Three-Dimensional Geologic Modeling of the Santa Rosa Plain, California

Home , Basement high, Merced Formation, Northern California

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.

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 Sweetkind et al. ed geologic map (modi- ed by total depth. Two Two ed by total depth. Section boundary; sections 25, T7N R9W 26, 35, and 36 in are discussed in text Location of deep basins in the by Santa Rosa Plain, as defined contour -14 mGal isostatic gravity (Langenheim and others, 2006) <100 m 100 - 200 m 201 - 300 m 301 - 500 m > 500 m Drill hole, classified by total depth ed from Saucedo et al., 2000; Graymer Saucedo et al., 2000; Graymer ed from Figure 1. Simplifi Figure fi et al., 2006) of the Santa Rosa Plain The Santa highlands. and surrounding Rosa Plain is bound on the west by Sebastopol fault and on the east by and Healdsburg faults. Rodgers Creek Ridge separates the Trenton The buried the south- Basin from Windsor northern ern Cotati Basin. Drill holes used in this classifi study are drill holes with high-quality lithologic and biostratigraphic data (Powell et labeled: OR—Occidental al., 2006) are Road well; SR—Sebastopol well. EXPLANATION Napa Cty 122°30' Plain Rosa Santa Marin Cty 123° Stream Mapped faults Mapped faults (after Graymer and others, 2006) Simplified trace of major faults used in 3D trace of major faults Simplified lithologic and stratigraphic models Pacific Ocean Pacific Area of map Quaternary alluvium Quaternary and Pliocene (Glen Ellen Formation) gravels Petaluma Formation Formation Grove Wilson rocks Neogene volcanic sequence Great Valley and Coast Range ophiolite Franciscan Complex rocks Ultramafic Sonoma Cty 38° Map units (Geology after Saucedo and others, 2000) 38°30' 37°30’ Cenozoic rocks Mesozoic rocks

Bennett Valley faultfault Rodgers Creek faultfault 530000 Santa Rosa 5 KILOMETERS Petaluma Cotati Rohnert Park

Maacama fault 010

Cotati Basin Basin Meacham Hill

520000 topol fault SR Basin Healdsburg fault Basin Windsor Windsor Trenton Ridge Ridge SebastopolSebas fault 25 Windsor 26 35 36 OR

Sebastopol Russian River Russian

Bloomfield fault fault Healdsburg 510000

10,000-meter grid based on Universal Transverse Mercator projection, Zone 10, North American Datum 1983. Shaded-relief base from 1:275,000-scale Digital Elevation Model; sun illumination from northwest at 30 degrees above horizon 4270000 4260000 4250000 4240000

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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. The Cotati Basin has a complex et al., 2010). There have also been studies on of these units unconformably overlie Mesozoic shape that suggests the presence of structurally exposed basin-margin stratigraphy and structure rocks (Fig. 1). This eastern margin of the Santa controlled subbasins. The Glen Ellen Formation (Fox, 1983; Davies, 1986; Allen, 2003), strati- Rosa Plain is highly deformed and cut by major is also considerably thinned within much of the graphic data from oil and gas wells (Wright and right-lateral strike-slip faults. West of the Santa Cotati Basin, as the basin fi ll is dominated by Smith, 1992; Zieglar et al., 2005), and detailed Rosa Plain, a broad, low topographic area is the Wilson Grove and Petaluma Formations. biostratigraphic and chronostratigraphic analy- underlain by Miocene to Pliocene, locally fos- sis of surface exposures and drill cuttings (Pow- siliferous marine sandstone formerly known Description of Principal Stratigraphic ell et al., 2004, 2006). This paper provides inte- as the Merced Formation (Cardwell, 1958), Units of the Santa Rosa Plain gration of these data sets with existing and new now referred to as the Wilson Grove Forma- well data to develop a modern context for sub- tion (Fox, 1983). These marine strata dip gen- Quaternary to Pliocene–Pleistocene surface analysis of the Santa Rosa Plain. tly northeastward beneath the Santa Rosa Plain nonmarine units (Glen Ellen Formation) In this paper we defi ne the subsurface stratig- and unconformably overlie Mesozoic rocks The Pliocene–Pleistocene (younger than raphy and lithologic heterogeneity of the four (Fig. 1). Interfi ngering of marine sandstone with 3.2 Ma) Glen Ellen Formation was fi rst principal aquifer units using compiled drill-hole transitional marine and nonmarine deposits is described by Weaver (1949) as exposures of data from the Santa Rosa Plain. A 3D model inferred to occur beneath the Santa Rosa Plain poorly sorted clays, sands, gravels, and cob- of lithologic variations within the basins that based on exposures at Meacham Hill immedi- bles near the town of Glen Ellen in the upper underlie the Santa Rosa Plain is developed by ately southwest of the Santa Rosa Plain (Powell Sonoma Valley. Exposures of similar rocks have extrapolating data away from drill holes using et al., 2004). However, this transition zone is since been mapped through most of the Santa a 3-dimensional gridding process (Rockware obscured by younger deposits beneath most of Rosa Plain, especially to the north and west of Earth science and GIS software: www.rockware the plain. Cross sections that accompanied pre- Santa Rosa. The unit consists of heterogeneous .com). Subsurface stratigraphy is defi ned through vious groundwater resource assessments of the mixtures of tuffaceous clay, mud, bouldery to the identifi cation of distinctive lithologic pack- Santa Rosa Plain (Cardwell, 1958; California pebbly gravel, and sand and silt deposits with ages, tied, where possible, to high-quality well Department of Water Resources, 1975, 1982) interbedded conglomerates. These sediments control. Available subsurface data provided suffi - portray most of the plain as being underlain by were deposited in a variety of nonmarine envi- cient detail to allow us to confi dently distinguish Glen Ellen Formation as much as ~300 m thick, ronments, including coalescing alluvial fans, major stratigraphic boundaries and enough inter- underlain, in turn, by an unspecifi ed thickness fan deltas, streams, and lakes. Cardwell (1958) nal detail within these units to develop a reliable of Wilson Grove Formation beneath the western referred to many of these deposits as Glen Ellen subsurface geologic model. Faults are incorpo- half of the plain, and fl anked by Neogene volca- Formation, but this terminology has been largely rated as discontinuities in structure contour and nic rocks on the east. The Petaluma Formation abandoned with the recognition of the existence isopach maps of the principal units; however; was inferred beneath the Petaluma Valley, but of a number of other named and unnamed interbasin structural complexities such as fold- not to the north in the Santa Rosa Plain. We rec- gravel-dominated sequences that overlap in ing and thrust faulting are not explicitly consid- ognize signifi cantly different stratigraphic rela- age and are derived from several different local ered by these models. This structural complexity tions and distributions between the Glen Ellen, source areas (McLaughlin and Sarna-Wojcicki, is partly accommodated in the model through Wilson Grove, and Petaluma Formations. 2003; McLaughlin et al., 2005). We retain the integration of unit interfi ngering and facies rela- The Santa Rosa Plain is bounded and tran- use of the term “Glen Ellen” to describe these tions changes. These 3D subsurface models sected by major faults, including the active diverse deposits in this study, mostly for consis- provide new insight into the confi guration of the northwest-striking, right-lateral Rodgers Creek– tency with earlier reports concerning the Santa basin-fi ll sediments, the relative importance and Healdsburg fault zone bounding the east side of Rosa Plain. lithologic character of each of the four principal the plain. The west and southwest side of the For our study we have combined all late basin-fi lling units, and a suitable hydrogeologic plain is bounded by a system of poorly defi ned Pliocene and younger nonmarine deposits in

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1 meter

B WE

Location of well profile DEPTH, m

Western end point of profile: EXPLANATION 122° 50.53' W; 38° 30.13' N Gravel Clay, sand and gravel Conglomerate Eastern end point of profile: Sand and gravel Clay, sand and trace gravel Volcanic conglomerate 122° 47.89' W; 38° 31.1' N Sandstone and gravel Clay and gravel Basalt Sand Clay and trace gravel Ash and/or tuff Sandstone Clay and sand Undifferentiated basement Sand and clay Clay and sandstone No data Sandstone and clay Clay

Figure 2. Characteristics of Pliocene–Pleistocene gravels (Glen Ellen Formation). (A) Surface exposure 5 km northwest of Santa Rosa. Height of outcrop is ~2 m. (B) Well proﬁ le showing typical lithologic logs from drill holes. Wells are hung from land surface; depth below land surface is in meters.

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the subsurface into a Glen Ellen unit, including the surfi cial Quaternary deposits, because the A younger surfi cial deposits are typically thin and diffi cult to differentiate in drill logs. Outcrop exposures of this unit typically consist of gen- tly to moderately tilted sections of stratifi ed, but poorly sorted heterogeneous mixtures of gravels and sands interbedded with more consolidated conglomerates (Fig. 2A). The unit is generally poorly sorted to unsorted, and unconsolidated to weakly cemented and consolidated. Although no drill-hole or outcrop data document such a thickness, the unit has been interpreted to be as thick as 1000 m (Cardwell, 1958; California Department of Water Resources, 1975). Typi- cal drill-hole lithologic descriptions of this unit (Fig. 2B) are notable for their overall heterogeneity, generally recording relatively thin beds (<5 m) of coarse and fi ne units, interspersed with coarse gravel intervals. Several nonwelded B tuffs occur in parts of this unit.

