Abstract

The competence of turbidity currents, that is, their ability to transport coarse material, is examined. The stress required to transport grains of diameter D (where D is greater than 7 mm) is evaluated with  
formula
where (ρs − ρt) is the density difference between the grains and the turbidity current. The bottom stress exerted by a flow of thickness h and density ρt on a slope sinβ is estimated from the relationship  
formula
where ρ is the density of water and the factor 1.50 takes into consideration the decrease in stress exerted on the bottom due to the drag on the upper interface of the flow by the overlying water.

These relationships are applied to modern turbidity currents confined to the Cascadia deep-sea fan channel. It is concluded that flows with expected dimensions yield sufficient bottom stress to transport the gravels observed in the channel. The equations are also applied to a cobble deposit contained within an ancient deep-sea channel (Doheny channel of the late Miocene lower Capistrano Formation, California). It is concluded that even these cobbles could be transported by turbidity currents of reasonable dimensions and density, flowing down the estimated paleoslope.

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