764 HEEZEN [chap. 27 



sweeping flows, such as the Grand Banks turbidity current, and as narrowly 

 channelized flows which create characteristic physiographic features, such as 

 natural leveed channels, deep-sea fans and deep-sea cones. AU investigators 

 are now convinced of the existence of turbidity currents, and their relatively 

 frequent occurrence in the deep sea. 



However, there is one point which is still quite controversial and that is the 

 capacity of turbidity currents to erode. It is held by several workers that 

 turbidity currents cannot erode into solid rock. Indeed, if turbidity currents 

 are to be considered the primary agent for the erosion of submarine canyons, 

 it would seem that they must have appreciable capacity to erode. In Chapter 12 

 on Submarine Topography and in Chapter 14 on the Abyssal Plains, physio- 

 graphic evidence for turbidity currents has been more extensively summarized. 



8. Biological Significance of Turbidity Currents 



The occurrence of carbonaceous deposits in the deep-sea alluvial cones led 

 to speculation on the effect of this material on the nutrition of the abyssal 

 fauna (Heezen, Ewing and Menzies, 1955, 1957). It was reasoned that turbidity 

 flows would have two radically contrasting effects on the abyssal fauna. Rapid 

 deposition of a thick bed of sediment over the abyssal floor must have a tre- 

 mendous destructive effect, smothering by burial much of the indigenous 

 fauna. In contrast, the large windfalls of organic debris brought to the deep sea 

 by turbidity currents must support a greater productivity than the normal, 

 slow, pelagic deposition. 



Repeated turbidity currents from an organic-rich source build an organic- 

 rich submarine deposit containing both the transported organic debris and the 

 trapped and entombed abyssal fauna. 



Another important effect of turbidity currents on petroleum occurrence is 

 independent of the bottom fauna. Orr, Emery and Grady (1958) found that 

 while the content of chlorophyll derivatives in sediment immediately below 

 turbidity-current-deposited sands was anomalously high, that of the sands 

 themselves was anomalously low. The turbidity current which brought in the 

 sands was apparently derived from a source low in organic matter and thus in 

 this case only the blanketing effect of the turbidity-current deposit is seen. 



Since the geologist's key to the past is the present, the implications of 

 submarine turbidity currents as regards paleographic reconstructions are tre- 

 mendous. No longer can scattered shallow-water fossils be considered evidence 

 of shallow-water deposition, nor does evidence of strong currents indicate 

 shallow depths. Even ripple marks, long considered an absolute proof of shallow- 

 water origin, have been found as deep as 2000 fm. 



9. Other Processes of Deep-Sea Erosion and Transportation 



A. Abyssal Ripples 



When photographs of the deep-sea floor were first taken in appreciable 

 numbers the frequent discovery of ripple marks and scour marks was simply 