Late Miocene and Pliocene Wilson Grove Formation The Pliocene and Late Miocene Wilson Grove Formation is exposed over a broad area on the west side of the Santa Rosa Plain, extending from Petaluma in the south to the Russian River on the north, and from the west edge of the Santa Rosa Plain westward to the Pacifi c Ocean coastline between Bodega and Tomales Bays (Fig. 1). The formation consists of consolidated to weakly consolidated deposits of massive or thick-bedded, gray to buff, fi ne-grained to very fi ne grained fossiliferous sand or sand- 25 cm stone (Fig. 3). The unit includes local beds of mollusk and gastropod shell hash, pebble to boulder conglomerate, and local pumiceous tuff (Fox, 1983; Blake et al., 2002; Powell et C al., 2004). The Wilson Grove Formation has a maximum exposed thickness of ~150 m; well logs indicate as much as 300 m thickness. The Wilson Grove Formation is marine, deposited in dune, littoral, and shelf settings. Distal western parts of the formation that are inset into the Mesozoic rocks may represent the head of a submarine canyon (Allen, 2003; Powell et al., 2004). The formation interfi ngers with the Peta- luma Formation in exposures near the town of Cotati and at Meacham Hill immediately southwest of the Santa Rosa Plain. Interfi ngering of marine facies rocks with transitional marine and nonmarine deposits is inferred to occur beneath the Santa Rosa Plain as well. Outcrop and drill-hole data suggest that the Wilson Grove Formation can be divided into Figure 3 (continued on following page). Characteristics of the Wilson Grove Formation. three distinct marine environments represented (A) Surface exposure 5 km north-northwest of Sebastopol of massive fi ne-grained sand with by lateral variations in lithology (Powell et al., scattered shell fragments. (B) Surface exposure 15 km southwest of Sebastopol of very fi ne 2004). The fi rst environment includes fi ne- grained marine sandstone facies with distinct shell layer. (C) Surface exposure southeast of grained marine sandstones (Fig. 3B) that were Sebastopol showing gravelly sand facies.

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probably deposited in water depths characteris- The third environment is represented by a southeastern extent of outcrop (e.g., south and tic of upper bathyal or outer shelf settings (Pow- transitional marine and/or continental facies east of Sebastopol). ell et al., 2004) and most commonly occur well commonly composed of medium- to coarse- Compared to overlying Quaternary and Pleis- to the west of the Santa Rosa Plain. The sec- grained, angular sandstone beds interbedded tocene units and to interfi ngered facies of the ond environment includes well-sorted fi ne- to with very well rounded pebble conglomerate Petaluma Formation, the Wilson Grove Forma- medium-grained sandstone (Fig. 3A) deposited beds (Fig. 3C). The transitional unit is interbed- tion is distinguished in drillers’ lithologic logs in shallow-marine settings. This facies repre- ded with local well-sorted, well-rounded, and by its overall homogeneous sorting, presence of sents much of the exposed Wilson Grove For- polished cobble to pebble gravel (pea gravel) shells, and massive bedding. Drillers typically mation, especially north of Sebastopol (Fig. 1). that increases in abundance at the eastern and described the fi ner-grained marine sand of the

D SW NE DEPTH, m

EXPLANATION

Gravel Clay, sand and gravel Conglomerate

Sand and gravel Clay, sand and trace gravel Volcanic conglomerate

Sandstone and gravel Clay and gravel Basalt Sand Clay and trace gravel Ash and/or tuff Sandstone Clay and sand Undifferentiated basement Sand and clay Clay and sandstone Location of well profile No data Sandstone and clay Clay

Western end point of profile: 122° 53.1' W; 38° 23.7' N Eastern end point of profile: 122° 50.88' W; 38° 26.22' N

Figure 3 (continued). (D) Well proﬁ le showing typical lithologic logs from drill holes. Wells are hung from land surface; depth below land surface is in meters. Drillers typically describe the ﬁ ne-grained marine facies of the Wilson Grove Formation as clay and sand.

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Wilson Grove Formation as clay or clay and Grove Formations, whereas the younger parts of the Wilson Grove Formation. The Petaluma For- sand (Fig. 3D). The presence of fossil shells the Sonoma Volcanics overlie the Petaluma For- mation is predominantly fi ner grained then the serves as an important marker in the recognition mation and are interbedded with, or underlie, Glen Ellen Formation. Although coarse gravelly of the Wilson Grove Formation in the subsur- the Pliocene–Pleistocene Glen Ellen Formation facies exist in the Petaluma Formation, these face. Although shells are infrequently described (Wagner et al., 2005). Drillers typically have coarse beds are thinner (usually <10 m), more in the Petaluma Formation, the Petaluma is distinguished volcanic rocks, although they may poorly sorted, and usually interbedded with generally considerably fi ner grained, typically not have reliably noted the degree of welding in fi ne-grained clay that lacks a gravel component consisting of a silty to clayey mudstone, and tuffaceous units. The term “volcanic conglom- (Fig. 5C). is usually easily distinguished from the Wil- erate” was often used by drillers due to its typi- son Grove Formation in drillers’ descriptions. cal association with sections of volcanic rocks. Pre-Miocene Rocks, Undivided The Wilson Grove Formation is mostly poorly We interpret this unit to be volcanic in origin, Pre-Miocene rocks (Eocene? and Cretaceous– cemented; some beds are cemented with cal- consisting of fl ow breccia or volcanic agglomer- Jurassic) consist largely of Franciscan mélange cium carbonate and iron and are reported by ate, rather than sedimentary conglomerate dom- of the Central belt, Eocene and older rocks of drillers as hard ledges. The unit contains beds of inated by volcanic rock clasts. In places, volca- the Franciscan Coastal belt, the Jurassic Coast soft white tuff as much as 3 m thick in outcrop nic rocks directly overlie Mesozoic rocks. The Range Ophiolite, and the Cretaceous and Juras- west of Sebastopol; some of these tuff beds have thickness of the volcanic rocks is highly variable sic Great Valley Group (Blake et al., 1984, 2002; been identifi ed as the Late Miocene Roblar tuff and, in general, water wells do not penetrate the McLaughlin and Ohlin, 1984). This unit forms (Sarna-Wojcicki, 1992; Bezore et al., 2003), an entire thickness of the formation (Fig. 4C). For the base of active groundwater fl ow. important time-stratigraphic marker. the purpose of the subsurface lithologic and Pre-Miocene rocks are characterized by a stratigraphic modeling of the Santa Rosa Plain, variety of consolidated rock types, including Neogene Volcanic Rocks and in the absence of age or stratigraphic con- penetratively sheared shale (mélange matrix), Volcanic rocks exposed in the general vicinity trol, the various volcanic rocks are not differ- graywacke, blocks of blueschist, chert, green- of the Santa Rosa Plain and present within the entiated as separate packages, and therefore are stone, thinly interbedded shale and sandstone, basin fi ll include the 3–8 Ma Sonoma Volcanics, combined in a single unit as Neogene volcanics and mafi c to ultramafi c ophiolitic rocks. Drillers the 8.5–9.5 Ma Tolay Volcanics, and the 10.6– in the 3D models. typically recognize serpentinite; other rock types 11.2 Ma Burdell Mountain Volcanics (Wagner et are given a variety of descriptions (Table 1). All al., 2005). The Sonoma Volcanics dominate the Pliocene and Miocene Petaluma Formation of these consolidated rock types are assigned east side of the Santa Rosa Plain. These volcanic The Pliocene and Miocene Petaluma Forma- to a single general lithologic class, i.e., undif- rocks are well exposed to the east of the Rodgers tion is dominated by deposits of moderately to ferentiated basement. The top of pre-Miocene Creek–Healdsburg fault zone and are complexly weakly consolidated silty to clayey mudstone rocks was picked in a drill hole at the highest imbricated by faulting along the southwest side (Fig. 5A), with local beds and lenses of poorly occurrence of one of the above-described con- of the fault zone, where they project beneath, sorted sandstone (Fig. 5B). The Petaluma For- solidated rocks, especially where additional and probably correlate with, volcanic units in the mation is as thick as 900 m in outcrop (Weaver, intervals of similar rocks occurred below. In subsurface of the Santa Rosa Plain (McLaughlin 1949) and as thick as 1200 m in the subsurface rare cases, intervals that could be interpreted et al., 2005, 2008). in Petaluma Valley (Morse and Bailey, 1935; as part of the Cenozoic section were reported The Tolay Volcanics and the Burdell Moun- Allen, 2003). The unit is intercalated with Neo- underlying undifferentiated basement. In these tain Volcanics are exposed in outcrops to the gene volcanics (andesitic to rhyolitic) around the cases, the drill-hole intercepts were compared to southeast and southwest of the Petaluma Val- margins of the Santa Rosa Plain that have radio- the interpreted depth to high-density geophysi- ley, respectively, and have been intercepted in metric ages ranging from ca. 5.0 to ca. 10 Ma cal basement (Langenheim et al., 2006, 2010; the subsurface in the valley based on 40Ar/39Ar (Wagner et al., 2005). The Petaluma Formation McPhee et al., 2007) to help guide subsurface dates from oil and gas wells (Wagner et al., consists of transitional marine and nonmarine stratigraphic interpretation. 2005). The Tolay Volcanics also are exposed in sediments that were deposited in estuarine, the fault-bound anticline at Meacham Hill that lacustrine, and fl uvial depositional settings METHODOLOGY FOR USE OF separates the Santa Rosa Plain from the Peta- (Allen, 2003; Powell et al., 2004). The upper DRILL-HOLE DATA luma Valley to the south (Fig. 1), and are present part of the Petaluma Formation is contempora- in the uplifted southeast corner of Cotati Basin neous with the Wilson Grove Formation. Where Drill-hole data were compiled from a variety north and northwest of the town of Rohnert Park the two formations interfi nger, they represent an of sources, including USGS water resources (Clahan et al., 2003; Wagner et al., 2005). Sev- oscillating Miocene–Pliocene shoreline (Powell reports (Cardwell, 1958) and drill-hole compi- eral drill holes in the vicinity of Cotati intercept et al., 2004). lations (Valin and McLaughlin, 2005), oil and volcanic rocks at depth that may represent bur- Petaluma Formation deposits interpreted gas exploration holes (California Department ied equivalents of these older volcanic units. from drill-hole lithologic data mostly consist of of Conservation Division of Oil, Gas, And Geo- All of the volcanic units include a wide vari- monotonous sequences of clay with occasional thermal Resources, www.conservation.ca.gov/ ety of volcanic rock types including basaltic, interbeds of sand, probably representing distrib- dog [July 2008]), data provided by local water andesitic, dacitic, and rhyodacitic fl ows, fl ow utary channels and gravel bars (Fig. 5C). The agencies, and water wells drilled by indepen- breccias, avalanche or talus breccia, tuffs, and Petaluma Formation is more diverse texturally dent entities and compiled as proprietary data several andesitic to rhyodacitic tephra units than the Wilson Grove Formation. The Peta- by the California Division of Water Resources (Figs. 4A, 4B). Many of the units have relatively luma Formation contains more clayey layers, (CDWR). Drill-hole data in USGS water limited lateral extent and appear to have erupted and is fi ner grained and generally less perme- resources reports (Cardwell, 1958) typically are from local volcanic vents. The older volcanics able, with sandy and coarser-grained units being summaries derived from the original CDWR are interbedded with the Petaluma or Wilson more poorly sorted than coarse units found in records. We used the original CDWR data, even

