SECT. 3] TUKBIDITY CURRENTS 751 



shelf covered the Mid-Atlantic Ridge northwest of St. Paul's Rocks and became 

 submerged during a catastrophe of seismic-volcanic character a few hundred 

 thousand years ago, material like that found in the Albatross cores might have 

 become distributed over the adjacent sea bottom." 



C. Creep 



Lombard (1956), referring to the transportation of ancient sediments, and 

 Koczy and Burri (1958), referring to the present deep-sea environment, favor 

 as an alternative to turbidity currents a slow creep which might require 

 hundreds or thousands of years to traverse a distance that a turbidity current 

 would cross in only a few hours. That slumps, slides and other slower and more 

 massive slope failures occur in the ocean is clear from the evidence of topo- 

 graphy, of sediments and of sedimentary structures. But the results of such slow 

 movements are quite different from the graded, well-sorted deposits laid down 

 by turbidity currents ti'avelling at express train speeds over the ocean floor. 

 Koczy (1954) notes that: "Near the shelf the surface is often undulating 

 because of the presence of the sediment sliding down the continental slope. 

 This movement can be assumed to be very slow." 



This slow creep, though certainly of importance over wide areas, cannot be 

 considered as a substitute for turbidity currents, but rather as an additional 

 and independent process (Kuenen, 1956). 



D. Tsunami 



Bucher (1940) formerly thought that tsunami waves set up by the Grand 

 Banks earthquake had caused the later sequence of cable breaks. Indeed there 

 seems to be a relationship between tsunami and cable breaks; however, they 

 are probably not cause and effect but both result from the motion of the slump 

 and ensuing turbidity current. Heezen and Ewing (1952) concur with Guten- 

 berg's (1939) view that the "principal cause of each tsunami is submarine 

 slumping". However, the turbidity currents themselves probably are also 

 capable of generating tsunami. That tsunami could not be the direct result of 

 earthquakes was proved by the fact that not all large submarine earthquakes 

 produce tsunami. In order to prove that turbidity currents and slumps alone 

 could produce a tsunami, we clearly need to find a tsunami and a related series 

 of cable failures not preceded by an earthquake; or to disprove the concept, to 

 discover a turbidity current lacking an associated tsunami. 



4. Physiographic Evidence 



Following World War II continuously recording echo-sounders were first 

 extensively used in deep water. One of the first discoveries was that, while 

 most of the deep-sea floor is covered by hills and mountains, large areas in the 

 deepest portions of the ocean are covered by abyssal plains (Heezen, Ewing and 

 Ericson, 1954). The origin of these vast flat areas posed a real problem until the 

 concept that turbidity currents were filling the oceanic depressions offered a 



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