54 • Marine Minerals: Expioring Our New Ocean Frontier 



Figure 2-6.— Potential Hard Mineral Resources of the Atlantic, Gulf, and Pacific EEZs 



SOURCES; Office of Tecfinology Assessment, 1987; U.S. Department of ttie Interior, "Symposium Proceedings— A National Program for the Assessment and Develop- 

 ment of the Mineral Resources of ttie United States Exclusive Economic Zone," U.S. Geological Survey Circular 929, 1983. 



PUERTO RICO AND THE U.S. VIRGIN ISLANDS 



Puerto Rico and the U.S. Virgin Islands are part 

 of an island arc complex with narrow insular shelves. 

 The geologic environment of this type of active plate 

 boundary suggests that sand and gravel deposits 

 would not be extensive and that placer mineral as- 

 semblages would be relatively immature. 



Sand and Gravel 



Modern and relict nearshore delta deposits are 

 the main source of offshore sediment for both 

 Puerto Rico and the U.S. Virgin Islands. Further 

 offshore the clastic sediments contain increasing 



amounts of carbonate material. In general, the is- 

 lands lack large offshore sand deposits because wave 

 action and coastal currents tend to rework and 

 transport the sand across the narrow shelves into 

 deep water. Submarine canyons also play a role in 

 providing a conduit through which sand migrates 

 off the shelf. The outer edge of the shelves is at a 

 water depth of around 330 feet. 



Three major sand bodies are located on the shelf 

 of Puerto Rico in water depths of less than 65 feet. 

 As one might expect in an area of westward mov- 

 ing wirids and water currents, all three deposits are 

 at the western ends of islands. Inferred resources 



