360 HEEZEN AND LAUGHTON [CHAP. 14 



produced the characteristic shape of the abyssal plains and, of course, bottom 

 currents would not be expected to produce beds of sand or silt exclusively of 

 continental origin. One would expect that if a bottom current produced sand 

 by winno\\ing, shells of pelagic organisms would ])lay a large part. However, 

 pelagic components are less abundant in turbidite deep-sea sands than in 

 pelagic oozes. 



G. Burial of Original Topography by Turbidity Currents 



The hypothesis that abyssal plains were formed by deposits of turbidity 

 currents, which have gradually covered the original topography, was made by 

 Heezen, Ewing and Ericson (1951, 195")). This hypothesis was based on (1) 

 the existence of flat, nearly level surfaces sloping away from areas adjacent to 

 sediment sources; (2) the occurrence of graded sand layers, shallow-water 

 benthonic Foraminifera, gray clays and silt layers in cores from the abyssal 

 plains, ail indications of turbidity-current transportation ; (3) the partially 

 buried appearance of seamoiuits protruding from the plain. Heezen and 

 Ewing (1952) interpreted the submarine cable breaks which occurred for 13 h 

 following the 1929 Grand Banks earthquake as the result of turbidity currents 

 running down the continental rise and depositing a layer of graded sand on 

 the abyssal plain to the south. In 1952 the prediction of an uppermost layer of 

 graded sediment was confirmed by the results of five coring locations on the 

 northern part of the Sohm Abyssal Plain south of Cabot Strait. Locher (1954), 

 from a study of five cores taken from the abyssal plain of the western equatorial 

 Atlantic, concluded that the deep-sea sands and shallow-water Foraminifera 

 (Phleger, 1954) contained in these cores were transported from the South 

 American continent to the plains by turbidity currents. He further suggested 

 that the flatness of the plains was the result of this process. 



Turbidity currents not only account for the existence of abyssal plains but 

 provide explanations for many of the characteristic features found on the 

 continental rise. The submarine canyons, deep-sea channels, deep-sea fans and 

 great abyssal cones of the continental margin can best be explained through 

 turbidity-current transportation, erosion and redeposition of sediment. Areas 

 of the sea floor which turbidity currents can reach are in general smooth. Areas 

 which turbidity currents cannot reach, because basins or trenches form sedi- 

 ment traps between the deep-sea floor and the continents, are areas of rough 

 topography. The smooth to])ography of the continental rise and the abyssal 

 plains does not necessarily slope away from the continent. In some areas the 

 slope is parallel to the continental margin in places where the outward flow of 

 turbidity currents is obstructed by mountains which act as dams. The basins 

 with irregular bottoms are common in places which are shielded from turbidity 

 currents by ridges or troughs. Similar basins have not been found in places 

 which are not shielded from turbidity currents originating on the continental 

 margin. 



If one considers the highly hypothetical ocean basin which is just beginning 



