Primary Sedimentary Structures Produced By Turbidity Currents: ABSTRACT

Abstract

Turbidity currents produce numerous primary sedimentary structures, depending on mechanism of particle movement and sediment load-current-bottom interaction. Because the sediment is responsible for the current, depositional loss of load systematically changes conditions and structures produced. Only a few sediment-current-bottom combinations are unique to turbidity currents; many combinations, hence sedimentary structures, also occur in other current-sediment systems. Two types of collective behavior in cohesionless sands are inferred to occur in turbidity currents: (1) high-velocity sheet flow, where grains, probably not in true suspension, shear over the bottom below and underneath the truly suspended load above, outrunning the latter; and (2) traction-carpet flow, in which the grain layer may become relatively passive with respect to underlying bottom, but is subjected to shear, with or without sand fallout, from the overlying current with suspended sediment. Drag relationships between turbidity currents and traction carpet have not been investigated in the laboratory, but probably could be. Bagnold's analysis indicates that drag may trigger an abrupt increase in current velocity. This possible auto-acceleration might explain the otherwise eni matic increase in current velocity without change in grain size, which the writer previously invoked in analysis of convoluted laminae. End_of_Article - Last_Page 545------------