HR: 0800h
AN: T11D-1289    [Abstracts]
TI: Preliminary 3-Dimensional Geologic Map of the Santa Rosa Plain, Northern California
AU: * McCabe, C A
EM: cmccabe@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AU: McPhee, D K
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AU: Valin, Z C
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AU: McLaughlin, R J
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AU: Jachens, R C
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AU: Langenheim, V E
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AU: Wentworth, C M
AF: U.S. Geological Survey, 345 Middlefield Road M/S 973, Menlo Park, CA 94025 United States
AB: We have constructed a preliminary 3-dimensional geologic map of the Santa Rosa Plain as a tool to address earthquake hazard and groundwater issues. The map allows integration of diverse datasets to produce a stratigraphic and structural architecture for the region. This framework can then be used to predict pathways of ground water flow and potential areas of enhanced or focused seismic shaking beneath the Santa Rosa Plain. The 3D map also allows us to identify relations which will require further refinement to develop a coherent 3D image of the crust. The 3D map, built using EarthVision 3D geologic mapping software, consists of three bounding components: fault surfaces, stratigraphic surfaces, and a basement upper surface. Fault surfaces are derived from geologic mapping, subsurface projection of fault dips from the surface geology and earthquake hypocenters. Stratigraphic surfaces are derived from the mapped geology, a digital elevation model and stratigraphic information from wells. A basement surface, predominantly composed of Mesozoic rocks of the Franciscan Complex, the mafic Coast Range Ophiolite and strata of the Great Valley Sequence, is derived from inversion of regional gravity measurements and constrained by well data. The preliminary 3D map of the Santa Rosa Plain area highlights two large basins ($>$2 km deep): the Windsor and Cotati basins. These basins are divided by a structural high associated with the W-NW-trending, NE-dipping Trenton thrust fault. The Cotati basin is further subdivided by a deeper basement ridge subparallel to the Trenton fault, which separates the basin beneath Cotati from the basin of Petaluma Valley to the southeast. Neither of the basement ridges breaks the surface, yet faults associated with the ridges could displace or truncate aquifers, provide channelways for groundwater flow between aquifers, or create zones of impermeability that disrupt the vertical and lateral continuity of groundwater flow. The complex configuration of the buried basins and their crosscutting ridges could also enhance local ground shaking during earthquakes on the active Rodgers Creek, Healdsburg and Maacama faults, and might explain unexpectedly severe damage suffered at Santa Rosa in the 1906 San Francisco earthquake and in the 1969 earthquakes on the Rodgers Creek-Healdsburg fault system. A major challenge in refining this model will be to develop other stratigraphic surfaces within the blocks framed by the major fault surfaces, such as the base of the Holocene, the base of the Tertiary sedimentary section, and the bases of Tertiary volcanic units. Additional gravity, magnetic, seismic, material property and well data, coupled with inferences from the geology exposed around the margins of the Plain will be used in subsequent versions of this 3D map to improve constraints on the sedimentary architecture of the basins, the shape of the basement surface beneath the water-bearing units, the location, shape and offset of faults within and bounding the Santa Rosa Plain, and the distribution of mafic volcanics rocks.
DE: 9350 North America
DE: 7223 Seismic hazard assessment and prediction
DE: 8015 Local crustal structure
DE: 8105 Continental margins and sedimentary basins
DE: 8122 Dynamics, gravity and tectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting