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

Open-File Report 2008-1009
By: , and 



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.

Study Area

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic and Geophysical Framework of the Santa Rosa 7.5' Quadrangle, Sonoma County, California
Series title Open-File Report
Series number 2008-1009
DOI 10.3133/ofr20081009
Edition Version 1.0
Year Published 2008
Language ENGLISH
Publisher Geological Survey (U.S.)
Contributing office(s) Western Earth Surface Processes
Description Report: iv, 51 p.; 3 Sheets: each 54 x 36 inches; Data Files
Scale 24000
Online Only (Y/N) Y
Additional Online Files (Y/N) Y
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