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A B

1 meter

NW SE C DEPTH, m

Western end point of profile: 122° 43.79' W; 38° 30.21' N Eastern end point of profile: Location of well profile 122° 40.46' W; 38° 28.66' N

EXPLANATION

Gravel Clay, sand and gravel Conglomerate

Sand and gravel Clay, sand and trace gravel Volcanic conglomerate

Sandstone and gravel Clay and gravel Basalt Sand Clay and trace gravel Ash and/or tuff Sandstone Clay and sand Undifferentiated basement Sand and clay Clay and sandstone No data Sandstone and clay Clay

Figure 4. Characteristics of the Neogene volcanics. (A) Surface exposure east of Santa Rosa, showing rhyolite lava ﬂ ow. Height of exposure is ~5 m. (B) Surface exposure north of Santa Rosa, showing pumice-rich, reworked nonwelded tuff. (C) Well proﬁ le showing typical lithologic logs from drill holes. Wells are hung from land surface; depth below land surface is in meters. Note that only one well intercepts pre-volcanic basement.

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A B

NS C Western end point of profile: 122° 38.66' W; 38° 18.23' N Eastern end point of profile: 122° 34.89' W; 38° 15.95' N DEPTH, m

Location of well profile

EXPLANATION

Gravel Clay, sand and gravel Conglomerate

Sand and gravel Clay, sand and trace gravel Volcanic conglomerate

Figure 5. Characteristics of the Petaluma Formation. (A) Surface exposure east of Petaluma, showing mudstones. (B) Surface exposure southeast of Petaluma, showing thick sandstone within a lens-shaped channel deposit. (C) Well proﬁ le showing typical lithologic logs from drill holes. Wells are hung from land surface; depth below land surface is in meters.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 Sweetkind et al. is described by any of these terms any is described other than serpentine by and the underlying intervals is not are not described as basement, this zone Although undifferentiated basement. described as undifferentiated on rare occasion basement is only used at depth in a drill hole, such as Glen Ellen Formation, is thrust Formation over Franciscan near the Russian River. fault Trenton along the rocks, but the distinction cannot be made. but rocks, Only units at depth are considered. When an interval at shallow depth When an interval at shallow Only units at depth are considered. Clay and gravel, in some cases, may be weathered ash and volcanic be weathered ash and volcanic may in some cases, and gravel, Clay In some cases this unit may be volcanic in origin, not sedimentary. be volcanic In some cases this unit may Typical drillers’ description drillers’ Typical Comment TABLE 1. GENERALIZED LITHOLOGIC CLASSES BASED ON DRILL-HOLE DESCRIPTIONS 1. TABLE basalt and shale; black rock with quartz; blue and/or green rock or clay when surrounded or clay and/or green rock blue with quartz; rock black basalt and shale; shale with quartz rock or green rock, and blue oily; shale and/or serpentine; graywacke, by shale with serpentine serpentine; or clay; rock green and blue or clay; ash; lava ash; fractured rocks; multicolored rock; pumice rock; rock (red or black, or when (red or black, rock pumice rock; multicolored rock; fractured rocks; ash; lava ash; or sandstone (when surrounded rock that are described as volcanic); rocks surrounded by volcanic clay; volcanic ash; volcanic tuff and basalt; tuff; shattered rock; ash or tuff); by rock ash; black volcanic rock; cinders; diorite; lava rock; porous volcanic rock porous volcanic rock; lava diorite; cinders; rock; volcanic black ash; (brown, blue, green, orange and yellow); sandy clay; silt; silty clay; shale; shale and sand shale; silty clay; silt; sandy clay; green, orange and yellow); blue, (brown, clay with gravel stringers with gravel clay conglomerate; clay and (with) rock; clayey sand and gravel; embedded clay or gravel; or gravel; embedded clay sand and gravel; clayey and (with) rock; clay conglomerate; hardpan clay; gravelly clay; gravel and clay; gravel gravel shale and rock; topsoil and gravel or rock topsoil and gravel shale and rock; sand; surface; topsoil surface; sand; is <5 ft (~1.5 m) thick]; rubble is <5 ft (~1.5 m) thick]; conglomerate shelly or sandstone layers); sand and ledges; sandy rock sand and ledges; shelly or sandstone layers); gravel and, with, or with streaks of clay; gravel with sand and clay; sand and gravel; sand sand and gravel; with sand and clay; gravel and, with, or with streaks of clay; gravel and gravel with clay; sand and rock; silty gravel; surface and boulders surface silty gravel; sand and rock; with clay; and gravel All units that underlie an interval described as serpentine; serpentine; basalt and serpentine; basalt and serpentine; All units that underlie serpentine; an interval described as serpentine; Altered ash; ash (blue, brown, gray, red, white, yellow); ash-flow tuff; broken rock; clay and yellow); red, white, gray, brown, ash (blue, Altered ash; Clay and sandstone; gray rock when surrounded by shale; sandy claystone; siltstone sandy claystone; shale; when surrounded by rock gray and sandstone; Clay shale and limestone and limestone; Clay conglomerate volcanic rock; Conglomerate and volcanic black basalt boulders; basalt and sand; basalt and lava; basalt and cinders; basalt; Andesite; Basin deposits; clay and sand; clay and shale; clay and silt; mud; mudstone; sand clay sand clay mudstone; mud; and silt; clay and shale; clay and sand; clay Basin deposits; Clay and little gravel; clay and some boulders; clay and some gravel; clay and streaks gravel; and streaks gravel; clay and some gravel; clay and some boulders; clay and little gravel; Clay Boulders in clay; cemented gravel and clay; clay and boulders; clay, gravel; clay with clay gravel; clay, and boulders; clay and clay; cemented gravel Boulders in clay; Sandy clay and streaks of gravel; shale and boulders; shale and gravel; silty clay some silty clay shale and gravel; shale and boulders; and streaks of gravel; Sandy clay Clayey sand with ledges; sandstone and clay; sandy rock sandstone and clay; sand with ledges; Clayey shale and gravel; and gravel; sandy clay sand and shale; gravel, sand and gravel; Clay, Boulders; cemented gravel; gravel; loose rock; rock [if surrounded by clay and/or gravel and and/or gravel clay [if surrounded by rock loose rock; gravel; cemented gravel; Boulders; clayey and conglomerate; clay cement; conglomerate gravel; conglomerate; Agglomerate; Adobe; clay sandstone; loam; quicksand; sand and clay; sand and shale; soil; silt; sticky sticky silt; soil; sand and shale; sand and clay; quicksand; loam; sandstone; clay Adobe; Alluvial deposits; clayey sand and gravel; clayey sand with minor gravel or streaks of gravel; or streaks of gravel; sand with minor gravel clayey sand and gravel; clayey Alluvial deposits; Cemented sand and gravel; sandstone and gravel Cemented sand and gravel; (when surrounded by rock cemented sand; sandstone; sand rock; graywacke; Blue rock; basement limestone conglomerate (sandstone) gravel trace gravel gravel and clay and gravel No data units and > 5 ft (~1.5 m) thick] volcanic [when not surrounded by No data, rock Undifferentiated Undifferentiated Ash and/or tuff Clay sand and Clay, Volcanic Basalt green, tan, or yellow gray, brown, blue, typically described as black, Clay, Clay and sand Clay Clay and sand Clay Clay and trace Clay Clay and gravel Clay Clay, sand, and Clay, Lithology class Clay, sand, and Clay, Sand (sandstone) Gravel Conglomerate Sand and clay Sand and gravel Sand Sand (sandstone) shells (as only descriptor of interval) sand; sand; red, or yellow gray, Blue, Sand (sandstone)

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if it was later published by the USGS, because depths vary from 6 to 1811 m; only 25 holes maries at 10 ft increments, to, most commonly, these data included information from all down- are >250 m in total depth (Fig. 6). The deepest generally short phrases that accompany a sig- hole intervals, rather than summaries or general- holes were drilled for oil and gas exploration. nifi cant lithologic change. Typically, descrip- izations of subsurface lithologic data. The average or mean total depth is 102 m and tions range from between 1 and 10 words that A digital database of lithologic information the median is 90 m. describe a change recognized by the driller as from drill holes was compiled by manually All of the 2683 wells in this fi nal compilation they penetrate a different unit; for example, entering lithologic data from the above sources. were located to either a specifi c street address, descriptions may include information on grain We culled the immense number of records to the center of a quarter-quarter section, or to size, presence or absence of gravel and/or large obtained from CDWR by selecting ~10 repre- the center of a county assessor’s parcel. Of the rocks, degree of consolidation, rock type, and/or sentative drill holes from each of the 36 sec- wells placed at a specifi c street address, 1883 abrupt color changes. This study relies heavily tions within a township and range, or ~10 holes were located using address geocoding in a on lithologic information from water well data, within each ~1.6 km2 (i.e., square mile) of the geographic information system, and 435 were which are usually assumed to be poor sources of study area. In parts of the study area where the located using mapping resources available on geologic and lithologic information. However, population density is low, our drill-hole distribu- the internet. Of the remaining wells, 295 were some previous studies have shown that drillers’ tion is correspondingly less. The drill holes that located to the center of a quarter-quarter section, logs can provide valid geologic information if were used represented those that contained the 54 were located at the center of a county asses- the logs are classifi ed and screened on the basis greatest amount of detail in the description of sor’s parcel, and 10 were located using drillers’ of the degree of detail provided (Laudon and each interval, had a large number of downhole written descriptions. Wells that could only be Belitz, 1991; Sweetkind and Drake, 2007; Faunt intervals described (as opposed to a single long located to the nearest section were deleted and et al., 2010). interval of “sand and gravel” or “alluvium”), not used in this study. Wells with fragmentary In an attempt to evaluate the reliability of were representative of downhole lithology of street addresses or addresses that could not be drillers’ logs and their usefulness in character- nearby holes, and represented a distribution of found to exist in Census Bureau data or internet izing the subsurface, we selected 186 drill holes holes that were not clustered but were approxi- geocoding services were likewise deleted. Wells for analysis in four sections (25, 26, 35, 36) mately equally distributed over the study area. that could only be located by assessor’s parcel located in the southeast corner of T7N R9W, The study area includes an area encompassed number were deleted if the parcel information northeast of Sebastopol (Fig. 1); 40 drill holes by three ranges in the east-west direction (R7W, listed on the log did not appear in the parcel data would have been selected in these four sections R8W, and R9W) and fi ve townships in the from the Sonoma County Assessor’s records. for the 3D subsurface modeling. By examining north-south dimension (T5N, T6N, T7N, T8N, in detail a dense concentration of drill holes in and T9N). In all, 2683 drill holes were compiled Interpretation of Drillers’ an area where the geology was relatively con- within this area (Fig. 1). Lithologic Descriptions stant over a small area, we hoped to evaluate When available, we selected drill holes with differences in the drill-hole lithologic descrip- the most detailed logs that were at least 100 m Drillers’ descriptions vary from detailed tions related to different drilling companies and deep, but most important, we selected holes that lithologic descriptions collected by an onsite their methods, rather than real variability in the could be defi nitively located. Total drill-hole geologist at the time of the drilling, to brief sum- geology. The selected 186 wells were completed

