Geologic Map and Map Database of Western Sonoma, Northernmost Marin, and Southernmost Mendocino Counties, California By M.C. Blake, Jr., R.W. Graymer, and R.E. Stamski Pamphlet to accompany MISCELLANEOUS FIELD STUDIES MAP MF-2402 Version 1.0 2002 U.S. Department of the Interior U.S. Geological Survey Contents Geologic explanation and acknowledgements 1 Introduction 1 Stratigraphy 1 Mesozoic and Tertiary terrane complexes 1 Description of Terranes 2 Tertiary Stratigraphy 10 Quaternary Surficial Deposits 11 Paleontology 11 Radiometric Ages 11 Structure 12 Structural History 12 Description of Map Units 15 Acknowledgements 23 Digital publication and database description 24 Introduction 24 For those who don’t use digital geologic map databases 24 MF-2402 Digital Contents 24 PostScript plotfile package 25 PDF plotfile package 25 Digital database package 25 TAR files 26 PostScript plotfiles 27 PDF plotfiles 27 Obtaining the Digital Database and Plotfile Packages 28 To obtain TAR files of database or plotfile packages from the USGS web pages 28 To obtain TAR files of database or plotfile packages by ftp 28 Obtaining plots from a commercial vendor 28 Obtaining plots from USGS Map On Demand Services 28 Revisions and version numbers 28 Digital database format 29 Converting ARC export files 29 Digital compilation 29 Base maps 29 Faults and landslides 29 Spatial resolution 29 Database specifics 30 Lines 30 Areas 32 Points 33 References Cited 35 ii Geologic Explanation and Acknowledgements Introduction The map in this report is modified from and supercedes USGS Open-File Report 71-44 (Blake and This report contains a new geologic map at 1:100,000 others, 1971). Two factors led to the decision to remap scale and an associated set of geologic map databases (Arc- the area. First, there has been almost 30 years of new Info coverages) containing information at a resolution geologic information accumulated for the map area. Two associated with 1:62,500 scale. The map and map areas within the map, in particular, have received much database, along with this interprative text, are based on the attention since the earlier work: Cazadero/Ward Creek in integration of previous work with new geologic mapping the central part of the map, where the medium- to high- and field checking by the authors (see Sources of Data grade blueschist rocks and minerals have been studied in index map on the map sheet or the Arc-Info coverage wso- detail (Liou and Maruyama, 1987; Maruyama and Liou, so and the textfile wso-so.txt). The descriptive text 1988, 1989; Erickson, 1995; Wakabayashi, 1992a), and (below) contains new ideas about the geologic structures the Geysers geothermal field in the northeast corner of the in the area, including the active Hayward-Rodgers Creek- map, which was carefully studied in relation to the Healdsburg Fault system and other active faults in the production of geothermal steam for generating electricity area, as well as the geologic units and their relations, (McLaughlin, 1978, 1981; McLaughlin and Ohlin, 1984; including the origin of the Franciscan Complex mélange Eberhart-Phillips and Oppenheimer, 1984; Oppenheimer, and the Coast Range ophiolite. 1986; Dalrymple, 1992; Hulen and Nielson, 1993; Together, the map (or map database) and unit Stanley and others, 1998). Second, tectonic theories for descriptions in this report describe the composition and the California Coast Ranges have changed since the distribution of geologic materials and the orientation and publication of the earlier map, and the new tectonic distribution of geologic structures within the study area at models have a strong influence on the depiction of regional scale. Such regional geologic information is through-going active faults, volcanic fields, folds, and important for analysis of earthquake shaking, landslide other mapped features. susceptibility, engineering materials properties, mineral The digital nature of the geologic map resources and hazards, and groundwater resources and information available in this publication is important for hazards. It is also vital for answering questions about the three reasons. First, the geologic map data can be geologic history and development of the California Coast digitally combined with other map datasets (topography, Ranges. groundwater information, landslide distribution) for rapid The information, however, is not sufficiently and complex regional analysis of geologic resources and detailed for site-specific evaluations. Those seeking hazards. Second, digital maps are much more easily detailed information of that kind should consult the more updated than traditional printed maps. Third, digital detailed earthquake-hazards maps produced by the publication provides an opportunity for regional planners, California