Combining Seismic Geomorphology and Physical Sedimentology to Resolve Depositional Processes within Submarine Channel Bends

Turbidity currents display exaggerated super-elevation at the outer banks of channel bends because they have low excess densities relative to the ambient sea-water. Low-velocity zones form where flows separate from the inner banks of bends. Depositional processes associated with the development of bank-attached bars in these flow-separation zones are evaluated using: a) measurements of geometries of bank-attached bars and their positions along channel curvature from two buried submarine channels, imaged in a high-resolution seismic data-set from the continental margin of West Africa, and b) detailed measurements of bed geometry, sedimentary structures and grain-sizes of a bank-attached bar in an exposed submarine channel complex from the Permian Brushy Canyon Formation, in west Texas. The 226 bar surfaces mapped in the seismic volume have high median slopes (10-11 degrees), occupy less than 30% of channel width, and are associated with weak channel incision and small amounts of lateral migration equal to less that 70% of a channel width. The mapped bank-attached bar outcrop shows a 17m-thick set of steeply inclined (median dip=10 degrees) fine-grained sandstone beds. Sub- to super-critically climbing ripple-lamination is abundant with paleo-transport oriented at 20-120 degrees relative to the dip azimuths of interpreted bar surfaces. Sedimentary structures, grain-size distributions and paleo-transport reconstructions from deposits in the bar and those filling the main channel reveal that this bar was constructed from fully-suspended sediment. These observations suggest that bank-attached bars constructed from fully-suspended sediment in submarine channels have been under-recognized on passive continental margins thus far. This data also defines the connection between the large scale morphologies of depositional elements, the processes responsible for their construction and the fine-scale attributes of the resulting stratigraphy.