Structureless, coarse-tail graded Bouma Ta formed by internal hydraulic jump of the turbidity current?

Abstract

Turbidity currents are commonly super critical if they travel on slopes steeper than 0.001. Hence, upon deceleration, such flows must pass a hydraulic jump. It is, therefore, surprising that sedimentary features related to the hydraulic jump are so poorly known and not clearly related to the Bouma sequence. To study this missing link, a stability diagram of bed forms generated by decelerating, initially super critical turbidity currents is presented here to sketch the position of hydraulic jump deposits relative to super and sub-critical bed forms observed in turbidites. The axes of the diagram are formed by the densiometric Froude number, the grain size and the particle fall-out rate. Experimental evidence shows that the hydraulic jump is hardly manifested in the deposit if turbidity currents pass the jump as a low-concentration, single-phase suspension flow. Only those turbidity currents that develop two-phase suspension flow with a super critical flowing traction carpet cause vigorous erosion and liquefaction of the substrate at the jump locality to produce rip-up of the substrate, rip-up clasts and flame structures, which are trapped in structureless, coarse-tail graded deposits characteristic of the classic Bouma Ta unit. The location of the jump is sensitive to density/velocity variations of the traction carpet and can shift rapidly up and down the slope. This would explain why structureless, coarse-tail graded deposits, such as Bouma Ta develop sheet-like.