Experiments on Wedge-Shaped Deep Sea Sedimentary Deposits in Minibasins and/or on Channel Levees Emplaced by Turbidity Currents. Part II. Morphodynamic Evolution of the Wedge and of the Associated Bedforms

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Rapidly decelerating sediment-laden flows typically emplace confined sedimentary deposits. In the fluvial environment, when sediment-laden rivers reach lakes the decelerating flow emplaces a subaerial delta with distinctive topset, foreset, and bottomset deposits. In the submarine environment, turbidity currents undergoing rapid deceleration commonly emplace sedimentary wedges (i.e., deposits thinning in the downstream direction). Froude-supercritical turbidity currents have an intrinsic self-regulating mechanism for deceleration, in that the faster they flow, the more they incorporate ambient sea water through mixing at their interface. In addition, special topographic configurations, such as the entrance into a zone of much lower slope and/or lateral confinement, or the passage into a confined minibasin, may trigger sudden flow deceleration by forcing a transition to subcritical flow through an internal hydraulic jump. The present paper and its companion present experiments on a generic configuration aimed at studying the emplacement of wedge-shaped sedimentary deposits by continuous supercritical density currents. The deceleration is achieved both by natural entrainment of ambient water and by the presence of an obstructing barrier downstream. Lightweight plastic sediment was used as an analog for sand, and was transported mostly as bedload, but with some suspension, by a saline underflow. The saline underflow served as a surrogate for a turbidity current driven by fine mud that does not easily settle out. The companion paper is focused on the flow patterns associated with the decelerating current. The present paper focuses on the depositional sequences. The decelerating supercritical flows produced a wedge with a distinct pattern of aggradation and progradation. In addition, a foreset-like structure is attributed to the presence of an internal hydraulic jump forced by the downstream barrier. Although they do not reproduce any specific field-scale setting, the experiments are deemed a good generic model for several wedge-shaped submarine deposits in various settings, from slope aprons to deposits in minibasins or on the external flanks of channel levees. The paper also documents the regimes of bedforms associated with the diverse flow regions. It provides the first evidence for the formation of trains of the upstream-migrating sediment waves known as cyclic steps, similar to those commonly observed on channel levees and also along the thalwegs of some steep canyons. In addition, the experiments provide convincing evidence for the formation of downstream-migrating antidunes as well.