Abstract

Redondo Submarine Fan infills the northern end of the San Pedro basin, one of the inshore fault-controlled basins of the continental borderland off southern California. An analysis was made of detailed echo soundings and sub-bottom seismic profiles by techniques used by geo-morphologists to study segmentation and channel morphology of alluvial fans. This revealed a threefold division of the submarine fan along its axis. These segments are indicated by changes in fan gradient and a channel pattern unique to each segment. The upper fan is steepest, and the channel is entrenched. Intersection of channel with fan surface and meandering characterize the middle segment, while a slightly steeper lower fan gradient results in a braided channel. These geomorphic divisions revealed both proximal and distal facies in the middle segment.

Basin slope fault movement is responsible for steepening the upper fan segment to which the active channel responds either by entrenchment or abandonment and creation of a new channel. Turbidity currents are confined solely o t the channel of the upper fan segment, but below the intersection zone they can continue to flow longitudinally within the channel confines or spread laterally over the fan surface. Associated internal sedimentary structures reveal these different deceleration environments.

Consideration of the total geometry of sand units, lateral and longitudinal changes in sedimentary facies, and channel morphology provides a model which has greater scope and flexibility than previous sedimentary longitudinal facies models. It can be applied to the vertical changes observed in stratigraphic sections and the distribution of surface sediments on present submarine fans.

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