USE OF FORAMINIFERAL BIOSTRATIGRAPHY IN MITIGATING POLLUTION AND SEISMIC PROBLEMS, SAN-FRANCISCO, CALIFORNIA

The distribution of an easily recognized assemblage of foraminifera in upper Pleistocene estuarine deposits beneath San Francisco Bay is a u seful correlation tool for engineers and geologists to address two env ironmental problems: groundwater pollution in the south Bay and failur e of the Cypress Freeway Structure and Bay Bridge in the 1989 Loma Pri eta earthquake, As a result of these and related problems, interest in the stratigraphy of San Francisco Bay has increased significantly in the past few years. The stratigraphy is complicated by highly varied a nd discontinuous sediments beneath the Bay, Glacially controlled fluct uations of sea level resulted in deposition of alternating alluvial an d estuarine sediments which inter-finger complexly at the margins of t he Bay, The coarse-grained alluvial deposits are widely used as aquife rs for municipal and agricultural water supplies around the Bay, Pollu tion seriously threatens this supply in several areas, Difficulty in c orrelating the alluvial deposits hinders pollution remediation efforts and engineering geologic site characterizations for seismic hazard re mediation. The estuarine deposits, which are thick, laterally continuo us, and widespread, provide a valuable time and marker horizon, The de posits of the last interglacial estuary, the Yerba Buena mud, are espe cially useful because sea level was approximately 6 m higher than at p resent, and low-lying margins of the present Bay were hooded, In the S outh Bay, the Yerba Buena mud extends 8 km or more inland from the Bay , Distinctive foraminifera and other microfossils in the Yerba Buena m ud permit correlation of the discontinuous alluvial units beneath the Bay and its margins, Thus, foraminifera contribute to the understandin g of the complex stratigraphy beneath the Bay needed for preventing an d remediating environmental problems in this highly populated urbanize d estuary.