. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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 e< ' 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 A179-4 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 peri
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