SECT. 3J TURBIDITY CURRENTS 761 



7. Tectonic Significance of Turbidity ('urrents 



A. Modern Geosynclines 



Turbidity currents are gradually filling the modern deep-sea trenches. 

 Turbidity-current smoothed trench (abyssal) plains have now been reported 

 from every trench which has been sounded in detail. Trenches bound most of 

 the circumference of the Pacific Ocean, but in the Atlantic such marginal 

 topographic trenches are exceedingly rare. However, wherever seismic-refrac- 

 tion studies have been made at the base of the continental slope, a deep trough 

 of lower velocity material ascribed to unconsolidated sediments and sedimentary 

 rock has been found paralleling the base of the continental slope (Fig. 16). 

 These former trenches were probably filled largely by turbidity-current sedi- 

 ments. They constitute in all probability the modern geosynclines which quietly 

 await the process of orogeny which, through deformation and uf)lift, may 

 eventually turn them into folded mountains. 



The effect of turbidity currents over a long period of geologic time is to fill in 

 the hollows of the deep-sea floor by building vast abyssal plains, and to erode 

 canyons into the continental slopes. Of course, if the rate of subsidence of the 

 marginal trenches exceeds the rate of deposition, broad basin-floor abyssal 

 plains would not form since all turbidity-current deposits would be trapped in 

 the trenches. The effect of turbidity currents in the deep sea produces a topo- 

 graphy essentially similar to that produced by periodic downpours in desert 

 mountains. The coalescing alluvial fans resemble the continental rise, and the 

 play as resemble the abyssal plains. 



Unfortunately, the turbidity-current concept has been occasionally over- 

 worked even in regard to recent deposits. By invoking turbidity-current action 

 Rigby and Burckle (1958) attempted to explain the emplacement of thin beds 

 (millimeters thick) of freshwater diatoms found in cores of mid-equatorial 

 Atlantic eupelagic sediment. This explanation is no less ridiculous than the 

 theory proposed by Malaise (1957) who believes them to be deposits laid down 

 in ponds during a recent and complete emergence of the Mid- Atlantic Ridge. 

 The Congo River at present carries freshwater diatoms hundreds of miles 

 offshore. Kolbe (1957, 1957a) points out that they are also blown hundreds of 

 miles to sea by the wind. To emplace diatoms on the Mid-Atlantic Ridge as 

 suggested by Rigby and Burckle (1958) would require turbidity currents to 

 climb over 3000 m in 500 miles, travelling all the way over an intensely rugged 

 bottom, an ability not attributable to turbidity currents. Rigby and Burckle do 

 not doubt that surface currents, or perhaps the wind, can transport diatoms as 

 far as the core stations on the Mid-Atlantic Ridge, but they do not believe, 

 for some unspecified reason, that ocean currents could deposit and concentrate 

 beds of freshwater diatoms. 



The geological record contained in the ancient sediments is sufficiently com- 

 plex for other misapplications undoubtedly to occur. 



Turbidity-current deposits have been described from the modern basins of 

 sedimentation off southern California by Gorsline and Emery (1959). These 



