LATE QUATERNARY TURBIDITE SYSTEMS IN SANTA MONICA BASIN, OFFSHORE CALIFORNIA

Santa Monica Basin is a tectonically active depression that is filling more rapidly than other offshore basins in the California Continental Borderland. Sediment accumulation rates during the last glacial maximum exceed 4 m/10³ yr, which is about an order of magnitude greater than for other basins. Four main submarine canyons feed sediment to the basin --- from west to east, Hueneme, Mugu, Dume and Santa Monica Canyons. An extensive multichannel and single-channel seismic-reflection profiling data set allows evaluation of the relative contribution of sediment from these four sources since about Marine Isotope Stage 12. In addition, high-resolution deep-tow boomer profiles, and recently available radiocarbon ages for sediment cores including those at ODP Site 1015, provide an opportunity to construct a detailed stratigraphic evolution of deep-water depositional systems in the basin during the last 30 ka. The dominant source for sediment in the Santa Monica Basin is the Santa Clara River and adjacent drainages, which feed both Hueneme and Mugu Canyons as well as three smaller delta-edge canyons between them. These canyons feed channel-levee complexes featuring high (>50-m relief) muddy levees and sandy floors. In contrast, Dume Canyon receives sediment from littoral drift and feeds a small channel with low (15-m relief) sandier levees; the sandy, prograding wedges of Dume Fan are generally buried by sediment coming from the Santa Clara river delta via Hueneme Canyon. Santa Monica Canyon, which is the longest canyon feeding the basin, has apparently not been a major conduit and its associated fan has the most limited expression of any of the turbidite systems in the basin. Rivers crossing the present Los Angeles Basin area appear to preferentially feed sediment farther south toward the Long Beach shelf area.