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 useful correlation tool for engineers and geologists to address two environmental problems: groundwater pollution in the south Bay and failure of the Cypress Freeway Structure and Bay Bridge in the 1989 Loma Prieta 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 and discontinuous sediments beneath the Bay, Glacially controlled fluctuations of sea level resulted in deposition of alternating alluvial and estuarine sediments which inter-finger complexly at the margins of the Bay, The coarse-grained alluvial deposits are widely used as aquifers for municipal and agricultural water supplies around the Bay, Pollution seriously threatens this supply in several areas, Difficulty in correlating the alluvial deposits hinders pollution remediation efforts and engineering geologic site characterizations for seismic hazard remediation. The estuarine deposits, which are thick, laterally continuous, and widespread, provide a valuable time and marker horizon, The deposits of the last interglacial estuary, the Yerba Buena mud, are especially useful because sea level was approximately 6 m higher than at present, and low-lying margins of the present Bay were hooded, In the South Bay, the Yerba Buena mud extends 8 km or more inland from the Bay, Distinctive foraminifera and other microfossils in the Yerba Buena mud permit correlation of the discontinuous alluvial units beneath the Bay and its margins, Thus, foraminifera contribute to the understanding of the complex stratigraphy beneath the Bay needed for preventing and remediating environmental problems in this highly populated urbanized estuary.