Patterns of deposition at the continental margin 305 



What factual data exist at present are all on the dcpositional side. In areas of low 

 gradients we have the deep sea plains, crossed by the mid-ocean channels with their 

 leveed banks (Menard, 1955; Dietz, 1955; Heezen et ai, 1955), and the same t\pc 

 of channels are found in various Swiss lakes (Daly, 1936). Graded bedding. long a 

 somewhat obscure dcpositional process, can be best explained by turbidity flows. 



So much attention has been focused on the supposed potential energy of the 

 turbidity current by their proponents that the factors that will dissipate this energy 

 such as friction of turbulent flow along boundaries, entraining of clear water, and 

 settling of particles have all but been lost sight of. One source of confusion is the 

 difference between a true turbidity current and a submarine slump or slide. Doubtless 

 a slide starting on a steep slope would eventually grade into a turbidity current, but 

 there must be a great diff'erence in energy at the start as well as in the mechanics of 

 their motion and their erosive power. Witness the conflict of opinion over the veloci- 

 ties and even the type of the Grand Banks " flow " which parted the Western Union 

 cables following the earthquake of 1928 (Kullenberg, 1954). Possibly this may 

 seem academic, but when the term " turbidity current " is mentioned many people 

 visualize a suspension current such as flows along the bottom of Lake Mead or in the 

 Swiss lakes. These patently have no erosive power; and it is not a question of having 

 a super Colorado River and producing a turbid suspension of greater volume. 

 Possibly a mud or sediment slide or flow would be a better term to apply to the initial 

 slump of unstable sediment failing under gravitational stress, restricting turbidity 

 current to the transporting and depositing agent. When looked at objectively, the 

 dynamic role assigned to density currents by their advocates may be seen to have 

 more than a tinge of wishful thinking induced by an attempt to escape from the 

 horns of a dilemma. Obviously an enormous amount of vigorous erosion must have 

 taken place to cut the submarine canyons and to make matters worse, during the late 

 glacial period, which is only yesterday. Conventional rivers would require more 

 relative displacement of land and sea than most geologists will admit is probable 

 or possible. To circumvent this, the turbidity current has been hailed almost over- 

 night as the answer to all vexing problems of submarine erosion. 



The topography of the continental slope in the northwestern Gulf of Mexico is 

 very complicated as Gealy (1955) has pointed out, and, according to her analysis, 

 has probably been produced by a variety of causes. Sub-aerial erosion, the failure 

 of unstable sediments which are under shearing stress, salt domes and crustal faulting 

 have all played a part. An extensive scarp bounds the Sigsee Deep and continues 

 around the continental slope off" the west Florida platform, where it is found to be 

 over 7000 feet high (Jordan, 1951 and Fig. 3). Pronounced normal faulting could be 

 the only cause for cuff's such as these. Higher up on the slope the troughs and ridges 

 and hummocky topography is thought to be the surface expression of lesser shearing 

 ai depth plus erosion by sediment flows and slumps. The material is largely furnished 

 by the thick Pleistocene deposits lying on the upper parts of the slope. 



MOBILE CONTINENTAL MARGINS 



As an historical background to the existing terraces, consider the - long succession 



of geosynclines of diff-ering kinds and trends ^ with their attendant island arcs, such 



as has been postulated by many authors (Kay, 1951) lying along the Atlantic and 



Gulf margins during Palaeozoic time. In a situation such as this the supply of detritus 



