Development of a Comprehensive Oxygen-Deficient Marine Biofacies Model: Evidence from Santa Monica, San Pedro, and Santa Barbara Basins, California Continental Borderland

Abstract

An extensive data base of box core x-radiographs, bottom photographs, and box core faunal data has been employed to establish the relationships between dissolved oxygen content and (1) sediment fabric, (2) biogenic structure distribution and size trends, and (3) faunal composition and species richness in three anoxic silled basins of the California continental borderland. Sediments of anaerobic environments are generally well laminated, whereas those of aerobic and dysaerobic environments are completely bioturbated. Intermediate, partially bioturbated sediments are produced near the present dysaerobic-anaerobic boundary by short-term temporal fluctuations in the position of the dysaerobic-anaerobic boundary. Although distribution of burrow types observed in x-radiographs does not show significant control by variations in dissolved oxygen, average size of burrows decreases markedly with decreasing oxygen. In terms of surface biogenic structures, most aerobic and upper dysaerobic sea floors are dominated by tracks and trails, but lower dysaerobic sea floors are characterized by burrow openings. No surface biogenic structures were observed on anaerobic sea floors. Regular and irregular echinoids dominate sea-floor surfaces in aerobic and upper dysaerobic environments, whereas polychaetes, ophiuroids, holothurians, small arthropods, and gastropods are common on lower dysaerobic sea floors. No surface-dwelling organisms were visible on anaerobic bottoms. Box core faunal analysis indicates that species richness generally decreases with increased depth and decreasing oxygen until it reaches near zero at the dysaerobic-anaerobic boundary. The most abrupt decrease in species richness occurs at the shelf-slope transition. These observations are highly supportive of criteria cited by Rhoads and Morse (1971) and Byers (1977) for distinguishing anaerobic facies from more oxygenated facies in ancient strata. However, their criteria for distinguishing between ancient aerobic and dysaerobic facies are not as definitive. Paleontologic criteria, biogenic structure size trends, and other rock characteristics are useful for establishing paleo-oxygen gradients but not for determining specific paleo-oxygen concentrations as there are no abrupt oxygen-controlled changes in these parameters above the anaerobic-dysaerobic boundary.