Abstract: Pleistocene Diatom Assemblages from Around Tomales Bay and Comparison to Holocene Diatoms from San Francisco Bay, CA

Marianne Binkin, Lisa White, Karen Grove

The Millerton and Olema Creek Formations are located near Tomales Bay in the valley formed along the San Andreas fault zone. The Millerton Formation is exposed as moderately dipping beds along the east side of Tomales Bay. The Olema Creek Formation is exposed as broadly folded beds in the fault valley south of Tomales Bay along the incised banks of Olema Creek. The ages of the Millerton and Olema Creek Formations, based on the thermoluminescence dating technique, correspond to the 125,000 yr. high stand of sea level. Samples from both locations were processed to recover siliceous microfossils, mostly diatoms. Qualitative analysis and comparison to Holocene diatom assemblages from San Francisco Bay suggest that sediments from the sampled sites were deposited in an estuarin environment. The most abundant species in both formations are Cyclotella striata, Thalassionema nitzchoides and Coscinodiscus sp. They represent a salinity range from brackish to marine. Our results contrast with Galloway's (1977) freshwater interpretation of the Olema Creek Formation diatom assemblages and provide the first evidence for a marine incursion into the valley south of Tomales Bay. An estuarine interpretation of diatom assemblages from the Millerton Formation is consistent with interpretations of previously collected macrofossil assemblages.

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