A survey of occurrences of Holocene laminated sediments in California Borderland Basins: Products of a variety of depositional processes

Laminated sediments are found in several contemporary depositional settings in basins of the California Continental Borderland. These primary structures are preserved either as a result of anoxia (or near anoxia) in the bottom water when oxygen demand is greater than the oxygenation rate, or as a result of sedimentation rates in excess of bioturbation rates. The controlling processes in the anoxic settings include cyclic seasonal variation in the composition of hemipelagic deposition (hemipelagic laminae), and/or formation and destruction of bacterial mats as a result of periodic bottom water flushing of normally low-oxygen bottom waters (cyclic organic-rich laminae). Non-bioturbated hemipelagic laminations occur in San Pedro, Santa Monica and Santa Barbara Basins. Low-oxygen bottom waters and exclusion of macrobenthos have been characteristic of San Pedro Basin for a few decades. These conditions have existed in Santa Monica Basin for a few centuries and in Santa Barbara Basin for most of the Holocene. Physical sedimentation processes include rapid deposition from turbidity currents. Fine grained contemporary distal turbidites occur in Santa Monica, San Pedro, San Diego, Santa Cruz and San Nicolas Basins. Turbidity current flows into the deep basin floor occur at century scale in Santa Monica Basin and at somewhat longer frequencies in San Pedro Basin. Frequency of such flows is at the kilo-year level in San Diego Trough, Santa Cruz Basin and San Nicolas Basin. Such events are of interest because they show that submarine canyons will remain active through a high sea-level period. In narrow-shelf, active margins, canyons may not be shut off as generally occurs on passive margins when sea level rises rapidly, as is typical of climatically driven sea level fluctuations.