The channel fills of the Miocene Alikayasi submarine channel system in the south of Turkey offer an extraordinary opportunity to improve our understanding of the physical properties of gravity-driven gravelly sand-laden flows and their resulting deposits on the seafloor. This study describes and analyses a particular area of an ancient channel-fill that is interpreted as the preserved deposits of relatively high-relief crescent-shaped bedforms consisting of conglomeratic sands (named the Guredin palaeo-bedforms). The mechanism of formation of these types of bedforms in submarine channel systems is largely speculative and normally explained by the ‘cyclic step paradigm’. A comparative analysis of the Guredin palaeo-bedforms with experimental flume studies and data from modern analogues supports a new model for the construction of relatively high-relief crescent-shaped bedforms. This model proposes the formation of this type of bedform as the result a two-stage process: 1) the formation of a crescent-shaped scour by an erosional hydraulic jump (from the abrupt transition from supercritical to subcritical flow regimes), and 2) the entry of a hyperconcentrated basal flow with a basal traction carpet, which develops a separation bubble in the scour that sorts out clasts and controls their deposition (creating the final dune-like crescent-shaped morphology). This new model deals with critical questions in the design of classification systems of bedforms for gravity-driven sediment-laden flows as well as with regard the dynamics and structure of the flow events that occur in submarine channels transporting gravelly sands.
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