- relationship is demonstrated in Figure sounders operate on exactly the same 2.2 and two observations can be made: principles. The important difference in these two systems is in their sound fre- " With a typical wide-beam echosounder quencies. Seismic reflection profilers (300 half-angle) a 30* gradient is utilize pulses of low frequency sound theoretically the steepest slope that that have the important advantage over would produce an echo trace. For high frequency pulses (used by echo- steeper slopes there is no point of sounders) of being less attenuated by tangency with the wave front (unless traveling through sediment or rock. the half-angle is increased). In Thus, seismic reflection profilers have reality, however, this is not the the ability to display thick sediment case. The geometry of the beam pattern accumulations and deep structure, there- produced by a transducer is such that fore providing a means of determining side lobes exist that are capable of the geometric relationships between the producing bottom echoes. Moreover, sediments and underlying basement struc- slope irregularities produce scatter, ture. The cost of this ability is reso- which is detected by the transducer. lution loss of fine-scale features such As a result of these factors, slopes as thin sediment cover. in excess of 30* can be "seen" but their echo trace may be weak and dif- In general, as the thickness of a layer fuse. becomes less than 1/4 wavelength of the " The point of tangency occurs upslope; sound energy, it is likely to escape thus, there is a downslope horizontal detection. Thus, a thin sediment cover displacement of the recorded depths overlying a rock (basement) surface relative to the ship's position. For would not be seen or, alternately, a example, a depth of about 3000 m is thin layer within a thick sequence of recorded when the ship is actually in sediment would not be seen. The limiting 4000 m of water. This results in an resolution of layer thickness for the apparent horizontal displacement of typical 3.5 kHz echosounder is about 12 approximately 1750 m in the downslope centimeters (cm). Seismic reflection direction for the 3000 m contour. As profilers, however, operate over a range water depth or bottom slope decreases of frequencies, with the most power the point of tangency moves closer falling in the 50-100 kHz range for a toward the vertical, resulting in more "sparker" sound source. As a result, the accurate water depth measurements. theoretical resolution is approximately Clearly, these considerations will 15-30 m. Under optimum conditions (pre- have their greatest effect on the cisely tuned equipment, flat bottom) south wall of the VIT where steep 25 m resolution has been achieved in the gradients are encountered. field. Routinely, however, it is reason- able to expect about 50 m resolution. Nares [2.15] and de Vanssay de Blavous Thus, an area may appear sediment-free [2.16] have also shown that for a sea in a reflection profile but could have floor of constant gradient, the echo an appreciable sediment cover. 2.7 In addition to this, there are numerous 2.4 Results and Interpretations distortions which can occur because of wide beamwidth and the compressional The following discussion presents the wave velocity structure of the sediment principal observations and conclusions cover. An excellent treatment of these of this investigation. distortions can be found in Tucker and Yorston [2.17]. 2.4.1 Bathymetry 2.3.4.3 Navigational Errors The geomorphology of the survey area is summarized by the bathymetric map shown Ship's position was continuously moni- in Figure 2.3 (also see pocket insert), tored using the Racal-Decca Trisponder and selected bathymetric profiles are navigational system, that consisted of shown in Figure 2.4. The profiles, which ship-mounted master unit and two remote are oriented north-south, clearly reveal transponder units on the island of St. the asymmetric cross-sectional shape of Croi:. Calibration of the master and the VIT. The extreme eastern end of the remote units was done on the island trough begins as a narrow, V-shaped prior to survey. This was accomplished cleft (profile I). At about 64*39'W by placing each remote unit a known dis- (Fig. 2.3) the trough, although still tance from the master unit and then mak- narrow, changes into a more U-shaped ing the necessary calibrating adjust- feature with a flatter floor (profile ments. 2). An asymmetrical U-shaped form is maintained for the remainder of the VIT The two remote transponder units were although it is considerably wider west positioned at the east end of the island of about 64*52'W (profile 11). and at the Ham's Bluff lighthouse, re- spectively (Fig. 2.1). The transponders The most impressive feature of the area were omni-directional with a line-of- is the steep submarine slope off the sight range of 35 miles. This was suf- north side of St. Croix. According to ficient for dual transponder coverage of Shepard [2.9], the slope is remarkably all the survey area (Fig. 2.1) discussed steep having an average declivity of 50% in this report except for the "shadow (about 260). Slope calculations, based zone" where reception of the east point only on the uppermost and lowermost transponder was masked by land obsta- slope contours in Figure 2.3, yield a cles: in one case Buck Island and in the somewhat lower range of values (18°- other case the projection of land at 230). These values, however, reflect a Salt River. Within the shadow zone, the smoothly sloping surface with no irregu- ship's position was determined by using larities. Clearly, this is not the case; the Ham's Bluff transponder in conjunc- the narrowing and widening of the con- tion with visual sitings and radar sit- tours is indicative of local slope ir- ings. In those areas where both trans- regularities. To obtain a more realistic ponder signals were simultaneously re- appraisal of the slope, gradients were ceived, position accuracy is estimated calculated at 23 locations over horizon- to be 10-30 m. Where only one transpon- tal distances ranging from 0.2 nm to der signal was received, position accur- 3.25 nm. These calculations reveal con- acy could be as poor as 1/4 mile. In the siderable local variability in gradient, shadow zone directly north of the is- with values ranging from a low of 50 to land, where good visual and radar fixes a high of 470; the upper slope generally were obtained, position accuracy is exhibits gradients in the 25o-35 ° range, somewhat improved, whereas the lower slope has values in 2.8 2 7T -I F- - ---1-; E cn - S I I I Tj - I ~ :r I ~ ~ ~ ,.c E ci E~ I' 92.9 the range of 8-19*. Generally speaking, the upper slope west of Christiansted Harbor is the steepest along the island slope, except off the extreme western end of the island where the contours indicate a steep N-S trending escarpment (perhaps steeper than 500). The steepness of the south wall is a strong indication that it is a fault scarp. Presently, the only direct evi- / / /dence of faulting was reported by Dill \ _ / /[2.8] who observed fault gouging along the south wall. S-East of Christiansted Harbor, the slopes are generally less steep than those to /the west. At approximately 64*40'W the lower slope contours turn gently to the / -north marking the presence of an exten- sive submarine bank (1900 m) that sepa- -rates the VIT from the St. Croix Basin 8 jto the east (Fig. 1.1). North of 18*05'N 7 ,/ the bank shoals (1062 m), narrows, and trends toward the northeast as the Bar- -- racuda Ridge, which forms the southern boundary of the Anegada Passage (Fig. ,\1 1.1). ,The entrance to the Anegada Passage is marked by a prominent northeast-trending reentrant in the contours west of about / 64*45'W and north of 18 0 0'N. A similar, but southeast-trending reentrant or "bight" is formed where the contours of ,/ the south wall turn northward. Unlike the Anegada Passage reentrant, which 3/ marks a major deep water access to the North Atlantic, this southern reentrant shoals rapidly and appears to lose major expression at about 1000 m. S,3,, The northern wall of the VIT is clearly not as impressive as the southern wall, which is steeper and intersects the floor of the VIT at a more acute angle (Fig. 2.4). The trough floor (generally outlined by the 4000 m contour) achieves its major Figure 2. 4. North-south topographic width and deepest depths (nearly 4500 m) profiles across the Virgin Islands Trough. west of about 64*52'W. East of this The numbers correspond to the numbered longitude the floor narrows rapidly and north-south lines in Figure 2.1. shoals, gradually losing expression at 2.10 about 64°40'W where the bottom rises A much smaller, but similarly discontin- from 3800 m to 1900 m over a horizontal uous canyon-like indentation also occurs distance of about 6 nm. directly off Salt River Bay. As shown by Shepard and Dill [2.101, Salt River Can- The regional downslope gradient of the yon is well-developed in its upper trough floor is from east to west and reaches, but our contours show that it north to south. Thus, the axis of deep- loses expression by about 1700 m. est depths is situated at the base of the south wall. These gradients suggest Examination of the bathymetric profiles that the Virgin Islands Shelf may be the reveals considerable fine-scale relief, dominant source of sediment for the VIT generally in the range of a few meters and that sediment trrnsport along the to several tens of meters, which may trough floor is generally east to west. represent shallow gullies cut into the St. Croix slope. This impression is The generally flat floor of the trough fostered by the fact that N/S profiles (1*-20) is interrupted by a prominent typically show a hyperDolated but gener- * topographic high (3743 m) situated at ally smooth, sloping surface, whereas about 17*54'N, 64*48'W (Figs. 2.3, 2.4; E/W profiles (across slope) reveal a profiles 9, 10). The sea floor to the very hummocky, dissected topography. east of this feature is about 300 m This is to be expected since N/S survey shoaler than to the west, indicating lines would essentially parallel linear, that sediment has been trapped behind downslope-trending features, generally the high. missing them and, therefore, show "smooth" slope profiles; E/W survey The contours along the north wall of the lines would cross such features and show VIT do not reveal any significant geo- irregular topography. Hyperbolae, which morphologic features. In contrast, the appear in the N/S profiles, may indicate south wall (i.e., north slope of St. oblique crossing of gullies or, alter- Croix) has a prominent canyon-like in- nately, outcropping rock ledges. dentation at approximately 64*43'W, di- rectly off Christiansted Harbor. Near In most profiles (Fig. 2.4), the south shore the walls of the upper canyon form slope of the VIT maintains nearly the an asymmetric profile, with the west same steep angle and sharply intersects wall sloping more steeply (28*-33 °) than the floor of the trough. This is also the east wall (180). Downslope the walls true for the north wall west of about slope more gently as the canyon broadens 64*52'W (Fig. 2.4, profiles 12 and 13). into a shallow valley. Correspondingly, East of this longitude, however, a clear the floor of the canyon exhibits a steep demarcation between the floor and north upper gradient (11) and a gentler gra- wall of the VIT is not always obvious. dient (5*) below the 2600 m contour. In profiles 7-11 the wall-to-floor tran- These values are in general agreement sition is obscured by irregular, hum- with Shepard [2.9]. mocky topography. Eastward (profiles 1-6) the hummocky area becomes progres- Although Christiansted Canyon can be sively more defined to the extent that traced shoreward cutting through the it forms a topographic entity separate fringing reef off the harbor entrance from the floor and wall. In this re- * [2.101, it is curiously discontinuous in spect, the hummocky area does not appear the downslope direction and shows no to "belong." Indeed, if the north wall topographic expression below a depth of in profiles 1-6 is projected downward to about 2800 m. In this respect, Chris- about the level of the trough floor and tiansted Canyon is atypical relative to then across to the south wall, the pro- other canyons that extend to the base of files resemble profiles 12 and 13 with * the slope that they dissect. the hummocky area clearly appearing 12.11 - .-.- ~ - ---- --. -y anomalous. In addition to this, it is Sediment thickness on the lower north coincident with a dramatic narrowing of wall appears to be about 0.1-0.15 sec, the VIT (Fig.2.3). These considerations but thins to zero as the slope shoals suggest that the hummocky area may rep- and steepens. At the base of the slope, resent a mass of slumped material that in the area of hummocky topography (Fig. has partially covered the floor and 2.3), sediment thickness ranges between lower north wall of the VIT, thus alter- 0.15-0.50 sec, with most values falliva ing its normal configuration as express- in the 0.2-0.3 sec range. In this area, ed farther west (profiles 12 and 13). and the trough in general, it is impos- sible to establish sediment thickness 2.4.2 Seismic Reflection with confidence because of the inability to clearly define the basement reflec- Acoustic basement is usually defined as tor. the deepest continuous reflector ob- served in seismic reflection profiles. The southward tilt of the trough sedi- In most situations, it represents vol- ments is most pronounced in profile L'M' canic or "true" basement and usually (Fig. 2.6) where the subbottom reflec- marks the base of the unconsolidated tors appear to onlap deep structure be- sediments. The seismic reflection pro- neath the area of hummocky topography. files collected during this investiga- This suggests that tectonic movement may tion are presented in Figures 2.5-2.9 have uplifted this part of the trough and are keyed to the track chart shown floor, effectively narrowing the trough in Figure 2.10. The profiles clearly as well as producing the irregular sur- show a variable sediment distribution in face topography. As previously noted, a the VIT. The south wall appears to be large basement high does exist in the virtually sediment-free (see Part axial part of the trough, and there are 2.3.4.2) whereas the north wall shows a suggestions of smaller basement protrus- thin, slightly hummocky sediment accum- ions (profiles L'M' and N'O'), indicat- ulation. In contrast, the axial portion ing tectonism at one time or another. of the trough contains a thick, acousti- Although this provides an alternative to cally laminated accumulation of sedi- massive slumping as the main reason for ment. Cross-sectional profiles of the the anomalous nature of the VIT in this VIT (Figs. 2.6, 2.9) show that the axial area, it does not completely eliminate sediments in the trough form a wedge- slumping. Tectonic movements could shaped body with the subbottom reflec- easily dislodge sediment from the walls tors dipping and thickening southward, of the trough. where they terminate abruptly against the south wall of the trough. This geo- Surprisingly, there is no evidence of metry strongly suggests, once again, major slumping in the western protions that the sediments are derived from the of the VIT (west of 64°52'W) as indi- north, i.e., the Virgin Islands Shelf. cated by the sharp contact the base of East of the topographic high situated in the south escarpment makes with the the trough axis (Fig. 2.8), the trough trough floor (Fig. 2.4). A similar con- sediments range between 0.5-1.0 (0.1 tact is often observed for the north second [sec] approximately equals 100 m) wall as well (Fig. 2.4, profiles, 12 and sec in thickness, but thin as they onlap 13); however, irregular relief usually the high and VIT margins. West of the characterizes the trough floor at the high, the sediments achieve a thickness base of the north wall. Although this of at least 1.5 sec. In general, for the relief, in some cases, resembles slump area west of the high, one could expect topography, more often its form suggests the sediments to be routinely 1.0 sec deep-rooted sediment disturbance related thick or more, except near the margins perhaps to seismicity (see following where the sediments onlap the walls of section). the trough. 2.12 II - 46rJ IPs C 7j rr 00 CC U; E - ~-oj 2.-1 100 L -- . r II - j 1 / -- -. 0 k00' 00 U0 .- ,, 3 -. ,t . .10 "2. ,- 2.14 0*71 0 L E c_~ I--3 cj) r o I-. .. 0 o j 2. 1 2.1W, 6600 55' 50' 45' 40' 35' 30' 64025' 18007' _ _ I .11.111 -. X 50' 14|1 55' Figure 2. 10. Ship's track showing the location of seismic profiles in Figures 2. 5-9. The lines are shown in different patterns as a visual aid. The letters correspond to the letters in Figures 2. 