MARINE GEOLOGY OF THE SOUTHERN VIRGIN ISLAND PLATFORM 



Dr. Charles Holmes and Dr. Louis E. Garrison 

 Office of Marine Geology, Geological Survey, 

 U. S . Department of Interior 



ABSTRACT 



Data for these studies were gathered aboard the R/V ADVANCE 

 II in two phases. The first phase consisted of high-resolution 

 seismic profiles made from April 7-14 by Louis Garrison and 

 Jack Lee of the USGS. The second phase consisted of water 

 samples and grab samples of the bottom sediments. These were 

 collected from April 14 to 18 and from October 18 to 23 by 

 Charles Holmes of the USGS. All data collected are being 

 analyzed under a cooperative program between the USGS and 

 the Caribbean Research Institute. A final report will 

 be made in 1971. 



Objective: To study the structural geology of the Virgin Island 

 Platform. 



To construct a preliminary map of the distribution of 

 certain trace elements in the water and bottom sediments 

 around St . Thomas . 



To map and date submerged features around the Virgin Island 

 Platform and St. Croix which relate to former glacially 

 lowered sea levels . 



Equipment: High resolution seismic equipment, and water and bottom 

 sampling equipment. 



Procedures: Seismic and sampling profiles were conducted aboard the 

 ADVANCE II. 



Results; 



Some preliminary results of interest are: 



1) an apparent geochemical anomaly in the waters south 

 of St. Thomas, 



2) a widespread distribution of calcareous nodules which 

 may be an important source of sediment on the platform, 



3) the subsurface division of the southern part of the 

 Virgin Islands platform into a) a northwestern area 

 of incoherent reflections and small scale roughness 



on the surface, and b) a southeastern area of distinct, 

 well-layered reflections and an undulatory surface. 

 A rather sharp boundary zone running southwesterly 

 from Bovocoap Pt., St. John divides the two areas. 

 This could be interpreted as a fault displacement 

 which downdropped the southeastern block, allowing 



VII-41 



