NEW GEOLOGIC MAPPING OF THE NORTHERN SAN FRANCISCO BAY REGION; MARIN, SONOMA, NAPA, AND SOLANO COUNTIES, CALIFORNIA

The California Geological Survey is conducting detailed geologic mapping in the tectonically active northern San Francisco Bay region and has released preliminary versions of thirteen 7-1/2 minute quadrangles within Marin, Napa, Sonoma, and Solano counties. The mapping is done in cooperation with the U.S. Geological Survey through a series of STATEMAP funded projects. The northern San Francisco Bay Area has not been previously mapped regionally at the scale of 1:24,000. This new mapping provides a level of detail that allows for greater understanding of the regional geology. The level of detail, coupled with the supporting digital databases allows others to use the geologic mapping to make derivative maps showing geologic hazards or resources.

An assortment of 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 unconformably overly the Mesozoic basement. The oldest Cenozoic units include the Eocene marine Domengine, Nortonville, and Markley formations located immediately west of the Napa Valley and southwest of the town of Cordelia. In the western part of the map area near the town of Cotati, 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 east of Petaluma on Sonoma Mountain. The Sonoma Volcanics were previously mapped as a single volcanic 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 Fault System. These three sequences are from oldest to youngest, the Burdell Mountain Volcanics, Tolay Volcanics, and Sonoma Volcanics. In addition, new mapping also includes defined locations of the Pliocene Huichica Formation and the Quaternary deposits of the Petaluma, Napa, and Sonoma valleys. The improved age and distribution relationships of the Cenozoic rocks will provide the framework for a better understanding of the regional structure and kinematics of this geologically complex area.