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Geologic Map and Map Database of Eastern Sonoma and Western Napa Counties, California

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Geologic Map and Map Database of Eastern Sonoma and Western Napa Counties, California Geologic Map and Map Database of Eastern Sonoma and Western Napa Counties, California By R.W. Graymer, E.E. Brabb, D.L. Jones, J. Barnes, R.S. Nicholson, and R.E. Stamski Pamphlet to accompany SCIENTIFIC INVESTIGATIONS MAP 2956 2007 U.S. Department of the Interior U.S. Geological Survey Contents Geologic explanation and acknowledgments 1 Introduction 1 Stratigraphy 2 Mesozoic terrane complexes 2 Tertiary Stratigraphy 3 Structure 3 Description of Map Units 6 Acknowledgments 14 Digital publication and database description 15 Introduction 15 For those who don’t use digital geologic map databases 15 SIM 2956 Digital Contents 15 PostScript plotfile package 16 PDF plotfile package 16 Digital database package 16 TAR files 17 PostScript plotfiles 18 PDF plotfiles 18 Obtaining the Digital Database and Plotfile Packages 19 Obtaining plots from a commercial vendor 19 Obtaining plots from USGS Map On Demand Services 19 Revisions and version numbers 19 Digital database format 19 Converting ARC export files 20 Digital compilation 20 Base maps 20 Faults and landslides 20 Spatial resolution 20 Database specifics 20 Lines 21 Areas 22 Points 23 References Cited 25 ii Geologic Explanation and Acknowledgments Introduction digital geologic map data can be combined with other digital map datasets (topography, groundwater This report contains a new 1:100,000-scale geologic information, landslide distribution) for rapid and map, derived from a set of geologic map databases complex analysis of geologic resources and hazards. (Arc-Info coverages) containing information at Second, digital maps are more easily updated than 1:62,500-scale resolution, and a new description of the printed maps. Third, the digital data is readily geologic map units and structural relations in the map available to regional planners, local, state, and federal area. Prepared as part of the San Francisco Bay agencies, teachers, consultants, and others who are Region Mapping Project, the study area includes the interested in the geologic data of the region. north-central part of the San Francisco Bay region, The map area includes most of eastern and forms the final piece of the effort to generate new, Sonoma County and western Napa County, and small digital geologic maps and map databases for an area parts of Lake and Yolo Counties to the north. The map which includes Alameda, Contra Costa, Marin, Napa, area is dominantly north-northwest trending valleys and San Francisco, San Mateo, Santa Clara, Santa Cruz, ridges of mountains of modest height (mostly less than Solano, and Sonoma Counties. Geologic mapping in 800 m); there are low rolling hills in the southwestern Lake County in the north-central part of the map extent corner. The northern and eastern mountains are largely was not within the scope of the Project. The map and uninhabited but the valley lands are the heart of the map database integrates both previously published California premium wine industry (see Swinchatt and reports and new geologic mapping and field checking Howell, 2004, for a discussion of the relationship by the authors (see Sources of Data index map on the between wine and geology). The southwestern part of map sheet or the Arc-Info coverage eswn-so and the the map area is undergoing rapid suburban textfile eswn-so.txt). This report contains new ideas development. about the geologic structures in the map area, This map is modified from and supercedes including the active San Andreas Fault system, as U.S. Geological Survey Miscellaneous Field Studies well as the geologic units and their relations. Map MF-483 (Fox and others, 1973). Two factors led Together, the map (or map database) and the to the decision to remap the area. First, there is new unit descriptions in this report describe the geologic information gathered in the area over almost composition, distribution, and orientation of geologic 30 years, and second, tectonic theories for the materials and structures within the study area at California Coast Ranges have changed since the regional scale. Regional geologic information is publication of the earlier map. The new tectonic important for analysis of earthquake shaking, theories have a strong influence on the depiction of liquifaction susceptibility, landslide susceptibility, through-going active faults, volcanic fields, folds, and engineering materials properties, mineral resources and other mapped features. Changes in the map include: hazards, as well as groundwater resources and hazards. 1) an updated depiction of the active Rodgers Creek, These data also assist in answering questions about the West Napa, and Maacama Fault Zones, as well as geologic history and development of the California other dormant faults; Coast Ranges. 