Average depth = 102 m Median depth = 90 m

10 Number of drill holes

Total Depth (m)

Figure 6. Frequency distribution of total drilled depth, in meters, for drill holes used in the Santa Rosa Plain subsurface mapping. Inset diagram shows frequency distribution for the holes drilled to 350 m or deeper.

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by 18 different drilling contractors; ~92 of the identifi ed (Fig. 7C). A wide variety of lithologic of the relationship between the mapped geologic holes were drilled by a single company. Most descriptions was used, but this is to be expected units and their lithologic characteristics. Strati- of the holes are shallow. With the exception of 2 of the Glen Ellen Formation and equivalents, graphic tops were picked interactively by view- holes drilled to a depth of ~450 m, the average and does not necessarily indicate inconsistency ing lithologic logs from 10–20 wells in a profi le. depth is ~50 m. between drillers’ descriptions. Below the 150 ft Contacts were picked in an iterative fashion from We counted the number of downhole intervals (~45 m) interval, sands and shells dominate the numerous cross sections of varying orientations in each of the 186 drillers’ logs and the number lithologic descriptions, consistent with the inter- with combinations of wells examined to elimi- of unique descriptive phrases to quantify the preted Wilson Grove Formation (Fig. 7C). It is nate spurious picks and maximize the consis- level of detail present in the logs. For example, important that few other lithologic categories tency of the stratigraphic interpretation. Subsur- if a driller described four intervals as clay, sand, are described, lending confi dence to the drillers’ face interpretation began with wells spudded in clay, and sand, respectively, that would con- overall interpretation. known outcrop or correlations to higher quality stitute only two unique descriptive phrases in Once the initial selection criteria of selecting data to condition the rest of the data set. four downhole intervals. There is no observed deep holes with enough descriptive subdivisions Map relations show that over most of its out- correlation between the number of downhole in the lithologic log to be useful and reliable crop area the Wilson Grove Formation uncon- intervals and the total depth of a drill hole (r2 = locations were met, the drill-hole lithology data, formably overlies pre-Cenozoic rocks, so the 0.0817) (Fig. 7A). Deeper holes do not neces- or drillers’ nomenclature, were then simplifi ed. unit could be confi dently assigned in the subsur- sarily have more downhole intervals than shal- If the physical characteristics of the major rock face. Some complexities arose in the far south- lower holes. This result indicates that holes are formations exposed at the surface are mapped, west part of the study area where volcanic rocks, described based on the units intersected rather these geologic criteria can be used to help inter- probably related to the Tolay or Burdell Moun- than some random criteria, such as equally pret and standardize the various descriptions tain Volcanics, were reported near the base of spaced description intervals. In addition, there submitted by numerous well drillers. By com- the penetrated section in several wells. Based is no correlation between the number of unique bining observations made at surface exposures on known facies and age variations within the descriptive phrases and the total depth of a drill with known or inferred facies relations, alluvial Wilson Grove Formation (Powell et al., 2004) hole (r2 = 0.05) (Fig. 7A). Deeper holes do not units can be distinguished from fi ne-grained and Petaluma Formation (Davies, 1986; Allen, have more unique descriptive phrases used than marsh and/or palustrine deposits, proximal 2003), we initially made stratigraphic picks of a shallow holes. However, there is a signifi cant coarse-grained deposits can be distinguished number of subdivisions of each formation based correlation between the number of downhole from fi ne-grained distal deposits, and interfi n- on grain size, sorting, and bedding characteris- intervals and the number of unique descriptive gering of major lithologic packages can be rec- tics. This fi ne-scale subdivision was effective phrases (r2 = 0.70) (Fig. 7B). The more subdivi- ognized in the subsurface data. This technique where well data could be tied to outcrop con- sions the driller made, the greater the number was used to simplify the drillers’ descriptions trol, especially for the Wilson Grove Formation. of descriptive units used. This indicates that (Table 1). However, such fi ne-scale subdivision was dif- descriptions tend to be unique and not repeated fi cult to maintain throughout the Cotati Basin, in a single drill hole. Interpretation of Stratigraphy from where the units interfi ngered but outcrop control As another test of evaluating the internal Drill-Hole Data was lacking. consistency of drill-hole data, we compiled the In a similar fashion, subsurface stratigraphic lithologic units described in all of the 186 holes Because one of the overall goals of this exer- picks of the Glen Ellen Formation were fi rst at 25 ft (~7.5 m) depth intervals (Fig. 7C). The cise was to create a geologic framework suit- assigned in drill holes in the Windsor Basin near compiled drillers’ descriptions were simplifi ed able for groundwater resource assessment, the outcrops of the formation. The unit was identifi - to the same 19 units used in the 3D modeling complex Neogene stratigraphy was simplifi ed able as a relatively thin bedded, heterogeneous of the entire Santa Rosa Plain (Table 1). We to four principal units: Glen Ellen Formation, package that contained gravels with a clayey or normalized the data from each depth interval Wilson Grove Formation, Petaluma Formation, fi ne-grained matrix, a unit often called clay and so that the numbers of lithologic keywords are and Neogene volcanics, all underlain by a gen- gravel by the drillers. The Glen Ellen Formation reported as percent of the total for that depth eralize unit, called undifferentiated basement, was readily identifi ed to the north and east of interval (Fig. 7C). Based on detailed lithologic that includes all pre-Cenozoic rocks. When the Trenton fault, but was more diffi cult to iden- descriptions (Powell et al., 2006) from the numerous drillers’ logs were viewed and inter- tify to the south, where both the Wilson Grove nearby Occidental Road and Sebastopol Road preted together, it became clear that each of the and Petaluma Formations are more gravel rich. drill holes (Fig. 1), the subsurface geology in principal stratigraphic units had a reasonably Heterogeneous, gravel-rich sediments that over- the four sections was expected to consist of an distinct mappable character in the subsurface lie volcanic rock on the east side of the Wind- upper sequence of clayey to pebbly silt, sand, such that they could often be distinguished from sor Basin, near the city of Santa Rosa, and in and gravel of dominantly nonmarine distal fl u- each other. Assignment of stratigraphic tops Rincon Valley were also assigned to the Glen vial, lacustrine, and deltaic deposits (the Glen was fundamentally lithology based and, as such, Ellen Formation. Ellen Formation and correlative strata) over- was rock-stratigraphic rather than being a true For wells drilled on or near an outcrop of vol- lying a lower sequence of silt, sand, and peb- stratigraphic assignment based on timelines or canic rocks, we selected the fi rst intercept of vol- bly sand with mollusks of dominantly shelfal sequence boundaries. canic rocks as the top of the Neogene volcanics marine affi nities (the Wilson Grove Formation). Mappable lithologic sequences were identi- unit. In certain areas, the volcanic units are inter- The expected geology is borne out with reason- fi ed in well data by analyzing numerous serial bedded with sediments and in those cases this able consistency by the 186 drillers’ lithologic cross sections across the Santa Rosa Plain and entire interval was called Neogene volcanics. logs. High in the section, clay and sandy gravel making stratigraphic interpretations based on The Petaluma Formation was consistently dominate the lithologic descriptions, the section rock type, bedding and sorting characteristics, described by drillers as being mostly monot- as a whole is poorly sorted, and no shells are stratigraphic succession, and an understanding onous sequences of clay with occasional

248 Geosphere, June 2010

25 20 15 10

N=54 200 ft NUMBER OF UNIQUE DESCRIPTIVE PHRASES PHRASES DESCRIPTIVE UNIQUE OF NUMBER N=62 175 ft DEPTH INTERVAL N=82 150 ft Number of intervals Number of Descriptive phrases used N=98 125 ft = 0.0817 2 R 100 ft N=112 = 0.05 2 R NUMBER 75 ft N=148 50 ft N=179 A 25 ft N=182 0 5 10 15 20 25 30 35 40 C 0 50

200 150 100

5 0

TOTAL DEPTH, IN M M IN DEPTH, TOTAL 35 30 25 20 15 10 45 40 NORMALIZED FREQUENCY NORMALIZED Figure 7. Plots showing statistical analysis of drillers’ lithologic descriptions. (A) Number of intervals and number of descri of intervals and number lithologic descriptions. (A) Number 7. Plots showing statistical analysis of drillers’ Figure of lithologic units occurring in 25 ft (~7.5 m) intervals. of descriptive phrases. (C) Normalized frequency vals versus number

