72 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



In discussing continental shelves, Pratt (1947, 

 p. 661) observed that "modern investigations 

 have also confirmed Nansen's pioneer observation 

 that the inland portion of the continental shelf is 

 a surface of degradation." Umbgrove (1946, 

 p. 249) stated, "it appears that the history of the 

 shelf was rather complicated. Sedimentation, 

 abrasion, and denudation played their role. The 

 area was subjected to changes of sea-level and 

 movements of the bottom. Wind-waves and 

 tidal currents acted upon the sediments of the 

 shelf. The influence of each of these and still 

 more factors in the building of the submerged 

 part of the continental margin is still an open 

 question." He also believed that the landward 

 part of the shelf may have resulted from plana- 

 tion when the sea was some 300 feet lower than 

 at present. 



Many workers believed that the topography 

 of the shelves resulted from subaerial erosion. 

 Dana (1863, p. 441) stated this was accomplished 

 by the elevating of the land. Long coast lines 

 would have to be uniformly elevated to such 

 heights that most geologists agree the hypothesis 

 has too many difficulties to be acceptable. The 

 lowering of sea level could also produce conditions 

 for subaerial erosion. Shepard and Emery (1941, 

 p. 154) found that the formation of Pleistocene 

 ice could account for lowering sea level 2,200 

 feet; Veatch and Smith (1939, p. 41) believed 

 sea level was lowered 12,000 feet and restored 

 in the last 25,000 years; Fisk (1944, p. 68) found 

 evidence for a drop of sea level of 400 to 450 

 feet; and Carsey (1950, p. 375) suggested that if 

 sea level was lowered 420 to 480 feet "the origin 

 of the shelves could be attributed' largely to wave 

 planation." 



The irregularities of the bottom of the shelves 

 and the great valley-like notches along the out- 

 ward slopes of the shelves are also unsolved prob- 

 lems. Umbgrove (1946, p. 249) believed "the 

 phenomena of the continental margin are corre- 

 lated with other periodic events occurring in the 

 earth's crust and its substratum." 



Daly (1936, p. 401) introduced the idea of 

 density currents or "bottom streams of sea water 

 containing mud in suspension and therefore tem- 

 porarily endowed with density greater than that 

 normal to the clean water overlying the respective 

 continental terraces. It is further supposed that 



the conditions for the formation of such bottom 

 currents were specially developed at certain stages 

 of the Glacial Period ..." This heavy mass of 

 mud and water would naturally move into the 

 depressions on the continental shelf, and in places 

 it would flow over the margin of the shelf and 

 down the continental slope with accelerated mo- 

 tion and force. 



A new hypothesis for the origin of continental 

 slopes and submarine canyons has been suggested 

 by Emery (1950, pp. 102-104). He proposed 

 that "thrusting along a shear plane at the con- 

 tinental margins may result in a temporary up- 

 bulging of the margins above sea level. During 

 the time of exposure erosion by streams should 

 have incised canyons which now, after isostatic 

 readjustment of the margins, constitute the widely 

 distributed submarine canyons. Known down- 

 warped peneplains below the surface of con- 

 tinental shelves may have been developed on the 

 bulged margins by long-continued erosion. The 

 margins may, thus, have served as sources of 

 some sediments now found on land and believed 

 to have been derived from a seaward direction." 



Kuenen (1950, p. 497) adhered to the beUef 

 that "the action of turbidity currents, especially 

 during the ice ages" cut the submarine canyons 

 along the edge of the shelf and slope of the 

 continents. 



An examination of the maps of the topography 

 of the outer shelf and slope of the northern Gulf 

 of Mexico shows many features which suggest an 

 origin due to density currents and the deposition 

 of the mass of mud. Also, continental shelf fauna 

 dredged from the Mexican Basin may have been 

 transported from the shelf by turbidity currents. 

 Furthermore, these currents may have carried 

 sediment to the central Gulf and, therefore, aided 

 in developing the rather flat floor of the Mexican 

 Basin. 



GEOMORPHOLOGY BY AREAS 



Soundings in only a few areas of the Gulf are 

 adequate to permit the drawing of accurate maps 

 of the surface of the continental slope. More 

 information is available concerning the northern 

 Gulf; therefore, this area is discussed in some 

 detail starting with the Straits of Florida and 

 progressing in a counterclockwise direction. 