2) Quaternary surficial deposits throughout the map; The spatial scale of the map or map database, 3) new mapping in Aetna Springs, Walter Springs, St. however, is not sufficiently detailed for site-specific Helena, Chiles Valley, Rutherford, Kenwood, and investigations. Those seeking detailed information of Cotati quadrangles; and that kind should consult the earthquake-hazards maps 4) reinterpretation of the Early Cretaceous section in produced by the California Geological Survey or the northeastern part of the map area. contact a licensed geologist or engineering-geologist. This pamphlet also includes enhanced unit The digital nature of the geologic map descriptions, and a discussion of the stratigraphy and information is important for three reasons. First, the structure of the map area not present in the earlier MF. 1 Stratigraphy crops out just south of the map area (Blake and others, 2000). The youngest parts of the Franciscan Complex Mesozoic terrane complexes don’t crop out in the map area, but are well exposed to the northwest in Sonoma and Mendocino Counties. The rock units in the San Francisco Bay region are These younger terranes include Eocene and younger made up of two components: 1) older rocks comprised sedimentary rocks (the Coastal Belt of the Franciscan of amalgamated, highly deformed tectonostratigraphic Complex), so accretion of Franciscan Complex terranes that are displaced, at least in part, from terranes in the region must have continued into hundreds to thousands of kilometers from their position Cenozoic time. However, the original relationship of origin (allocthonous or parautochthonous); and 2) between the Coastal Belt Franciscan Complex terranes younger, less deformed rocks that overlie the to the west and older Franciscan Complex and Great amalgamated terranes and are roughly in their original Valley complex rocks in the map area is not well position (except for San Andreas Fault system offsets understood. Because the Great Valley complex, and smaller dislocations described below). Throughout including the Coast Range ophiolite, formed the the map area, the older set of rocks are Mesozoic and hanging wall of the subduction zone where the the younger are Cenozoic. Franciscan Complex terranes accreted, the contact The amalgamated older terranes are grouped between the two Mesozoic complexes is everywhere into three related rock complexes, two of which crop faulted (Bailey and others, 1964), and the Franciscan out in the map area. One of these, the Great Valley Complex presumably underlies the entire San complex, is made up of the Coast Range ophiolite and Francisco Bay area east of the San Andreas Fault. the Great Valley sequence. The Jurassic Coast Range The third older rock complex, the granitic ophiolite in the map area consists mostly of rocks of the Salinian complex, crops out only west of serpentinite, serpentinite-matrix mélange, gabbro, the San Andreas Fault Zone west of the map area. diabase, and basalt. The Great Valley sequence is Both the Franciscan Complex and the Great composed of Jurassic and Cretaceous age sandstone, Valley complex have been further divided into a conglomerate, and shale. Although the sedimentary number of fault-bounded tectonostratigraphic terranes rocks and ophiolite have been tectonically separated (Blake and others, 1982, 1984). When the terranes were first established, the prevailing methodology was almost everywhere in the map area, the Great Valley to identify separate terranes if there was any doubt sequence was originally deposited on top of the about stratigraphic linkage between structurally ophiolite. This depositional relationship is preserved separated entities. As a result of further research, locally in the Chiles Valley and St. Helena additional data, and, in particular, new fossil quadrangles. The Great Valley complex represents the localities, the distribution and nature of the originally accreted and deformed remnants of arc-related Jurassic mapped terranes have been greatly modified (see oceanic crust with a thick sequence of overlying Blake and others, 2000, 2002). turbidites, at least in part, related to the North Terranes in the map area are shown in an American fore arc (parautochthonous). index map on the map sheet (and in the map database The second set of amalgamated terranes, the eswn-terr.e00). See Blake and others (2002) for a Franciscan Complex, is composed of weakly to detailed description of the Mesozoic terranes, a recent discussion of the origin of the Coast Range ophiolite strongly metamorphosed graywacke, argillite, basalt, and the Franciscan mélange, as well as a description serpentinite, chert, limestone, and other rocks. The of the terranes listed above. rocks of the Franciscan Complex in the map area are Strata of two terranes in the Great Valley mostly derived from Jurassic to Cretaceous oceanic complex crop out within the map area. The strata and crust and pelagic deposits overlain by Late Jurassic to ophiolitic rocks in the Napa, Sonoma, and Rutherford Late Cretaceous turbidites. Most Franciscan Complex quadrangles are probably part of the Del Puerto rocks are little metamorphosed, but high-pressure, low- terrane, based on the presence of large masses of temperature metamorphic minerals are common in keratophyre in rocks of similar structural position along rocks that crop out as mélange blocks (Bailey and strike southeast of the map area (Graymer and others, others, 1964) and in several fault-bounded lenses 2002).
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