GIS-BASED RECONSTRUCTION OF A BURIED PLEISTOCENE LANDSCAPE, SAN FRANCISCO BAY, CALIFORNIA

Our GIS-based interpretation of compiled subsurface data shows the elevation of the buried top of Pleistocene deposits and thicknesses of Holocene deposits along the San Francisco Bay margins. The top of buried Pleistocene deposits likely represents a surface exposed prior to the rise of seawater through the Golden Gate at the beginning of the Holocene. The primary basis for identifying the top of Pleistocene unconformity, now generally covered by Holocene sediments, is based on the consolidation of Pleistocene deposits during subaerial exposure following lowering of water levels in the Bay during the last sea level low stand. This period of exposure of a relatively stable landscape is distinguished by the development of a distinct pedogenic soil horizon. Soil development within Pleistocene deposits, now buried, is associated with a notable increase in density, and changes in color and texture, that are identifiable in standard geotechnical boring logs and are consistent across varied deposits formed within different geologic environments along the Bay margins. The buried ‘top of Pleistocene’ corresponds closely to the base of potentially liquefiable sediments. Thus, reconstruction of the Pleistocene landscape provides valuable information for assessment of liquefaction hazards as well as information on the subsurface structure and stratigraphy of basins along the margins of the San Francisco Bay. Contour maps of the top of Pleistocene deposits show irregularities in the inferred Pleistocene surface that likely correspond to buried former stream channels, based on historic locations of streams along the Bay margins. In addition, reconstruction of this laterally extensive surface provides a unique and important strain gauge for evaluating the location and style of Holocene deformation along the Bay margins and, therefore, may contribute to our understanding of the structural origin of the San Francisco Bay.