Inverse Grading Resulting from Coarse-sediment Transport Lag

Abstract

Inverse grading in turbidites is conventionally ascribed to dispersive pressure caused by mutual collisions among grains within a traction carpet or grain flow. The shearing traction carpet is supposed to become stratified, with particle size increasing upward; freezing of this traction carpet yields an inversely graded layer. An alternative mechanism, proposed here, emerges from the fact that coarse sediment fractions are transported more slowly than fine fractions. In a turbidity current, a sufficiently heterogeneous mix of sediment sizes should undergo streamwise size segregation as the coarsest fractions lag behind. If deposition begins before the coarsest fraction that will reach a particular station has arrived, the lower part of the resulting deposit should be inversely graded. A similar mechanism may produce inversely graded intervals in pyroclastic flow deposits. Because the head of a turbidity current typically travels only about 0.83 as fast as flow velocity in the body a short distance behind the head, the coarsest grains able to keep up with the front of the flow are those having transport velocity V sed nearly equal 0.83 U body . If coarser size fractions are available, they will arrive later than the head, permitting the development of inverse grading. The proposed mechanism will not operate when the coarsest size fraction travels as fast as (or faster than) the head of the flow, or when onset of deposition is delayed until after the coarsest size fraction has arrived. Assuming a logarithmic velocity profile below the maximum-velocity streamline, transport velocity can be calculated for particular sizes of suspended sediment, and expressed as a fraction of flow velocity. An upper limit for flow strength (shear velocity) can then be derived from the fall velocity of the coarsest grains at the base of an inversely graded interval: u * <= 4 w init . Where deposition of an inversely graded turbidite began immediately after passage of the head of the current, u * nearly equal 4 w init .