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interbeds of sand or gravel bars. We initially tal dimension over the vertical (Fig. 8B). Also, the Windsor Basin (Williams et al., 2008) show attempted to identify the following three sub- the method preserves the local variability of the a progressive increase in refl ector dip beneath divisions within the Petaluma Formation: lithology in each drill hole with no smoothing ~100–200 m. Several more complicated models (1) Petaluma Upper, assigned to intervals of Pet- or averaging. Thus, where data are abundant, were constructed that incorporated stratal tilt or aluma Formation near Santa Rosa above thick local lithologic variability is incorporated. One folding. However, the 3D gridding approach is sequences of Sonoma Volcanics; (2) Petaluma limitation of this type of numerical interpola- very sensitive to the choice of dip or the magni- Middle, assigned to most of the unit beneath tion is the sensitivity to the distribution of the tude and style of the fold chosen as a bounding the Santa Rosa Plain; and (3) Petaluma Lower, data, where values from an isolated drill hole surface; none of the more complicated models assigned where there were signifi cant amounts tend to extrapolate outward to fi ll an inordinate yielded results that were higher quality than the of volcanic rocks, typically basalts in the sec- amount of the model area. The effect is particu- simple horizontal model. tion, that were inferred to be older volcanic larly noticeable where a small number of deep An initial test of the strength of the sub- units such as the Tolay Volcanics. However, due drill holes are interspersed with shallower holes. surface 3D lithologic model is to compare to structural complexity and lack of correlatable Data from the deepest drill holes in this case the mapped surface geology to that predicted horizons, we eventually abandoned attempts to tend to overextrapolate over the entire model at land surface by the 3D model. The density subdivide the Petaluma Formation. area. A second limitation of this method is that of drill-hole lithologic data is greatest at the it is purely deterministic and data based. Alter- surface, so resolution of the resultant model 3D MODELING RESULTS natively, it may be possible to use a stochastic should be highest. When the solid lithologic approach where the drill-hole data are used as model is trimmed with a DEM, the resulting 3D Lithologic Model a guide to predict subsurface lithologic vari- upper model surface compares favorably to ability (e.g., Weissmann et al., 1999). Such an the geologic map; for example, compare the Drillers’ descriptions were simplifi ed to a approach would have the benefi t of being able to general distribution of sand and volcanic-rock small number of internally consistent lithologic incorporate depositional process and facies rela- lithologic classes in Figure 8B with the map classes (Table 1) for all 2683 drill holes. When tionships by evaluating the tendency of specifi c distribution of Wilson Grove Formation and these drill-hole data were viewed together, the lithologic units to be adjacent to each other in Neogene volcanics, respectively, in Figure 1. 19 lithologic units derived from the drillers’ specifi c geologic environments. Because of the The sand-dominated marine deposits in the descriptions fell into distinct spatial groupings large-scale nature of the Santa Rosa Plain, the south and west, the fi ne-grained basin-axis (Fig. 8A) that were amenable to stratigraphic presence of multiple depositional environments, deposits capped by younger, coarser and thin- classifi cation with some confi dence. The stan- and resource limitations, stochastic modeling ner alluvial fans, and the volcanic highlands dardized subsurface lithologic data were then approaches were not applied. to the north and east are all well expressed in used to construct a 3D lithologic model of Faults were not explicitly included in the the 3D model (Fig. 8B). Although no faults the study area (Fig. 8B). Interpreted drill-hole creation of the 3D lithologic model, owing to were used in the construction of the lithologic lithologic data were numerically interpolated the limitations of the software package used. model, due to the density of well data the con- between drill holes by using a cell-based, 3D However, the interpolation methods used here tacts between lithologic units are relatively gridding process using the RockWorks 3D mod- produce lithologic variations that approximate abrupt and are coincident with the major basin- eling software package (Rockware Earth Sci- fault truncations of lithologic units where data bounding faults (Figs. 8B, A1–2, and A1–4). ence and GIS software: www.rockware.com). density is high. In this method, a solid modeling algorithm is Cell dimensions for the 3D interpolation were 3D Stratigraphic Model used to extrapolate numeric codes that repre- 250 m in the horizontal dimensions and 10 m in sent a lithologic class. Grid nodes between drill the vertical dimension. The vertical discretiza- In order to tie the basin-fi ll lithology to a holes are assigned a value that corresponds to tion was chosen as a compromise between pre- stratigraphic context and to mapped surface a lithologic class based on the relative proxim- serving geologic detail, such that thin geologic exposures, we created a 3D stratigraphic model ity of each grid node to surrounding drill holes. units are not averaged out, and computational of the Santa Rosa Plain. In contrast to the 3D The interpolation routine looks outward hori- effi ciency, such that model runs could be com- lithologic model, which used just a single type zontally from each drill hole in search circles of pleted in a reasonable time. The model ranges of data, interpreted drill-hole lithologic data, ever-increasing diameter. Initially, the algorithm in elevation from 400 m to −400 m, for a total to populate a 3D volume, the 3D stratigraphic assigns a lithology class to grid nodes imme- thickness of 800 m, before being trimmed at model was built using multiple geologic data diately adjacent to each drill hole, at a vertical the surface and base. We trimmed the resulting sets including geologic maps, surface traces of discretization defi ned by the modeler. Then the model at the top using a digital elevation model faults, interpreted subsurface stratigraphic con- interpolation moves outward from the drill hole (DEM) to represent land surface elevations tacts from drill-hole data, and the results of geo- by one node and assigns the next circle of grid and at the base by a grid of the top of the geophysical models. The 3D stratigraphic model, nodes a lithology class. The interpolation con- physically modeled high-density geophysical built using EarthVision (Dynamic Graphics, tinues in this manner until the program fi nds a basement that represents the elevation of pre- Inc., http://www.dgi.com/) and Rockworks 3D cell that is already assigned a lithology class Miocene rocks (Langenheim et al., 2006, 2010; (Rockware Earth science and GIS software: (presumably interpolating toward it from an McPhee et al., 2007). www.rockware.com) geologic mapping soft- adjacent drill hole), in which case it skips the For the 3D lithologic model presented here, ware consists of three bounding components: node assignment step. strata were assumed to be horizontal. The fault surfaces, stratigraphic surfaces, and a A strength of the 3D gridding process is assumption of horizontality is likely more valid modeled surface representing the top of pre- that the interpolated data in the resulting 3D for the younger, upper parts of the basin fi ll than Cenozoic rocks. grid have the appearance of stratigraphic units, for the deeper parts of the alluvial section. Seis- Fault surface traces were generalized from with aspect ratios that emphasize the horizon- mic refl ection profi les across the eastern side of published geologic maps (McLaughlin et al.,

250 Geosphere, June 2010

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MF 510,000

HF BVF

TR 4,270,000 RCF Explanation of symbols Gravel SF Sand and gravel Northing (UTM) 4,260,000 Sandstone and gravel Sand 4,250,000 Sandstone Sand and clay

4,240,000 Vertical exaggeration is 10x. Sandstone and clay Clay, sand, and gravel BVF; Bennett Valley fault HF; Healdsburg fault Clay, sand, and trace gravel MF; Maacama fault Clay and gravel RCF; Rodgers Creek fault SF; Sebatopol fault Clay and trace gravel 530,000 B TR; Trenton Ridge Clay and sand Easting (UTM) Clay and sandstone 520,000 Clay Conglomerate MF 510,000 Volcanic conglomerate Basalt Ash and (or) tuff HF BVF Undifferentiated basement TR RCF 4,270,000 No data

SF Northing (UTM)4,260,000

4,250,000

Vertical exaggeration is 6x. 4,240,000

Vertical sections cut through the solid volume 3D lithology model. Sections are hung from land surface. Land surface is transparent; as a result, the sections have the appearance of hanging in space. Tops and bottoms of each section appear irregular because the model was clipped at the top by a digital elevation model and at the base by the modeled elevation of pre-Cenozoic bedrock.

Figure 8. Perspective views of drill-hole lithologic data and resultant three-dimensional (3D) lithology model. View is from above and the southwest, looking northeast. UTM—Universal Transverse Mercator. (A) Perspective views of drill-hole lithologic data. (B) Perspective 3D view of vertical sections cut through the solid volume 3D lithology model. For a fully interactive 3D image, see Supplemental Figure 11 in Appendix 2.

1Supplemental Figure 1. Zipped ﬁ le containing a RockPlot3D (http://www/rockware.com/downloads/trialware.php#R [February 2010]) image of the three-dimensional (3D) lithologic model. This 3D image corresponds to Figure 8B and presents vertical sections cut through the 3D solid lithologic model in three dimensions. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00513.S1 or the full-text article on www.gsapubs.org to view Supplemental Figure 1.

