Ch. 2— Resource Assessments and Expectations • 45 



lution mining) and, tlius, represent potentially re- 

 coverable mineral commodities from offshore de- 

 posits, although at present they would not be likely 

 prospects in the Atlantic region. 



While they have potential for oil and gas forma- 

 tion and entrapment, the bulk of these sedimen- 

 tary rocks are not likely to be good prospects for 

 hard minerals recovery because of their depth of 

 burial. Exceptions could occur in very favorable cir- 

 cumstances where a sufficiently high-grade deposit 

 might be found near the surface in less than 300 

 feet of water or where it could be dissolved and ex- 

 tracted through a borehole. Better prospects, par- 

 ticularly for locating potentially economic and 

 mineable placer deposits, would be in the overly- 

 ing Pleistocene and surficial sand and gravel. 



The igneous and metamorphic basement rocks 

 of the continental shelf, although possible sites of 

 mineral deposits, would be extremely unlikely pros- 

 pects for economic recovery because of their depth 

 of burial. The oceanic crust that formed under what 

 is now the slope and rise also probably contains ac- 

 cumulations of potential ore minerals, but these too 

 would not be accessible. The best possibility for lo- 

 cating metallic minerals deposits in bedrock in the 

 Atlantic EEZ probably would be in the continen- 

 tal shelf off the coast of Maine where the sediments 

 are thinner or absent and the regional geology is 

 favorable. There are metallic mineral deposits in 

 the region and base-metal sulfide deposits are mined 

 in Canada's New Brunswick. 



One other area that may be of interest is the 

 Blake Plateau located about 60 miles off the coasts 

 of Florida and Georgia. It extends about 500 miles 

 from north to south and is approximately 200 miles 

 wide at its widest part, covering an area of about 

 100,000 square miles. The Blake Plateau is thought 

 to be a mass of continental crust that was an ex- 

 tension of North America left behind during rift- 

 ing. There is some expectation that microcontinents 

 such as the Blake Plateau might be more mineral- 

 ized than parent continents or the general ocean- 

 floor, and, because they have received little sedi- 

 ment, their bedrock mineral deposits should be 

 more accessible.' 



*K.O. Emery and B. J. Skinner, "Mineral Deposits of the Deep- 

 Ocean Floor," Marine Mining, vol. 1 (1977), No. 1/2, pp. 1-71. 



Sand and Gravel 



Sand and gravel are high-volume but relatively 

 low-cost commodities, which are largely used as ag- 

 gregate in the construction industry. Beach nourish- 

 ment and erosion control is another common use 

 of sand. Along the Atlantic coast most sand and 

 gravel is mined from sources onshore except for a 

 minor amount in the New York City area. For an 

 offshore deposit to be economic, extraction and 

 transportation costs must be kept to a minimum. 

 Hence, although the EEZ extends 200 nautical 

 miles seaward, the maximum practical limit for 

 sand and gravel resource assessments would be the 

 outer edge of the continental shelf. However, the 

 economics of current dredging technology neces- 

 sitate relatively shallow water, generally not greater 

 than 130 feet, and general proximity to areas of high 

 consumption. While these factors would further 

 limit prospective areas to the inner continental shelf 

 regions, they could potentially include almost the 

 entire nearshore region from Miami to Boston. 



Sand and gravel are terms used for different size 

 classifications of unconsolidated sedimentary ma- 

 terial composed of numerous rock types. The ma- 

 jor constituent of sand is quartz, although other 

 minerals and rock fragments are present. Gravel, 

 because of its larger size, usually consists of multi- 

 ple-grained rock fragments. Sand is generally de- 

 fined as material that passes through a No. 4 mesh 

 (0.187-inch) U.S. Standard sieve and is retained 

 on a No. 200 mesh (0.0029-inch) U.S. Standard 

 sieve. Gravel is material in the range of 0.187 to 

 3 inches in diameter. 



Because most uses for sand and gravel specify 

 grain size, shape, type and uniformity of material, 

 maximum clay content, and other characteristics, 

 the attractiveness of a deposit can depend on how 

 closely it matches particular needs in order to min- 

 imize additional processing. Thus the sorting and 

 uniformity of an offshore deposit also will be de- 

 terminants in its potential utilization. 



The Atlantic continental shelf varies in width 

 from over 125 mUes in the north to less than 2 miles 

 off southern Florida. The depth of water at the outer 

 edge of the shelf varies from 65 feet off the Florida 

 Keys to more than 525 feet on Georges Bank and 

 the Scotian Shelf. A combination of glacial, out- 

 wash, subaerial, and marine processes have deter- 



