Continental slope morphology and sedimentary processes at the mouth of an Antarctic palaeo-ice stream

Continental-slope and shelf-edge morphology off Marguerite Bay, western Antarctic Peninsula, is investigated using swath-bathymetric data and parametric sub-bottom profiler records, together with sediment cores. Marguerite Bay has a well-defined cross-shelf trough, and a relatively steep continental slope. The slope beyond the trough mouth is convex in longitudinal profile, whereas to the north and south it is concave and reaches a maximum of 12degrees. There are no deep canyons cutting into the prograding outer shelf and slope. Instead, a series of gullies runs down the upper slope, reaching depths of >200 m south of the trough mouth but <120 m deep beyond the trough. The mid and lower slope appears to be relatively smooth and downslope sediment transfer is probably by small-scale slides, slumps and debris flows. The continental rise contains dendritic channels related to turbidity currents, and sediment drifts produced by southwest-flowing bottom currents from the fine-grained component of the turbidity currents. Elongate sedimentary bedforms indicate that a fast-flowing ice stream occupied the trough under full-glacial conditions, and transferred deforming subglacial till rapidly to the shelf edge. By contrast, on either side of the trough mouth, ice is inferred to have been slower-moving and probably cold-based, delivering little sediment to the upper slope. The steepness of the continental slope results in rapid downslope sediment transfer by debris flows, slumps and turbidity currents and accounts for the lack of a well-developed trough-mouth fan, which is typical of many lower-gradient glacier-influenced margins.

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