SLOAN, Doris, Dept. of Geology and Geophysics, University of California, Berkeley, CA 94720-4767; McGANN, Mary, U.S. Geological Survey, Menlo Park, CA 94025 and Museum of Paleontology, University of California, Berkeley, CA 94720-4780; and WAN, Elmira, U.S. Geological Survey, Menlo Park, CA 94025.
As part of a project to delineate fault offsets beneath San Francisco Bay, eleven cores were examined which had been drilled to bedrock by the California Department of Transportation (Caltrans) in preparation for the seismic retrofit of the San Francisco-Oakland Bay Bridge. The goal was to develop a detailed stratigraphy of the late Pleistocene and Holocene deposits, primarily by using foraminifers, and to correlate these data with the stratigraphy developed by Caltrans utilizing the geotechnical information it had collected. In our study, Bay mud to a depth of 97.5m was recovered and examined for microfossils. The preserved microfossils indicate deposition under shallow to deep estuarine conditions, at water depths of ~0-20m.
The late Pleistocene deposits contain an abundant estuarine and nearshore marine fauna. Dominant species of foraminifers include Ammonia beccarii, Elphidiella hannai, Elphidium excavatum, and Trochammina inflata. Three associations, defined on the basis of occurrence of these foraminifers, record a long transgressive episode. A subsequent drop in sea level or a tectonic event is suggested in the uppermost samples in several cores.
The use of microfossils in this study makes possible a more detailed stratigraphy at the Bay Bridge transect than was previously available, which relied on engineering properties alone. By using foraminifers and diatoms as indicators of estuarine environments, we are able to place contacts between estuarine and alluvial units with greater precision than was previously possible. For example, in Core 94-11, engineering data suggest that the base of the Yerba Buena mud is at ~44m. However, the presence of estuarine microfossils indicates that the contact should be placed 3m lower. Where no microfossil evidence is available, engineering data can provide the necessary control.
Thus, microfossil and engineering data are both essential to developing a detailed sub-Bay stratigraphic framework. Both types of information are needed by engineers as they site bridges in the nation's estuaries or undertake seismic retrofits of bridges in areas with active faulting, such as the San Francisco Bay Area.