5-9. The floor of the VIT strongly resembles that the trough has probably experienced a turbidite plain. This is indicated not a long, relatively uneventful period of only by its generally smooth surface, pelagic deposition randomly punctuated but particularly by the thick and con- by eposides of turbidity current deposi- formably bedded sequence of reflectors tion. that make up the sedimentary section and is characteristic of regions of known It is important to add that each "re- turbidite deposition (abyssal plains). flector" seen in the profiles of the One can surmise, therefore, that the trough sediments does not necessarily axial sediments of the VIT consist of correspond to an individual turbidite sand/silt layers alternating with layers layer. If a sound pulse produced a of fine-grained terrigenous clay miner- single wavelet, then each turbidite als and calcium carbonate. In those layer would indeed produce a single re- areas where the floor of the trough ap- flector. The sound pulse, however, pro- proaches 4500 mn, lower carbonate con- duces a complex wave train. Therefore, tents can be expected (relative to the interaction of sound with a turbid- shoaler areas) because this is approxi- ite layer results in several "reflcc- mately the depth at which major dissolu- tors." For this reason, the extensive tion of calcium carbonate occurs. The sequence of reflectors seen in the seismic reflection profiles reveal no trough sediments is only indicative of major acoustic discontinuities that the presence of numerous turbidite might represent periods of erosion, non- layers. The same explanation applie, to deposition, tectonism, etc., indicating any major reflecting surface such as the 2.16 seafloor itself. Indeed, even steep A distinctive gully pattern does not slopes appear to have a veneer of sedi- appear in the seismic profiles that cut ment because of this artificial layering across-slope west of Christiansted effect. Canyon. Some gullying of this part of * the slope is indicated in the 3.5 kHz The seismic reflection profiles also echograms; however, downslope movement indicate, like the bathymetric profiles, is probably more uniformly distributed that the south wall of the VIT is along the slope, with the result that creased by gullies. Cross-sectional pro- gullies (if any) are probably small and files show a uniformly smooth slope widely spaced. Of particular signifi- whereas across-slope profiles show a cance is the absence of a well-defined hummocky topography. This is well illus- gully pattern off Salt River Canyon, trated by Figure 2.6. Profile H'I'; is implying that this canyon is not a major oriented N/S and shows a smooth, steep conduit for sediment transport. escarpment. Profile I'J' marks an abrupt change in direction to E/W and shows a The system of gullies on the lower slope * correspondingly abrupt change to hum- off Christiansted Canyon can be attrib- mocky topography. Two hummocky areas on uted to the premature termination of the the lower slope (Fig. 2.3) eventually canyon at midslope. The gullies can be merge into one area extending to the compared, in a sense, to the network of trough floor. The westernmost area is turbidity current channels that develop clearly associated with Christiansted off the mouth of a major river. Erosion- Canyon. The last profile (Z'A' Fig. al down-cutting of both canyons was 2.7), showing the final vestige of the probably stopped, or at least was dra- canyon, occurs at 17*51.5'N (Fig. 2.3). matically reduced, by the existence of The next traverse (profile VW, Fig. 2.5) more resistant material at this point on reveals a broad area of low, hummocky the slope. topography directly in front of the * canyon. Successive profiles show that 2.4.3 Seismicity the hummocky area broadens in a fan- shaped pattern and merges with the Holcombe et al. [2.6] have summarized eastern hummocky area. This last area, existing information concerning the coincides with the prominent reentrant, seismicity of the St. Croix area. Since or bight, formed by the contours off the then, a considerable amount of addition- eastern end of the island (Fig. 2.3). al information has been gathered due to the establishment of a network of seis- It is apparent that sediment and rock mic stations (2.191. Although the basic debris are being transported downslope tectonic interpretation of the area re- via a few large, well-defined conduits mains unchanged, the network has allowed on the upper slope, but on the lower the seismicity of the Virgin Islands * slope, this material is transported to region to be resolved with an accuracy the floor of the VIT by a series of and sensitivity previously unavailable. small gullies. The possibility that the hummocky topography actually represents Most of the seismic activity occurs slumps seems unlikely because one could north of the islands and is associated then expect rough topography to appear with the boundary between the North * in N/S as well as E/W profiles. American and Caribbean lithospheric plates. Superficially, this boundary is It is evident (Fig. 2.3) that the gul- expressed as the Puerto Rico Trench; at lies feed sediment directly to the depth it is manifest as a downward- narrow, eastern portion of the VIT where sloping (45° ) zone of shallow to inter- it has obviously been contained (Fig. mediate focus earthquake foci that ex- * 2.8, profile FG) by the topographic high tends beneath the Virgin Islands Shelf. that "plugs" this part of the trough. 2.17 - V Earthquake foci associated with the VIT The results from the seismic network and are all shallow events (less than 20 km earlier analyses [2.23] indicate that deep) and represent intraplate seis- the north wall is the major focus of micity not directly related (if at all) seismicity for the VIT area and that it to the downgoing seismic zone. The VIT, is presently active. as noted, appears to be a manifestation of north-south extension, however, it 2.4.4 Dredging may also accommodate an important com- ponent of strike-slip motion [2.201 During the LYNCH cruise, seven attempts along the north wall, which is coinci- were made at dredging the north slope dent with the Anegada Fault Zone. off St. Croix to determine the nature of the slope (i.e., rock outcrops, loose Holcombe et al. (2.6) concluded that rock debris or talus, mud, etc.); Only weak events in the VIT region probably two attempts yielded samples. This ma- occur daily, but are detectable only terial consisted of a few cobble-sized, with sensitive instrumentation; that dark rock fragments and occasional moderatly strong events (intensity 4-6) coral. The angularity of the rock frag- occur infrequently (perhaps 3 or 4 per ments indicated that they had experi- 200 years); and that major events (in- enced little reworking (such as in tensity 7 or more) are rare. The VIT has shallow-water, high energy environments) a history of destructive seismic activi- and may have been derived from submarine ty as shown by the two major earthquakes outcrops. Although the ship was firmly (intensity 7-7.75) it produced in 1867 anchored by the dredge on two occasions [2.21]. Significantly, in nearly 400 (indicating that the dredge was snagged years of recorded history there is no on an outcrop of rock), wire tensions record of a great earthquake (intensity were generally low as the dredge was 7.8 or more) in the area of the plate dragged across the bottom, suggesting margin between 63°-67*W, leading Frankel that rock outcrops were not abundant. In et al. [2.22] to speculate on whether addition, the meager sample recovery such an earthquake can ever occur in suggests that large debris (10-20 cm and this area. more) is not abundant on the slope. These conclusions are generally support- Perhaps the most significant observation ed by the visual observations discussed revealed by the seismic network was the in the following section. frequent occurrence of swarms of earth- quakes, that is, several events (5- 2.4.5 Visual Observations 100's) produced within a small source area (less than 10 km) over a few days Bottom photographs taken during the [2.22]. Most swarms occurred along 19*N BARTLETT cruise and principally by the between 64*W-65.2°W and were associated submersible ALVIN (January, 1980) pro- with the plate boundary. Major swarms, vide the most direct information con- or sequences whose largest event was of cerning the seafloor environment of the magnitude greater than 4, only occurred VIT. Camera stations and dive sites are in this area. located in Figure 2.3. Three swarms were associated with the Photographs taken of the trough floor VIT. One occurred south of Vieques at (Fig. 2.11A) are monotonously similar in about 17.90N, 65.50*W and a second about appearance, all conveying the impression 15 km to the southwest. The third, and of a flat, soft bottom that has been most active swarm area, was centered at disturbed only by the activity of bottom about 18.1*N, 64.9*W south of St. dwelling creatures, as shown by the pre- Thomas, roughly at the base of the north sence of numerous small mounds, depres- wall. Events with intensities of 3.2 sions, tracks, and trails (lebensepur- were recorded. en). None of the photographs show any 2.18 0L. I zz. L. -z C -z 9 2.19 sign of the sedimentary microrelief as- gullied. N/S trending ridges and valleys sociated with bottom current activity; were frequently observed and produced a thus, tranquil bottom conditions are swale-like, or undulatory, topography on indicated. some areas of the slope. Estimated height of the ridges was 2 m or less Moderate to abundant lebensspuren are with a spacing of about 10 m, and they displayed in the trough photography, appeared to be whitish outcrops of clay- suggesting a large animal population. like material. Owens et al. [2.24] have noted, however, that in areas of low sedimentary deposi- Large outcrops of massive clay-like ma- tion, animal signs may be preserved. terial produced a hummocky cliff/terrace Thus, bottom photographs may show the topography between water depths of 3850- sum of animal activity over a very long 3530 m. Slopes on the smooth terrace period of time. Because the lebensspuren areas appeared to be gentle (10*) and on the trough floor are plentiful and sediment covered, whereas the terrace biota is sparse, many of the lebensspur- faces were considerably steeper, forming en may be quite old. Some lebensspuren cliff-like escarpments (Fig. 2.11C) sev- are well-defined, while others have a eral meters high in some cases. Attempts smoothed appearance. Such selective to sample fragments of lightcolored mat- smoothing by bottom currents is unlike- erial similar to the outcrops showed ly. It is reasonable to conclude, there- that it was soft and easily broken. fore, that many of the lebensspuren are indeed old, and have been gradually Visual observations indicated that the smoothed by slow pelagic deposition, and bottom sediment was fine-grained. Analy- that destructive turbidity currents have ses of collected samples by Hubbard et not been recently active. al. [2.11] showed that sand-sized mat- erial generally accounted for about 25% The most extensive photographic coverage of the total sediment. Mean grain-size focused on the south wall of the VIT varied between 0.0156-0.0625 m, indicat- where the submersible dives were made ing that the sediment was primarily a off Christiansted Canyon, Salt River medium to coarse silt. The existence of Canyon, and Cane Bay (Fig. 2.3 ). appreciable finer-grained sediment, how- Approximately two hours of continuous ever, is indicated because the submer- videotape (black and white) coverage was sible frequently raised a cloud of sed- collected at each site, in addition to iment which remained suspended. Predict- numerous still color photographs. The ably, grain size generally decreased visual information and the written logs toward the deep basin. The concentration are condensed in the following para- of shallow-water constituents (coral, graphs. coralline algae fragments, ostracods, benthic foraminifera, molluscs, and 2.4.5.1 Christiansted Canyon rounded rock fragments) ranged between 7-43%, increasing toward the island. Site A, located off Christiansted Canyon occurred in water depths of 3950-3503 m. Very coarse-size debris generally con- A typical photograph of the bottom is sisted of pebble (a few millimeters) to shown in Figure 2.1lB. In general, vis- boulder-sized (up to 60 cm) rock and ual estimates of the slopes ranged from coral fragments widely scattered over 10* to 30* with most observations in the the slope. Occasionally, this debris 12*-20* range. These estimates are in occurred in slightly greater concentra- good agreement with the slope calcula- tions, and heavy accumulations were ob- tions based on the bathymetric map (Fig. served in what appeared to be channels 2.3). The observations also confirm that or gullies. the lower slope off the canyon is indeed 2.20 r 0 .21 0 C,, 1.0 0 0 0~ C. 0 0 E 0. r 0 C) C', '-4 -4 2. 2i 2.4.5.2 Salt River and gravel also form a significant por- tion of the sediments (Fig. 2.11F). Several features off the Salt River Can- Sediment analyses [2.111 show that silt yon area (Site B) distinguished this and clay-sized material comprised * part of the slope from the Christiansted roughly 50-75% of the total sediment on Canyon area. Although the maximum esti- the lower third of the dive transect mated slope was only 5* greater off Salt with mean grain-size falling between River Canyon (35* versus 30*), the 0.0156-0.125 mm. Their results for the slopes were consistently steep (20*- remaining two-thirds of the transect 300). This was also evidenced by fre- show a sharp increase in coarse consti- * quently observed zones of sediment tuents, with sand and gravel together cracking, suggesting slope instability, comprising roughly 80-90% of the sedi- The cracks appeared as deep, narrow, ment and mean grain-size ranging between linear features. Slope instability was about 1-16 mm. Shallow water constitu- further demonstrated by the submersible ents were consistently more abundant touching bottom and causing the sediment (50-93%) than off Christiansted Canyon. * to crack. Much of the gravel consisted of pebbles, fist-sized cobbles, and boulders as Another striking feature is the much large as 50-100 cm (Fig. 2.11G). In greater amount of coarse debris (Fig. areas of abundant cobbles the highest 2.11D), noticeably, a gravel component concentrations were estimated at 2-3 (up to 7 mm) comprised primarily of cobbles/meter; in areas of high concen- coral rubble and rounded, terrigenous traLion, boulders were spaced about 8-10 clastic pebbles [2.111. An appreciable m apart. number of cobble and boulder-sized fragments were also observed (maximura Estimated slope gradients were generally size 80 cm x 150 cm). intermediate (15*-250) to the Salt River and Christiansted Canyon slopes. Al- Perhaps the most significant feature of though the Cane Bay slope appeared rela- the area is the existence of a hard sub- tively uniform and topographically mon- strate or "crust" (Fig. 2.11E) that is otonous, it was marked in certain areas generally covered by about 10-15 cm of by a swale-like topography produced by sediment. Attempts to retrieve sediment low ridges. Most impressive was the oc- cores (by ALVIN as well as BARTLETT and currence of a large outcrop of bedded LYNCH) were largely unsuccessful because (layered) rock at 3085 m that formed a of the coarse nature of the sediment and vertical escarpment of 7-10 m (Fig. also the hard, impenetrable crust. In 2.11H). The bedding planes dipped toward outcrop the crust appears to be cemented the island, which is consistent with the calcareous material at least 10 cm regional dip of the exposed island rock. thick. The existence of the crust indi- The dark color and bedded appearance of * cated that the "failure" cracks previ- the outcrop suggest that it could be a ously noted could not be too deep unless part of the thick Caledonia Formation the crust has cracked as well. that outcrops on St. Croix Island. The presence of small ridges indicated A hard substrate or crust, covered with that this portion of the south slope may a thin sediment veneer, was also ob- * also be gullied; however, this was not served in this area. Several cores suc- as apparent as off Christiansted Canyon. cessfully broke through, suggesting that the thickness and/or hardness of the 2.4.5.3 Cane Bay crust to variable; soft sediment occur- L red beneath the crust. The slope off Cane Bay (Site C) resem- bles the Salt River area because sand 2.23 jII 14) CL ri. 0 2.24 • . 2.25J 040 4.4 2.26 p I * 0 U, 0~~ C * 0 I C L 4. C * 4-aC I~ C. * C.) U) p $4 -.4-4 C.,' I L iz I I 2.27 75 JAL 2.4.5.4 Overview fragments, as well as sea grass) is con- vincing evidence of their shallow water The previous discussion highlights the origin. Hubbard et al. [2.22] indicate major features observed at each dive that the sands and gravels are trans- * site. Consequently, one can easily de- ported off-shelf as bedload during velop an exaggerated mental image of the storms. Gravitational settling dictates slope environment. As a general overview that these components deposit preferen- and with the intent of establishing per- tially on the upper slope area although spective, it is important to note that with time they could work downslope. the visual impression of the bottom con- Hubbard et al. [2.11] also suggest that * veyed by the videotapes is that of a transport of large blocks of material generally monotonous, subdued terrain occurs only during major storms or per- covered by predominantly muddy, biotur- haps were carried off during the low sea bated sediment (similar in appearance to level stands of the Pleistocene. Once on Fig. 2.11B.). This is true even off the slope, however, it is not clear Christiansted Canyon where clay outcrops whether these larger materials roll or * and gullied topography were most appar- slide continuously down slope before ent. The most imposing topographic fea- coming to rest, undergo gradual down- ture was the large outcrop off Cane Bay slope creep, or are carried down in (Fig. 2.11H), although some of the clay slumps. It is reasonable to suspect all outcrops off Christiansted Canyon were of these mechanisms. impressive. Most surprising, however, is that large outcrops were not encountered Many of the larger blocks, perhaps most, more frequently. It was difficult to are not necessarily derived from shallow distinguish in the videotapes coarse water areas, but rather from submarine debris smaller than about 10 cm. Bould- outcrops at various places on the face ers and large cobble-size debris, how- of the slope. Although outcropping clay ever, appeared generally sparse and ledges were observed on the lower slopes widely scattered. off Christiansted Canyon, outcrops of hard rock formations were conspicuously 2.4.6 Processes absent over the sea floor traversed by ALVIN (except for the single, large out- It is apparent that a variety of sedi- crop off Cane Bay). Outcrops would prob- ment processes are, or have been, active ably occur more frequently on the upper in the survey area. The visual observa- slopes, such as off Salt River Canyon, tions and sediment analyses clearly show where gradients are steeper. The angu- that the steep south wall of the VIT is larity of many of the rocks retrieved covered by a veneer of relatively fine- during dredging operations is more indi- grained sediment (predominantly silt) as cative of material having "calved" off well as appreciable amounts of coarse the face of an outcrop (perhaps during debris consisting primarily of sand and seismicity) than material that resided gravel, and occasional large, massive in a shallow water area where it could blocks of material, undergo significant reworking before reaching the shelf edge. Indeed, it Pelagic deposition of fine-grained bio- would be difficult to imagine some of genic and terrigenous material can ac- the larger blocks of material being count for some of the sediment found in moved to the shelf edge even by large the deep basin and on the slopes. How- storms. ever, this mechanism does not account for the sands, gravels, and coarser ma- Slumping, together with turbidity cur- terials which occur within the area. The rents, represent obvious and catastro- nature of these materials, (coral, cor- phic mechanisms to distribute coarse as alline algae fragments, rounded rock well as finer sediment downslope. Cracks 2.29 observed in the slope sediments are on a global scale. Since then, erosion highly indicative of slope failure or (and turbidity current activity, slump- incipient slumping although, as noted in ing, etc.) has greatly diminished. In Part 2.4.2, evidence indicates that this regard, the gullied face of the St. slumping is not a major process. Despite Croix slope (and perhaps the canyons the steepness of the slope off St. themselves) may be a relict of the Croix, the sediment thickness may not be Pleistocene as well. great enough to develop sufficient shearing stress for large-scale, sponta- neous slumping to occur. On the other References for Part 2 hand, slumping may be a significant process on the north wall of the VIT, which is also steep, has a relatively thick sediment cover and is the locus of [2.1] Hess, H. H. (1933). Interpreta- seismicity in the VIT. Indeed, the seis- tion of Geological and Geophysi- mic reflection profiles of the north cal Observations. U.S. Hydro- wall (Figs. 2.6 and 2.9) show terr- graphic Office Navy-Princeton ace-like steps which may be slump scar Gravity Expedition to the West features. Indies, 1932, p. 27-54. Turbidity currents and small-scale chan- [2.2] Shurbet, G. L., J. L. Worzel, and nelized flows may be an important sedi- M. Ewing, (1956). Gravity Measur- meait transport mechanism on the slope ements in the Virgin Islands. off St. Croix as indicated by the exist- Geol. Soc. Amer. Bull., V. 67, p. ence of gullies and canyons, and by the 15291536. observation that the floor of the trough is a turbidite plain. [2.3] Shurbet, G. L. and J. L. Worzel, (1957). Gravity Anomalies and Despite the abundant evidence of down- Structure of the West Indies, slope sediment transport, two facts sug- Part III. Geol. Soc. Amer. Bull., gest that sediment from St. Croix and V. 68, p. 263-266. its surrounding shelf area does not con- tribute as significantly to the basin [2.4] Officer, C. B., J. I. Ewing, J. sediment as one might expect. First, F. Hennion, D. G. Harkrider, and only a small depositional fan occurs at D. E. Millor, (1959). Geophysical the base of the slope off Christiansted Investigation in the Eastern Car- Canyon. Second, the tendency for the ibbean--Summary of 1955 and 1956 trough floor and subbottom reflectors to Cruises, p. 17-109. In: Ahrens, slope southward strongly implies north- L. H. et al. (Editors), Physics to-south infilling of the trough. St. and Chemistry of the Earth, V. 3, Croix clearly contributes some sediment London, Pergamon Press, 464 p. to the VIT, however, the amount is prob- ably vastly overwhelmed by material de- [2.5] Stalcup, M. C., W. G. Metcalf, rived from the Virgin Islands Shelf to and R. G. Johnson, (1975). Deep the north. The shallow water area of Caribbean Inflow Through the Ane- this shelf (200 m or less) provides an gada-Jungfern Passage. J. Mar. order of magnitude greater source area Res. (Supplement), p. 15-35. than the St. Croix shelf. 