GEOMETRIES AND INFERRED FORMATIVE PROCESSES OF BANK-ATTACHED BARS IN SINUOUS SUBMARINE CHANNELS

Sinuous submarine channels often show bank-attached bar-forms, associated with lateral or downstream migration of channel bends. Bar accretion surfaces were mapped at a high level of detail, from nine channel bends of a buried sinuous channel imaged by high-resolution seismic data from offshore West Africa. Three-dimensional geometries and temporal evolution of barforms are synthesized in order to estimate the patterns and processes of sediment transport associated with the evolution of this sinuous channel.

This channel has an average depth of 44m, average width of 430m and sinuosity of 2.36. Channel bends have maximum curvatures of 0.15-0.35 degree/m and are separated by straight channel segments with lengths 1.5-3 times average channel width.

Most of the bends in this channel show migration distances equal to less than one channel width. Systematic spatial changes in the heights of accretion surfaces indicate that bar and bend growth occurred during phases of weak channel incision. This study has been the first to document the common occurrence of concave-bank benches positioned downstream from high-curvature bends in submarine channels. These observations suggest that sedimentation in low velocity zones occurred in these areas, analogous to concave-bank accretion in fluvial channels, caused by flow separation from the inner bank down-stream of strongly accreting bends.

The median accretion slope is 11 degrees, with values as high as 18 degrees. Median bar height is 40% of channel depth, with values up to 80% channel depth. Cross-channel bar slopes occupied 25-30% of cross-channel width. In contrast, bedload-dominated bank-attached bars in rivers have slopes ranging from 4-7 degrees and cross-channel widths equal to 70-90% of the total channel width.

Observed bar geometries and their relationship to channel curvature are consistent with construction from suspension-dominated deposition, rather than from bedload transport. Results of this study contribute to our understanding of the evolution of sinuous channels in submarine environments and the reservoir properties of deposits of migrating submarine channels. The connection between morphology and depositional process, is a useful tool in the analysis of remotely sensed data in modern and ancient environments on Earth and other planets.