Three-dimensional geologic modeling of the Santa Rosa Plain, California Donald S. Sweetkind* U.S. Geological Survey, Denver Federal Center, Mail Stop 973, Denver, Colorado 80225, USA Emily M. Taylor U.S. Geological Survey, Denver Federal Center, Mail Stop 980, Denver, Colorado 80225, USA Craig A. McCabe ESRI, 380 New York Street, Redlands, California 92373, USA Victoria E. Langenheim U.S. Geological Survey, Mail Stop 989, 345 Middlefi eld Road, Menlo Park, California 94025, USA Robert J. McLaughlin U.S. Geological Survey, Mail Stop 973, 345 Middlefi eld Road, Menlo Park, California 94025, USA ABSTRACT lence of the clay-rich Petaluma Formation Formation (Fig. 1). Although the outcrop dis- and its heterogeneous nature. Isopach maps tribution of each of these formations has been New three-dimensional (3D) lithologic and of the Glen Ellen Formation and the 3D mapped (e.g., Blake et al., 2002; Wagner et stratigraphic models of the Santa Rosa Plain stratigraphic model show the infl uence of the al., 2006; Graymer et al., 2007), the degree of (California, USA) delineate the thickness, Trenton Ridge, a concealed basement ridge subsurface interfi ngering and overlapping age extent, and distribution of subsurface geo- that bisects the plain, on sedimentation; the relations of the Miocene and Pliocene marine logic units and allow integration of diverse thickest deposits of the Glen Ellen Formation and nonmarine units have only recently been data sets to produce a lithologic, strati- are confi ned to north of the Trenton Ridge. recognized and have important signifi cance for graphic, and structural architecture for the the hydrogeologic system. The large increase region. This framework can be used to pre- INTRODUCTION in population and concomitant changes in land dict pathways of groundwater fl ow beneath use within Sonoma County requires a reassess- the Santa Rosa Plain and potential areas of Sonoma County is in the northern part of the ment of the hydrogeologic system, including the enhanced or focused seismic shaking. San Francisco Bay region of northern Califor- thickness, extent, and three-dimensional (3D) Lithologic descriptions from 2683 wells nia, an area that has undergone rapid popula- distribution of each of these important aquifers. were simplifi ed to 19 internally consistent tion growth and accelerated urbanization in The distribution, subsurface extent, and inter- lithologic classes. These distinctive lithologic response to economic expansion over several fi ngering relations between the four principal classes were used to construct a 3D model decades. Approximately half of the popula- formations refl ect the geomorphologic devel- of lithologic variations within the basin by tion of Sonoma County resides on the Santa opment of the basins that underlie the Santa extrapolating data away from drill holes Rosa Plain (Fig. 1), a northwest-trending topo- Rosa Plain, the history of uplift and subsid- using a nearest-neighbor approach. Subsur- graphic and structural low. Water supply in this ence, tectonic activity, including offset along face stratigraphy was defi ned through the area is provided by a combination of surface major basin-bounding faults, and the interaction identifi cation of distinctive lithologic pack- water delivered via aqueduct from the Russian between continental and marine sedimentation. ages tied, where possible, to high-quality well River and groundwater from beneath the Santa The complexity in stratigraphic and structural control and to surface exposures. The 3D Rosa Plain. The Santa Rosa Plain is known relations across faults bounding the Santa Rosa stratigraphic model consists of three bound- to be underlain by four Miocene and younger Plain makes it diffi cult to project the geology ing components: fault surfaces, stratigraphic formations, each of which has distinct aquifer exposed in the uplands surrounding the plain surfaces, and a surface representing the top properties, including: (1) Pliocene–Pleistocene directly to the subsurface, making 3D subsur- of pre-Cenozoic basement, derived from gravels that have been referred to in part as the face analysis from well data essential. An under- inversion of regional gravity data. Glen Ellen Formation (Fox, 1983); (2) domi- standing of the extent and 3D geometry of these The 3D lithologic model displays a west nantly marine sands of the Miocene and Plio- formations bears on an understanding of basin to east transition from dominantly marine cene Wilson Grove Formation; (3) various types evolution, the timing of movement of faults the sands to heterogeneous continental sedi- of Miocene and Pliocene volcanic rocks; and bound and transect the basins that underlie the ments. In contrast to previous stratigraphic (4) dominantly fi ne-grained continental sedi- Santa Rosa Plain, and the relation to volcanism studies, the new models emphasize the preva- ments of the Miocene and Pliocene Petaluma in the nearby Sonoma volcanic fi eld. *[email