Western Earth Surface Processes

U.S. Geological Survey
Open-File Report 2008-1009

Geologic and Geophysical Framework of the Santa Rosa 7.5’ Quadrangle, Sonoma County, California

By R.J. McLaughlin, V.E. Langenheim, A.M. Sarna-Wojcicki, R.J. Fleck, D.K. McPhee, C.W. Roberts, C.A. McCabe, and Elmira Wan


thumbnail view of geologic map

The geologic and geophysical maps of Santa Rosa 7.5’ quadrangle and accompanying structure sections portray the sedimentary and volcanic stratigraphy and crustal structure of the Santa Rosa 7.5’ quadrangle and provide a context for interpreting the evolution of volcanism and active faulting in this region. The quadrangle is located in the California Coast Ranges north of San Francisco Bay and is traversed by the active Rodgers Creek, Healdsburg and Maacama Fault Zones. The geologic and geophysical data presented in this report, are substantial improvements over previous geologic and geophysical maps of the Santa Rosa area, allowing us to address important geologic issues. First, the geologic mapping is integrated with gravity and magnetic data, allowing us to depict the thicknesses of Cenozoic deposits, the depth and configuration of the Mesozoic basement surface, and the geometry of fault structures beneath this region to depths of several kilometers. This information has important implications for constraining the geometries of major active faults and for understanding and predicting the distribution and intensity of damage from ground shaking during earthquakes. Secondly, the geologic map and the accompanying description of the area describe in detail the distribution, geometry and complexity of faulting associated with the Rodgers Creek, Healdsburg and Bennett Valley Fault Zones and associated faults in the Santa Rosa quadrangle. The timing of fault movements is constrained by new 40Ar/39Ar ages and tephrochronologic correlations. These new data provide a better understanding of the stratigraphy of the extensive sedimentary and volcanic cover in the area and, in particular, clarify the formational affinities of Pliocene and Pleistocene nonmarine sedimentary units in the map area. Thirdly, the geophysics, particularly gravity data, indicate the locations of thick sections of sedimentary and volcanic fill within ground water basins of the Santa Rosa plain and Rincon, Bennett, and northwestern Sonoma Valleys, providing geohydrologists a more realistic framework for groundwater flow models.

Files for Viewing and Printing

Download Sheet 1 as a 54" x 36" PDF file. This map sheet that can be used for viewing map in a browser, as well as for plotting at full scale (geologic map, list of map units, and correlation of map units) (of2008-1009_sheet1.pdf; 79.9 MB)

Download Sheet 2 as a 54" x 36" PDF file. (cross sections, comparative stratigraphy, station-location figure, and structure figures) (of2008-1009_sheet2.pdf; 36.6 MB)

Download Sheet 3 as a 54" x 36" PDF file. (Isostatic gravity map and Aeromagnetic map) (of2008-1009_sheet3.pdf; 46.7 MB)

Download the Pamphlet to accompany the map as a 51-page PDF file. This pamphlet contains a chapter on the geologic framework (chronostratigraphy, description of map units, structure) and a chapter on the geophysical framework (gravity, magnetics, anomalies). (of2008-1009_pamphlet.pdf; 13.5 MB)

Files for GIS

Readme file that lists and describes the digital dataset files linked below (of2008-1009_readme.txt; 16 kB)

FGDC compliant metadata (of2008-1009_metadata.txt; 88 kB)

Geodatabase as a compressed file. (of2008-1009_geodatabase.zip; 33.2-MB compressed file that expands to 220 MB)

Shapefiles as a compressed file. (of2008-1009_shapefiles.zip; 9.9-MB compressed file that expands to 24.9 MB)

For questions about the content of this report, contact Bob McLaughlin

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