Introduction

This report contains a new geologic map at 1:100,000 scale and an associated set of geologic map databases (Arc-Info coverages) containing information at a resolution associated with 1:62,500 scale. The map and map database, along with this interprative text, are based on the integration of previous work with new geologic mapping and field checking by the authors (see Sources of Data index map on the map sheet or the Arc-Info coverage wsoso and the textfile wso-so.txt). The descriptive text (below) contains new ideas about the geologic structures in the area, including the active Hayward-Rodgers Creek- Healdsburg Fault system and other active faults in the area, as well as the geologic units and their relations, including the origin of the Franciscan Complex mélange and the Coast Range ophiolite.

Together, the map (or map database) and unit descriptions in this report describe the composition and distribution of geologic materials and the orientation and distribution of geologic structures within the study area at regional scale. Such regional geologic information is important for analysis of earthquake shaking, landslide susceptibility, engineering materials properties, mineral resources and hazards, and groundwater resources and hazards. It is also vital for answering questions about the geologic history and development of the California Coast Ranges.

The information, however, is not sufficiently detailed for site-specific evaluations. Those seeking detailed information of that kind should consult the more detailed earthquake-hazards maps produced by the California Division of Mines and Geology or contact a licensed geologist or engineering geologist.

The map area includes about two-thirds of Sonoma County and a small part of Marin County west of longitude 122°45’W and north of latitude 38°15’N, plus a tiny part of southern Mendocino County along the north edge of the map. This area includes the Pacific coastline in the western part, low rolling hills in the southern part, rugged mountainous terrain in the northern and central parts, and the broad flatlands of Santa Rosa and Alexander Valleys in the eastern part. The northern and central mountains are largely uninhabited, but the eastern valleys are undergoing rapid development.The map in this report is modified from and supercedes USGS Open-File Report 71-44 (Blake and others, 1971). Two factors led to the decision to remap the area. First, there has been almost 30 years of new geologic information accumulated for the map area. Two areas within the map, in particular, have received much attention since the earlier work: Cazadero/Ward Creek in the central part of the map, where the medium- to high-grade blueschist rocks and minerals have been studied in detail (Liou and Maruyama, 1987; Maruyama and Liou, 1988, 1989; Erickson, 1995; Wakabayashi, 1992a), and the Geysers geothermal field in the northeast corner of the map, which was carefully studied in relation to the production of geothermal steam for generating electricity (McLaughlin, 1978, 1981; McLaughlin and Ohlin, 1984; Eberhart-Phillips and Oppenheimer, 1984; Oppenheimer, 1986; Dalrymple, 1992; Hulen and Nielson, 1993; Stanley and others, 1998). Second, tectonic theories for the California Coast Ranges have changed since the publication of the earlier map, and the new tectonic models have a strong influence on the depiction of through-going active faults, volcanic fields, folds, and other mapped features.

The digital nature of the geologic map information available in this publication is important for three reasons. First, the geologic map data can be digitally combined with other map datasets (topography, groundwater information, landslide distribution) for rapid and complex regional analysis of geologic resources and hazards. Second, digital maps are much more easily updated than traditional printed maps. Third, digital publication provides an opportunity for regional planners, local, state, and federal agencies, teachers, consultants, and others who are interested in geologic data to have the new information long before a traditional paper map could be published.