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2005; Graymer et al., 2006). A limited number (predominantly composed of Mesozoic rocks Neogene volcanics was forced to the elevation of faults was included in the framework model of the Franciscan Complex and the mafi c Coast of the depth-to-basement model, the 3D map- to bound the major basin elements, including Range Ophiolite) and less dense Quaternary– ping incorporates uncertainty that is inherent (Fig. 1) a combined Bennett Valley–Maacama Tertiary sedimentary rocks and Neogene volca- in the depth-to-basement model. Specifi cally, fault trace that offsets Neogene volcanics to the nics. The inversion method allows the density of the variation of density with depth in the volca- east of the Santa Rosa Plain, a combined Rodg- bedrock to vary horizontally as needed, whereas nic units can dramatically infl uence the model ers Creek–Healdsburg fault trace that generally the density of basin-fi lling deposits is speci- results. For wells in the center of the Santa Rosa bounds the eastern side of the Santa Rosa Plain, fi ed by a predetermined density-depth relation- Plain where the top of the Petaluma Forma- a generalized trace of the Sebastopol fault that ship (Jachens and Moring, 1990). The resulting tion was interpreted, the base of the Petaluma bounds the western side of the Santa Rosa Plain, model of depth to pre-Cenozoic bedrock for the Formation was defi ned as the elevation of the a generalized fault that bisects the Santa Rosa Santa Rosa Plain defi nes both the overall basin depth-to-basement surface, forcing the forma- Plain and approximates the Trenton Ridge as a geometry and the confi guration of subbasins tion to be very thick and fi ll the deepest parts of single structure, and a generalized trace of the that are bounded by internal faults. Locally, the the basins. This method forces any older Ceno- Bloomfi eld fault that offsets the Wilson Grove modeled depth to geophysical basement from zoic rocks that might be present beneath the Formation to the southwest of the Cotati Basin. the gravity inversion may not exactly match the Petaluma Formation and Neogene volcanics to All faults are presumed vertical for this study; depth to pre-Cenozoic rocks observed in every be included in those units. this is probably an acceptable simplifi cation drill hole because of the resolution of the grid The Glen Ellen Formation was a special case for the major faults with strike-slip motion, but model from the inversion or in areas of large where wells, especially in the Windsor Basin, may be less applicable to faults bounding the gravity gradients. did not penetrate the entire thickness of the for- Trenton Ridge, which have been interpreted as The 3D geologic framework of the Santa mation, but some other unit would be expected being reverse faults with gentle dip (Fox, 1983) Rosa Plain was constructed by standard subsur- to underlie it. The formation therefore could not or steep dip (Williams et al., 2008). These faults face mapping methods of creating isopach maps be reasonably expected to extend down to the were incorporated into the structure contour and (Fig. 9) and structure contouring for each of the pre-Cenozoic rocks. In order to contour the for- isopach maps of each of the major units, serving four principal stratigraphic units. The structural mation, the thickness was arbitrarily picked at to bound and truncate contoured thickness and elevation of stratigraphic tops and thickness for ~15 m below the base of wells that bottomed unit extents. each of the four major units were contoured from in the Glen Ellen Formation. In areas where the Stratigraphic surfaces are derived from strati- map and well data using simplifi ed fault traces to entire thickness of the formation was not pen- graphic information from wells, described in the bound contoured regions. Data were contoured etrated, these data give a minimum thickness to previous section, along with point data derived using an inverse distance algorithm with a mod- the Glen Ellen Formation. by combining the mapped geology and a DEM. erate smoothing routine. Data were considered Computer-generated isopach maps were A generalized hydrogeologic map (Fig. 1) was to be suffi ciently numerous that no prefi ltering, reviewed to identify anomalous data points. constructed by merging geologic map data from regridding, or declustering of the original data These data points were evaluated and, in many several sources (Saucedo et al., 2000; Blake was done prior to contouring. Attempts at con- cases, reinterpreted to create more consistent et al., 2002; Graymer et al., 2007) to portray touring the data using a pre-kriging routine were isopach trends. Isopach maps were fi ltered to four distinct Cenozoic formations (Pliocene– computationally intensive and did not provide remove extremely thin parts of units, and thick- Pleistocene gravels, the Wilson Grove Forma- signifi cantly different results. nesses of 2 m or less typically were set to zero. tion, Neogene volcanic rocks, and the Peta- One challenge in creating the isopach maps Isopach maps were hand-edited in selected luma Formation) and a fi fth unit representing is that few of the wells, being for the most places to remove outliers that were well outside undifferentiated pre-Cenozoic rocks. The 3D part shallow water wells, penetrated the entire the main part of the unit. The fi nal grids used geometry of outcrops of each of the fi ve units thickness of a formation. To provide some 3D to create the 3D framework represent a hybrid was defi ned by intersecting the hydrogeologic control on unit thickness, the modeled depth- approach that combines (1) unit thickness that map with a DEM, resulting in x, y, z coordinate to- basement surface was used to defi ne unit incorporates the depth-to-basement model, as locations within each outcrop area that were thickness where it could be reasonably inferred described above, with (2) unit thickness defi ned exported for use as input data in the stratigraphic that the stratigraphic unit would be expected to by interactive assignment of stratigraphic tops modeling. Where possible, interpreted strati- directly overlie pre-Cenozoic rocks. For exam- from well data. In places the grids show abrupt graphic surfaces were tied to high-quality well ple, where the Wilson Grove Formation crops transitions where the regions in which the two control where biostratigraphic information was out to the west of Sebastopol, the base of the unit methodologies were used abut each other. available (Powell et al., 2006), or tied to previ- is exposed and is nearly everywhere unconform- The 3D stratigraphic framework was con- ously identifi ed formation picks in wells (Valin able on pre-Miocene rocks with no intervening structed initially in EarthVision and later in and McLaughlin, 2005). units. So, for the Wilson Grove Formation to the Rockworks 3D modeling packages by import- The surface representing the top of the geo- west of the Sebastopol fault, the modeled depth- ing gridded surfaces to defi ne the top and base physically modeled high-density geophysi- to-basement surface was used to defi ne the base of each stratigraphic horizon that were then cal basement was derived from inversion of of the formation (Fig. 10). This method was also stacked in stratigraphic sequence to form a 3D regional gravity measurements (Langenheim et used for the Neogene volcanics in wells that digital solid. The 3D stacking was guided by al., 2006, 2010; McPhee et al., 2007), as con- penetrated a thick sequence of volcanic rocks rules that controlled stratigraphic onlap, trunca- strained by outcrop data and well data. This uninterrupted by sediments. The base of the vol- tion of units, and minimum thickness. Stacked surface is inferred to represent the elevation of canic section is rarely exposed in outcrop near grid models for the upper and lower surfaces pre-Miocene rocks. This depth-to-basement the Santa Rosa Plain. However, volcanic rocks of each of the stratigraphic units are then dis- inversion takes advantage of the large density unconformably overlie pre-Cenozoic rocks east played on multiple 3D cross section fence pan- contrast between dense pre-Cenozoic rocks of Napa Valley. In cases where the base of the els (Fig. 11).

252 Geosphere, June 2010

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 3D geologic modeling—Santa Rosa Plain Figure 9. Isopach maps of Figure principal aquifer the four formations, Santa Rosa Plain. (A) Glen Ellen For- Wilson mation map. (B) Formation. (C) Neo- Grove gene volcanics. (D) Peta- and B A luma Formation. at different contoured are intervals than C and D. > 900 850-900 800-850 750-800 700-750 650-700 600-650 550-600 500-550 450-500 400-450 350-400 250-300 200-250 150-200 100-150 50-100 2-50 0-2 > 2,800 2,600-2,800 2,400-2,600 2,200-2,400 2,000-2,200 1,800-2,000 1,600-1,800 1,400-1,600 1,200-1,400 1,000-1,200 800-1,000 600-800 400-600 200-400 2-200 0-2 Thickness, m Thickness, m 530000 Easting Easting

520000 520000 530000

200 200 4270000 4260000 4250000 200 200 4270000 4260000 4250000

Northing Northing > 2,800 2,600-2,800 2,400-2,600 2,200-2,400 2,000-2,200 1,800-2,000 1,600-1,800 1,400-1,600 1,200-1,400 1,000-1,200 800-1,000 600-800 400-600 200-400 2-200 0-2 > 180 160-180 140-160 120-140 100-120 80-100 60-80 40-60 20-40 2-20 0-2 Thickness, m Thickness, m Easting Easting 520000 530000 520000 530000

Glen Ellen Formation Wilson Grove Formation Neogene volcanics Petaluma Formation

200 200 4270000 4260000 4250000 200 200 4270000 4260000 4250000

Northing Northing AB CD

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3D Subsurface Mapping of Textural Classes iment texture was developed from the 3D lithol- tion, these discretized stratigraphic and textural ogy model to help characterize grain-size varia- data are preserved in a form amenable for guid- In addition to building a 3D geologic frame- tions of the aquifer system. Textural classes still ing assessment of vertical and lateral hydraulic work of stratigraphic units, it is important to need to have a stratigraphic context, however, conductivity and storage property distributions assess geologic factors that could affect con- because each formation largely represents a dis- for the Santa Rosa Plain. ductivity and storage properties of the aquifer tinct depositional setting and would be expected A 16 layer scheme of grids was devised for system for characterization of groundwater to have different permeability characteristics the study area as required by the anticipated fl ow. Lateral and vertical variations of sediment than surrounding units. The combination of tex- discretization of a planned groundwater fl ow texture, including grain size, sorting, and bed- ture and stratigraphy was accomplished by sam- model; the fl ow model is being constructed in ding, may affect the direction and magnitude of pling the 3D texture and stratigraphy models California State Plane feet coordinates; as a groundwater fl ow and the amount of compaction and imprinting this geologic information onto result the texture model was constructed in feet, and potential subsidence. A 3D portrayal of sed- nodes in a predefi ned grid. Through this opera- rather than in metric units. The tops and bottoms

Drill hole that Wilson Grove Formation intercepts base of A Outcrop of Wilson Wilson Grove Grove Formation in subsurface beneath younger alluvium Formation and top of underlying Petaluma Formation Drill holes spudded in Drill holes intercept top Wilson Grove Formation of Wilson Grove but do not penetrate Formation in the base of unit subsurface but do not penetrate base of unit

Wilson Grove Formation Top of ? Petaluma Formation ? ? Without any constraints, Petaluma Formation tends to ? overextrapolate beneath Wilson Grove Formation Petaluma Formation

Transitional area where Area where base of B base of Wilson Grove Wilson Grove Formation Area where base of Formation is not is controlled by the Wilson Grove Formation constrained by elevation of the top of forced to elevation of pre-Cenozoic basement the underlying Petaluma pre-Cenozoic basement or deep well control Formation

Wilson Grove Formation Interpreted base of Wilson Grove Formation

Modeled top of pre-Cenozoic basement Petaluma Formation Beneath outcrop area thickness Where enough well control of Wilson Grove Formation is exists, thickness of Wilson Grove controlled by the elevation of Formation may be controlled by pre-Cenozoic basement the elevation of the top of the underlying sedimentary unit, e.g., Petaluma Formation

Figure 10. Example from the Wilson Grove Formation of the use of the modeled depth-to-basement surface to help constrain unit thickness. (A) Outcrop and drill-hole lithologic data. Without drill-hole or outcrop data deﬁ ning the base of the Wilson Grove Formation, the underlying units such as the Petaluma Formation tend to be overextrapolated beneath the Wilson Grove Formation. (B) Constrain- ing unit thickness with the depth-to-basement surface. Beneath surface outcrops of the Wilson Grove Formation, the elevation of the unit is forced to equal the modeled elevation of geophysical basement.