protected]. Geosphere; June 2010; v. 6; no. 3; p. 237–274; doi: 10.1130/GES00513.1; 11 fi gures; 5 tables; 3 plates; 8 appendix fi gures; 2 supplemental fi gures. For permission to copy, contact [email protected] 237 © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 on 30 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf 238 Geosphere, June2010 510000 520000 530000 Figure 1. Simplifi ed geologic map (modi- 123° 122°30' fi ed from Saucedo et al., 2000; Graymer Area of map et al., 2006) of the Santa Rosa Plain Sonoma Cty and surrounding highlands. The Santa Healdsburg Maacama fault Rosa Plain is bound on the west by the 38°30' Napa Cty Santa Sebastopol fault and on the east by the Rosa Plain Rodgers Creek and Healdsburg faults. Healdsburg fault fault The buried Trenton Ridge separates the northern Windsor Basin from the south- 4270000 Marin Cty ern Cotati Basin. Drill holes used in this 38° study are classifi ed by total depth. Two Windsor drill holes with high-quality lithologic Russian River and biostratigraphic data (Powell et al., 2006) are labeled: OR—Occidental Pacific Ocean Road well; SR—Sebastopol Road well. Windsor 37°30’ Basin EXPLANATION Map units (Geology after Saucedo and others, 2000) 4260000 Trenton Ridge Cenozoic rocks Sweetkind etal. Location of deep basins in the Quaternary alluvium Santa Rosa Plain, as defined by Santa Quaternary and Pliocene -14 mGal isostatic gravity contour Rosa gravels (Glen Ellen Formation) (Langenheim and others, 2006) Bennett Valley fault fault OR Petaluma Formation 26 25 Drill hole, classified by total depth 35 36 Cotati Wilson Grove Formation SR <100 m Neogene volcanic rocks SebastopolSebas faultBasin 100 - 200 m Sebastopol Mesozoic rocks t 201 - 300 m 4250000 opol fault Great Valley sequence and Coast Range ophiolite 301 - 500 m Franciscan Complex > 500 m Rodgers Creek fault fault Ultramafic rocks Rohnert Park Bloomfield fault Section boundary; sections 25, Meacham Hill Mapped faults Cotati 26, 35, and 36 in T7N R9W (after Graymer and others, 2006) are discussed in text Simplified trace of major faults used in 3D lithologic and stratigraphic models 4240000 Stream Petaluma 10,000-meter grid based on Universal Transverse Mercator projection, Zone 10, North American Datum 1983. 0105 KILOMETERS Shaded-relief base from 1:275,000-scale Digital Elevation Model; sun illumination from northwest at 30 degrees above horizon 3D geologic modeling—Santa Rosa Plain Studies of the Santa Rosa Plain that have framework for groundwater resource assess- Pliocene and younger normal faults, here gener- focused on water availability (Cardwell, 1958; ments of the Santa Rosa Plain. alized as the Sebastopol fault (Fig. 1). California Department of Water Resources, Inversion of gravity data indicates that the 1975, 1982) used drill-hole data to develop GEOLOGIC SETTING OF Santa Rosa Plain is underlain by two main geologic cross sections and to help estimate the SANTA ROSA PLAIN structural basins, the Cotati Basin to the south transmissivity of various rock types. However, and the Windsor Basin to the north (Fig. 1). these previous subsurface interpretations largely The southern part of the Santa Rosa Plain These depositional troughs are 2–3 km deep were based on limited borehole information is covered by Quaternary alluvial deposits and fi lled with Tertiary and younger deposits from a small number of oil and gas wells and (Fig. 1). The northern part features low, slightly (McPhee et al., 2007; Langenheim et al., 2010). water wells, augmented by projection of surface dissected exposures of late Pliocene and Qua- These two basins are separated by a shallow exposures to the subsurface. Since these water ternary (Pleistocene and Holocene) fl uvial, west- northwest–striking bedrock ridge (the availability studies, much new work has been lacustrine, and alluvial plain deposits that have Trenton Ridge) that bisects the Santa Rosa Plain conducted, including new geologic maps pub- in part been referred to as the Glen Ellen Forma- (McPhee et al., 2007; Williams et al., 2008; lished by the California Geologic Survey (Wag- tion (Fox, 1983), along with younger alluvium Langenheim et al., 2010) (Fig. 1). The Wind- ner and Bortugno, 1982; Bezore et al., 2003; within stream channels (Graymer et al., 2007) sor Basin to the north is ~9 × 12 km, centered Clahan et al., 2003; Wagner et al., 2003, 2006) (Fig. 1). The highlands to the east of the Santa near the town of Windsor, and is located near and geologic maps and other studies published Rosa Plain are underlain by various types of many of the thickest outcrops of the Glen Ellen by the U.S. Geological Survey (USGS) (Blake Miocene and Pliocene volcanic rocks, in part Formation in the Santa Rosa Plain. The Cotati et al., 2002; Graymer et al., 2007; McPhee et interbedded with the largely nonmarine and Basin to the south is larger, 10 × 18 km, and al., 2007; McLaughlin et al., 2008; Langenheim estuarine strata of the Petaluma Formation; both 2.5–3 km deep.