Abstract: Evolving Depositional Styles in California Margin Basins: Miocene to Recent

Jon R. Schwalbach, Kevin M. Bohacs, Don S. Gorsline

Basins along the southwestern California continental margin demonstrate varying depositional styles as the margin evolved from Miocene to Recent. We focus on the Monterey and Sisquoc Formations of the Santa Maria Basin for the Miocene to Pliocene, and the basins of the California continental borderland for the Pliocene to the present day. Sediment gravity flows contribute large quantities of material to these steep-sided basins if they are proximal to major mainland drainages and coastal littoral cells. Paleooceangraphy determines the character of sediments in basins without detrital influx through its control of biological productivity, water-column chemistry, and current-circulation patterns.

The response of these depositional systems to forcing mechanisms, such as relative changes in sea level, varies significantly. For example, paleoceanographic signals are dominant in outer-tier margin basins detached from the mainland drainages. During deposition of the Miocene Monterey Formation, lower sea levels drove more vigorous oceanographic circulation, enhanced upwelling, and higher rates of biosiliceous productivity. These conditions deposited thick packages of very siliceous sediments in these basins and overwhelmed the slightly enhanced influx of fine-grained detritus during the lowstands. In contrast, basins adjacent to mainland drainage systems received large volumes of clastic-rich sediments during lowstands, mostly by sediment-gravity flows. Data from the Holocene indica e that sediment-gravity-flow mechanisms remain active even during transgressions to highstands of sea level, although at a reduced frequency and magnitude. These basin-distribution systems are fed by canyon systems that extend across the present-day shelf to intercept some coastal littoral cells.

Both these settings differ substantially from distal marine settings in the Cretaceous of the US Western Interior. Distal marine sediments are distributed more uniformly in these gently sloping ramp margins; they also respond differently to changing accommodation.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California