254 Geosphere, June 2010

Northing (UTM) 540,000 4,250,000 1,000

4,240,000 , IN METERS IN ,

-1,000 ELEVATION -2,000

EXPLANATION View is from the southeast looking to the northwest from an elevation 30 degrees above Glen Ellen Formation the horizon. Vertical exaggeration is 4x. Colors appear variable due to the effects Wilson Grove Formation of illumination from above and northeast. Neogene volcanics 4,270,000 540,000 Petaluma Formation

Undifferentiated 4,260,000 Easting (UTM) basement 530,000 Northing (UTM)

4,250,000 B View from northwest. 520,000

4,240,000

510,000 1,000

-1,000

-2,000 ELEVATION, IN METERS IN ELEVATION,

Vertical sections are cut through the 3D lithology View is from the northwest looking to the model at 2,500-foot intervals. Upper surface of the southeast from an elevation 45 degrees above the horizon. Vertical exaggeration is 4x. model clipped with a DEM at the elevation of land surface. Colors appear variable due to the effects of illumination from above and northeast.

Figure 11. Perspective views of multiple vertical sections cut through solid volume three-dimensional (3D) stratigraphy model of the Santa Rosa Plain. UTM—Universal Transverse Mercator. DEM—digital elevation model. (A) View from southeast. (B) View from northwest. For a fully interactive 3D image, see Supplemental Figure 22 in Appendix 2.

2Supplemental Figure 2. Zipped ﬁ le containing RockPlot3D (http://www/rockware.com/downloads/trialware.php#R [February 2010]) image of the three-dimensional (3D) stratigraphic model. This 3D image corresponds to Figure 11 and presents vertical sections cut through the 3D solid stratigraphic model in three dimensions. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00513.S2 or the full-text article on www.gsapubs.org to view Supplemental Figure 2.

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of the 16 layers are parallel to the land surface discretizations of 50 ft, 100 ft, and 500 ft (~15, bined attribute called “strat_text” (Tables 4 and as defi ned by a DEM. Within each layer, grid ~30, and ~150 m) thickness, respectively. Using 5). This attribute combines stratigraphy and cells were 660 ft (~198 m) in the x, y dimension, a geographic information system (GIS), each texture class such that gravels in the Petaluma and of a thickness defi ned for each layer. The grid cell for each of the 16 layers was attributed Formation can be distinguished from gravels in top 4 layers are 50 ft (~15 m) thick, the next 8 with texture class by intersecting the layering the Glen Ellen Formation or Wilson Grove For- layers are 100 ft (~30 m) thick, and the bottom scheme with the classifi ed data from a textural mation. Because each formation largely repre- 4 layers are 500 ft (~150 m) thick, such that the model. Layers 1–4 [each 50 ft (~15 m) thick] sents a distinct depositional setting, and gravels base of the model volume is 3000 ft (~900 m) were populated with texture by sampling the may have different sorting characteristics and below land surface. This discretization scheme texture model with 50 ft vertical discretization; presence of fi ne matrix, this distinction is of use created 26,376 grid cells in each layer; a total layers 5–12 [each 100 ft (~30 m) thick] were in identifying permeability differences between of 422,016 cells composed the model domain. populated with texture by sampling the texture units. The results of sampling the 3D solid vol- Textural classes were defi ned by grouping the model with 100 ft (~30 m) vertical discretiza- ume texture and stratigraphy models and com- lithologic classes used in constructing the 3D tion; and layers 13–16 [each 500 ft (~150 m) bined attribute strat_text are shown for layers lithologic model. Textural classes were based thick] were populated with texture by sampling 1–5 (Plate 1), layers 6–10 (Plate 2), and layers on the percentage of coarse-grained lithologic the texture model with 500 ft (~150 m) vertical 11–16 (Plate 3). classes and on degree of sorting; the relative discretization. Stratigraphic units were assigned proportion of clay matrix was considered an a numeric code (Table 3). Using a GIS, each IMPLICATIONS OF SUBSURFACE important variable. Resultant texture classes grid cell for each of the 16 layers was attributed MODELING TO BASIN EVOLUTION (Table 2) included coarse grained, intermediate, with stratigraphic unit by intersecting the layer- and fi ne grained; volcanic rocks did not fi t into ing scheme with the 3D stratigraphic framework An understanding of the extent and 3D geom- this scheme and were retained as two additional and assigning the stratigraphic unit that the cen- etry of the major lithologic types and interpreted classes. Using this classifi cation, three texture troid of each cell is within. formations yields insight into the evolution of models were constructed with grid cells 660 ft Numeric values for textural class and strati- the Cotati and Windsor Basins, the timing of (~198 m) in the x and y dimension and vertical graphic unit were added to create a new com- movement of faults that bound and transect the

TABLE 2. DESCRIPTION OF TEXTURE CLASSES Texture Descriptor Lithologic classes* included in texture class number 1 Coarse grained Conglomerate; sandstone and gravel; gravel; sand and clay; clay, sand, and gravel; sand and gravel; sand; sandstone 2 Intermediate Clay and gravel 3 Fine grained Clay and sand; clay; sandstone and clay; clay and trace gravel; clay and sandstone; clay, sand and trace gravel 4 Tuff Ash and/or tuff 5 Basalt Basalt 0 Not classiﬁ ed Undifferentiated basement; volcanic conglomerate; shells; undeﬁ ned 98 No data Areas where layer is above of below the texture model *Lithologic classes from three-dimensional lithology model.

TABLE 3. NUMERIC CODES USED FOR STRATIGRAPHIC UNITS Numeric code Stratigraphic unit 1000 Glen Ellen Formation and equivalents and Quaternary alluvial deposits 2000 Wilson Grove Formation 3000 Neogene volcanics 4000 Petaluma Formation 5000 Mesozoic basement rocks, undivided

TABLE 4. NUMERIC CODES USED FOR STRATIGRAPHIC UNITS Texture class Stratigraphic unit (texture number) (numeric code) Coarse grained Intermediate Fine grained Tuff Basalt (1) (2) (3) (4) (5) Glen Ellen 1001 1002 1003 1004 1005 (1000) Wilson Grove 2001 2002 2003 2004 2005 (2000) Neogene volcanics 3001 3002 3003 3004 3005 (3000) Petaluma Formation 4001 4002 4003 4004 4005 (4000) Basement, undivided 5000 5000 5000 5000 5000 (5000)

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basins that underlie the Santa Rosa Plain, and the Pliocene–Pleistocene gravels (Glen Ellen indicate deposition in west-southwest–fl owing the relation to volcanism in the nearby Sonoma Formation) and the confi nement of the thickest fl uvial systems that were nearly orthogonal to volcanic fi eld. Without strict age control on deposits to north of the Trenton Ridge. Beneath the Rodgers Creek–Healdsburg and Maacama many of the wells, analysis of basin evolution Pleistocene fans of the Cotati Basin the Glen fault zones (McLaughlin et al., 2005), consis- depends upon interpretations of lithologic pat- Ellen Formation is thin and irregularly distrib- tent with the isopach trends. Obsidian pebbles terns tied to a limited number of wells with age uted, but in the Windsor Basin the formation is in gravels of the Glen Ellen Formation within control and relatively rare deep well penetra- at least 165 m thick. Between 100 and 150 ft the Windsor Basin came from a 2.8 Ma obsid- tions into the basin. (~30–45 m) below land surface, the Pliocene– ian source area in northwestern Napa Valley Pleistocene gravels are interpreted to exist only (McLaughlin et al., 2005). In contrast, gravels Structural Controls on Depositional Trends north of the ridge (Plate 2). The thickest deposits in the Cotati Basin contain obsidian clasts that and Thickness in the Glen Ellen Formation appear to occupy sinuous depocenters that have correlate to 4.5 Ma volcanic source areas in general northeast-southwest trends (Fig. 9A). northwestern Sonoma Valley and the volcanic The contoured thickness of the Glen Ellen Paleofl ow data and the distribution of chemi- uplands to the east of Santa Rosa (McLaughlin Formation (Fig. 9A) shows distinctly greater cally fi ngerprinted obsidian clast suites show et al., 2005). This suggests that drainage across thickness in the Windsor Basin, to the north of that Glen Ellen gravels in the Windsor and Cotati the Rodgers Creek–Healdsburg fault zone from the Trenton Ridge, than in the Cotati Basin to Basins were deposited by separate west-fl owing, northwestern Sonoma Valley into the Windsor the south. Stratigraphic maps of the shallow- interfi ngering fl uvial systems. Paleofl ow direc- Basin was blocked by the Trenton Ridge during est layers (Plate 1) highlight the dominance of tions collected from the Glen Ellen Formation the deposition of these gravels between 3 and

