NEW GEOLOGIC MAPPING IN WESTERN SONOMA COUNTY, CALIFORNIA

The California Geological Survey recently completed geologic mapping of the Camp Meeker 7.5’ quadrangle in western Sonoma County. This culminates a multiyear STATEMAP effort, in cooperation with the USGS, to create a geologic GIS database for the Napa 30’ x 60’ quadrangle. The goal was to provide basic geologic data to use in developing Seismic Hazard Zone Maps for liquefaction and seismically induced landsliding, as well as an improved understanding of the framework geology in this tectonically active area. Work began with the digital compilation of bedrock geology from 5 previous studies published at different scales. Unit differences were resolved to fit with current correlations, and contacts adjusted to reflect additional details added from aerial photo interpretation and new field mapping. Quaternary mapping by Witter and others (2006) was digitally integrated with the bedrock and contacts adjusted. Because the map is intended for use in developing regulatory hazard zones for liquefaction, particular care was taken to accurately portray the bedrock/Quaternary unit contacts. Feature locations were refined by digitizing over DOQQ’s for horizontal control. The DRG, and where available, higher resolution orthophotos and topographic data from the County of Sonoma were used for additional spatial reference. Basement rocks exposed in the Camp Meeker study area consist largely of highly deformed and diverse rocks of the Jurassic to Early Tertiary age Franciscan Complex. Tectonically interleaved with the Franciscan are fault-bounded slivers and blocks of the Great Valley complex, including serpentinite derived from the Jurassic Coast Range ophiolite and Jurassic to Cretaceous age sedimentary rocks of the Great Valley sequence. The basement rocks are overlapped by less deformed late Miocene to late Pliocene marine sandstone and pebbly sandstone of the Wilson Grove Formation. Interbedded with the sandstone are lenses and more persistent marker beds of pumiceous tuff breccia and reworked tuff. Ongoing efforts, using tephrochronology to correlate volcanic ash found here and in non-marine deposits further east, offer an opportunity for new insights on the late Tertiary stratigraphy and structural evolution of this dynamic region between the San Andreas fault and Rodgers Creek-Healdsburg-Maacama fault system.