12.6] Holcombe, T. L., A. M. Einwich, Much of the basin fill is probably a F. A. Bowles, and J. Egloff, Jr., product of Pleistocene deposition. It is (1977). The Geological Environ- generally acknowledged that existing ment West of St. Croix. NORDA turbidite plains formed largely during Report 5, Naval Ocean Research the Pleistocene when erosion increased and Development Activity, Bay St. Louis, Miss., 82 p. 2.30 - - ., , '* ? [2.7] Frassetto, R. and J. Northrop, ing. Hydrographic Rev., v. 7, p. j (1957). Virgin Islands Bathymet- 50-63. ~ric Survey. Deep-Sea Res., V. 4, p. 138-146. [2.171 Tucker, P. M. and H. J. Yorston, p 3 4(1973). Pitfalls in Seismic In- [2.8] Garrison, L. E., R. G. Martin, terpretation. Monographic Series, Jr., H. L. Berryhill, Jr., M. W. No. 2, Society for Exploration Buell, Jr., H. R. Ensminger, and Geophysicists, Tulsa, OK., 50 p. R. K. Perry, (1972). Preliminary Tectonic Map of the Eastern [2.18] Dill, R. F. (1977). Deep Water * Greater Antilles Region. U.S. Erosional Feature Bedrock of Geol. Survey, Map 1-732. Geology of St. Croix, U.S. Virgin Islands as seen from the research [2.9] Shepard, F. P. (1979). Submarine submersible, ALVIN. Abstracts, Slopes and Canyons on North Side VIII Caribbean Geological St. Croix Island. Mar. Geol, v. Conference, Addendum, Geologisch * 31, p. M69-M76. Instituut (Amsterdam), Curacao, Dutch West Indies. [2.10] Shepard, F. P. and R. F. Dill, (1977). Currents in Submarine [2.19] Murphy, A. and W. McCann, (1979) Canyon Heads Off North St. Croix, Preliminary Results from a New U.S. Virgin Islands. Mar. Geol., Seismic Network in the Northeast v. 24, p. M39-M45. Caribbean. Bull. Seismol. Soc. Amer., v. 69, p. 1497-1513. [2.11] Hubbard, D. K., T. H. Suchanek, I. P. Gill, S. Cowper, J. C. [2.20] Hess, H. H. and J. C. Maxwell, Ogden, J. R. Westerfield, and J. (1953). Caribbean Research pro- Bayes, (in press). Preliminary ject. Geol. Soc. Amer. Bull., v. Studies of the Fate of Shallow- 64, p. 1-6. * Water Detritus in the Basin North of St. Croix, U.S.V.I. Proceed. [2.21] Reid, H. and Taber, S., (1920). 4th Internat. Coral Reef Sympo- The Virgin Islands Earthquakes of sium, Manila, 1981. 1867-1868. Bull. Seismol. Soc. Amer. v. 10, P. 9-30. [2.12] Hoffman, J. (1957). Hyperbolic * Curves Applied to Echo Sounding. [2.22] Frankel, A., W. R. McCann, and A. Internat. Hydrographic Rev., V. J. Murphy, (1980). Observations 34., p.45-55. From a Seismic Network in the Virgin Islands Region: Tectonic [2.13] Krause, D. C. (1962). Interpreta- Structures and Earthquake Swarms. tion of Echo Sounding Profiles. J. Geophys. Res., v. 85, p. 2669- Internat. Hydrographic Review. V. 2678. 39, p. 65-123. 39 [2.23] Sykes, L. R. and M. Ewing, [2.14] Flood, R. D. (1980). Deep-Sea (1965). The Seismicity of the Sedimentary Morphology: Modeling Caribbean Region. J. Geophys. and Interpretation of Echo-Sound- Res. v. 70, p. 5065-5074. ing Profiles. Mar. Geol. v. 38, * p. 77-92. [2.24] Owens, D. M., H. L. Sanders, and R. R. Hessler, (1967). Bottom [2.15] Nares, 3. D. (1933). Echo Sound- Photography as a Tool for Estima- ing. Hydrographic REv., v. 10 p ting Benthic Populations. In: J. 38-40. B. Hersey (Editor), Deep-Sea Pho- * [2.16] de Vanssay de Blauous, P. (1930). tog. Johns Hopkins, Baltimore, Slope Corrections for Echo Sound- MD. p. 229-234. 2.31 PART 3 GEOTECHNICAL INVESTIGATIONS R.H. Bennett D.N. Lambert G.F. Merrill F.L. Nastav 3 S $ S S] 3. 1 I PART 3. GEOTECHNICAL INVESTIGATIONS Geotechnical investigations were carried 3.2 Area of Investigation out in a selected area on the northern * margin of St. Croix, Virgin Islands, Station locations (Table 3.1) were integrating geology and geophysics chosen to collect representative sedi- (Part. 2) to provide fundamental data ment samples from the shelf, slope, for offshore engineering applications on trough, and "high" within the area of a carbonate island margin, interest (Fig. 3.1). Limited ship time precluded a higher sampling density, The general nature of the geotechnical which would be suggested for site spec- properties of carbonate oozes and their ific areas of engineering concern. variability are of interest in this in- vestigation. Very limited geotechnical 3.3 Types of Sediment Sampling, Data studies of carbonate sediments occur in Collected, and Limitations the literature, particularly for depos- * its found on steep submarine slopes. Not Seven piston cores, 14 hydroplastic only are these properties important to gravity cores, and 19 Shipek grab sam- understanding the nature of the sedi- ples were recovered for geotechnical and ments, but this and the morphological sedimentological testing and study. The setting of the sedimentary deposits are piston and hydroplastic corers used pol- shoegierng aciiyutlzn the and.*L.Te V ubswr used a important aspects for virtually any off- yvinyl chloride (PVC) tubes (See Part shore engineering activity utilizing the 3.4.2.ih. The PVC tubes were used as sea bed. liners in a modified Ewing Piston Corer and were used as the coring sleeve 3.1 Earlier Investigations (barrel) for the hydroplastic device [3.4, 5]. These coring tools recovered Two earlier investigations addressed the high quality sediment samples as re- * geology and sedimentology of the north vealed by x-radiographs made aboard slope of St. Croix [ 3.1, 2 ]. Geotechni- ship. Details of the sedimentary struc- cal measurements were reported [ 3. 3 1 for ture and bioturbation can be observed sediments on the Fredericksted Plateau readily in the radiographs. Soil testing off the west end of the island, but such procedures applicable to submarine sedi- studies do not exist for the remaining ments can be found in earlier reports offshore areas of the island or the VIT. [3.6, 7] and numerous textbooks on soil This investigation is concerned with the mechanics [3.8, 9]. Shipek grab samples engineering properties of the surf icial were used at sites where core recovery sediments in relationship to the morpho- was not possible because of thin sedi- logical characteristics and setting. ment thickness or coarse sediment tex- tures. These conditions often prevented Figure 3.1 depicts the geological envi- corer penetration. Shipek grab sampling ronments of interest including the outer of muds often recovered relatively in- shelf and slope off the north side of tact samples but disturbance prohibited St. Croix Island and the adjacent VIT testing of the mechanical properties where bottom sampling and coring opera- such as shear strength. Of the 40 sample tions were conducted. Water depths range stations occupied (Table 3.1), the high- from a few tens of meters on the shelf est quality samples were tested in de- to greater than 4000 a in the trough. tail for mass physical and mechanical _ 3 .3 i M MnM 6A M , M N i U - -> * - L.%1 1706N 3743 , MAO0 43B e a IT 3e 01 1~ 553,0 02 04 1,%1 05 01111012 64W64050' 64045' 64040' Figure 3. 1. Bathymetric map of north slope, St. Croix Island and adjacent Virgin Islands Trough; contours are in uncorrected meters; contour interval is 100 m; bottom sample sites are indicated by solid dots and indexed by numbers. properties as reflected in Table 3.2. shear (consolidated-drained) tests and Other samples having sufficient sediment one triaxial (consolidated-undrained) volume, such as grabs and short hydro- compression test were made on selected plastic core samples, were tested only VIT sediment samples. Soil sensitivity for grain size and, in some cases, was determined from natural and remolded Atterberg limits were determined. The vane shear tests for only the upper 1 to various figures throughout this report 2 m of core lengths. Water content, po- reflect the extent of the soil testing rosity, wet unit weight, grain specific during this study. Miniature vane shear gravity, and Atterberg limits analyses tests were made aboard ship and numerous were performed on numerous samples with subsamples were collected for shorebased the greatest subsampling and testing testing. A limited number of direct also in the upper meter. Selected data 3.4 Table 3.1. Core locations. Sample Number Lat(N)/Long(W) Sample Locations Sample Device 1 17048.2, 64054.7 1885 Grab 2 17047.5, 64052.9 1389 Grab 3 17049.0, 64046.0 1441 Grab * 4 17047.6, 64046.7 100 Grab 5 17046.9, 64041.0 42 Grab 6 17048.2, 64041.3 1262 Grab 7 17048.0, 64041.5 1071 Grab * 8 17048.0, 64044.6 25f Grab 9 17047.2, 64047.9 81 Grab 10 17047.5, 64044.8 70 Grab 11 17047.4, 64044.7 57 Grab * 12 17046.3, 64045.4 68 Grab 13 17042.4, 64044.6 44 Grab 14 17048.0, 64044.7 88 Grab 15 17048.3, 64044.4 760 Grab 1 16 17048.8, 64045.0 1006 Hydroplastic 17 17049.6, 64045.2 1745 Hydroplastic 18 17048.8, 64044.4 1293 Hydroplastic 19 17048.4, 64044.2 1289 Hydroplastic £ 20 17050.0, 64044.6 2651 Hydroplastic 21 17049.2, 64044.3 1568 Grab 22 17056.2, 64 044 .9 3819 Piston 23 17054.3, 64043.8 3859 Piston $ 24 17054.3, 64043.8 3859 Hydroplastic 25 17055.4, 64048.0 4005 Piston 26 17055.4, 64048.0 4005 Hydroplastic 27 17053.6, 64047.0 3724 Hydroplastic S 28 17048.0, 64047.5 730 Grab 29 17050.0, 64048.3 3131 Arab 30 17054.5, 64048.2 4098 Piston 31 17054.5, 64048.2 4098 Hydroplastic - 32 17054.3, 64047.8 3707 Piston 33 17054.3, 64047.8 3707 Hydroplastic p IP 3.5 , 44LA - Table 3. 1, continued. Core locations. Sample Number Lat(N)/Long(W) Sample Locations Sample Dcvice 34 17053.5, 64047.9 3911 Hydronlastic 0 0 35 17 53.5, 64 47.9 3911 Piston 36 17 052.4, 64 047.8 3949 Hydroplastic 00 38 17051.9, 640 44.7 3659 Hydroplastic 39 17 054.2, 64044.7 3868 Hydroplastic 40 17 054.2, 6,f44. 7 3868 Piston 41 17050.8, 650 00. 8 4354 Hydroplastic 42 17 050.8, 60 00. 8 4354 Piston '13 17 051.4, 640 56.7 4371 Hydroplastic 44 17051.4, 640 56.7 4371 Piston 45 17050. 3, 640 55. 1 4334 Hydroplastic 46 17 050.6, 64055.3 1334 Piston 47 17 047.5, 65 001.0 900 Hydroplastic 48 1075 6001.0 900 Piston 49 17 045.1, 6!? 06.2 1150 Hydroplastic 50 17045.1, 65006.2 1150 Piston 51 17 053.7, 6005.4 4350 Piston 52 17 048.1, 60 05. 3 3261 Hydroolastic 53 17048.1, 6005.3 3261 Grab 54 17 048.6, 650 03.6 3915 Hydroplastic 55 17 048.6, 650 03. 6 3915 Grab 56 17 048.6, 6002.4 2926 Hydroc~lastic 00 57 17048.6, 65 02.4 2926 Grab 00 58 17049.0, 64 59.9 2155 Hydroplasti c 59 17 047.6, 64 058.4 1396 Grab 60 17 047.8, 640 56.2 2495 Hydroplastic 61 17 058.2, 65053.9 20 Grab 62 17 058.2, 60 53.9 20 Grab 63 17 058.2, & 53.9 17 Grab 64 17 057.7, 65 055.2 35 Grab 65 17 057.7, 65 055.2 28 Grab 3. 6 Table 3. 1, continued. Core locations. S Sample Number Lat(N)/Long(W) Sample Locations Sample Dev.ce 66 17054.2, 65057.9 45 Grab 67 17 057.9, 65 054.2 91 Grab 68 17058.0, 6& 5 4 . 2 34 Grab 69 18000.5, 65053.9 417 Grab 70 17057.6, 65053.5 825 Grab 71 17057.4, 65053.8 822 Hydroplastic 72 17057.2, 65053.6 1006 Hydroplastic 73 17056.8, 65052.8 1354 Grab 74 17056.8, 65052.8 1354 Hydroplastic 75 17056.0, 65051.9 1750 Grab 76 17055.9, 65052.0 1790 Hydroplastic 77 17055.8, 65054.2 1500 Hydroplastic 78 17056.3, 65054.5 1207 Hydroplastic 79 17056.9, 65°53.1 890 Hydroplastic 80 17056.9, 65053.6 1200 Hydroplastic 81 17052.2, 65051.8 1920 Hydroplastic 82 17052.2, 65051.8 1920 Piston 83 17053.9, 65049.7 1899 Hydroplastic 84 17053.9, 65049.7 1899 Piston 85 17056.3, 65052.1 1674 Piston 86 17057.2, 65053.9 1100 Hydroplastic 87 17057.2, 65053.9 1100 Piston C Footnote - Samples 41 through 60 were taken from the West St. Croix Margin. Samples 61 through 87 were taken from the south Puerto Rican Margin. Data from these samples are not included in this report. C * 3.7 .-- -- . ~j- :ro on un c A n LA .c Ln LAI Ln -c Ln Ln Ln C%.i C%4 J l ~ cici ci 0 CC - = LAA ~ D CL4 c~ m NJ dcs l l CD m -n m On NJ m t m -l Cl J N .ID 11: ID ID rl- LA % D L D I D - D LA LA LA %D ko LA ID- LA ko D to WD L p- to LA LA k. N ND % O, U- ID koI ~ . N C r C O N. m . c w D NJ C' LO m D w L, (3 E -- w LA ID ID ko -D Ln ID ID N.%D -cr ID ID C- ccc -j -) 0 %D- -- - - -cc 3m co ko (7% -- O %0 - .2 r ;t to LAL IDLAI LA L ID4AL I iD Zr - - - l w - MI. N.0 0 L A -: - In e ! C J rl ID N.I C-1; LJ u) 31 2w C)f 0 V)V n.-k 0 k N. N. NJ &n %0 L cc LID L.Jn . N. L N . . I~ LA ' - N , LA N.N.N ~~e r- LA 00 L.J L ' L ~ c Zr ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ c C).L AL Ae~(~ AI d A(~ A L Cj LA '0 1A ID l +J Z3. are reported for the upper 5.5 m of sur- bathymetric profiling in very shallow ficial sediments, however, greatest em- water near shore (see Fig. 3.1 for sam- phasis in this report is on the upper 1 ple locations). m. Large diameter piston cores, 20- and 3.4 Technical Approach, Equipment, 40-feet long, were handled using a Procedures 20,000 ton A-frame and 10,000 pound ca- pacity crane on the aft boat deck. Pis- Soil samples were collected aboard the ton cores were assembled, deployed, and NOAA Ship RESEARCHER in August 1981. recovered using a piston core cradle Concurrently geophysical (narrow beam [3.10] mounted on the starboard side and echo sounding and seismic reflection) aft area of the ship. Short PVC hydro- profiling was completed and used for plastic cores and ship grab samples were establishing sampling locations. (It is handled from the port oceanographic important to note that the narrowbeam winch. Upon recovery, soil samples were data could not be incorporated into Fig- numbered, sectioned, capped, sealed, and * ures 2.3 and 3.1. It is not believed, moved into the oceanographic laboratory however, that its inclusion would have aboard ship for further processing. Sam- greatly altered the contours.) Samples ples were kept in cold storage aboard were recovered from soil deposits repre- ship (when not being processed) for sentative of the major morphological shipment to the shore-based laboratory features from the area of investigation (NOAA, AOML) in Miami, Florida. * on the northern margin of St. Croix, V.I.: the narrow shallow water carbonate 3.4.2 Equipment and Techniques shelf; the steep slope; and the deep-sea basin. Shallow water reef material and The following subsections provide spe- carbonate debris occurs along the shelf. cific information relating to the equip- A thin carbonate ooze overlies the steep ment and procedures used for this in- slope where occasionally rock outcrops vestigation. occur. The trough sediments are clayey silts and turbidite deposits. 3.4.2.1 Field Equipment and Shipboard Techniques 3.4.1 Surveying and Sampling Sediment coring devices used during the A 210 nm2 reconnaissance survey was con- field work consisted of two types: a ducted at approximately 500 m trackline large diameter gravity corer called the spacings. This area selected for geo- hydroplastic corer and a modified technical investigation is part of a "Ewing-type" large diameter piston large geological study area discussed in corer. Part II of this report. The high degree of precision (theoretically ±1 m) of the The hydroplastic corer was originally Racal-Decca model 540 navigation system described by Richards and Keller [3.4] and the precision of the narrow beam and later modified by Lambert and (-2-2/3 half-angle beam) enabled the Merrill [3.51. Figure 3.2 is a line RESEARCHER to accurately relocate and drawing of this corer showing the weight position over seafloor features for bot- stand with finned shroud and stackable tom sampling, weights, which allows the variability of driving force of up to 450 pounds by the The large sail area and overall size of addition of 50 pound lead weights. The the RESEARCHER made maneuvering diffi- core barrel is made of standard 160 psi cult in shallow waters on the St. Croix rated 3 inch IPS PVC plastic pipe, which shelf. This precluded the collection of has a 3.25 inch ID and bolts onto the large numbers of bottom samples and weight stand with two machine screws. * 3.9 BAIL CHECK VALVE PVC VALVE SEAT SHROUD RE EASE ARM PISTON 2 ZCOb WEIGHT STAND cOgs HEAD LOCK RING LEAD WEIGHTS MYOROPLBTIC STAINLESS STEEL BOLTS PILOT CORE CUTTER P.V.C. CORING TUBE a RTAIER NN PISTON] Figure 3. 