Sea floor bedforms and their influence on slope accommodation

In deep-water settings, the accommodation for sediment transported by turbidity flows relates to the difference between the elevation of the depositional surface and its equilibrium profile. As a consequence, accommodation creation, or disruption, may depend from changes in the physiography of the receiving basin, or changes in the flow properties. In topographically complex slopes, such where salt-withdrawal intra-slope basins occur, three different types of accommodation have been recognized. Among other parameters, the ratio between flow thickness and depth of the intra-slope basin controls the partial, or full, ponding of the sediment in suspension, and consequently, the lithology distribution within the deposit. On a smaller spatial scale, the behavior of bottom-hugging sediment-laden flows can be affected by topographic variations of the sea floor associated with the presence of km-scale bedforms. In this work, we show that ponded lobes accumulate on the convex-up stoss side of pre-existing large-wavelength bedforms (length up to 103, and height up to 102), and that their lithology distribution depends on the flow characteristics respect to the bedform height. In detail, when partial ponding of turbidity currents occurs, flow stripping promotes the accumulation of the coarse-grained fractions on the stoss side of the bedform, while the fine-grained cloud over-spills the lee side, affecting deposition basinward. By introducing the concept of stoss-side accommodation, this work suggests a new mechanism for the formation of ponded coarse-grained facies in slope settings due to the trapping effect large-wavelength bedforms with convex-up stoss sides.