Division of Mines and Geology or contact a local, state, and federal agencies, teachers, consultants, and licensed geologist or engineering geologist. others who are interested in geologic data to have the new The map area includes about two-thirds of information long before a traditional paper map could be Sonoma County and a small part of Marin County west published. of longitude 122°45’W and north of latitude 38°15’N, plus a tiny part of southern Mendocino County along the north Stratigraphy edge of the map. This area includes the Pacific coastline in the western part, low rolling hills in the southern part, Mesozoic and Tertiary terrane complexes rugged mountainous terrain in the northern and central parts, and the broad flatlands of Santa Rosa and Alexander The bedrock units in the San Francisco Bay region are Valleys in the eastern part. The northern and central made up of two components: amalgamated, highly mountains are largely uninhabited, but the eastern valleys deformed tectonostratigraphic terranes that are displaced, at are undergoing rapid development. least in part, hundreds to thousands of kilometers from 1 their position of origin (allocthonous or relationship to the older Fransiscan complex rocks and the parautochthonous); and generally younger, less deformed Great Valley complex is not well understood. Because rocks that overlie the amalgamated terranes and are much of the Franciscan Complex was accreted beneath the roughly in their original location (except for transport Great Valley complex, the contact between the two along the San Andreas fault system). In most of the Mesozoic complexes is everywhere faulted (Bailey and region, the older set of rocks is Mesozoic and the younger others, 1964), and the Franciscan Complex presumably is Tertiary, but in the map area the amalgamated terranes underlies the entire San Francisco Bay area east of the San include some rocks as young as Miocene. The young age Andreas Fault. Subsequent deformation, described below, of rocks within these displaced terranes reflects additional has obliterated the original subduction contact almost complexity in the geologic and structural history of the everywhere. map area that is not found in any other part of the region. A third complex crops out in the map area only west of the San Andreas Fault Zone at Bodega Head, The amalgamated terranes can be grouped into which is composed of granitic rocks of the Salinian three related rock complexes. One of these, the Great complex. This is the northernmost outcrop of the Valley complex, is made up of the Jurassic Coast Range Salinian complex, which extends southwards between the ophiolite, which in the map area consists mostly of San Andreas and Sur-Nacimiento Fault Zones about 500 serpentinite, gabbro, diabase, basalt, and keratophyre km to the Big Pine Fault east of Santa Barbara (Ross, (altered silicic volcanic rocks), and the Jurassic and 1972). The complex in the San Francisco Bay region is Cretaceous Great Valley sequence, composed of sandstone, made up of Late Cretaceous (81.6-91.6 Ma, Curtis and conglomerate, and shale. Although the sedimentary rocks others, 1958; 80-117 Ma, Kistler and Champion, 1991) and ophiolite have been tectonically separated almost granitic rocks intruded into high-grade metamorphic rocks, everywhere in the map area, the Great Valley sequence was Late Jurassic (161 Ma, James and others, 1993) gabbroic originally deposited on the ophiolite. This depositional rocks, and Late Cretaceous (Campanian to Maastrictian) relationship is extrapolated from contacts exposed outside sedimentary rocks (Branner and others, 1909; Hall and the map area: in the Berkeley Hills (Jones and Curtis, others, 1959; Clark, 1981; Brabb and others, 2000). 1991; Graymer and others, 1996) and Solano County in Both the Franciscan and the Great Valley the San Francisco Bay region, and elsewhere in California. complexes have been further divided into a number of This complex represents the accreted and deformed fault-bounded tectonostratigraphic terranes (Blake and remnants of arc-related Jurassic oceanic crust with a thick others, 1982, 1984). When the terranes were first defined, sequence of overlying turbidites, at least in part related to the prevailing methodology was to identify separate the North American forearc (parautochthonous). terranes if there was any doubt about stratigraphic linkage The second set of amalgamated terranes is the between structurally separated entities. As a result of Franciscan Complex. These terranes are composed of further research, many additional data, in particular new weakly to strongly metamorphosed graywacke, argillite, fossil localities, are known and the distribution and nature basalt, chert, limestone, and other rocks. The rocks of