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Geohydrology of Coast Ranges basins

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Research Goal 2—northern California Coast Ranges basins

Cenozoic rocks in the northern California Coast Ranges between San Francisco Bay and Cape Mendocino record the tectonic effects of the northward migration of the Mendocino triple junction and the far-field effects of the evolving plate margin boundary. Basins and valleys, from Petaluma Valley on the south to Ukiah Valley and Willits area on the north, record much of this tectonic history as well as contain important aquifers. Faults and related folds near the north end of the Petaluma basin, and extending northwestward towards the San Andreas fault provide evidence of a “proto- Hayward” fault system and are intimately involved in the evolution of the Petaluma and Santa Rosa basins and the overall development and integration of the fault systems in the North Bay. Characterization of these structures will help answer the questions of how slip on the North Bay system interacts with the San Andreas fault proper.

The large increase in population and concomitant changes in land use within Sonoma County requires a reassessment of the hydrogeologic system, including the thickness, extent, and 3D distribution of each if these important aquifers. The Water Resources Discipline of the U.S. Geological Survey has established a long-term cooperative program with the Sonoma County (CA) Water Agency to address water-availability, quality, and management issues countywide. The ground-water basins of interest are: Sonoma Valley, Alexander Valley, Santa Rosa Plain, and Petaluma Valley. Geologic studies by GD staff under the “Understanding Geohydrology” project resulted in 3D lithologic and stratigraphic maps that served as primary geologic input into a ground-water flow model of the Santa Rosa Plain. We anticipate that the geologic studies in Petaluma Valley will be similarly used.

This work is coordinated closely with and benefits greatly from ongoing work in the 3D/4D Geologic Mapping Project.

Research Abstracts for Goal 2

Langenheim, V.E., Phelps, G.A., and Jachens, R.C., 2009, Mapping layers in the Franciscan Complex and faults of the San Andreas system with new aeromagnetic data in the northern Coast Ranges, California: Geological Society of America Abstracts with Programs, v. 41, no. 7, p. 282.

Research Products for Goal 2

Sweetkind, D.S., and Taylor, E.M., 2010, Digital tabulation of geologic and hydrologic data from water wells in the northern San Francisco Bay region, northern California: U.S. Geological Survey Open-File Report 2010-1063, 125 p. (available at

Sweetkind, D.S., Taylor, E.M., McCabe, C.A., Langenheim, V.E., and McLaughlin, R.J., 2010, Three-dimensional geologic modeling of the Santa Rosa Plain, California: Geosphere; June 2010; v. 6; no. 3; p. 237–274; doi: 10.1130/GES00513.1.

Geologic map of the northern Coast Ranges


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