247 



A broad regional study of the geology of the continental shelf and sloi)e off 

 the Atlantic coast of the United States was begun in late 1962 as a joint program 

 of the U.S. Geological Survey and the Woods Hole Oeeanographic Institution 

 (Emery and Schlee, 1963). The study includes investigations of topography sedi- 

 ments, lithology, and geological structure. By the end of 11M;4 a topographic 

 compilation was completed, with a series of four general charts and many more- 

 detailed small ones ( Uchupi, 19(>5 ; R. M. Pratt, report in preparation). General 

 sampling of the sea floor also was nearly finished, with about 1,800 large surface 

 samples from an 18-kilometer grid that extends from shore to depths as great as 

 4,000 meters. In addition, about 8,000 km of continuous seismic profiles were run 

 at 50- to 100-km spacing. Preliminary examination of the results permits the 

 outlining of the shallower portions of the region that are most likely to contain 

 mineral deposits of economic value (fig. 1). Additional detailed dredging and 

 study of geological and geophysical data should narrow the areas of maximum 

 interest and possibly reveal additional areas not shown by figure 1. 



SAND AND GRAVEL 



The most widespread resource to be exploited from the surface of the con- 

 tinental shelf is probably sand and gravel for construction use as concrete 

 aggregate, road material, and material for beach widening. As shown by D'Amieo 

 (1964), the value of sand and gravel quarried on land in the United States during 

 1963 was $849 million, or about 20 percent of the total value of all nonmetallic 

 minerals other than fuels. For sea-floor operations, this relation is apt to be more 

 heavily weighted in favor of sand and gravel because of the wide distribution, 

 great thickness, lack of overburden, and cheap barge transportation of tliese 

 materials. The samples that have been obtained to date show relict sand to 

 present throughout most of the length of the continental shelf, extending from 

 an average depth below sea level of about 20 m to a depth of 80 to 140 m near 

 the shelf edge (fig. 1). Thickness of the sand is as much as 60 m. The sand 

 consists mainly of quartz and feldspar, and it is coarser jjrained than the modem 

 sediment occurring both landward and seaward of it. Sediment on the shelf just 

 north of Miami is highly calcareous and thus is unsuitable for many construction 

 purposes. Surface sediment in the Gulf of Maine (off Boston and Portland, fig. 1) 

 consists chiefly of silty clay and poorly sorted mixtures of clay, silt, sand, and 

 gravel. 



The relict nature of the sand on most of the continental shelf is indicated 

 by a grain size coarser than that of sand nearer shore, and by the typical 

 presence of iron stain and solution pits. The relict sand also contains oc- 

 casional shells of the common edible oyster that normally lives at depths of 

 only a few meters within coastal estuaries and lagoons. Radiocarbon ages ob- 

 tained for several specimens range from 8,000 to 11,000 years (Merrill and 

 others, 1965). In summary, the relict sand appears to consist of ancient shore 

 deposits that were formed as the ocean transgressed the continental shelf 

 at the end of the latest glacial epoch. Supporting evidence for that environ- 

 ment of deposition is given by the presence of submerged terraces and beach 

 ridges on the shelf. The independence of the offshore relict sand from the 

 inshore modern sand means that the relict sand probably can be mined without 

 disturbing the shoreline equilibrium. 



Gravel is subordinate to sand on the continental shelf, it may be abundant 

 in two general areas. One of these is a large gravel fan off New York City, 

 where gravel must have been deposited by the Hudson River as a sheet per- 

 haps in a manner similar to that of the widespread gravels on the Atlantic 

 Coastal Plain ( Schlee, 1957 ; 1964) . The other area is atop the northern margin 

 of Georges Bank (the shelf projection southeast of Boston, flg. 1), where re- 

 sidual gravels from glacial till, glacial outwash, and Tertiary strata have 

 been concentrated by the winnowing action of tidal currents. 



