ABSTRACT

Santa Monica Basin, an inner basin of the California Continental Borderland, is a deep margin basin presently receiving sediments under high sea-level conditions. Since the slowing of the last transgression, the basin has been receiving primarily suspended load contributions with rare turbidites except in the most headward parts of the two principal canyons. One turbidite, found throughout the basin, was laid down about 200 years BP, and was probably deposited by a turbidity current generated by mass movement of slope sediments in the slope apron adjacent to the major active canyons.

Similar basin-wide events have occurred throughout Holocene time at intervals of about 500-1000 years and these represent the effects of big earthquakes, major floods, or other exceptional happenings not commonly recorded in historic time. Except for these exceptional events, the basin has received fine clastic sedimentation derived from the mainland rivers and transported by nepheloid plumes near bottom and in the surface mixed layer.

Biogenic carbonate and organic carbon input from areas of high productivity fed by upwelling is masked by the fine detrital influx over the major fans by dilution from the rain of particulates from the major surface and near-bottom turbid plumes that pass over the basin center. A maxima of carbonate and organic content occurs in the basin floor sediments most distant from the combined diluting effects of these two major detrital inputs.

The basin, slope-fan, and shelf-bank top sediments are differentiated on the basis of mean particle size and skewness. Changes in sand and silt content are related to the pattern of the prevailing surface and near-bottom water circulation with their reservoirs of fine suspended particulates. The principal process removing the fine particles from the surface plumes is biological filtering and pelleting. Sedimentation rates of major sediment components show that near-bottom nepheloid plumes are trapped by the closed deep basin, and their load settles out as the turbid waters circle over the central basin floor.

Sedimentation rates vary from 40-80 cm/1000 years. Average sediment delivery is about 10⁶ tons, but exceptional floods can deliver several-fold more sediment at 10-30 year intervals.