Morphology and Sediments of Redondo Submarine Fan, Southern California
Abstract The following abridged article is included because of the strong similarity between the modern San Pedro basin with the associated Redondo Canyon submarine fan system and the Plio/Pleistocene Ventura basin and sedimentation. Both basins were tectonically active during cycles of sediment influx with each being constrained by an outboard series of topographically positive seamounts. The interrelationship of tectonic activity during sediment influx and outboard resulted in juxtaposition of sedimentary depositional features not normally observed in the “Classic” unconstrained submarine fan model. Through Haner’s work in the modern fan setting, a better understanding of associated sedimentary features observed in ancient basin environments is now available (Haner, 1971). The Redondo Submarine Fan fills the northern end of the San Pedro basin, one of several inshore, fault-controlled basins of the continental borderland of southern California. Fan segmentation and channel morphology were analyzed in detailed echo soundings, sub-bottom seismic profiles, and box cores. They reveal a threefold segmentation of the submarine fan along its axis as indicated by changes in fan gradient and channel patterns 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, whereas a slightly steeper lower fan gradient results in a braided channel in the lower segment. The recognition of these geomorphic divisions reveals the that both proximal and distal facies exist together in the middle segment. Movements along faults in the basin slope are responsible for steepening of the upper fan segment; he active channel responds to the displacement either by entrenchment or abandonment and creation of a new channel. Turbidity currents are confined solely to the channel of the upper fan segment, but below the intersection zone (mid fan) they flow longitudinally within the channel confines or spread laterally over the fan surface. Associated internal sedimentary structures reveal these different current deceleration environments. The total geometry of sand units, lateral and longitudinal changes in sedimentary facies, and channel morphology provide a model which has greater scope and flexibility than previous sedimentary longitudinal facies models which other writers postulated for unconstrained fan development. The model can be applied to the vertical stratigraphic changes and to the distribution of sediments on the submarine fan surface.