GEOLOGIC MAP OF THE NAPA AND BODEGA BAY 30’X 60’ QUADRANGLES, CALIFORNIA

The Preliminary Geologic Map of the Napa and Bodega Bay 30’x 60’ Quadrangles, and Adjacent Areas, California was compiled from new and existing geologic mapping. New geologic mapping by the CGS was completed for twenty-five of the thirty-five 7.5’ quadrangles. This map is a product of a cooperative geologic mapping effort in the northern San Francisco Bay Area between the CGS and the USGS, as well as graduate students from San Jose State and San Francisco State universities. This map covers an area of approximately 2,000 square miles of the Coast Ranges geomorphic province. Most of the map area is north and west of the San Francisco Bay estuary, which consists of the San Pablo Bay, the Sacramento River delta and wetlands near Napa, and the Fairfield-Cordelia area. A compilation of the offshore geology will be added to the map for the final publication.

Major scientific contributions resulting from the new geologic mapping are: 1) confirmation that volcanic rocks west of the Rodgers Creek Fault that were previously thought to be part of the Sonoma Volcanics are actually allocthonous, having been displaced up to 175 km along the East Bay Fault System, 2) the total displacement along the Rodgers Creek Fault is 28 km, and 3) the Sonoma Volcanics young to the northwest, consistent with models of northwest propagation of the San Andreas Fault System.