Depositional events in Santa Monica Basin, California Borderland, over the past five centuries

The southern California coastal climate is characterized by decadal and generational floods that discharge one to three orders of magnitude more sediment than the average annual contribution. These can produce turbidity currents directly or load the canyons and slopes with sediments that then may fail and form turbidity flows. Seismic activity is high on this transform margin and can trigger slope failures where slope sedimentation rates are high and sediments are unstable. These events produce slope failures, and slides and slumps in canyon heads, which evolve into turbidity currents of sufficient volume to reach the adjacent margin basin floors. Such events occur at approximately century frequency. This paper describes six basin-floor turbidites that have been mapped in box cores sampling roughly the top 40-50 cm from the basin floor of Santa Monica Basin in the California Borderland. These deposits were laid down during the past 500 + years of record. Their depositional areas have been conservatively estimated to range from 500 to almost 1500 km(2), and contain masses of sediment ranging from 10,000,000 to 100,000,000 tons. This compares with a long-term average annual terrigenous contribution rate of about 4,000,000 tons/yr from the adjacent major streams, although annual rates vary from 0 to 60,000,000 tons over the last century of record. Three of the six events have been related to two major floods (1884-6, 1969) and to one major seismic event (the Mission Earthquakes of 1822-1812). The older turbidites were deposited before European entry to the area and cannot be assigned to an historic event but, from their size, were probably seismically triggered and are the largest of the six events. It is likely that other smaller events are masked by the more recent turbidites, particularly in the area where box cores are dominantly composed of turbidites. These features exhibit average 'outcrop' thicknesses of 2 to 7 cm with maxima of the order of 10 cm thickness, yet represent major events and significant volumes of sediment. They also document that the two major sources for such events, floods and seismic events, produce deposits of very different mass and can probably be separated in the record on that basis. Thus the frequency of major floods and of major earthquakes in the region could be defined from study of such deposits in longer cores. These deposits also provide examples of continued turbidity current activity to the basin floors at a time of high sea level. This is contrary to the passive models for sequence stratigraphy depositional patterns.