Structural Geology for Petroleum Geologists

By William L. Russell | Go to book overview

CHAPTER 14
STRUCTURE OF CONTINENTAL SHELVES

The interest of oilmen and petroleum geologists in the continental shelves is occasioned primarily by the realization that they contain great reserves of oil. However, entirely aside from the commercial developments, the continental shelves are of considerable interest because of their bearing on the general structural problems of geosynclines, continents, and oceans. In their search for oil fields on the continental shelves, geologists and geophysicists look for local structures. A knowledge of the general or regional structure of the continental shelves would doubtless be an aid in prospecting for these local structures.


GEOGRAPHY

Near the shores of the continents the sea bottom slopes very gently toward the oceans for some distance. At a depth of roughly 450 ft the angle of slope increases abruptly. This change marks the seaward boundary of the continental shelf and the landward edge of the continental slope. The depth of the water at the edge of the continental shelf varies from 400 to 600 ft, and is therefore fairly constant all over the world. The width of the continental shelf varies from a few miles to over 300 miles, and the angle of slope of the shelf varies in a corresponding manner. In the United States the continental shelves are narrow off southern California and wide off Louisiana, western Florida, and eastern New England. The continental shelves are about 250 miles wide off eastern Maine, and even wider in the region of the Grand Banks off southeastern Canada. Wide continental shelves are also found in the region of the East Indies, east of China, between New Guinea and Australia, in the North Sea, near Bering Strait, and off the shores of the Arctic Ocean.

The height of the continental slope may average around 12,000 ft, its width 10 to 75 miles, and its angle of slope generally 2 to 15 per cent. According to Shepard,1 the average angle of slope of the continental slopes is 4° 17′ for the first 1,000 fathoms of descent. In some areas, for example off the western coast of Florida, there are definite cliffs on the continental slopes which must be fault scarps. Between the steep continental slopes

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1
Francis P. Shepard, "Submarine Geology," Harper & Brothers, New York, p. 187 ( 1948).

-335-

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