3. Operation of the free- fall method of coring using the modified "Ewing-type" large dia- meter piston corer with a hydro- plastic corer used as the trip weight. CORE RETAINER CORE CUTTER together with sleeved and threaded coup- lings for cores up to 58 ft long. The core cutters and retainers (Fig. 3.4) Figure 3.2. Hydroplastic corer, are produced from hardened steel and phosphor bronze, respectively. The pis- High quality 10 foot cores can be col- ton (Fig. 3.5) is simply two 0.5 inch lected easily with this corer in rela- thick rubber discs compressed between a tively soft sediment. The hydroplastic steel block and backing plates. The corer with a 5 ft barrel is also used amount of suction created by the piston routinely as a pilot (trigger) for the piston corer. The hydroplastic corer has been used successfully to collect high- quality samples for trace metal analysis when fitted with plastic core cutters and retainers [3.5]. The large diameter piston corer (Fig. 0 3.3) is a modified version of the stand- ?O1EW ard 2000 pound "Ewing-type" piston corer T 4------ manufactured adEsenby Ocean/Seismic/Survey,Copr- o Inc., and Eastern Instrument Corpora- POPOFNI tion. The modified corer uses the stand- ard core head and 3.5 inch IPS (4.0 inch " OD, 3.55 inch ID) black water pipe with ' threaded ends as the core barrel and 125 3--&a--I psi, 3 inch IPS (3.5 inch OD, 3.25 inch SEC?,LSVI ID) PVC plastic pipe as the core liner. CORE RETAINER CORE CUTTER The core barrels are shortened (in- house) to 19 ft, 4 inch lengths (from 21 Figure3.4. Core retainers and cutters ft standard lengths) and can be coupled used with the large diameter piston corer. 3.10 v - - -5. WW CONCTOR, I" DIA. STEEL BACKING PLATE -/2" MAWKE CISC 0 -SET SCREW STEEL BLOCK 3" IA Figure 3.6. Shipek grab sampler. Immediately after collection, all cores were cut into 12 inch sections and x- radiographed aboard ship, using a Hewlett-Packard Faxitron x-ray machine. -ITHICx HAND MSER This instrument is equipped with an DISC, 3.25" DIA. automatic exposure control system for TSTEEL ACKWO PLAT consistency of film exposure. Since the cores were exposed to x-radiation while IA" 5. MAR~lESOLTin contact with the film, exposures are 1:1 and depth within the core was deter- Figure 3.5. Modified piston used with mined on the film by a lead scale lying the large diameter piston corer. beside the core section on the film. The radiographs of the cores were used to can be adjusted by compressing the rub- assess any disturbance that may have ber discs with the backing plates, thus occurred during coring operations, to enlarging the outside diameter of the delineate the presence or absence of discs and increasing the sidewall fric- sedimentary structures, and to describe tion on the core liner. The piston corer the cores initially, prior to laboratory is free-fallen approximately 15 ft by analysis. Using the initial core de- using a standard trigger release arm and scriptions, a general sampling pattern a hydroplastic corer, with a 5 ft barrel was established for geotechnical testing used as the trigger weight (Fig. 3.3). and subsampling before the cores were opened. This procedure permits the best * The grab sampler used in this project is selection of the core sections and in- the Model 860 Shipek sediment sampler tervals for geotechnical analyses. manufactured by Hydro Products (Fig. 3.6). This spring loaded sampler col- Shipboard laboratory analyses included lects approximately 0.5 ft by 4 inch x-radiography, natural and remolded min- deep samples of soft oozes or coarse iature vane shear, torvane, and core sand. Samples collected with the Shipek description. Subsamples were removed for grab are relatively undisturbed compared water content, grain specific gravity, to other types of grab samples. Because Atterberg limits, carbonate analysis, of the horizontally oriented cylindrical total organic carbon, size analysis, shape after closure, this sampler is direct shear and triaxial shear. All virtually immune to sample washout dur- subsamples removed for shorebased lab- Sing retrieval. oratory analyses were refrigerated at 3.11 4*C immediately after collection except for the organic carbon samples which TORQUEaDEGREES RTATIO were frozen. The water content samples TOQ INDICATED BY POINTER were placed in pre-weighed glass vials, MOTOR--IATED YPIN capped, and inserted into moisture tight -- CARATED SPRING polethylene bags along with moist paper towels to retain a high humidity envi- - VANE ronment around the vials until the water content analyses could be completed. Torvane measurements were made on se- lected ends of core sections using an instrument manu'i-ctured by Soiltest, Figure 3.8. Minature vane shear Inc. The torvane consists of a hand-held apparatus. wheel with pointer, and a calibrated spring through which torque is trans- [3.11], Richards [3.6 ] and the American mitted to the multi-bladed vanes (Fig. Society for Testing Materials [3.12]. 3.7). Basically, the vane is inserted "Natural" shear strength measurements into the sediment (a smooth surface is were made on both ends of the 12 inch required), and torque is applied through core sections by gently clamping the the movable hand-held wheel until fail- complete section in the instrument (Fig. ure occurs. Numerous measurements can be 3.9) where sediment conditions and carried out quickly. Unfortunately, the quality permitted. Samples for water speed and consistency at which the content and grain specific gravity were torque is applied depends on the opera- removed from the immediate vicinity of tor, with potentially varying results. the vane measurement. Remolded shear During shipboard testing, if miniature strength measurements were selectively vane measurements were made on the same made by scooping adequate material out core section end, the torvane measure- of the core section ends after the ment was completed first using either natural "undisturbed" test was deter- the standard vane (0 to 1.0 TSF) or the mined, remolding the sample and recon- sensitive vane adapter (0 to 0.2 TSF). stituting the sample by placing it in a Torvane results are given in Appendix A, 1-1/4 inch by 2-3/4 inch long, thin-wall Table A-6.7.3. brass tube. The sample was then clamped in the vane apparatus and the remolded Miniature vane shear tests were accomp- strength determined. The sample used for lished aboard ship using a Wykeham Far- the remolded test was then containerized rance motorized laboratory vane appar- and later used for shore based Atterberg atus (Fig. 3.8) using a 0.5 inch wide by limits tests. 1 inch long vane rotated at 60*/min. Details of vane shear testing of soils can be found in Evans and Sherratt TOP -SEDIMENTARY BEDS HORIZONTAL -CORE LINER OR CYLINDER SENSITIVE VANE CORE SAMPLE ADAPTER /LBOTTOM STANDARD VANEI- Figure 3.9. Typical arrangement for in- Figure 3. 7. Torvane device and sensitive serting mini-vane into short core section. vane adapter. Vane is parallel to core length. 3.12 -4 Immediately after testing the ends of above and below by porous stones for each core section for miniature vane pore water drainage. Prior to applying shear, torvane, and removing samples for the shear stress a normal load was water content, grain specific gravity applied to the sample and allowed to and Atterberg limits, each of the upper consolidate (load P, Figs. 3.10 and four core sections (upper 4 ft) were 3.11). When a sufficient magnitude of extruded and split using an osmotic shear stress is applied to t'.e movable knife (3.13]. Each split section was retainer, the soil sample deforms and additionally sampled for water content ultimately shears along a fixed surface at 2 inch intervals or where structural (Fig. 3.11, see shear plane). Electronic or textural changes occurred. One-half monitoring devices (load cell and two of the split core sections was described linear displacement transducers), along in detail and saved for archive pur- with a data logger, were used continu- poses. Core sections below the first ously throughout the test to monitor four were not extruded and split aboard shear stress applied, and horizontal and ship but were sampled only at the ends vertical movement. With the standard of each section. direct shear test, drained or undrained * (consolidated or unconsolidated) testing Special core sections or portions of can be carried out [3.14]. Details and sections were saved for direct shear and well-documented discussions on the triaxial shear testing. The unextruded theory and testing can be found in core sections were packed in padded con- numerous textbooks on soil mechanics. tainers and stored vertically in a re- frigerated van until transferred to cold Triaxial tests were performed on a very storage onshore. limited number of VIT samples (cores 22 and 35) using Wykeham Farrance equip- 3.4.2.2 Shore-Based Laboratory Testing ment. The triaxial test is a more rig- and Techniques orous strength test than the direct shear test and is often referred to as Limited soil strength (consolidated- the "cylindrical compression test" drained) testing was accomplished using [3.14]. It utilizes a cylindrical spec- a modified direct shear apparatus manu- imen and chamber fluid which produces a factured by Soiltest, Inc. The tests stress in two principal planes (Figs. were performed using a lightweight 3.12 and 3.13) when pressure is applied acrylic plastic split ring soil sample through the confining pressure unit. retainer (Fig. 3.10). One-half of the Thus, a confining stress is produced in sample retainer is held fixed (lower) which 02 -03 and a uniaxial shearing while a shear stress is applied to the stress i.s produced by a movable piston adjoining but slightly separated re- or ram. tainer (upper). The sample was confined P PNORMAL STRESS SEAR STRESS LINES REPRESENTING TOPMOVSOX -SHEAR TREsMs N-UNIFORM DISTRISUTIO-., -- - .-- SHEAR PLANE OF SHEAR STRAINS AND BOTTOM POROUS STONES * IN SAMPLE BOTTOM Box D RAINAGlE T-LMA FIXED 1" D @O"T0M SHEAR CONFIGURATION AT FAILURE PRINCIPAL STRESS AT FAILURE Figure 3. 11. Example of shear configur- Figure 3. 10. Sediment sample shown in ation at failure in the direct shear test. a split ring soil sample retainer for use Note direction of normal load, shear in the direct shear test. Principal plane, and non-uniform distribution of * stresses at failure shown (al and 03). shear strains. 3.13 * concepts, theory, and practice of PISTON strength testing of soils is beyond the PRESSURE scope of this report and can be easily GAUGE CONINING, found in numerous textbooks on soil mechanics and in numerous papers. SSOIL SAM L Atterberg limits tests were performed on MEMBRANE- the VIT samples generally following the CNFINDG - -PISTON ASTM standard methods for liquid limit PRESSURE- ESURE (LL) (D 423-66, 1972) and plastic limit CONTROL (PL) (D 424-59, 1971) with a minor ex- ception. A representative portion of the Figure 3. 12. Typical configuration of entire sample, including all grain triaxial test apparatus and cylindrical sizes, was used for the Atterberg limits soil sample. From Bishop and Henkel, tests. The ASTM method recommends remov- 1962. al of all grain sizes larger than silt. Removal of small percentages of sand The triaxial test is time consuming and size particles in fine-grained submarine requires considerably more sample prepa- sediments is considered unnecessary ration than the direct shear test. It [3.7]; consequently, our techniques for has advantages because the complete the determination of Atterberg limits state of stress is known at all stages followed those prescribed by Richards, of testing. Specimen volume changes are [3.7]. Samples containing large percent- more accurate to determine, and pore ages of sand size material were not pressure can be monitored and control- tested for Atterberg limits. Plasticity led. Readers with further interest in index was calculated from the liquid these tests are referred to a good re- limit and plastic limit: PI - LL-PL. view by Lambe [3.14, chapter XI] of the Liquidity index (LI) was calculated from advantages of the triaxial and direct the plastic limit, plasticity index, and shear test. An excellent book on tri- w -PL axial testing is available elsewhere water content (w); LI - LL- PL The [3.15]. A book giving further informa- liquid limit device used in these tests tion regarding the shear strength of was a motorized type with automatic cohesive soils is available through the counter (Model CL-205) manufactured by American Society of Civil Engineers Soiltest, Inc. Measurements of water [3.16]. Details of shear strength test- content (w) followed ASTM standard pro- ing of soils can be found in Lambe and cedures (D 2216-71) and values reported Whitman [3.9]. Further details of the are uncorrected for salt content. Values of wet unit weight and porosity were determined using methods described by Bennett and Lambert [3.17]. Values of void ratio and dry unit weight were cal- culated using simple formulas that can be found in soils engineering textbooks. 0207 3 0-2,03 Grain specific gravity values were de- termined using an in-house designed he- lium pycnometer (Fig. 3.14). The prin- ciple of this instrument is based on the ideal gas laws and uses a single cylin- der piston to compress a relatively Figure 3. 13. Typical example of principal large and constant volume of dry gas in- stresses on cylindrical soil sample in tri- to a small chamber containing the dry axial test. 3.14 EAL STOP NMETAL SlOP PRESSURE CYL IWIER "I - - - - - - - - - - -. fIRSIC 200RONE - I lCROSALTCH TRIP LEVER Ii DIGITAL READOUT CON UT9 PANEL LIGHT ) _ BAITE Y COMPRESSED AIR BPESEEDA HELRE A BLEEDER VALVE LICONEDBYPASS VALVE COMPRESSED AIR [ SUPPLY TO Ft PSI gnShT-OFF VALVE HELIUM SUPPLY S PEIjLl pAs CHAMBE vole oi r d i --" TO 200 PSI i JMICRO-NEEDLE VALVE COMPRESSED AIR SUPPLY THREL-gAY BALL VALVE TOGGLE VALVE TWO-WAY BALL VALVE I HELIUM LINE MICRO-NEEPLE VALVE COMtPRESSED AIR LINE | Figure 3. 14. Schematic of single piston helium pycnometer. powdered sediment. As the volume of sed- technique for determining grain density iment in the chamber increases, the has been thoroughly checked against the final pressure registered after the com- ASTM standard method [3.18]. pression of the gas also increases pro- portionately. Calibration of the instru- Size analysis was accomplished using a ment was accomplished by using a combination of sieving (particles >63 standard sediment of known grain density jam) and pipette analysis (particles <63 and varying the sample volume to deter- 1m) following the Wentworth [3.19] size mine a corresponding pressure. The in- classification. The samples were first strument is sensitive to small changes air dried then wet sieved to remove in atmospheric pressures; however, this grains >63 ;m. The silts and clays (<63 is easily compensated for by determining um) were caught in a one liter graduated the pressure of a standard volume before cylinder during wet seiving. The coarse each sediment sample is tested and nor- fraction, after oven drying, was dry malizing its final pressure to a set seived and components divided into 4.75 standard determined in the instrument mm, 4.75 mm-2.0 mm, 2.0 mm-75 jm, 75 calibration. Once the volume of the sam- pm-63 pm. Any residue left in the sieve ple Is determined using the pycnometer, stack pan (<63 pm) was then added to the average grain density is easily calcu- graduated cylinder for pipette analysis. lated by the ratio of the weight of the The pipette analysis follows the method sample, in grams, to the sample volume described by Krumbein and Pettijohn * in cubic centimeters. Preliminary cal- [3.20]. One gram of sodium hexametaphos- culation of instrument precision (re- phate was added to each cylinder as a peatability) is approximately 0.004 cc dispersent prior to testing. Settling in volume or 0.01 g/cc in average grain times for each pipette draw were calcu- density for a sample with a grain den- lated using the average grain specific sity of 2.70 g/cc. Reliability of this gravity of sample and density of the 3.15 S demineralized water. Pipette draws were of the water can be used for the calcu- made corresponding to the 3.9 pam and 2.0 lation of the volume occupied by the jm size boundaries, salts in standard seawater having a salinity of 35 ppt (3.17]. 3.4.5 Techniques and Data Quality The test procedures for the geotechnical Equations used for the calculations of properties closely followed the methods the mass physical properties are found described by others [3.14, 23] and de- in various textbooks on soil mechanics tails of special techniques are discus- and other publications [3.8, 9, 17, 19, sed in Sections 3.4.2.1 and 3.4.2.2 in 201. No corrections were made for salt this report. content of the interstitial water with the exception of specific gravity A detailed discussion of testing proce- determinations. The interested reader dures and limits of reproducibility can can use a value of 63.9 lb/ft3 (1.024 be found elsewhere [3.26] but for con- n uvenience of the reader the limits of Mg/m ) for the density of seawater (@35 analytical precision are summarized in ppt, 4C) and for most practical pur- Table 3.3 as follows: purposes, a value of 1.2% of the volume Table 3. 3. Reported limits of reproducibility for selected geotechnical tests and analyses. Type of Test/Analysis Reported Reproducibility* Reference Atterberg Limits +2-5% 3.24, 25 Vane Shear Tests +O.O1PSI(0.7 g/cm ) 3.26 Vane-Triaxial Apparatus t1% 3.27 Vane Shear Test +.003-0.011PSI 3.28 (O.2-0.8g/cm 2) Grain Size t2%-+4% 3.29 +1%-+2% a.28 Water Content, and t1% 3.28 Wet Unit Weight Grain Specific Gravity _0.5% 3.28 Porosity <+2% 3.28 Void Ratio <+5% 3.28 *percent of observed value except where absolute value is given 3.16 3.5 Results and Interpretation although there is a general fining of the sediments with increased water Tables and plots of the geotechnical depth. properties for individual soil samples (cores and grabs) are found in Appendix Shelf sediments are characterized by * A. Detailed discussion of each sample is large sand fractions (2-0.075 mm) aver- beyond the scope and purpose of this aging 75% and gravel fraction (>2 mm) report, but the interested reader can varying from 3-38%. Silts (0.075-0.002 find detailed results by sample number mm) and clays (<0.002 m) typically con- and by depth in core (sample) in the stitute only 2-18% of the sample. All of appendix. The following discussion of the shelf sediments classify as sands as the geotechnical properties analyses indicated in Figure 3.15 (3.30]. results is limited to the suite of data as a whole rather than as individual A thin veneer of sediment occurs on the discussions of samples. Where necessary, slope reaching only one to two meters in individual samples are noted and the thickness near the base. Upper slope reader can quickly refer to the appendix sediments have highly variable textures * as required. ranging from clayey silts to gravely sands and rock fragments. Silt content 3.5.1 Sediments averages approximately 60% for the upper slope sediments, generally decreasing Sediments within the study area are cal- downslope while the amount of clay in- careous oozes composed primarily of bio- creases proportionally. Sample 22 in * genic debris with terrigenous material Figure 3.15 is an example of the high usually forming a minor part of the con- variability of sediment textures on the stituents. The texture of these soils lower slope (3131 m) and contained 23% displayed in Figure 3.15 is highly vari- gravel, 50% sand, 18% silt and only 9% able over the shelf, slope and trough, clay. Cores taken from the slope are generally structureless, which implies * ,oo% considerable reworking by benthic fauna. Figure 3.15 shows that overall slope * SHELF sediments classify as sandy-clay silts. CLAY UPPER SLOPE MID SLOPE LOWER SLOPE SASIN Basin sediments generally classify as . CORE 22 sandy-clay silts; however, high vari- CLAY CLAYability exists here due to the common 0. SILTV- SAWV- 0% presence of turbidity sequences as ob- served in several cores. Core 22 is a )good example (see labeled samples on &Fig. 3.15) with textures ranging from aclayey silts to sands due to the grading a of these sediments within the turbidite A 1 0 sequences. Graded turbidite sequences ,oo 1& ,o0% are evident in cores 22 (Fig. 3.16), 23, 0% 24, and 25 and to a minor extent in Figure 3. 