Linking Vertical Grain Size Sorting in Turbidity Currents to Grain Size Distribution of Submarine Channel Deposits

Abstract

Turbidity currents carry sediment of different grain sizes in suspension. In various field datasets it is found that suspended sand is only deposited by turbidity currents at a limited height above the channel thalweg. This is thought to be a result of vertical sorting in the flows. Coarse grains are concentrated at the base of the flow whereas fine grains are distributed more uniformly. This has significant impactions for the distribution of facies. Deposition at the base of the flow will be coarser grained than the mean composition of the sediment load of the flow. On the other hand, overflow from a confinement will mainly result in fine-grained deposition (i.e. deposition on levees). Thus, vertical sorting in turbidity currents controls which grain sizes are deposited where along turbidite systems. The distribution of suspended sediment in a turbulent flow results from a balance between turbulence, which acts to homogenise the current, and sediment settling. Here we present a simple model that predicts the vertical distribution of suspended sediment for turbidity currents that carry multiple grain sizes in suspension. The model shows that the grain size distribution near the base of field-scale turbidity currents is significantly different from the mean flow composition. Perhaps a surprising result is that decelerating flows may start to deposit more coarse-grained deposits as a result of the reduced mixing, which causes the coarse grains to get concentrated at the base of the flow. This in contrast to the intuitive decrease in deposit grain size with decreasing flow velocity. Finally, the model is applied to an outcrop example provided by a slope channel of the Tres Pasos Formation (Chile). The outcrops provide exposure of massive sandstone deposited on the channel floor as well as more fine-grained deposits from elevated terraces at the channel margin. The difference in grain size distribution between these deposits provides constraints on the vertical sorting in the turbidity currents that created these channels. The diffusion model is here used to link the vertical sorting to the velocity structure of the flow.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016