Late Quaternary Depositional Systems and Sea Level Change--Santa Monica and San Pedro Basins, California Continental Borderland

Abstract

A suite of seismic reflection data that provides different degrees of resolution and penetration was used to map the depositional systems that have developed in Santa Monica and San Pedro basins during the late Quaternary. Submarine fan growth, particularly at the mouths of Hueneme and Redondo Canyons, has been the dominant mode of basin filling. Mass movement processes, ranging from creep to large-scale catastrophic slumping, have been important locally. In general, large-scale fan growth fits Normark's model in which the suprafan is the primary locus of coarse sediment deposition. Smaller scale morphologic and depositional patterns on the Hueneme and Redondo fans (e.g., distributary channels and coarse sediment concentrations basinward of the inner suprafan) suggest that a significant amount of coarse sediment presently bypasses the suprafans, however. Long-distance coarse sediment transport was particularly pronounced during late Wisconsinan lowstand of sea level and resulted in progradation of lower mid-fan and lower fan deposits. As sea level rose rapidly between 18,000 y.B.P. and 10,000 y.B.P., shelves trapped much of the sediment supply. Consequently, rates of basin sedimentation decreased markedly and the area of fan developmen was restricted. As the rate of shelf sedimentation decreased concomitantly with a decrease in the rate of sea level rise 10,000 y.B.P., sediment transport to the basins was renewed and modern depositional patterns were established. Fan development also has been influenced by large-scale mass movement, extensive mid-fan overbank flow and the consequent development of sediment waves, deflection of turbidity currents by bathymetric obstacles, and local tectonism. Correlations between sand layer characteristics and high-resolution echo types for fan and basin-plain deposits show trends similar to some of those observed in deep-sea depositional environments. However, because Santa Monica and San Pedro basins are small, enclosed, tectonically active basins that receive sediment from relatively large drainage areas, proximal-to-distal changes in sand layer thickness, abundance, and frequency are less pronounced than those observed in deep-sea environments.