Sedimentology of Shelf and Slope in Santa Monica Bay, California

Ronald L. Kolpack

Grain-size parameters and values for organic carbon, nitrogen, sulfur, and calcium carbonate were derived from analyses of the top 2 cm from 200 box core and 80 grab samples. Near-bottom water currents measured by tripod-mounted current meters and more than 500 vertical profiles of light transmission provided supplementary information. Sediment supply is presently limited to nearshore sediments transported southward from Santa Barbara Channel and to a small amount of material from shoreline erosion. About 6.4 × 10⁴ MT of particulate material, primarily organic detritus, are also discharged yearly from sewage effluent and sludge outfalls.

An isolated region near the outer edge of the central shelf is dominated by a relict deposit of gravel. Carbonate values within this area range from 10 to 50%, whereas values on the remaining shelf area are generally < 1.5%. Sulfur values on the shelf are between 0.1 and 0.2%, but there is an aureole of higher values (up to 1.0%) around the gravel deposit. Organic carbon and nitrogen values are elevated in small areas near the outfall sites and increase progressively from the shelf to base-of-slope domains. Organic carbon/nitrogen ratios display a distinct cross-shelf pattern that coincides with a similar pattern in the distribution of sewage effluent in the water column. The overall distribution of sediment parameters is related to wave-associated currents and is characterized by non-depositional regime near the central part of the outer shelf, transport of fine-grained detritus off the shelf, and a secondary transport toward the southeast from the sewage outfall sites.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.