15. Textural classification of cores 34 and 35. Cores revealing mot- sediments from the northern margin of tling, indicative of bioturbation or St. Croix using Link's (1966) method. reworking of sediment due to burrowing Samples are shown according to their organisms, include 30, 31, 32, 34, 35, physlographic location on the shelf, up- 36, and 38. These cores were taken on, per, mid or lower slope and basin. Note or in the vicinity of, the topographic the high variability in the texture classi- "high" in the trough with the exception fication of Core 22 (labeled) due to the of core 38. One piston core cutter was presence of several turbidite sequences. 3.17 badly damaged at site 40 and only re- covered a few freshly broken rock frag- CORE 22 ments, indicating exposed bedrock or the -- existence of a displaced boulder. 3.5.2 Geotechnical Properties 145 results of the measurements made for 2 sediment mass physical and mechanical CY properties. These results are also sum- - marized in Table 3.2. W 3.5.2.1 Water Content, Porosity, Void - Ratio 0 150 Co With few exceptions, average water con- M O tents range between 55-65 % (percent dry - weight). The highest value was found for a grab sample in the upper 1-4 cm (aver- W age value-10l%). The average porosity Ir" values are remarkably similar (59-65%) o with the exception of two grab samples of surface sediments (samples No. I and - 2; see Table 3.2). Although differences Z 15 W in average values for water content and L) porosity are not large among samples, Z maximum and minimum values vary signifi- 1 W cantly as shown in Table 3.2. Some of the "D highest values and greatest ranges of W W porosity, void ratio, and water content - L) occur in sediment associated with the "high" in the VIT. This high variability W probably is due largely to bioturbation 160 and reworking of the sediments as noted 16 in the previous section (3.5.1). The j variability and general trends in poro- sity and void ratio, which decreases I- with depth below the sea floor, are clearly depicted in Figures 3.17 and 3.18 for all samples. 165 Detailed tabulations of water content, SLporosity, and void ratio as well as ad- ditional parameters pertaining to spe- cific samples are given in Table A-6.7.2 Figure 3.16. X-radiograph of a section of Appendix A. The corresponding plots of Core 22 showing one complete turbidity are shown in Plot A-6.8.1 of Appendix A. sequence and the lower portion of 3.5.2.2 Wet Unit Weight another. Wet unit weight (wet bulk density) dis- plays significant variability due to 3.18 4,, 0. 0 (gr' 1.3 15 17 19 i. 5 ... .22 * .. 15 6 55- S. 7 20 6 25 40 50 60 70 80 7 o 90 100 1;0 120 WET UNIT WEIGHT (pcfl Figure 3. 17. Plot of porosity (%) versus depth in core for all samples. Figure 3.19. Plot of wet unit weight (pcf) versus depth in core for all localized differences in grain size and samples. degree of consolidation (Fig. 3.19). The average values shown in Table 3.2, how- These data (all "undisturbed" cores) are ever, are not remarkably different among plotted as a function of depth in Figure cores. The grab samples display low 3.20. In addition, maximum, minimum, and values of wet unit weight and high water average values of shear strength and content due to negligible overburden. sensitivity are reported and summarized The large range in wet unit weight is for all available data in Table 3.2. A revealed by the relatively large "enve- very wide range in shear strength is lope" encompassing the data of Figure observed. Shear strength and variability 3.19. The relatively uniform nature and increases significantly with depth below mineralogy of the solid grains compris- the sea floor. Over the 5 m depth inter- ing the sediment matrix is displayed by val, the minimum shear strength increase the consistent specific gravities for (vane data only) is approximately 0.7 all samples in Table 3.2. psi (5 kPa) using the "envelope" of the minimum strength values in Figure 3.20. 3.5.3 Shear Strength Parameters High variability of geotechnical proper- ties is common in carbonate oozes Numerous miniature vane (undrained) [3.28]. shear tests were determined aboard ship on core samples shortly after recovery. (APo) 0 , 20 30 4.0 5 60 70 80 0.0 0r - . 0 ... I _ - .1 0 . . . . 1I0 4~ 5 20' ~20, 6 7 0 1 2 3 4 5 6 7 8 9 10 11 25-09 2 15 I's 21 24 27 30 33 NATURAL SHEAR STRENGTH (pn) VOID RATIO Figure 3. 20. Plot of natural shear Figure 3.18. Plot of void ratio versus strength (psi) versus depth in core for depth in core for all samples. all samples. 3.19 S 4 • " '" ' ' .... .."- --' * ' " -" Z ' ' ': ': -' ' " I " '" " .. .i "''''" "':-I'-.. ., , .. ,. I F. : Sensitivity (ratio of natural to remold- o 0 ed shear strength determined by minia- P ture vane shear tests) is also highly 5 2 variable. The sensitivity is an indica- 2 tion of strength loss upon remolding or a, -3 severe disturbance of the soil mass. Of 4 significance is that sediments (general- '5 E ly clays) with high sensitivities may F flow on gently slopes if severely dis- 20- - -6 turbed [3.203. Thus knowledge of the -7 sensitivity is an aid in assessing the 25- potential behavior of a deposit if sub- 0 3 6 9 12 15 is 21 24 jected to disturbance or remolding in SENSTIVITY natural environments (i.e. sediments Figure 3.22. Plot of sensitivity versus subjected to earthquakes). Shear Fe 3. 22. Pot of ses strengths of remolded material are low depth in core for all samples. in the upper 1 m. Values are generally less than 1.4 psi (10 kPa) with most values falling lower than 0.5 psi (3.5 with deposits on the sides and top of kPa) as depicted in Figure 3.21. The the topographic "high" and the deposits sensitivity data shown in Table 3.2 and in the northern portions of the VIT Figure 3.22 reveal that the sediments (samples 22, 25, 30, 35, and 38). See range from normal (sensitivities of 2.4) Table 3.1 and Figure 3.1 for sample to extra-sensitive (sensitivities of locations. 8-16). Two sets of direct shear tests (consoli- In general, sensitivities appear to in- datid drained: rate of shear - 1.78 x crease downslope with increasing water 10 inch/min or 4.52 x 10 - 3 mam/min) depth. Sediments collected from the St. were completed for cores 22 and 35 re- Croix Island margin slope display sensi- covered from the VIT. Core 22 was col- tivities in the normal (2-4) to sensi- lected from the northern portion of the tive range (4-8). Those slope sediments trough in approximately 3900 m of water. in the sensitive range generally have Core 35 was recovered from the southwest values of only 4-5. The highest sensi- slope of the "high" in about 3800-3900 m tivities (greater than 5) are associated water depth. Both samples show good stress-strain characteristics as depict- (P 5 ed in Figures 3.23 and 3.24. Despite the 0 0o 20 30 40 506 0 6 0 . 0 differences in depositional environment (trough floor versus slope of the W. "high") the drained angles of internal -2 friction agree within 0.3 (see Figs. 10. -o 3.23 and 3.24, Sample 22, iCD=37.6°; Sample 3 5 , #CD- 37 .9 0). The rather large 4 c-- angles of internal friction probably are z 15-, -5 due to the high carbonate content of the 920. .6 samples. Table 3.4 gives the gasometric carbonate analyses of subsamples of each 25 core and reveals carbonate contents of 0 1 2 3 4 5 6 7 e 9 10 If 82-83% (Core 22) and 63-76% (Core 35). REMOLDED SHEAR STRENGTH (psi) Triaxial tests of deep-sea sediments reveal increasing angles of internal Figure 3.21. Plot of remolded shear friction with increasing carbonate con- strength (psi) versus depth in core for tent, and those sediments having carbon- all samples. ate contents greater than 40% exhibit 3.20 WON granular behavior [3.21]. Angles of in- 3.5.4 Atterberg Limits and Indices ternal friction of 31.3 were reported for sediments having greater than 60% Liquid (LL) and plastic limits (PL) were calcium carbonate [3.31]. Angles of in- determined on selected subsamples. Fig- ternal friction (undrained triaxial ure 3.26 gives the data from calcula- tests) of 370 for fine calcareous ooze tions of liquidity index, (LI-- wPL and 33 ° for coarse calcareous ooze were fLpL reported earlier [3.32]. Note that in where w - natural water content and in- both tests performed in samples 22 and dicates values of greater than one for 35, a small cohesion intercept (c) was most samples tested. Some of the highest S found to be approximately 5 kPa. values of Liquidity Index (LI-9, sample 16; LI-6, sample 38) are found for sedi- Triaxial data (one set) for sample 35 ment samples from the slope off St. are shown in Figure 3.25. Angles of in- Croix. As noted earlier [3.8), remolding ternal friction for total stress and of natural sediments having water con- effective stress were OCU-19.7* and tents greater than their liquid limits OCU=37 .5, respectively. Note that the (liquidity index greater than 1.0) angle of internal friction for effective transforms the deposit into a viscous stress analysis compares well with an- slurry. Thus liquidity index is an im- gles of internal friction determined by portant soil property for evaluating direct shear drained testing ( N * sediment behavior and seafloor stabil- (G 37.6; 37.90). ity. ( tPe ) so 0 10 20 30 40 50 C 7 .50 7 so o -40 40 0 0 - 4- 0. 3.6 1o 4- 3o0o- - S - 20 O.to IA 0 0 a 10 1S 20 Lo .. 0 0 0 3 4 5 6 7 0 HORIZONTAL DISPLACEMENT (mm) ROOM:, STRESS (pil S.. CORI 2? INTERVAL SHEAR SHEARING EFFECTIVE SHEAR WATER WET UNIT (in) LOAD STRESS AT S4ORMAL DISPLACEMENT CONTENT WEIGHT (Ibs) FAILURE STRESS AT FAILURE W It 1*1 V,, rT (3. ) (pcf (psi) (psi) (in) 1 9.9-20.5 6.30 1.28 0.54 2.3P 0.175 50.3 102.4 20.5-21.6 17.15 3.50 3.67 .953 0.153 53.5 13R.O 21.6-23.6 32.01 6.52 7.23 .;96 0.1743 51.21 IOR.O • Figure 3.23. A. Stress-strain plot from consolidated-drained direct shear tests-- Core 22. B. Plot showing the drained angle of internal friction for Core 22. SII3.21.. Although the liquidity indices are high, were used to generate a bathymetric map the &tterberg limits (liquid and plastic at a scale of approximately 1:65,000 and limits) reveal that the sediments are is included in this report as a pocket inorganic silts of low plasticity. Using insert. This bathymetric map was used to the plasticity chart of Figure 3.27, produce a slope map of the study area [3.331, these sediments are considered for the purpose of graphically depicting to behave as silts having medium corn- five slope zones, or gradients, showing pressibility. These St. Croix samples major morphological features of the sea have considerably lower plasticity in- floor. Major seafloor features are read- dices and liquid limits than the carbon- ily apparent in the slope map. The ate sediments described by others steepest gradients (>35 ° ) are found pre- [3.311. This can be attributed to the dominantly at water depths of 2500 m or very high silt and carbonate contents less, with the exception of the very for the St. Croix samples as seen in steep escarpment at the western edge of Figure 3.15 and Table 3.4, respectively, the study area (-latitude 64*54'W) where these steep gradients extend from 2200- 3.5.5 Seafloor Gradies 4400 m water depth. The nature of these contours implies strong structural con- A bathymetric map (Fig. 3.1) was deve- trol on the bathymetry and slopes. In loped from earlier cruises aboard Navy contrast to the steep gradients, the survey vessels during geological and axial gradients of Christiansted Canyon geophysical investigations off the range from -5-11. Large portions of northern margin of St. Croix (see Part 2 the slope, however, are characterized by of this report). The bathymetric data 20- 120 0 s.o too I 150 o60I 40 so 20 A B 0o 0 a! .... I I... , .... .. .... 0 1 0. 15 20 0 a to is RIO . NORMAL STASIS (pll HORIZONTAL DISPLACEMENT (mm) CORE 35 INTERVAL SHEAR SHEARING EFFECTIVE SHEAR WATER WET UNIT (in) LOAD STRESS AT NORMAL DISPLACEMENT CONTENT WEIGHT (Ibs) FAILURE STYSS AT FAILURE W 7 Orfn i7i, £ E (Pef) ________(psi) (psi) _____ -in) ____ 12.2-13.3 44.71 9.11 10.85 .839 0.194 58.8 104.9 13.6-14.8 84.9 17.29 21.58 .801 0.222 57.3 106.1 15.7-16.9 29.3 5.96 7.25 .822 0.176 55.6 105.5 Figure 3. 24. A. Stress-strain plot from consolidated-drained direct shear tests-- Core 35. B. Plot showing the drained angle of internal friction for Core 35. 3.22 AJ Soo 500 To A To- C so- 400 - 0400 50- sof ',-u So 300 40, 40. 300 So. aoo 30 ff 20Ftoo 20- 2 o -too 1100 0 0 0 . . 0 0 4 , 2 It 20 24 a 1 S 2 14 20 24 AXIAL STRAIN % AXIAL STRAIN lW (, -500 a- To. B | o- -400 -30 fo , vi 50. 30 200 A B C cell pressure. a. . ? psi S. 3 psi ' 3 P 20- back pressure, bp 'I.? psi "(9 psi I 3 psi o o :' i dl stra tin , C ! ' 1 11.2 11.2 10- effective cell 14.5 psi ?.4 PsI 3,t.q psi *pressure. I.= a3 coMpreSsiOn 31.A psi 13.9 ps, 1.7 Ps' 0 0 0 sress 0 -i03 0I12mom Principle 46.0 psi 42.3 ps, 65.1 pS5 AXIAL STRAIN MW total Stress. *, excess pore water 9.S PSI 2?. pti 43.;, psi pressure. V 51501 ! prlcIple 3'. 1 19.4 Psi "1.9 oSI e;ffective stress, 61 :minimum Principle 5. I psi . psi P psi effective Stress. , pore pressure 0.30 1,..? 3 cofflc lent. A effective angle of Internal friction. 3u "3"0 total angle of Internal fricton, #.- 9 7 0 Figure 3. 25. Plots of triaxial shear tests results for Core 35. 3.23j II II I I I. - I ll [ p_ ' •I ... .L II I J - . - 0 . , ,. 5," • . . . . . . . . 0 f00 - -so- 10 10 s2 1 o Is. 2 4 6 O S ' 1 0 4060 ; O ; LICUITY INE IUDLIMIT L Figure 3.26. Plot of liquidity index Figure 3. 27. Plasticity chart for all versus depth in core. samples tested. gradients of 1-20* and 20-35, which The "top" of the topographic "high" is are substantial compared with other essentially flat. A few patches of 5* to types of margins [3.36]. In contrast to 100 gradients are found in the northern the slope off St. Croix the VIT is char- portion of the VIT. The steep slopes acterized by gradients of only 50 and along the St. Croix Margin must be care- less. The topographic "high," however, fully considered in any offshore engi- displays areas of rather steep gradients neering construction or activities plan7 extending up to 35* (see map insert). ned in the area. Table 3.4. Calcium carbonate analyses. Percent (dry weight) Sample Number Below Mud Line (cm) CaCO 3 16 (0-7) 81.8% 16 (33-38) 83.7% 16 (60-66) 83.5% 17 (0-6) 81.3% 18 (0-6) 83.1% 18 (26-32) 84.6% 18 (59-65) 84.7% 20 (0-7) 82.4% 22 (40-45) 81.7% 22 (75-80) 82.4% 25 (8-12) 78.5% 25 (48-52) 77.2% 25 (102-108) 63.6% 30 (55-61) 77.5% 30 (85-91) 77.2% 32 (0-5) 75.8% 32 (49-56) 75.7% 32 es(112-116) 74.5% 35 (13-18) 76.3% 35 (58-65) 72.6% 35 (112-117) 63.2% 36 (18-24) 79.1% 38 (9-13) 59.7% 38 (56-62) 66.3% 3.241 tyef a ,n [.. .n.nrs o 0 rdet ar-ond. h 2 - -.. m o U LU >0 0 0 -LO >- - . CL M a- L L >. CD" "l 0 -J' "ID M,. Z (/) D C A I) (NJ (W )r-. V) ( . C) V) U v 1 0.001 - 0.CS6 0.i 30.4 2.1 47.2 3.9 16.2 GRAB I -r Z r LL, <. ZO .. .< 0) < WZ U) - (D A (OOj ()1--. U 0 )r) ) v 1 0.001 - 0.06& 0.4 23.4 5.3 45.8 3.0 22.1 GRAB 2 LU< LU 2- JLl ,~i- I 4 a...1 a_ L LU >0 0 o w .- L) >- 0 Z: Z m- LU < . zo mI _j . <) < < LD <(' xZ N < q ~- (NJ --j . - \ (/)Z ( n - -" 0 A (f')J (n 1-.. U)W Un Uo LV 1 0.001 - 0.066 7.1 50.6 0.9 33.2 1.8 6.3 GRAB 6 < X w W w >. _j I - I _. -2 ; tt __j Li __ - ..w N C' aLU - w 0 Im (\J L I I' r-Z X .- U LU Z0 0 m ILI <- 0) < i) < LLU x~ ZOj zi .. 0 03 0( to (Ni M r r - U <. O m mI m (D < = < m U x \ < . < 14 - COZ U) - D .A U) 'Cj U) r,. .. v 1 0.001- 0.066 16.3 76.0 1 1.2 6.4 GRAB 8 3N U U >- .. : _ 2 I - ILI -JLU -J w w- LU5( U).- a_ m 0. LU LU o >0 0 I 0( to UwCQ x: 2: LU <. - = Z I M (D - -.J t , - (C) _.JUJ .J C i.- ILut ('J Cl) 0- m C. 0 w o 3 0 U.JC X: : - w <. zo zi C)Z < < Z U- W- C\ < . < wr - C z (n) I CD A IUEnC\l (t- LL V I 1 0.001 - 0.066 15.4 71.0 2.3 11.3 GRAB 10 <,, ,- C J C a- m a. Ui u '-*1 00 0(0 lwiCN < D < m IL Q: C C' < . < v - En Z (1 -- - 0 A U) CJ Cl',. L1.L V 1 0.001 - 0.066 37.7 50.3 1.3 10.7 GRABII _Jt. _.J C I- ILl (X ()• z--' - rr' i- Lu . Z " U) Z /) - 0 (A (f) C\J L/Ot. LL V 1 0.001 - 0.066 15.3 82.4 0.4 1.9 GRAB 12 _.I L L u...J {:" i-- I. (J' ) 0.. . { L ub Lu ::OC 0 0(0 LCJ ' w - I- L J zO z i Z O D-< z U- Cr C, < * q. - W Z (f) - - DA W/)C~jI V) r-,. LL V 1 0.001 - 0.066 31.6 61.0 0.9 6.5 GRAB 13 3.32 . ... . . -.4 Table 6. 7. 1, continued. Size classification. LL, W U,. f -m Lj LU >0 L E X: X: Z LLU <: =0 2: <: 4: 2t LU x(\ <: I : <' (. L ) CD A I ) ( ) r- LL V 1 0.001 - 0.066 2.8 72.8 6.3 18.1 GRAB 14 <: I. rl LflU ..JLUI _. CY - Lu 2 C'J (1 • am w __" x LU LO 02 Z WCzi C' LI, 4:' 0 in I (.1)2w - I - o..M a-. Luj LJ 0 01 0 (D -() > < : < z , N <- <, -- Q _, _ . U) Z I) - (-A V)C'j .fU) . Ct,,0 CU. I ) v 1 0.001 - 0.098 0.1 14.2 3.1 60.8 3.9 17.9 2 0.460- 0.558 0.1 14.8 3.0 59.3 3.6 19.2 3 1.080- 1.180 12.8 1.9 62.9 3.0 19.4 4 2.070- 2.170 15.0 2.1 58.4 3.6 20.9 CORE 16 <~1: l -- . N 0 ,-- w, of Lu >-_j j-'I I. - ro ..j w -4 X~~ LU5 N' I J - . Lu . Lu >- 01 m 0(D ._) IN Q :u Z : LI < - m 0 Z -- . < <. 0 01 M~ (0 ~ .- - 1: X: LU 4. = 0 i Z .i 0) 4: U) M (I- CD A U1)0N Vf) t-- ('D Ur~ n .U V 11 0.00 1 -0.098 118.7 2.8 53.1 3.4 22.0 2 0.591 - 0.660 16.0 2.0 57.1 3.4 21.5 CORE 20 < Lu N .- J ,Z Z ~-LJ X . Ci >-_ III - . -\ - 0.] ._ 1JJ -1 wr - LLI j Q..rn . Lu Lu >0 0 0 LD - -- i O- X: Lu X:: ZJ < . 4=I j ) <: < : LL- C N<* < 'T -0j . < (f)Z () - CD A U cr-- L).0 U r,") ) v 1 0.001 - 0.066 17.3 2.5 59.7 4.8 15.7 GRAB 21 3 3.3 Table A-6. 7. 1, continued. Size classification. Wi~er Li.I .- . .J - I "-" "-r) "- " "LA ..w i W- J Cro '. \ a a- L w L 0 0(0 -l) >-- i0 < = < = U- m N < < l- _- j < * ()Z (/U) - - 0 A (/ (-,. ' ,- E f(0 vr) 1v 1 0.098- 0.197 27. 2.1 45.5 4.1 21.2 2 0.460- 0.560 0.1 40.6 1.4 36.7 3. i 18.0 3 1.480- 1.570 16.0 2.5 51.4 5.0 25.1 4 2.030- 2.070 5.7 1.6 57.3 4.3 31.1 5 2.230- 2.260 0.9 74.7 1.7 12.7 1.3 8.7 6 2.660- 2.690 21.7 2.0 49.0 3.3 24.0 7 3.380- 3.410 0.1 53.1 2.1 25.1 2.6 17.0 8 6.430- 6.500 0.1 19.9 2.3 42.4 5.0 30.3 9 9.380- 9.450 16.2 2.8 45.0 5.1 30.9 10 12.340 -12.430 0.2 27.3 3.9 38.2 3.6 26.8 I1 15.290-15.350 0.! 22.3 3.9 41.0 5.3 27.4 CORE 22 O.~ .. .. ~Liil.0 0~ t= -.. 0) =0 z: < x: rl ,,o . 'I* -JLLw -j W~ U- (NJ (N _ i w wi > 0 D i - --L >- I 0 1: X:Z I- w~ <: zO = 0 4:. <: <: m LLi ~ <: V-' .