NEW GEOLOGIC MAPPING BY THE CALIFORNIA GEOLOGICAL SURVEY IN THE SAN FRANCISCO BAY REGION, SUPPORTED BY STATEMAP

The California Geological Survey has been conducting geologic mapping in the northern San Francisco Bay region since 1997 as part of a continuing collaboration with the U.S. Geological Survey through a series of STATEMAP mapping projects. Thirteen 7.5-minute quadrangles have been completed and have been, or are near being published. The maps will be released in paper and digital formats. The northern SF Bay region is within the San Andreas Fault System so it is a tectonically active area. Diverse rocks of the Mesozoic Franciscan Subduction Complex and the Great Valley Sequence make up the basement of the mapped area. The Franciscan rocks are in fault-bounded terranes of differing ages and have different tectonic and metamorphic histories. Cenozoic volcanic and sedimentary formations overlie the Mesozoic basement. The oldest Cenozoic units are the marine Eocene Domengine, Nortonville and Markley formations. In the western part of the map area the Miocene to Pliocene Wilson Grove and Petaluma formations record a complex marine to nonmarine transition. The coeval Sonoma Volcanics interfinger with the Petaluma Formation. Previously, the Sonoma Volcanics were mapped as a single field but new mapping and isotopic data show that it consists of at least three volcanic sequences of different ages and eruptive sources now juxtaposed along active dextral faults of the San Andreas System. This improved interpretation of the lithology, structure and distribution of the various volcanic units will provide the framework for a better understanding of the hydrologic environment in the Napa and Sonoma valleys.