Effect of Suspended Sediment Concentration on Deposition From Suspended Sediment Dispersions

Arnott, R.W.C.¹ (1) University of Ottawa, Ottawa, ON

Sediment-gravity flows are a special kind of density current in which the density contrast with the ambient fluid is maintained by suspended sediment. However, despite the first-order control of suspended sediment on the very existence of these flows little is known of the effect(s) of sediment concentration on the depositional record from these flows. For example, dating from the work of Bouma (1962), the idealized model of a turbidite lacks a stratal layer related to deposition from subaqueous unidirectional dunes. Even more perplexing is the fact that in rare instances dune cross-stratification does occur where expected – sandwiched between upper-stage plane bed and ripple cross-stratified sandstone. Another common but puzzling observation is that in many turbidites the b-division is several times thicker than the c-division, which in many cases is a scant one to a few sets thick, or even more problematic absent.

Many of these puzzling issues may be explained by considering the deleterious effect of high suspended sediment concentration on the initiation and maintenance of dunes and ripples. In contrast, upper-stage plane bed appears to be unaffected by high sediment concentrations, at least up to 35 volume % sediment concentration (conditions bordering on hyperconcentrated flow). As a consequence, in many depositing sediment suspensions upper plane bed may remain stable bed even under flow conditions that in an otherwise clear-water flow would form dunes or ripples. This suggests that an important effect of high suspended sediment concentration is to depress the stability of upper plane bed to lower flow speeds compared to that in clear-water flows.