- C ,\ -j -jCN (.fl () C CA CLC U/)-. (D LO r) cv 1 0.001 - 0.098 27.6 2.5 41.9 4.7 23.3 2 0.460- 0.558 47.0 0.5 31.0 2.0 19.5 3 0.591 - 0.660 2.6 0.3 74.0 3.8 19.3 4 0.984 - 1.080 62.4 2.8 27.0 0.5 7.3 3 5 1.570- 1.640 14.8 1.0 47.8 5.3 31.1 6 2.490- 2.560 13.7 2.4 45.0 5.0 33.9 7 6.500- 6.560 12.9 2.5 36.8 7.5 40.3 8 9.450- 9.510 16.0 3.8 43.9 4.1 32.2 9 12.400 -12.470 16.2 5.4 48.5 3.4 26.5 10 15.350 -15.420 11.9 4.0 49.8 5.1 29.2 11 17.320-17.390 9.6 2.4 43.8 8.4 35.8 CORE 25 2 - ._LU __J t3- (N..- J I C'J- _- m a._ L w 0 0 0 o > >-D T- : - LU <: = 0 ZI < m:7 <4 < Z 2 L- w-- < z < - C' U)Z ) - (D A M) (J I )-. IU)c ) (._.) v 1 0.001 - 0.066 12.0 1.4 48.7 5.6 32.3 CORE 26 LiJ Li -j W - * ~.J ,.. _j t I- (NJ I ('J a.m C. LJ LLJ :0 C) 0' - l >-i - XZ X: I.- L <• zo zI -j . < C :< <:f 4:z CK N < -. IT. .- CNj _j. -jC"N /)Z I) (D A ])(N U p" UO (( Q_ ) C) v 1 0.001 - 0.066 31.5 3.3 38.7 4.4 2?.1 CORE27 3 Table A-6. 7. 1, continued. Size classification. w x~ LU > -s'r. o- W a Ll > C" i Ow L,) >- I >- 0 -- Q L-- LEi < = Z I < ,> 4 < u3 < L xC~cj < . <- *.-c' C ~ _j.* C):M (n - 1_ 0A IC WC-J t-- (/(0 41 r CjV 1 0.001 - 0.066 21.8 2.5 58.9 4.0 12.8 GRAB 28 i x,. w >-- . . _1LiLJ ._j X I-- NLl (L M .. L U LLJ > 0 0(0 -- Lf >- ) >- 0 M 1: 1:1- LUI <. O =) Z I _ .. < < . <-:I< 7 LL x( j < 4- < q* -C-j _j -jCj I)Z (I) - I CD A U() 0I (1IN- ()(0 L ) () v 1 0.001 - 0.066 23.4 48.7 1.5 17.8 0.9 7.6 GRAB 29 '-0) ._ xI. .1 : I- -- _j~ i N I 1 - a_. m O_ Lw wd >- 0 in - LNw - I >- 0 Z: X: : 1.- w, D0 0 I LA >--w >- >0 1- Z- IL < . z 0 z i _J. < < . < =- < z LL w C <. - I - F -r - X - ,iL < £ <- = ,J -j < 0) < . <: -< Z U- Cr N V- C\J s-r -i ._j CJn (3 - - A (/)( (n (') (D .)r) v * 1 0.001 - 0.066 0.1 15.8 1.5 46.0 5.7 30.8 GRAB 33 -J w~ -i M.a - - a_ (n 0- Ui <.l zo z C.4Z) <~ Z Z I _j <. <~-N .. J' >)Z (1 - 0 A ((n U , l". (-2to U-" "v 1 0-m001 - 0.033 26.2 1.6 47.6 6.8 17.8 $ 2 0.459- 0.492 36.1 2.4 3F.6 3.7 21.2 3 1.120- 1.150 18.5 3.2 46.9 5.0 26.4 4 1.610- 1.640 23.4 1.3 43.9 3.9 27.3 5 2.260- 2.300 0.1 14.5 2.0 1 48.0 4.9 3 0 CORE 34 3.39 Table A-6. 7. 1, continued. Size classification. w. w U ) - .0 ..JLjj J C I- LUJ I\ I\ ..J 0 ( L_ J Lu- :. 0a 0 ( I-- l >- I : X: LLU < . Z- 0 ' Z I n a_~ 0 Lw w >0 C 0 0'f w -( >- CD X:1 1: U < Co Zi - < 0) < - C m-O :z: - I- i <- zo z =- <0 <• < D < z W ,-,N < . <- ._j m [_ J)Z () - 0A (n DCJ (n. r_ G (0 " IU v 1 0.001 - 0.066 12.0 1.9 50.3 5.0 30.8 2 0.098- 0.;31 0.1 9.2 1.5 51.1 5.3 32.8 3 0.853 - 0.919 18.6 4.1 42.6 5.4 29.3 4 1.900 - 1.970 16.8 1.8 49.7 4.0 27.7 CORE 38 w -aj Uj -1af a_~ LU a_ w . Ui 0 *D m JL J c 1 - L__ . 0 M I __i -m , -V, Q .l L U L U ,I , > 0 0 O l"- D I --- l t> - I - :D- < = L < - zo z , - < < • V)Z I L/) - - CD A 0A) N 1t,.. V) 1.0 W C. C-) V 0.001 - 0.066 0.8 23.0 2.4 50.0 3.0 20.'7 CORE 40 3 3.141 ' C Table A-6. 7.2. Soil properties. wA I- :3 w -j = - ww-r: _ a- - - lj~nK LU Z . - z l_ .- Z X .J X: I - ljJU1: --- C) ,, < QCD Q_ LU LU LI. - r7 C_()> 0-7 (C r ' - LU I-Z I- O.JLU< --- rr >- 0 x 9. 2CD7 > w 56.>2 1 0.001 - 0.031 84.6 94.95 2.73 2.31 69.8 51.44 2 0.001 - 0.033 117.6 88.02 2.74 3.06 75.3 78.76 GRAB I 3. :2 < 16- LU LU > z>- z LL-- z .J LU -j Q: I- wt x l :D :D am M 0a Lu LU LU - 0 L C)' )- c C() X:~ LUJ IZ Q- C) < - (rQ-. > ) =) l < ,Z W L. OC X ) a; I V)l = () Dt L) 3 - :3 >- - (D CD W~ C L.~ 1 0.001 - 0.131 75.0 98.20 2.76 2.07 67.4 56.12 2 0.001 - 0.131 73.1 98.76 2.75 2 01 66.8 57.04 3 0.001 - 0.131 77.1 97.45 2.73 2.10 67.8 55.03 4 0. 00 1 - 0.131 84.0 196.14 2.82 2.37 _70. 3 52.26 GRAB 2 3.412 1 Table A-6. 7. 2, continued. Soil properties. IU LU~I W -- --i (y_ w. w om a_ U w W -CD 0 U- U - C) CD . w- ,~ -z U- c U <4 w~ >- U z IL < 0 -.z LUJ [.a a. X: C < 0 Q : a (n z U) : - V)J' D cI0 :> a- 0~- 1 0.001 - 0.033 80.0 92.14 2.73 2.18 68.6 51.77 2 0.;64- 0.i97 64.0 101.88 *2.74 1.75 63.7 62,13 3 0.328 - 0.361 62.9 102.26 *2.74 1.72 63.3 62.76 4 0.492 - 0.525 63.9 101.76 2.75 1.76 63.7 62.10 5 0.660- 0.690 60.5 103.07 *2.74 1.67 62.6 64.20 6 0.820- 0.853 72.9 98.64 *2.74 2.00 66.7 57.05 7 0.984- 1.020 56.1 105.32 *2.74 1.53 60.5 67.48 8 1.080- 1.120 60.3 03.63 2.75 1.66 62.4 64.66 9 1.310- 1.350 64.1 101.88 *2.74 1.75 63.7 62.07 0 1.480- 1.510 66.0 101. 13 *2.74 1.81 64.4 60-92 11 1.640- 1.670 59.6 103.51 *2.74 1.65 62.2 64.85 12 1.800- 1.840 62.0 102.69 *2.74 1.70 62.9 63.38 13 1.970- 2.000 79.4 96.76 *2.74 2.16 68.4 53.93 14 2,130- 2.I0 53.2 106.75 2.74 1.46 59.3 69.68 CORE 16 ,L- Z-- - Z J j a:CrLLJa w D - - C) U)n a-m a- LUJ LUJ LU M L L) :> D L < :D < LU -,Z LL- C)LU- -I. W: V- 0- w U)l 0 () - :x U- .3:- U(-,0 CD c a~' O'- 1 0.001 - 0.033 60.0 103.63 2.73 1.64 62,1 64.78 2 0.098- 0.131 53.0 106.75 *2.74 1.45 59.2 69. 76 CORE 17 * inferred value 3.143 -- -- - ' -: Z-, __ ;, ,.ii " Table A-6.7.2. continued. Soil properties. - _ - .-- L-j Lu - Z >- Z a._ --- wz < w- i ~ i rr" l.- <± .-z, IJ -0 - 0iL< -- " C 0 >.- ci_ 7) < . LLi.- - Z _.J jI.I _. ~r 1-- r~UL n, D .- - C (,23 ' _n _ Lu - - 3' a- n C C - _ I a- r- C 1 0.001 - 0.033 65.1 101.76 2.74 1.78 64.1 61.65 2 0.098- 0.131 55.7 105.50 *2.74 1.53 60.4 67.76 3 0.196- 0.230 55.2 105.50 *2.74 1.51 60.2 67.99 CORE 20 * inferred value 3.44 Table A-6. 7.2, continued. Soil properties. I< LU L > Z Z LLI- Z Q* JM a_ Q Uj LUQj I- M- 02 U) w~ o Lu mu Lu - QU X 0- 'ir - L I- "t- cr CL CL 0 D< C (n~ U) C - 3:L)- D3> U(n CD * CD:, L - - 1 0.098- 0.131 66.8 100.51 2.74 1.83 64.7 60.27 2 0.262- 0.295 53.8 106.38 *2.75 1.49 59.8 69.16 3 0.394- 0.427 50.5 107.88 *2.75 1.40 58.4 71.68 4 0.525- 0.560 37.8 116.12 2.76 1.04 51.1 84.24 5 0.591 - 0.623 36.5 117.55 *2.76 0.99 49.8 86.13 6 0.755- 0.787 45.9 110.81 *2.76 1.27 55.9 75.94 7 0.853- 0.886 45.9 110.81 *2.76 1.27 55.9 75.93 8 0.951 - 0.984 65.8 101.38 *2.76 1.82 64.6 61.15 9 1.120- 1.150 60.5 103.38 *2.76 1.68 62.7 64.40 10 1.280- 1.310 59.3 103.82 *2.76 1.63 62.0 65.16 11 1.440- 1.480 53.3 106.56 *2.76 1.46 59.4 69.52 12 2.000- 2.030 52.2 107.38 2.76 1.44 59.0 70.56 13 2.170- 2.200 66.0 100.5f 2.70 1.78 64.1 60.53 14 2.300- 2.330 66.5 100.63 *2-73 1.83 64.7 60.43 15 2.430- 2.460 50.5 107.56 *2.73 1.39 58.2 71.45 16 2.460- 2.490 59.1 103.82 *2.73 1.61 61.7 65.25 17 2.590- 2.620 54.2 106.75 2.76 1.50 59.9 69.23 18 2.760- 2.790 53.4 106.38 *2.75 1.49 59.8 69.33 19 2.920- 2.950 56.6 105.00 *2.75 1.57 61.1 67.05 20 3.080- 3.120 59.4 103.63 *2.75 1.62 61.9 65.00 21 3,250- 3.280 55.2 105.94 *2.75 1.51 60.2 68.28 22 3.440- 3.480 43.3 112.50 *2.75 1.18 54.2 78.52 23 3.480- 3.510 40.0 114.43 *2.75 1.10 52.4 81.74 24 4.460- 4.490 34.3 118.61 2.74 0.94 48.5 88.29 25 5.410- 5.450 62.0 102.69 *2.74 1.70 62.9 63.40 26 5.450- 5.480 60.7 103.07 *2.74 1.67 62.6 64.13 27 6.430- 6.460 62.2 102.69 *2.74 1.72 63.3 63.31 28 7.380- 7.420 70.6 99.32 *2.74 1.94 66.0 58.20 29 7.420- 7.450 71.0 99.26 2.74 1.94 66.0 58.06 30 8.370 - 8.4( 0 65.6 101.13 *2,74 1.81 64.4 61.07 CORE 22 * inferred value 3.45 Table A-6. 7. 2, continued. Soil properties. wci w Ui> z> z -L OL M . UJ LU -Q IL1.- 0- 0 2 X: M - LL - .- ) wL< k -~ >- U) 4:f 4 Z LL < 0 WI C0. 0.L w 0 < 0 cr< Q U)Z C/ - 30-' 3 ) (Df cr a- 0, ___ 31 8.400- 8.430 61.8 102.69 *2.74 1.70 62.9 63.46 32 9.350 - 9.380 62.5 102.26 *2.74 1.72 63.3 62.93 33 10.330 -10.370 65.1 101.51 *2.74 1.78 64.0 61.47 34 10.370 -10.400 64.0 101.88 *2.74 1.75 63.7 62.11 35 11.320-11-.350 55.6 105.32 *2.74 1.53 60.5 67.68 36 11.350 -11.380 .56.9 104.82 *2.74 1.56 61.0 66.79 37 12.300-12.340 42.8 112.31 *2.74 1.18 54.1 78.66 38 12.340-12.370 46.0 110.50 *2.74 1 .26 55.8 75.70 39 113.290 -13.320 47.2 109.94 *2.74 1.29 56.3 74.67 40 13.320-13.350 46.9 109.94 *2.74 1.29 56.3 74.82 41 14.270-14.300 47.9 109.37 *2.74 1.31 56.8 73.95 42 14.300-14,340 47.1 109.94 *2.74 f.29 56.3 74.75 43 15.260-15.290 54.9 105.75 *2.74 1.51 60.1 68.29 44 15.290-15.320 51.7 107.25 *2.74 1.43 58.8 70.69 CORE 22 (CONT'D) * inferred value 3.,46 - , Table A-6. 7. 2, conCinued. Soil properties. , , z >-- z L_ - Z LU W-0 LL C.)> i.- cc0 m= CL M Q- w w LI, M- 7) w< .- i- CL - M a-.. 1 0.001 - 0.033 80.5 96.14 2.74 2.21 68.8 53.27 2 0.164- 0.;97 69.2 100.00 *2.74 1.89 65.4 59.09 3 0.328- 0.361 62.2 102.69 *2.74 1.70 62.9 63.30 4 0.492- 0.525 59.9 103.50 *2.74 1.65 62.2 64.73 5 0.558- 0.590 663, 0 1-13 2.75 1.82 64.6 60.80 6 0.590 - 0.623 63. i 102.38 2.74 1 .73 63.3 62.79 7 0.722- 0.755 38.8 114.86 2.74 1.06 51.5 82.77 8 0.919- 0.951 55.8 105.23 *2.73 .53 60.5 67.49 9 1.020- 1.050 59.0 103.63 2,73 1.61 61.7 65.1 10 1.080- 1.120 59.1 103.82 *2.73 1.61 61.7 65.25 11 1.540- 1.570 60.2 103.38 *2.73 1.64 62.1 64.54 12 1.570 - 1.610 62.8 102.38 2.73 1.71 63.2 86.01 13 i.740- 1.770 58.1 104.25 *2.73 .58 61.3 65.92 14 1.900-- 1.940 58.4 104.25 *2.73 1.58 61.3 65.92 15 2.070- 2.100 60.8 132.94 *2.73 ;.66 62.5 64.00 16 2.230- 2,260 60.4 103.38 *2.73 >.64 62.1 6 4.44 17 2.390- 2.430 57.6 104.25 *273 .58 61.3 65.921 18 2.530- 2.560 57.9 104.25 2.73 .58 61 .2 66.04 19 2.560- 2.590 58.; 104.38 *2.74 1.59 61 .4 66.02 20 2.660-- 2-690 58.5 104.38 *2.74 1.59 61.4 66.02 21 2.820-- 2.850 63.0 132.26 *2.74 1.72 63.3 62.72 22 3.020- 3.050 64.2 101.88 *2.74 1.75 63.7 62.03 23 3. i80 - 3.220 66.3 i31. 13 *2.74 1.81 64.4 60.81 24 3.350- 3.380 60.9 103.07 *2.74 1.67 62.6 54.04 25 3.510- 3.540 59.4 103.94 *2.74 1.62 61.8 65.22 26 3.540- 3.580 60.4 103.51 *2.74 1.65 62.2 64.53 27 4.490-- 4.530 62.2 102.69 *2.74 1.70 62.9 3.231 28 4.530-- 4.560 62.0 102.69 *2.74 1.70 62.9 63.23 29 5.480-- 5,510 65. 101.51 *2.74 1.78 64.0 61.47 30 5.510 - 5.540 62.41102.70 *27 1 1.70f6. 63.2 CORE 25 * inferred value 3.47 £ Table A-6. 7. 2, continued. Soil properties. I-I C) -i LU -i x Z - Z 0- o Z Li mz: U i< l Li - < :- - C_ W 0 < 0 (_ r- r- i- ui - Z - C) LbJ --- n -'- C-) u) m C) - C)U- -- ! ( C CD >- x .C - 0;-- 31 6.460- 6.500 68.5 100.00 *2.74 1.89 65.4 59.34 32 6.500- 6.530 68.1 100.38 *2.74 1.87 65.1 59.73 33 7.480- 7.510 66.1 101.13 *2.74 1.81 64.4 60.90 34 7.780- 7.810 66.7 100.76 *2.74 1.83 64.7 60.45 35 8.430- 8.460 56.2 105.32 *2.74 1.53 60.5 67.42 36 8.460- 8.490 55.8 105.32 *2.74 1.53 60.5 67.42 37 9.420- 9.450 55.1 105.75 *2.74 1.51 60.1 68.20 38 9.450- 9.480 53.5 106.75 *2.74 1.45 59.2 69.56 39 10.400-10.430 54.7 105.75 *2.74 1.51 60.1 68.35 40 10.430-10.460 50.6 108.25 *2.74 1.40 58.3 71.88 41 11.380-11.420 52.7 106.75 *2.74 1.45 59.2 69.90 42 11.420-11.450 54.9 105.75 *2.74 1.51 60.1 68.27 43 12.400-12.430 55.3 105.75 *2.74 1.,.1 60.1 68.11 44 13.350-13.390 56.6 104.82 *2.74 1.55 61.0 66.92 45 13.390-13.420 60.4 103.51 *2.74 1.65 62.2 64.54 46 14.340-14.370 54.1 106.25 *2.74 1.48 59.7 68.96 47 14.370 -14.400 52.9 106.75 *2.74 1.45 59.2 69.84 48 15.320-15.350 50.6 107.75 *2.74 1.40 58.3 71.56 49 16.340 -16.370 54.4 106.25 *2.74 1.48 59.7 68.80 50 17.280-17.320 55.1 105.75 *2.74 1.51 60.1 68.18 51 17.320-17.360 45.2 111.06 *2.74 1.23 55.2 76.47 52 18.140-18.180 58.7 104.03 *2.74 1.62 61.8 65.57 CORE 25 (CONT'D) * inferred value 3.48 Table A-6. 7. 2, continued. Soil properties. LU >-.i a a-w U w -- .2 0' LUCK¢ LU > "> LL - - Z < ~ D 0 - - C) 1 0.001 - 0.033 85.0 94.89 2.73 2.32 69.9 51.30 2 0.131- 0.164 68.1 100.38 *2.74 1.87 65.1 59-71 3 0.262 - 0.295 67.0 100.76 *2.74 1.83 64.7 60.34 4 0.394- 0.427 63.2 102.38 2.75 1.74 63.5 62.72 5 0.787- 0.820 55.0 105.75 *2.74 1.51 60.1 68.23 6 0.820- 0.853 55.0 105.50 2.74 1.51 60.1 68.05 7 0.984- 1.020 58.5 103.94 *2.74 1.62 61.8 65.57 8 1.150- 1.180 59.9 103.51 *2.74 1.65 62.2 64.75 9 1.310 - 1.350 56.3 105.32 *2.74 1.53 60.5 67.39 10 1.480 - 1.510 58.2 104.38 *2.74 1.59 61.4 65.99 11 1.640- 1.670 59.2 103.94 *2.74 1.62 61.8 63.31 12 1.770- 1.800 58.3 104.38 *2.74 1.59 61.4 65.93 13 1.800 - 1.840 61.7 103.01 2.74 1.69 62.8 63.72 14 1.970 - 2.000 63.i 102.13 *2.73 1.72 63.2 62.61 15 2.130 - 2.170 61.2 102.94 *2.73 1.67 62.5 63.86 16 2.300 - 2.330 65.2 101.38 *2.73 1.78 64.0 61.35 17 2.460 - 2.490 64.2 101.76 *2.73 1.75 63.6 61.98 18 2.620 - 2.660 67.1 100.63 *2.73 1.83 64.7 60.21 19 2.790- 2.820 68.3 99.88 2.72 1.86 65.0 59.35 20 2.950 - 2.990 68.9 99.76 *2.72 1.87 65.2 59.08 21 3.120 - 3.150 65.9 100.82 *2.72 1.79 64.2 60.76 22 3.280- 3.310 63.0 102.01 *2.72 1.71 63.1 62.60 23 3.440- 3.480 67.2 100.45 *2.72 1.82 64.6 60.07 24 3.610 - 3.640 67.6 100.13 *2.72 1.85 64.9 59.73 25 3.740 - 3.770 70.6 99.26 2.72 1.92 65.8 58.17 26 3.770- 3.840 71.5 99.32 *2.74 1.94 66.0 57.92 27 4.720- 4.760 69.8 99.64 *2.74 1.92 65.7 58.67 28 4.760- 4.790 67.5 100.38 *2.74 1.87 65.i 59.91 29 5.710- 5.740 70.5 99.32 *2.74 1.92 66.0 58.24 L.30 5.740- 5.770 68. 100.38 *2.74 1.87 65.1 59.72 CORE30 *inferred value 3.49 C Table A-6. 7. 2, continued. Soil properties. I- _c j X- ..- Z . >- n ._ J -J Ur i w .- 0 0 M U- Q.A 0' W. LJ WUl -- " LL 0J . 0.),-., C _ -- i -- '- 1.- w ,-,I- L < - >- U) < : <. z LL.. <0 C) wJQ- Q- r 0 ~ 0 N w~ a-O En z (n - 3c..3 U - Q. (n0CDa 0;.-'- 31 6.690- 6.730 74.0 98.32 * 2.74 2.03 67.0 56.51 32 6.720- 6.760 73.8 98.32 * 2.74 2.03 67.0 56.58 33 7.680- 7.710 71.6 98.95 *2.74 1.98 66.4 57.65 34 7.710- 7.740 68.0 100.38 *2.74 1.87 65.1 59.74 35 8.660- 8.690 67.5 100.38 *2.74 1.87 65.1 59.92 36 8.690- 8.760 67.8 100.38 * 2.74 1.87 65.1 59.82 37 9.650- 9.680 56.9 104.82 *2.74 1.56 61.0 66.82 38 9.680- 9.710 57.8 104.38 * 2.74 1.59 61.4 66.16 39 10.630-10.660 66.1 101.13 *2.74 1.81 64.4 60.89 40 10.660-10.700 62.9 102.26 *2.74 1.72 63.3 62.80 41 11.610-11.650 59.7 103.51 *2.74 1.65 62.2 64.82 42 11.650-11.680 60.0 103.51 *2.74 1.65 62.2 64.70 43 12.600 -12.630 64.4 101.88 * 2.74 1.75 63.7 61.99 44 12.630-12.660 55.6 105.32 *2.74 1 .53 60.5 67.68 45 13.580-13.610 65.2 101.51 *2.74 1.78 64.0 61.44 46 13,610 -13.650 58.6 103.94 *2.74 1.62 61.8 65.55 47 14.570 -14.600 66.1 101.13 *2.74 1.81 64.4 60.87 48 14.600 -14.630 62.9 102.26 *2.74 1.72 63.3 62.77 49 15.550-15.580 56.6 104.82 *2.74 1.53 60.5 66.95 50 15.580-15.620 58.9 103.94 *2.74 1.62 61.8 65.41 51 16.800-16.830 60.9 103.07 *2.74 1.67 62.6 64.07 CORE 30 (CONTD) * inferred value 3.50 I * * , - A .2 i Table A-6. 7.2, continued. Soil properties. _ J ;2 - M -- CLrr m a- Lij w LL --C 0U_ ,.>,.- 0'm CD, _ 0.066- 0.098 61.1 103.07 *2.74 1.82 62.6 63.98 2 0.328- 0.361 61.3 103.07 *2.74 1.82 62.6 63.98 •3 0.660- 0.690 61.2 103.07 *2.74 1.82 62.6 63.98 4 0.984- 1.020 65.1 101.51 *2.74 1.78 64.0 61.48 5 1.310- 1.350 71.9 98.95 *2.74 1.98 66.4 57.54 CORE 31 I,-- -- < D -I .w U n.- i- LU n -- C CD x D <= < z ,,. - Z L.-- Z -jw -j x~ Q: Xrw :D Fm 1: Y w t- i C)> z~ < - L) < - - Z LL - -j L (-- Cr I.- XJ L-- X3 D L CL "-M a_ L-- ILU w- Z v-- ow~<: - 0. - S0.001 - 0.033 67.8 100.51 2.75 1.86 65.1 5990 2 0.164- 0.197 54.1 106.38 *2.75 1.49 59.8 69.03 3 0-328-- 0.361 54.2 106 38 2. 75 1.49 59.8 69 00 4 0.4-92- 0.525 55.4 105.94 *2. 75 1 .51 60.2 68.17 5 0.656- 0.6891 55.5 1105.32 *2,75 11.53 160.6 167.72 -6 0.755- 0.787 53.8 1106.13 _2.74 1 1.47 1 59.6 69.-0 0 CORE 36 I-x o.- 2J 0 001 - 0-3 6-- 1--- L4-3 LU 3 Z>- Z U0' -- - Z 50-49- 052L 5 64. 01- LL *2> r - 60 . 6 06 0- 09 7-8 13 * . 74 1 5 61. 66_16 1 0.001- 0.033 58.3 104.25 2.74 1.60 61.5 65186 2 0.001- 0.033 66.4 101.13 2.74 1.82 64.5 60.76 3 0.164- 0.197 52.5 106.75 2.74 1.44 59.0 70.00 4 0.328-- 0.361 58.7 103.94 *2.74 1.62 61.8 65.50 5 0.492-- 0.525 64.7 101.50 *2.74 1.78 64.0 61.62 6 0.660- 0.690 57.8 104.38 *2.74 1.59 61.4 66.16 7 0.820- 0.853 58.2 104.38 *2.74 1.59 61.4 66.16 8 0.853- 0.886 58.2 104.25 2.73 1.59 61.4 65.90 9 0.984- 1.020 62.1 102.69 *2.74 1.70 62.9 63.34 10 1.150- 1.180 60.9 103.07 *2.7"4 1.67 62.6 64.06 11 1.310- 1.350 63.4 102.26 *2.74 1.73 63.3 62.59 12 1.480- 1.510 58.0 104.38 *2.74 1.59 61.4 66.06 13 1.640- 1.670 63.8 101.88 *2.74 1.75 63.7 62.18 14 1.800- 1.840 66.6 101.13 2.74 1.82 64.6 60.70 15 1.970- 2.000 58.4 104.38 *2-74 1.59 61.4 65.90 CORE 38 * inferred value 3.56 0ii, i Table A-6. 7. 3. Strength measurements ::_- Z -j UJ i Or r- c::Z < n aLi L, < wi L E Y- z Lw w X nU WQ:: 2f: Q LUo - X: 1:1Z I- Lu ia wrln UjcrCn z ~c( - 1: X X:i- LUJ LUI XU) LUa:U) Z a: ( < D < Z- Li-L I- a_. I- U 01-- (n z U) - - )U) U)U)- U) U -) 1 0.001- 0.098 1.01 10.21 14.81 CORE 20 3.60 I-I Table A-6. 7. 3, continued. Strength measurements. iiT< MILU z i- . 9J 4 - 0w492 <.72 M Q. L L <0.82 X 1.Y -I--- 1crU x < 7 < z IL M FH--QM a'- wu CDI u)Z a, ) - - U0 U) (IiV)'- Cfl I- 1 0.394 - 0.492 1 .72 2 0.722- 0.820 1 .63 13 0.820- 0.853 2. 10 0. 15 13. 7 4 1 .710 - 1 .800 2. 11 5 1.800- 1.900 1.86 0.13 14.0 6 2.690- 2.790 2.11 7 2.790- 2.890 1.85 0.21 8.7 8 3.670- 3.770 2.33 9 3.770- 3.870 2.07 10 4.720- 4.760 2.20 11 4.760 - 4.860 2.57 12 5.710 - 5.740 2.40 13 5.740- 5.840 2.63 14 6.690- 6.730 2.20 15 6.720- 6.820 3.06 16 7.680 - 7.710 2.10 17 7.710 - 7.800 3. i2 18 8.660- 8.690 2.90 19 8.690 - 8.790 3.72 20 9.650 - 9.680 2.90 21 9.680 - 9.780 3.32 22 10.660--10.760 4.01 23 11.610 -1 .650 3.90 24 11.650 -11.750 4. 42 25 12.600 -;2.630 4.10 26 12.630 -12., 50 4.88 27 13.580 -13.6i0 4.00 28 13.610 -13.720 5.40 29 14.500 -14.600 5. 18 30 14.600--14.700 5.42 CORE 30 3.61 Table A-6. 7. 3, continued. Strength measurements. -MJ M< LU ui~ j- cr_ w-~,. a- lM Lj w w z Q2m Q r U LU w - Lii I w )> rL < D < z LA- M CLM 01- w CD- z LIn Z - U) UOL- n W 1 0.001 -- 0.098 0.30 2 0.328- 0.427 2.59 3 0.755- 0.853 2.88 4 0.853- 0.951 3.50 5 1.740- 1.840 4.22 6 1.840- 1.900 4.70 1.10 4.2 7 2.720- 2.820 5.13 8 2.820- 2.920 5.67 9 3.710 - 3.810 5.80 1.28 4.5 10 3.810 - 3.900 5.18 11 4.760 - 4.790 5.20 12 4.790- 4.890 5.6? 13 5.740- 5.770 5.60 14 5.770 - 5.870 6.54 15 6.730- 6.760 4.50 16 6.760- 6.860 6.84 17 7.380- 7.410 5.10 18 7.640- 7.740 5.67 19 7.710- 7.740 6.20 20 8.730- 8.830 5.50 21 9.680 - 9.710 4.90 22 9.680 - 9.780 6.94 23 10.630 -10.660 6.70 24 10.660-10.760 7.42 25 11.610 -11.650 6.90 26 1.650 -11.750 7.60 27 12.600 -12.630 7.1 28 12.630 -12.730 8.39 29 13.620--13.710 7.60 30 14.570 -14.590 7.20 CORE 32 3.63 Table A-6. 7.3, continued. Strength measurements. -J ccz iw 0 .- z '-c ,- L W~o W > - D (.9 I- z _J w o Q: a:. M a : z - < - a- n a _ L LL_ X- r Z- w (r (n wc X n) Z c Iz I-r X wL -Ji LU > C .C W M. C -- Z a-0 O- CL WL L < LU - < WL 3 b U iE : LL- Cru U)" LLU Xn z CC:(W MI J LL i--O_ a- x - a- L- 1 0.001 - 0.033 1.40 2 1.150- 1.180 2.30 3 2.260- 2.300 3.00 CORE 34 3.64 - -- , Table A-6. 7. 3, continued. Strength measurements. M < Mw > 2z 0-2- 0 w L W i w CD CD M- a- 0m a-- <1- CD CD L 31 115.720 -15.750 5.20 32 15.680-15.780 10502 33 16.630 -16.670 5.30 34 16.670 -16.760 7.68 35 17.450 -17.490 4.20 CORE 35 (CONT'D) <9 -1< LLiJ >j= x 1 0.001- 0.098 0.50 2 0.689- 0. 787 .,8 0.27 4.4 CORE 36 << L x 1~ LU:> -- 0. 0 z wL -j x I- X Or- CZ~- - > 0. 0.Q LU LU < LU- Yr- r I'- wU LU X C UU Z Cn z ( < :D < z LU- tI-0 a -0 LU C--a-- Un)z U) - - U)U)' - U)) LO --- ( - 1 0.001 - 0.098 0.98 2 0.853 - 0.951 1.28 0.26 4.9 3 1.8 0- 1.970 2.06 0.28 7.3 CORE 38 3.66 Table A-6. 7.4. Atterberg limits. A-- -i ~ ~ L.L r- - X 0- x am a: w u 3 I-- < - <0 C3 < < ___ ___ -n - C - c - - 3.01 -- 0.033 45.4 36 2 9.2 3.5 GRAB 2 <1 __j w _._j 0- z: a- X. CL x, _' G - . . I-- ac aw C) o i< n cj- c- CM S C)- a- 1 0.001- 0.164 43.9 34 3 GRAB 7 L U, L" - n - U- - w r- Z LU C:D < r- < X- - I LL, C) -I rL E E w~ C) <-fr~ E- . < Q - C < :D < Lu Lu D- (fl- __j M , . = _ ( .. -- - -- _ -.