SECT. 3] 



TURBIDITY CURRKNTS 



759 



this can be supplied by earthquakes, hurricanes impinging on the shore, high 

 bedload discharge of rivers; and, lacking a trigger mechanism, turbidity 

 currents probably can occur after a long-continued deposition simply by slope 

 failure resulting from gradual over-steepening of a depositional slope. Finally, 

 turbidity currents require a slope. The optimum conditions for the generation of 

 a turbidity current probably would be a large body of fine, well-sorted sand 

 triggered by an earthquake on a steep slope. 



20 



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' J ' 



INDEX CHART 



WESTERN CARIBBEAN 



Adapted From KO 5487 



Fig. 14. Sediment cores from the western Caribbean. Sediment cores indicated by open 

 circles contain only pelagic sediment, without any evidence of turbidity-current 

 deposition. Sediment cores A179-4 and V8-10 indicated by solid dots both contain 

 beds high in vegetal debris. The log of core A 179-4 is shown in Fig. 15. The core 

 V8-10 contained two beds almost entirely composed of twigs and leaves. Turbidity 

 currents are frequently initiated in the area off the Magdalena and Sixaola Rivers. 

 These areas are indicated by black rectangles. (After Heezen, Ewing and Menzies, 

 1955.) 



Heezen (1956), studying cable failures from various parts of the world, has 

 found evidence for several different trigger mechanisms. Besides earthquakes, 

 hurricanes have, in at least one instance, triggered turbidity flow off Cape 

 Hatteras. One of the most interesting types of trigger mechanism is that related 

 to the high bedload transport at the mouths of rivers. Potentially, such a source 

 should have a greater geologic importance since both the trigger effect and the 

 source of sediments are supplied by the river. In such an area repeated flows 

 from a single source could build up a submarine abyssal cone, and at the same 

 time repeated currents down the same path on the continental slope could 

 erode a canyon. Significant periodic changes in depth have been observed at 



