504 SHEPARD [chap. 20 



10. Pleistocene Canyon Cutting by Turbidity Currents 



Daly, Kuenen, and the Lament scientists have all considered that the cutting 

 of submarine canyons took place during the Pleistocene glacial epochs when 

 the sea-level was lowered so as to allow the rivers to carry sediments directly 

 to the edge of the shelf and hence supply materials for canyon cutting on the 

 slope below. Such a lowering certainly must have helped develo]) turbidity 

 currents but there are some serious objections to this idea. In the first place 

 such erosion should affect only the sloj^es, but the majority of the known 

 canyons penetrate deeply into the continental shelves inside. It seems obvious 

 that if the canyons were cut by turbidity currents, these currents must have 

 operated across the shelves and, in fact, this is well demonstrated by cable 

 breaks across canyon heads in various places. These breaks may be due to 

 laiidshdes but these in turn can be expected to produce turbidity currents. 



Further opposition to a Pleistocene cutting of the canyons comes from their 

 vast size, particularly where they have walls of granite or other hard rocks. If 

 we assume that these hard-rock canyons are submerged river valleys, we are 

 still confronted by the vast size of most other canyons. The walls of the east 

 coast canyons rise hundreds of fathoms and we know that some of them are cut 

 in moderately well consolidated rocks of Cretaceous age. To think that the 

 necessarily infrequent turbidity currents could have produced such vast cuts in 

 the small fraction of a million years during which the sea uncovered the shelf 

 is giving a role to turbidity currents that is certainly not indicated by their 

 typical fine-grained sediments nor by their a]:)parent failure to remove soft 

 underlying sediments. The turbidity currents have been estimated to come 

 down the southern California canyons about once in 100 years (Gorsline and 

 Emery, 1959) and, so far as we know, these do not erode the canyons 

 appreciably. 



If Menard (1960) is correct about the size of the fans along the west coast, it 

 would appear that the normal products of sand transportation along the shore 

 to tlie canyon heads would have had to have been in operation for many 

 millions of years to develop the fans, quite in addition to any erosion within 

 the canyons themselves. The turbidity currents coming into the Los Angeles 

 and Ventura basins were very likely directed along canyons which were, of 

 course, far older than the glacial stages. Whether we believe in a tiu-bidity- 

 current origin or not, it is far easier to explain the canyons as older than 

 Pleistocene. The investigations now in progress in Scripps Canyon (Dill. 19(52) 

 appear to lend considerable weight to erosion at the canyon heads by slumping 

 action. 



References 



Bourcart, J., 1959. Morphologie du precontinent des Pyrenees a la sardaigne. In La 

 Topographie et la Oeologie des Profondeurs Ocianiques. Centre Nat. Rec. Sci., Paris, 

 33-52. 



Buflfington, E. C, 1951. (iullied submarine slopes off southern California (abstr.). Bull. 

 Oeol. Soc. Amer., 62, 1497. 