- - -- - - - 1 0.001 - 0.164 48.9 32.1 16.8 2.1 2 1.610- 1.840 45.3 31.4 3.9 1.3 3 3.670 - 3.810 48.2 29.9 18.3 ;.2 4 5,774 - 5.938 50.4 36.2 14.2 1.6 5 8.727 - 8.891 55 1 42.0 13. 1 0.5 6 11 .646 -11 .844 59.8 43.81 16.0 0.6 7 i4.600-14.797 46.2 33._1 13. 0.9 CORE 32 LU~C - U >MC -j LUj -j cr_ - - - -J a-x JLuJ _J I- --- U i-- .- I-XcL ->< a- a L Lu Lu D- 0- o CflJ Lu EE Z - Luj C : <- <- C c/)Z U") - .-. - d -. J 1 0.001 - 0.164 51 .0 35.5 15.5 CORE33 I- wu Lu >f a- X - C) M a- w Luw C co -d S , , -- Lu.- < - r- . - X) :D < Z Lu -- i jN .z~ - Z r)z M C!) - CL~'- CLJ O-~ _ 1 0.001- 0.164 46.6 39.1 7.6 2.7 CORE34 $ 3.71 : , 'i Tuble A-6. 7.4, continued. Atterberg limits. < U- 4: X) C)J - -. j IAJJ -j -2 0 - i o-M a.. L LI - ()- (UJ Z- r 1- Lw c3E -. < - - DC C) cnz Icn - - - Q- Q. -- - -j 1 0.427- 0.591 42.5 30.1 12.4 2.0 2 1.900- 2.130 43.7 38.2 5.5 5.9 3 3.670- 3.840 56.5 44.0 12.5 2.0 4 5.839- 6.003 56.5 38.8 17.7 1.4 5 8.792- 8.956 44.6 36.2 8.4 2.0 6 1 1.745-11.909 46.9 33.4 13.5 1.8 7 14.697-14.862 73.6 50.5 23.1 0.7 8 16.666-16.830 44.8 33.9 10.9 1.8 CORE 35 .- : C - L LU w 0) i - w.. -- 0 J .J I (n- I--J .-- I C .-- I C-.- - . I 1 0.591 - 0.787 41.5 31.0 10.5 2.3 CORE 36 3.72 3 I~.. T - ii { "- Table A-6. 7. 4, continued. Atterberg limits. -j I ELI-- 1: E w: U, <~ D J _J LjJ .-- t' I.- - -- -J i-- - I1 !- *-3_ .X 0,295- 0.427 44.5 39. 3 5.2 3.7 2 0.853- 1.050 44. 1 31 7 12.4 2.3 3 -.840- 2.030 41 .7 ,8.7 3.0 6.61 CORE 38 _j ,Cl __j' 3 2 < Q Q- C) C)O - L/) Lr * 119i 0. 164 36' 3.0 5. CORE 40 3.73 _ *Awl Plot A-6.8. 1. Soil properties - - _- - _ [ . . .... .......... --- ~ WATER C0NTFN T -l LIQUIIY INDEX: WET UN'J WEIGHT liW PLASTIC LIMIT ,, I w x LIDLJID LINIT _I j1Y PCF Z 20 55 900 5 1090 i35 120 GRAB I '-20 55 906 5 1090 ;05 120 GRAB 2K 20 55 900 5 1090 05 120 2055900 1 90 13 0 zGRAB 15 - I-L 20 55 90 0 5 10O90 1 05 120 I . 3.73 I--I cr. z GRABI15 Wi 20 .515, . 900 ... 51 1090- 1135 120 CORE 16K 3.741 Plot A-6. 8. 1, continued. Soil properties. WATER CONTENT -- LIQU:DITY INDEX WET UNI WEIGH T I Il PLASTIC LIMIT " W LIOUID LIMIT I LLM. 0 z. PCF z - 20 55 900 5 1090 105 120 0 CORE 17 IL 4 20 55 900 5 1090 105 120 o CORE 18 - -J w 20 55 900 5 i090 i05 120 0 CORE 20 w 20 55 900 5 1090 05 120 a. GRAB 21 3.75 .. . . .. t5 . .... .. Plot A-6. 8. 1, continued. Soil properties. WATER CONTENT - LIQUIDITY INDEX WET UNIT WEIGHT -PLASTIC LIMIT iwi LIQUID LIMIT )t LL % PCF 20 55 900 5 1090 i05 120 - 0 a- 44 IL // o I00 w S I. CORE 22 41 I-v 3. 76 • Id. Plot A-6. 8. 1, continued. Soil properties. WATER CONTENT - LIQUIDITY INDEX WET UNIT WEIGHT PLASTIC LIMIT LIQUID LIMIT * l PCF w 55 900 5 1090 105 120 0 K *- / - / .1___CORE 25 _____ L3.77 i . . .. . . , . -w-.. . . ' 'l , . . Plot A-6. 8. 1, continued. Soil properties. WATER CONTENT - LIQUIDITY INDEX WET UNIT WEIGHT PLASTIC LIMIT , L LIQUID LIMIT )K ,IA % PCF LL 20, 515. 900 ... 51 1090 1105 120 0 0 z 0 0 L w I U) 0 -J w z 0 z I00 w CA. CORE 30 3.78 .. AM. A" Plot A-6. 8. I, continued. Soil properties. WATER CONTENT - LIQUIDITY INDEX WET UNIT WEIGHT PLASTIC LIMIT & - [ I 't i.- LIQUID LIMIT * W % PCF W 20 55 900 5 1090 105 120 _z" i .... . . . .. I ' Zj CORE26 K 0 20 55 900 5 1090 105 120 0o!_ ', ',. .. . . . ',,,. , - , * LL. 4 GRAB 28 550 900 5 1090 __105 120 0 0 * CORE 31 W 20 55 900 5 1090 105 120 Z w/ I (L * " CORE 34 K 3.79 Plot A-6. 8. 1, continued. Soil properties. WATER CONTENT - LIQUIDITY INDEX WET UNIT WEIGHT PLASTIC LIMIT & LIQUID LIMIT K Uj % PCF WL 20 5 15 900 51 1090 1.k05 120 LL oo z 0 0 -J LL. w U) 0 -J *J z 0 Wom- U I- W 00_ -0 z CORE 32 3.80 Af X Plot A-6. 8. 1, continued. Soil properties. wATER CONTFNT - LIQUIDITY INDEX WET UNIT WE!GH T PLASTIC LIMIT LIQUID LI M"IT K I'- PCF W 20 55 900 5 1090 05 120 H LL 0 o f / I 'I II- o I o- // o' 2 LAL W - z a. CORE 35 3.81 Plot A-6. 8. 1, continued. Soil properties. WATER CONTENT - LIQUIDITY INDEX WET UNIT WEIGHT PLASTIC LIMIT & w LIQUID LIMIT )K LL 7. - F 20 ..... 5 5, 900 5, 10 90 1105 1120 z 0 0 CORE 36 K 0 i.. w U) 20 55 900 5 1090 0 05 1.20 °0 # = CORE 38 z0 I.- = 20 55 900 5 1390 105 120 ICORE 40 0 3.82 Z j .. .. .. .,... ., .- L o= , . . . ... w. .. .. . Plot A -6. 8.1. continued. Soil properties. ~VOID RATIO POROSITY SPECIFIC GRAVITY1 C." 0 2 4 40 60 802.70 2.75 2.80 ', j p _ __ _ I-I w t L GRAB IK z - 0 2 4 40 60 802.70 2.75 2.80 OR -1 0 0 0 U. GRAB 2 w 0 2 4 40 60 80 2.70 2.75 2K8 0244 82.0 2.75 280 z 0 0 w COREI 17 W 0 2 4 40 60 802.70 2.75 2.80 0 a.. COREI8 IS t 3.83 Plot A-6.8. 1, continued. Soil properties. RATIOORO SPECIFIC GRAVITY V OI I D, 0 2 40 60 802.70 2.75 2.80 I )- r CORE 20 Z 0 2440 60 802.70 2.75 2.80 0 0-J I-- w i 3.844 r i z 0 w C1. CORE 22 G 3.841 I Plot A-6. 8. 1, continued. Soil properties. VOID RATIO POROSITY SPECIFIC GRAVITY e G, 0 2 4 40 60 802.70 2.75 2.80 S0 0 LfLo 'U LaL LL 0 -J 'J 0u I- I " w . S0 3.85 W. . . ,- .. Plot A-6. 8. 1, continued. Soil properties. VOID RATIO POROSITY SPECIFIC GRAVITY e G, 0 2 4 40 60 802.70 2.75 2.80 I- 0 0 .. 1. w o:: CORE 30 I- Ii240Q 60 80 2.7'0 2.75 2.80 I L CORE 31I 3.86 Plot A -6. 8. 1. continued. Soil properties. *VOID RATIO POROSITY SPECIFIC GRAVITY e G 0 2. 4 40 60 802.70 2.75 2.80 IP- z 0 3 0 0 4 0 w I- CORE 34 3.8 Table A-6. 8. 1, continued. Plot of soil properties. VOID RATIO POROSITY SPECIFIC GRAVITY e r) G 0 2 4 40 60 802.70 2.75 2.80 o 0 I-. w JULK z cc 0(0( 0 w z 0 P it I-Icc it z LJ CORE 35 3.88 I. mi Plot A-6. 8.1, continued. Soil properties VOID RATIO POROSITY SPECIFIC GRAVITY e G % 0 2 4 40 60 802.70 2.75 2.80 w CORE36 0 0 -j 0 2 4 40 60 80 2.70 2.75 2.80 LL. 0 I I- w CORE 38 0 I. wz w 3.89 a.. - .O Plot A-6. 8. 2. Strength measurements. DESCRIPTION SHEAR STRENGTH SENSITIVITY NATURAL = A S, REMOLDED = [!, (PSI) W~ 0 6 122 8 1 IL Lt. Gray olasyey silt osrbonate 7- Struturelems; Bioturbation Ct In- throughoutCO E1 0 0 0 6 122 8 4 0- Tan olaysy silt 01 .carbonate withi Wi foram & pteropod CO I spinss-several semi-angula r S Ipebbles near surface CORE 17 0 -j en 0 6 12 2 8 1 01 Lt. brownish gray z clayey silt with sd(foremi 0 pt'ood tests) ISlightly mottled 6 I. throughout Strue tc&ea CORE 18 Iz I~l. 0 622 814 Ton~ clayey silt L }carbonate with a. lsome forems Mottling through- I out Structurelea, CORE20 3.90 Plot A-6. 8. 2, continued. Strength measurements. DESCRIPTION SHEAR STRENGTH SENSITIVITY NATURAL = * I REMOLDED =m S LIA (PSI) LLi L. __ _ _ _ _ 06 122 8 t4 0 o Pale brown sandy- z clayey silt carb- 0 - onate; 1.0' Silty sand 1.0- Co - 2.01 o Clayey silt with esone sand L2.0- S 15.4' WSeveral turbidite Co- sequences with W f ine layering; texture ranging 'between coare 0 sand and silty _j a)- clay 0 z _- -j -- z 0 Li0 Z2_ 3; 00-0 C- .- 0. * CORE 22 . 3.91 r Plot A-6. 8. 2, continued. Strength measurements. DESCRIPTION SHEAR STRENGTH SENSITIVITY NATURAL =,& ST REMOLDED = S, Lii (PSI) Li 0 6 122 8 14 LL 0o- Surrace-paIle 1.0' brown clayey .z sndy silt carbona te Graded beds ranging from 0 - silty and to" 0 clayey silt; gray carbon- .1 ate Z 1.0- o '12.0' Li Stiff CClayey silt to silty clay Mottled through- out ~W 0 z 2 12.0' Hint of layer- ing and dense 0 silt lense with - small silt pock- I- eta 12.51 S- Sand and small pebble layer 12,5- Wi end Z Stiff clayey o silt with some cc mottling through- 0. out CORE 25 3.92 S . w 1..,I% ( .Z- . "..,, Plot A-6. 8. 2, continued. Strength measurements. DESCRIPTION SHEAR STRENGTH SENSITIVITY NATURAL & REMOLDED - in W (PS ) ,U 0 6 122 8 14 L0. Tt. v. . . "o z Z .3' brcvmf mui!: c' .-bcoi.::te sond Z U' 7C/ s. te 0 to- end 0 C :.ey * -rey to 'nale L brown carbon- 4 ate W tottling through- (Z C0 Occasional hint of layering below 12' Otherwise 0j structureless I-- wK 0 C CORE 30 3.93 Plot A-6. 8. 2, continued. Strength measurements. DESCRIPTION SHEAR STRENGTH SENSITIVITY NATURAL = S, REMOLDED -0, I- (PSI) 0- 6 122 8 14 U.00 Lt. yellois 2.7' brown clayey z silt to silty clay carbon- ate cc 27- 3 C' Fine layering 0 aslternting 0 with homogen- eous silty La. clay-possible turbidite S pulses C3.0- end Homogeneous silty clsy to 0 clayey silt; slightly mot- tled; rare hint of layering 0 0 r- w z CORE 32 (N 3.94 J Plot A-6. 8. 2, continued. Strength measurements. DESCRIPTION SHEAR STRENGTH SENSITIVITY NATURAL = S, REMOLDED 7 Sr LAJ (PS 1) WA0 6 122 8 14 0-. 0 - ILt. ye" 2o,,ish 0 end w y' claye Sed silt to clayey- - sandy silt carb- onate ; mott. d\ throughuut 0 - Below 12' there 0 is rare hint of _j layering - other- SI .vise structure- :1less co Lii z 0 I- 4w z CORE 35 3.95 -. Plot A-6. 8. 2, continued. Strength measurements. W"'1 LL. DESCRIPTION SHEAR STRENGTH SENSITIVITY Z NATURAL = S, n,- I REMOLDED =[ 0(PSI) 01 o. 69(P I 122 ..... 81 14 0 - Lt. yellowish 0 end brown clayey aili OE pteropod teat@ IMottled thrvugh- t Sructure- z0 0 6 122 8 14 edbrown clayey silit Im carbonate Mottled throu*h- zF W CORE 38 0- 3.96 ' I PART 4 I ENGINEERING SIGNIFICANCE W.J. Burton . 14 PART 4. ENGINEERING SIGNIFICANCE The purpose of this part of the report 4.1 Geological and Geophysical Considera- is to place in perspective the data tions gathered from the north margin of St. Croix and the VIT, particularly with The major geological and geophysical respect to the installation of under- aspects of interest center around the water structures and equipment in the slopes, sediment thicknesses, bottom area investigated. As previously indi- terrain, and the ongoing seafloor cated Parts 2 and 3) the area has processes. wide variations in seafloor topography and sediment geotechnical properties. 4.1.1 Slopes, Sediment, Terrain Accordingly, situations requiring the installation of structures and equipment As indicated (Part 2), the overall is- whose placement, orientation and settle- land slope is steep and local gradients ments are to fall within narrow limits, vary considerably. Moreover, the slope will necessitate further examination of is covered with a veneer of unconsoli- the sea floor and sediment properties at dated sediments, appreciable amounts of the specific locales of interest, coarse debris, and (less frequently) large massive blocks of material. Be- This part of the report relates the cur- neath the layer of unconsolidated sedi- rent findings to the siting and place- ment, a hard, calcareous material, ment of hardware on or beneath the sea occurs in some places. Occasionally sed- floor. Some typical applications are: iment-barren outcrops of rock or clay form cliff-like escarpments. This com- * Installing of structures to support bination of circumstances creates poten- hydrophones for an underwater tracking tially unstable situations, heightened range; by the fact that the area undergoes some " Anchoring mooring lines from floating level of frequent seismic activity, platforms or submerged bodies; which could trigger mud slides or shift- * Siting and constructing off-shore ing of boulders. towers; " Selecting routes for underwater cables Chern and Tudor [4.1] have defined a and determining the best methods for slope as potentially unsafe if it ex- their burial/protection anchoring; ceeds 40 in an area that is not in a * Placing and securing pipelines on or delta or seismic zone (where even gent- beneath the sea floor; etc. ler slopes are unstable). Herrmann (4.2], in specifying guidelines for de- For purposes of presentation, the geo- sign of small (maximum dimension less logical and geophysical characteristics than 15 feet), non-strategic structures, and the geotechnical properties of the restricted the applications of the pro- bottom will be discussed separately, cedures to sea floors with slopes of less although their effects are interrelated than 100. Based on these considerations, as far as foundation engineering for the it is readily apparent that hardware sea floor is concerned, placed on the north slope of St. Croix 4.3 .. ... ... .... '..I C. .. . . ' .. ... " _ .: L ' -_ L .. . . ..I. . . 2 will require that special attention be would not expect radical changes in the given to its location and to the design properties of the sediment as might of its foundation and anchoring system. occur if large amounts of intravoid Otherwise, there is potential risk of water were introduced into sediment the item sliding downslope. A similar interstices by grains (carbonate shells) risk is not expected for hardware placed crushing because of applied load. (From on the basin floor where gradients are a practical standpoifit, these considera- only a few degrees. tions are probably only of academic in- terest in view of the high liquidity Sediments in the basin, like those on indices of the material [Fig. 3.26].) the slope, exhibit high variability in texture and are also characterized as 4.1.2 Processes calcareous oozes. Lee and Clausner [4.3] state that calcareous oozes are very The investigation of the north St. Croix susceptible to failure when subjected to Margin revealed evidence of material repeated loading. The mass physical and being transported downslope to the basin mechanical properties (Part 3) reveal by a variety of processes. Moreover, that the sediments in the Virgin Island there are indications that the downslope Trough and on the north slope of St. movement of materials may have created Croix exist in a metastable state and gullies that scar the face of the slope thus are subject to becoming a viscous and thus contribute to the local morph- fluid when disturbed. Such disturbance ology. The presence of cracks in the could result from installation of a surficial slope sediments reveal failure structure, seismic activity, or wave- and incipient slumping. There is also induced bottom instability during evidence of active bioturbation that storms. As shown in Figure 3.26, the results in the reworking of the surfi- tendency for the material to liquify is cial sediments, keeping them in an un- most pronounced in the upper meter of consolidated state with high liquidity the sea floor; however, the data also indices. All of these factors must be indicate that the sediments are highly considered in designing and installing susceptible to being quick throughout underwater structures and equipment for the upper 18 ft, (5 1/2 m). use in the area. In addition, the steep- ness of the slopes will contribute to The dynamic behavior of calcareous oozes the generation of stresses in the sedi- is not well-understood. Furthermore, ment as it is loaded by the structure. there is little practical experience in This can only aggravate the potential assessing the performance of structure- for failure of the sediment around sediment interaction involving such foundations and anchors. oozes [4.4]. The placement of numerous low-capacity anchors or foundations to With respect to the so-called Salt hold a structure would probably be pre- River, it is important to point out that ferred to relying on a few large-capa- this is really an embayment and not a city supports, especially since the river in the true sense. There is no region is subject to disturbance by source for the "river" and, therefore, seismic activity [4.4, 5]. it is at best an intermittent stream. During periods of high rainfall, signif- Texturally, the slope and basin sedi- icant drainage does occur at the Salt ments are classified overall as sandy- River embayment and this flow transports clay silts. Thus, they are predominantly debris down the slope. cohesive, fine-gained carbonate sedi- ments. Because of this, the liquid Measurements of currents were not con- 4 limits and plasticity indices (Figs. ducted during the investigations covered 3.26, 27) can be considered representa- in this report. The presence of currents tive of the materials. Accordingly, one will induce hydrodynamic drag forces on 4.4 1 underwater structures and also give rise is evidence of bioturbation, the under- to scouring action of the sea floor mining of a structure by benthic fauna around the base of the structure. Some can be minimized by using a mechanical data from the literature are available barrier on the bottom edge of the foot- that may be germane to specific areas of ing [4.9]. This barrier takes the form the region investigated. Shepard and of a skirt or key that penetrates the Dill [4.61 report measurements of cur- sediment to a depth of 6 inches. The key rents 10 ft (3 m) above the bottom at three will also restrict lateral movement of locations along each of the axes of the the structure. The extremely low level Christiansted and Salt River Canyons in of organic carbon (0.5%) indicates that water less than 650 ft (200 m) deep. The the bottom materials are inorganic and measurements were made over a five-day therefore are not subject to weakening period in June 1976. Maximum current by the presence and action of organic velocities were downcanyon and amounted components. to approximately 0.95 and 0.6 fps for the Salt River and the Christiansted 4.2 Geoechnicol * Canyons, respectively. The gathering and analyses of the sedi- Another investigation reported by Under- ment samples (Fig. 3.1) were limited by wood [4.7] which involved a limited time and funding considerations. In view series of current measurements off the of this, the decision was made to con- eastern end of Vieques, showed current centrate on portions of the slope and P velocities to be approximately 2.2 fps basin that lie generally north from the near the bottom. This site is well out- Salt River area. Geographically, this r side the geographic area of this report, region is the most attractive setting but may be relevant from the standpoint for placement of structures and under- of a comprehensive investigation in- water equipment that require intercon- volving the region encompassing and in- nection of activities in the deepest cluding the VIT. From this latter point part of the basin with, say, monitoring of view, some other sources of informa- and ipport equipment situated on the tion that may be of interest are found St. Croix Island. elsewhere [4.6, 91. The selection of parameters and the ap- In cases where an engineer must work proach taken to analyzing and presenting * without the benefit of data on currents, the geotechnical data have been guided Herrmann and Valent 14.91 give some by the recommendations of the Naval * general guidelines for estimating the Civil Engineering Laboratory [4-3]. magnitude of the currents. They recom- mend that at water depths between 30 and 4.2.1 Mass Physical and Mechanical Properties 400 feet and at deeper locations near * bay entrances, narrow passages, etc., a maximum water current velocity of 5.1 The mass physical and mechanical proper- fps may be assumed. In deep water loca- ties (wet unit weight, undrained shear- tions, other than the above, a velocity ing strength, void ratio, etc.) have of 2.3 fps may be assumed. They also been used to establish the variability show a plot of threshold water velocity of the bottom sediments, their loading versus grain diameter for general use in characteristics, and other engineering predicting incipient scouring of