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Fine-scale relief related to Late Holocene channel shifting within the floor of the upper Redondo Fan, offshore Southern California



Erosional and depositional bedforms have been imaged at outcrop scale in the upper Redondo Fan, in the San Pedro Basin of offshore Southern California in ≥600 m water depths, using an Autonomous Underwater Vehicle developed by the Monterey Bay Aquarium Research Institute. The Autonomous Underwater Vehicle is equipped with multibeam and chirp sub-bottom sonars. Sampling and photographic images using the Monterey Bay Aquarium Research Institute Remotely Operated Vehicle Tiburon provide groundtruth for the Autonomous Underwater Vehicle survey. The 0·3 m vertical and 1·5 m lateral bathymetric resolution and 0·1 m sub-bottom profile resolution provide unprecedented detail of bedform morphology and structure. Multiple channels within the Redondo Fan have been active at different times during the Late Holocene (0 to 3000 yr bp). The currently active channel extending from Redondo Canyon makes an abrupt 90° turn at the canyon mouth before resuming a south-easterly course along the east side of the Redondo Fan. This channel is floored by sand and characterized by small steps generally <1 m in relief, spaced 10 to 80 m in the down-channel direction. A broader channel complex lies along the western side of the fan valley that was last active more than 850 years ago. Two distinct trains of large scours, with widths ranging from tens to a few hundred metres and depths of 20 m, occur on the floor of the western channel complex, which has a thin mud drape. If observed in cross-section only, these large scours would probably be misidentified as the thalweg of an active channel.

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