sedi- properties. The geotechnical properties ments in deep water. of the sediments consistently show wide variations in magnitude with depth as To minimize the effects of scour, the demonstrated by the values of wet unit footings for structures should have as weight and undrained shear strength low a profile as possible. Because there (Figs. 3.19, 20), In presenting the data, 4.5 envelopes have been constructed on the In applications requiring deflection plots of the parameters to indicate con- sensitive, downward-bearing foundations servative values when applying the para- for which settlements must be predicted, meters to design engineering. For exam- the data are inadequate. Such predic- ple, in using the wet unit weight to tions require knowledge of certain con- calculate the effective overburden pres- solidation parameters, namely: Cc; the sure on an anchor embedded in the sea compression index; C v the coefficient floor, one would use the values from the of consolidation; and Cs, the recompres- left envelope in Figure 3.19 to estimate sion swell index. These parameters were a conservative value of the holding not determined because of funding and strength of the anchor. time constraints. As previously noted, the behavior of calcareous oozes is not The measurements of geotechnical param- well understood, and because of the eters in the cores extend to a depth of paucity of the experience involving the about 18 ft in the sediment and may not interaction of structures and calcareous be adequate for some applications. Lee oozes, the prediction of such settle- and Clausner [4.3] state that for embed- ments may not be possible at present. ment anchors, the strength of the sedi- ment should be known to a subbottom 4.2.2 Index Properties depth of approximately 30 to 50 ft. In situations pertaining to surface-bearing The determinations of the Atterberg foundations, they also state that the Limits (Liquid Limit, Plastic Limit, and strength of the soil should be known to Plasticity Index) reveal the bottom sed- a depth of 1.5-2 times the width of the iments to be of low plasticity and to footing. On the island slope these behave as inorganic silts of medium com- guidelines are probably not applicable pressibility (Fig. 3.27). This finding because of the thin sediment cover. With supports the evidence of very low organ- respect to the basin floor, the applica- ic carbon (0.5%) in the bottom materi- tion of these guidelines for strength- als. In addition, since the calcium car- depth determination will have to be ap- bonate is already broken down into very plied with caution. Because the sedi- fine particles and thus free of intra- ments have sensitivity values in the particle water, the porosities and void "sensitive" and "extra-sensitive" range ratios as measured are considered reli- (Fig. 3.22), the sea floor is weak espe- able. Likewise, the liquid limits for cially after undergoing disturbance. the materials are not subject to being Thus, from the standpoint of load bear- Increased because of water being re- ing capability of the sediment, the de- leased from internal voids in fossil sign of foundation and mooring-and- components of the sediments when such anchoring systems will require special components are crushed under loading. considerations. The susceptibility to strength loss is Lee and Clausner 14.3] note that dis- also reflected in the plot of liquidity turbance of cohesive sediments almost index versus depth (?ig. 3.26). As men- always lowers their in situ strengths by tioned earlier, the values of liquidity about a factor of three or more. The index are greater than unity for most high sediment sensitivities obtained in measurements indicating that any dis- this investigation support their obser- turbance of the sediment results in a vation. As a result, large factors of significant loss of strength. Therefore, safety must be used in selecting sedi- the bottom might be unsuitable for the ment bearing capacities on which to base placement of structures. Details of soil designs of footings for structures. plasticity and other Atterberg limits are found in Appendix A-6.7.4. 4.6 4.3 Obstructions References for Part 4 A comprehensive determination of poten- tial obstructions to future engineering activities in the St. Croix area was not part of this investigation. During the [4.1] Chern, C., and W. Tudor, (1981). data collecting period, however, no sta- Ocean Thermal Conversion (OTEC) tionary floating objects existed in the Project, Bottom Cable Protection surface waters of the survey area except Study; Environmental Characteris- for an acoustic instrument buoy that is tics and Hazards Analysis. Ocean maintained by Tracor Marine. Some effort Engineering Command, Report No. was made during a previous study 14.10) FPO-I-81-(22), pg. 29. to estimate ship traffic in the area. [4.2] Herrmann, H. G. (1982). Founda- Because the data collection involved tions for Small Seafloor Instal- dredging the bottom, contact was made lations. Naval Civil Engineering with the Long Lines Division of the Laboratory, Technical Note American Telephone and Telegraph Company N-1246, p. ii. (AT&T). It was confirmed that no working communication cables existed on the [4.3] Lee, H. J. and Clausner, J. E.. north slope of St. Croix and, therefore, (1979). Seafloor Soil Sampling imposed no restrictions on the dredging and Geotechnical Parameter De- operations. A furnished map shows that termination - Handbook. Civil the nearest cables (emanating from St. Engineering Laboratory, Technical Thomas) pass St. Croix about 7-9 miles Report R873, pgs. 73, 5, 72, 77. east of the island and 11-13 miles west of the island. It is not known, however, [4.41 Winters, W. J., and H. J. Lee, whether or not old, out-of-use cables (1982). Evaluation of Geotechni- (or pipelines) exist on the north slope cal Properties and Slope Stabil- or in the. trough. In discussions with ity of Calcareous Ooze on the AT&T, Tracor Marine, and tracking range South-west Slope off Oahu, personnel, there was no knowledge of any Hawaii. U.S. Department of the obsolete cables leading to the island. Interior Geological Survey, Menlo Park, CA, Open File Report (Pre- The presence of plant and animal life, liminary) No. OTC 82.468B, p. 22. Iparticularly near shore, that might in- fluence biological fouling and attack of 14.51 Van Eepoel, R. P., W. Owen, and structures and underwater equipments A. E. Dammann,(1971). Notes on and, in turn, induce corrosion, were Some Oceanographic and Marine also not assessed other than in a curso- Factors in the U.S. Virgin Is- ry manner. The shallow water areas have lands. Caribbean Research Insti- some grass growing in them, but it did tute, Special Publication No. 2, not appear to be in great abundance. p. 22. Neither was there evidence of large amounts of grass having settled out down [4.6] Shepard, F. P., and R. F. Dill. the island slope [4.11]. The potential (1977). Currents in Submarine for large amounts of plant growth or Canyon Heads off North St. Croix, * coral becoming attached to a cable and, U.S. Virgin Islands. Mar. Geol., in turn, cause damage to the cable be- 24 (1977) M39-M45. cause of hydrodynamic forces interacting with the cable/plant-growth system, is 14.7] Underwood, J. W. (1967). Oceano- not believed to be of major signifi- graphic Cruise Summary, Salvops cance. However, such possibilities need Vieques, USS HOIST (ARS-40). Na- * to be more thoroughly evaluated before a val Oceanographic Office, IR No. definitive position can be taken. 67-16. 4.7 * t- [4.8] Ostericher, C., Jr. (1967). Ocean- (1980). Ocean Engineering and ographic Cruise Summary, Atlantic Construction Project Office, Fleet Tactical Underwater Range; Chesapeake Division, Naval Facil- Southeast Puerto Rico - 1967. Na- ities Engineering Command, Report val Oceanographic Office, IR No. No. FPO-1-80 (1), pgs. A-31 67-76. through A-36. [4.9] Herrmann, H. G., and P. J. Valent, [4.11] Hubbard, D. K., T. H. Suchanek, (1973). Interim Design Guidelines I. P. Gill, S. Cowper, J. C. for Seafloor Footing Foundations. Ogden, J. R. Westerfield, and J. Naval Civil Engineering Labora- Bayes, (in press). Preliminary tory, Technical Report R799, pg.. Studies of the Fate of Shal- 3, 18. low-Water Detritus in the Basin North of St. Croix, U.S. V.I. [4.10] Atlantic Undersea Test and Evalu- Proceedings of 4th International ation Center (AUTEC) Relocation Coral Reef Symposium, Manila, Study, Phase I - Site Selection 1981. 4.8 I' PART 5 * CONCLUSIONS AND RECOMMENDATIONS p S I 5. 1 PART 5. CONCLUSIONS AND RECOMMENDATIONS The following paragraphs summarize the sediment. As a result, the sea floor conclusions reached in the course of is 300 m higher on the east side of this investigatianand list recommenda- the "high" than on the west. tions arising therein. f) A major slump and or tectonic uplift 5.1 Conclusions has altered the configuration of the VIT such that the floor of the trough The significant findings resulting from is markedly narrower at its eastern this investigation are listed below, end (east of the topographic "high"). a) The submarine slope off the north g) Sediment thickness on the north slope side of St. Croix is comparable in of St. Croix is not seismically re- * steepness with most of the steepest solvable. Coring attempts indicate fault scarps of the sea floor. Its that the sediments may, in fact, only overall average gradient ranges be- measure a meter, or so, in thickness. tween 18-23, but local slopes vary In some areas, a thin, hard crust between 5-43*. In general, the steep- exists a few centimeters beneath the er gradients occur on the upper slope sediment surface. * area off Salt River. h) Visual observations show that the b) Submarine canyons cut the slope off north slope of St. Croix is sediment Christiansted Harbor and Salt River, covered and that rock outcrops are respectively, and are the pri..cipal generally infrequent (at least in the topographic features of the slope, areas of observations). This conclu- Both canyons terminate at midslope sion is also supported by dredging and only the larger canyon (Chris- attempts. tiansted) is a major conduit for transporting erosional debris. i) Pelagic deposition, downslope creep, slumping, and turbidity currents are c) The lower slope off Christiansted all active mechanisms that transport * Canyon is dissected by a system of sediment to the basin floor. Despite gullies which transport erosional the steepness of the slope, slumping debris from the canyon mouth to the does not appear to be a major mech- basin floor. Major gullying is not anism on the north slope of St. Croix associated with Salt River Canyon. perhaps because of the thin sediment cover. * d) The floor of the VIT is relatively a smooth, gently sloping (1-2 °) turbi- J) Sediments within the study area can dite plain with a sediment thickness be classified as calcareous oozes of at least 1.5 sec (1500 a) and a composed primarily of biogenic de- regional gradient to the west and bris. The texture of the sediments is south. Most of the sediment filling highly variable but generally falls * the VIT is probably derived from the within the sandy-clay silt range. In Virgin Islands Shelf to the north. general, sand and gravel form a sig- nificant portion of the slope sedi- e) The basin floor is interrupted at ments except off Christiansted Can- 1754'N, 64048'W by a prominent topo- yon. Large, massive blocks of coral graphic "high," which acts as a bar- and rock debris are widely dispersed r rer to the westward transport of on the slope. 5.3 1 U1ASO, M 0I k) With few exceptions, average water changes in their properties because contents range between 55-65%. Some of applied load. of the highest values and greatest ranges were associated with sediments r) Evidence of significant bottom cur- taken from the topographic "high" in rent action was not observed on the the VIT. north slope of St. Croix or in the basin. 1) Wet unit weight displayed significant variability due to localized differ- s) Obstructions (ships, buoys, cables, ences in grain size and degree of etc.) are not expected to be a major consolidation, consideration to future engineering activities. m) Shear strength measurements varied widely, with strength and variability 5.2 Recommendations increasing significantly with depth below the sea floor. The presence of It is felt that the information and in- sensitive and extrasensitive sedi- terpretations presented in this report ments and the high liquidity indices add significantly to existing knowledge (>1.0) indicate potentially unstable of the VIT and, particularly, of the seafloor deposits. This instability north slope of St. Croix. Nevertheless, was also supported by the visual ob- the environmental scenerio remains in- servation of cracks in the slope complete. The following recommendations sediments. address this deficiency. n) The high carbonate content of the a) Further resolution of fine-scale sediments, differing depositional morphologic features (gullies, environments (shelf, slope, trough), terraces, etc.) cannot be accomp- and bioturbation contribute to the lished by conventional bathymetric observed high variability in the geo- surveys. To significantly improve technical properties, sediment types, resolution, it will be necessary to and textures. employ side-scan sonar imagery. o) Steep gradient, sediment instability, b) Current measurements (at any level in and seismic risk combine to make the the water column) are nonexistent for north slope of St. Croix a potential- the most of the VIT. Such measure- ly hazardous environment for engi- ments will be necessary to assess neering applications. In contrast, current/sediment dynamics in the the floor of the VIT is a more stable trough axis and on the escarpments. environment because of much lower gradients. c) The demonstrated variability of the sediment geotechnical parameters p) Given the calcareous nature of the points to the need for further inves- sediments and considerable variabili- tigation of these parameters to ty of the geotechnical parameters, develop a definitive understanding of large factors of safety are advisable the sediments. when choosing bearing capacities for the design of structural footings. d) Additional bottom photographic infor- mation should be collected. Under- q) Atterberg Limits reveal that the sed- water photography provides the only iments are inorganic silts of low means of obtaining direct information plasticity and medium compressibili- concerning the seafloor. Ideally, the ty. As such, it is not anticipated photographic coverage should be co- that they will undergo radical ordinated with a side-scan survey to 5.14 I provide "ground truth" verification analyses, photographs, dredges, cur- of features resolved by the survey, rent meters, etc.) necessary to com- pletely determine the bottom morph- e) This investigation did not attempt an ology, sediment character, and sedi- exhaustive assessment of engineering ment dynamics at the site. obstacles (ships, buoys, cables, etc.). It is suggested that such an g) As in the case of the side-scan sonar assessment be performed prior to any imagery, the use of a submersible is engineering activities, highly recommended for future study of the slope off St. Croix. This is f) Site specific surveys should be con- especially true for site specific ducted at identified areas of engi- survey areas where "ground truth" neering importance. These surveys verification is especially critical. should incorporate all investigative The submersible should be instrument- elements (bathymetry, seismic reflec- ed for in situ measurement of geo- tion, side-scan, sediment cores and technical parameters. 5.5 PART 6. GLOSSARY AOML - Atlantic Oceanographic and decrease in gradient, i.e. base of a Meteorological Laboratories steep slope. ASTM - American Society for Testing Graben - an elongate crustal unit or Materials block that has been lowered by faulting CHESNAVFACENGCOM - Chesapeake Division, relative to the blocks on either side. Naval Facilities Engineering Command Hummocky - uneven, irregular. DSRV - Deep Sea Resarch Vessel Hyperbolae (hyperbolated) - downward NOAA - National Oceanic and Atmospheric arching echo trace produced by a depth Administration recorder and usually caused by some type of "sharp" projection on the sea floor NORDA - Naval Ocean Research and (e.g. outcropping rock ledge on a slope). Development Activity Outcrop - exposure ot rock. NSTL - National Space Technology Pelagic deposition - slow, continuous Laboratories deposition of fine-grained terrigenous * OTEC - Ocean Thermal Energy Conversion minerals and skeletal remains of orga- nisms that live in the surface waters of VIT - Virgin Islands Trough the ocean. Abyssal Plain - see Turbidite Plain Slumping - the sliding downslope of a Acoustic basement - the deepest, continu- mass of unconsolidated sediment. ous observable reflecting horizon in a seismic reflection profile. It may or may Swale - low relief ridge/valley not be "true" basement (see basement). topography; undulating. Acoustically laminated - layering ob- Turbidity current - Bottom-flowing, served in seismic reflection profiles and short-lived, powerful, gravity-driven produced by acoustic impedance differ- current laden with suspended sedime-t rences in the sediment columnr which moves swiftly down a submarine slope and spreads horizontally across the Basement - the primary crustal material basin floor. of the earth between the sedimentary Turbidite - The sediment layer, usually deposits. consisting of sand and silt, which is Bight - prominent bend or curve, deposited by a turbidity current. Escarpment - a long, more or less contin- Turbidite plain - A flat, very gently uous cliff or relatively steep slope fac- sloping region of the sea floor, usually ing in one general direction. at the base of a slope, that has derived Fan - a gently sloping, fan-shaped body most of its sediment from turbidity cur- of detritus normally forming at the ter- rents. Abyssal plains, which are formed * mination of a canyon (or similar feature) in this way have slopes less than 1:1000 or at a place where there is a notable and occur at the base of continental rises. 6.1 $ 4 , t -. ,, / 11 l o e M p f 17055 , 1-39 II--" ~ * - A Slope Map of Portion of St. Croix Margin A / / / / / / /AI / / / 7,7/ ~ / / '7, / / - // // /77 / / /7 / / "-7 // 'VI---,,, I 13 / 7 .7 7 / / 7 7 7 / 7 '7 777 r, / 7- / 7 /7 7 -'7', '', 7,7- 7, 7 7 7 ~~~~7~' .,-, flfldfY.'' ,77 7 '"A', 2.' HHt7 7 ,-,-/~,, -> 7 'H,. H,>,, '-~7 7 7,-' 7 'H, /01 . 17050' 'to 1705'N./~-- 6455 400 X/ / ST. CROIX 7 CONOU 7, / R QA/ . 7/, 50 640451 7 i I /. # ,, . [/ '"" /"/I //-*X " 7 II i~l ,ii.i7'. ii7ii '1,-' d I7 7 *..*I I' I.I '"' 7'.x., -x.- 7' 7. ,11 ,x1 '-"7'/ -,'..' "' ",'- ' ' 7 , , , ' 7 7, 7 / ' ' 7 7 7,'',,, 7' /., 7, 77' I X'/ 77I ,l.7 <-7" 7'/ ""7,"''o . 7' 7 (Gradie"7of77' '7'',-'Z////,",/.Z"I ll" il D I