TABLE 5. EXPLANATION OF STRAT_TEXT CODES strat_text Description code 1000 Glen Ellen Formation without a texture assignment; assigned as the texture most typical of the formation, and thus assumed to be similar to 1001 (Glen Ellen coarse gravels). 1001 Coarse-grained Glen Ellen Formation, dominantly gravel and sand + gravel as relatively thin beds. 1002 Intermediate-grained Glen Ellen Formation; poorly sorted mixtures of clay and gravel, as relatively thin beds. 1003 Fine-grained Glen Ellen Formation, typically as clay or clay, sand and gravel, as relatively thin beds. 1004 Ash or tuff defi ned by borehole data in the texture model, but within the Glen Ellen Formation in the three-dimensional (3D) stratigraphic model. Likely to be Neogene volcanics; common in Rincon Valley and Valley of the Moon where selection of top of Neogene volcanics is complicated by interbedding of sedimentary and volcanic rocks. 1005 Basalt defi ned by borehole data in the texture model, but within the Glen Ellen in the 3D stratigraphic model. 2000 Wilson Grove Formation without a texture assignment; assigned as the texture most typical of the formation, so assumed to be similar to 2001 (Wilson Grove mostly coarse grained). 2001 Coarse-grained Wilson Grove Formation, dominantly thick beds of medium-grained sandstone and sand + gravel. 2002 Intermediate-grained Wilson Grove Formation; relatively minor distribution, often occurs where Wilson Gove and Petaluma Formations interfi nger. 2003 Fine-grained Wilson Grove Formation, typically as thick beds of very fi ne-grained sandstone with shells; drillers typically call the lower marine facies of the Wilson Grove clay. 2004 Ash or tuff defi ned by borehole data in the texture model, but within the Wilson Grove in the 3D stratigraphic model. Includes two rock types; in the vicinity of Freestone, WSW of Sebastopol, unit is dominantly a nonwelded ash 10-15’ thick, at least some of which is the nonwelded Roblar tuff of the Neogene volcanics. In the southwestern part of the model domain, near Bloomfi eld and Two Rock, unit includes older volcanics, probably Burdell Mountain Volcanics, below the Wilson Grove Formation. 2005 Code would correspond to basalt defi ned by borehole data in the texture model, but within the Wilson Grove in the 3D stratigraphic model; this combination does not occur in any cell. 3000 Neogene volcanics without a texture assignment; assigned as the texture most typical of the formation, so assumed similar to 3004 (Neogene volcanics as ash and/or tuff). 3001 Neogene volcanics in which texture model assignment is coarse grained. In places, especially north of the city of Santa Rosa, top of volcanics was chosen at fi rst intercept of volcanic rock, even if there were sedimentary rocks beneath it. Thus, the Neogene volcanics hydrogeologic unit may include sedimentary rocks where they are interbedded with volcanics. This classifi cation can also arise as a model artifact; where borehole data are sparse, horizontal extrapolation of texture values may populate cells with a coarse-grained attribute whereas the irregular grid that defi nes top of volcanics may defi ne the cell as being within Neogene volcanics. 3002 Neogene volcanics in which texture model assignment is intermediate grained. 3003 Neogene volcanics in which texture model assignment is fi ne grained. 3004 Neogene volcanics in which texture model assignment is ash and/or tuff. 3005 Neogene volcanics in which texture model assignment is basalt. 4000 Petaluma Formation without a texture assignment; assigned as the texture most typical of the formation, so assumed similar to 4003 (Petaluma mostly fi ne grained). 4001 Coarse-grained Petaluma Formation, dominantly sand and sandy gravels that occur as lenses or channels. 4002 Intermediate-grained Petaluma Formation, poorly sorted mixtures of clay and gravel that occur as lenses or channels. 4003 Fine-grained Petaluma Formation, often present as thick, monotonous intervals. 4004 Ash or tuff defi ned by borehole data in the texture model, but within the Petaluma in the 3D stratigraphic model. Common near the contact of the Neogene volcanics and the Petaluma Formation, where the texture model may have horizontally extrapolated tuff lithology into cells assigned as Petaluma; also present in the lower model layers where Petaluma may be interfi ngered with Neogene volcanics or older volcanics such as Tolay Volcanics. 4005 Basalt defi ned by borehole data in the texture model, but within the Petaluma Formation in the 3D stratigraphic model. 5000 All Mesozoic basement, as defi ned by the 3D hydrogeologic framework model, was assigned a single value, without any textural classes.

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6325000 6350000 6375000 6400000 LAYER 1

Land surface 1975000 to -50’ below land surface 1950000 1925000 1900000 STRATIGRAPHY Glen Ellen Formation Wilson Grove Formation 1875000

6325000 6350000 6375000 6400000 Neogene volcanics LAYER 2 Petaluma Formation

-50’ to -100’ 1975000 Undifferentiated below land surface basement TEXTURE CLASS Not intersected with layer 1950000 or not classified Coarse-grained Intermediate-grained Fine-grained 1925000 Tuff Basalt STRAT_TEXT CLASS

1900000 1000; Glen Ellen Formation; undifferentiated 1001; Glen Ellen Formation; coarse-grained

1875000 1002; Glen Ellen Formation; 6325000 6350000 6375000 6400000 intermediate-grained LAYER 3 1003; Glen Ellen Formation;

-100’ to -150’ 1975000 fine-grained 1004; Glen Ellen Formation; below land surface tuff 1005; Glen Ellen Formation; basalt 1950000 2000; Wilson Grove Formation; undifferentiated 2001; Wilson Grove Formation; coarse-grained 1925000 2002; Wilson Grove Formation; intermediate-grained 2003; Wilson Grove Formation; fine-grained

1900000 2004; Wilson Grove Formation; tuff 3000; Neogene volcanics; undifferentiated

1875000 3001; Neogene volcanics; 6325000 6350000 6375000 6400000 coarse-grained LAYER 4 3002; Neogene volcanics;

-150’ to -200’ 1975000 intermediate-grained below land surface 3003; Neogene volcanics; fine-grained 3004; Neogene volcanics; tuff 1950000 3005; Neogene volcanics; basalt 4000; Petaluma Formation;

1925000 undifferentiated 4001; Petaluma Formation; coarse-grained 4002; Petaluma Formation; intermediate-grained

1900000 4003; Petaluma Formation; fine-grained 4004; Petaluma Formation; tuff 4005; Petaluma Formation; 1875000 basalt 6325000 6350000 6375000 6400000 LAYER 5 5000; Undifferentiated basement -200’ to -300’ 1975000 below land surface Simplified trace of major faults used in 3D lithologic and stratigraphic models 1950000 Drill hole that penetrates the layer top 1925000 1900000

1875000 25,000-foot grid based on Calfornia State Plane system; Model grid, shown as faint horizontal and vertical lines, consists Lambert Conformal Conic projection, North American of 168 rows and 157 columns of cells 660 feet on a side datum 1983.

Plate 1. Maps showing discretized results from three-dimensional (3D) stratigraphy model, solid volume 3D texture class model, and attribute strat_text (see text) for layers 1–5. Thin horizontal and vertical lines portray the grid cell discretization, which consists of 168 rows and 157 columns of cells 660 ft (~198 m) on a side. The locations of drill holes that are deep enough to penetrate the upper surface of each layer, and thus serve as a point of geologic information for that layer, are shown on the stratigraphy maps. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00513. SP1 or the full-text article on www.gsapubs.org to view the large-format ﬁ le of Plate 1.

258 Geosphere, June 2010

-300’ to -400’ 1975000 below land surface 1950000 1925000 1900000

STRATIGRAPHY Glen Ellen Formation

1875000 Wilson Grove Formation 6325000 6350000 6375000 6400000 Neogene volcanics LAYER 7 Petaluma Formation

-400’ to -500’ 1975000 Undifferentiated below land surface basement TEXTURE CLASS Not intersected with layer 1950000 or not classified Coarse-grained Intermediate-grained Fine-grained 1925000 Tuff Basalt STRAT_TEXT CLASS 1900000 1000; Glen Ellen Formation; undifferentiated 1001; Glen Ellen Formation; coarse-grained

1875000 1002; Glen Ellen Formation;

6325000 6350000 6375000 6400000 intermediate-grained LAYER 8 1003; Glen Ellen Formation; fine-grained -500’ to -600’ 1975000 1004; Glen Ellen Formation; below land surface tuff 1005; Glen Ellen Formation; basalt

1950000 2000; Wilson Grove Formation; undifferentiated 2001; Wilson Grove Formation; coarse-grained

1925000 2002; Wilson Grove Formation; intermediate-grained 2003; Wilson Grove Formation; fine-grained 2004; Wilson Grove Formation; 1900000 tuff 3000; Neogene volcanics; undifferentiated 3001; Neogene volcanics; 1875000 6325000 6350000 6375000 6400000 coarse-grained LAYER 9 3002; Neogene volcanics; intermediate-grained -600’ to -700’ 1975000 3003; Neogene volcanics; below land surface fine-grained 3004; Neogene volcanics; tuff

1950000 3005; Neogene volcanics; basalt 4000; Petaluma Formation; undifferentiated

1925000 4001; Petaluma Formation; coarse-grained 4002; Petaluma Formation; intermediate-grained 4003; Petaluma Formation; 1900000 fine-grained 4004; Petaluma Formation; tuff 4005; Petaluma Formation; basalt 1875000

6325000 6350000 6375000 6400000 5000; Undifferentiated LAYER 10 basement

-700’ to -800’ 1975000 Simplified trace of major faults below land surface used in 3D lithologic and stratigraphic models

1950000 Drill hole that penetrates the layer top 1925000 1900000 1875000 25,000-foot grid based on Calfornia State Plane system; Model grid, shown as faint horizontal and vertical lines, consists Lambert Conformal Conic projection, North American of 168 rows and 157 columns of cells 660 feet on a side datum 1983. Plate 2. Maps showing discretized results from three-dimensional (3D) stratigraphy model, solid volume 3D texture class model, and attribute strat_text (see text) for layers 6–10. Thin horizontal and vertical lines portray the grid cell discretization, which consists of 168 rows and 157 columns of square cells 660 ft (~198 m) on a side. The locations of drill holes that are deep enough to penetrate the upper surface of each layer, and thus serve as a point of geologic information for that layer, are shown on the stratigraphy maps. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/ GES00513.SP2 or the full-text article on www.gsapubs.org to view the large-format ﬁ le of Plate 2.

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STRATIGRAPHY

1875000 Glen Ellen Formation 6325000 6350000 6375000 6400000 Wilson Grove Formation LAYER 12 Neogene volcanics -900’ to -1000’ 1975000 Petaluma Formation below land surface Undifferentiated basement 1950000 TEXTURE CLASS Not intersected with layer or not classified 1925000 Coarse-grained Intermediate-grained Fine-grained

1900000 Tuff Basalt STRAT_TEXT CLASS 1875000

6325000 6350000 6375000 6400000 1000; Glen Ellen Formation; LAYER 13 undifferentiated

1975000 1001; Glen Ellen Formation; -1000’ to -1500’ coarse-grained below land surface 1002; Glen Ellen Formation; intermediate-grained

1950000 1003; Glen Ellen Formation; fine-grained 1004; Glen Ellen Formation; tuff

1925000 1005; Glen Ellen Formation; basalt 2000; Wilson Grove Formation;

1900000 undifferentiated 2001; Wilson Grove Formation; coarse-grained 2002; Wilson Grove Formation; 1875000 6325000 6350000 6375000 6400000 intermediate-grained LAYER 14 2003; Wilson Grove Formation;

1975000 fine-grained -1500’ to -2000’ 2004; Wilson Grove Formation; below land surface tuff

1950000 3000; Neogene volcanics; undifferentiated 3001; Neogene volcanics; coarse-grained

1925000 3002; Neogene volcanics; intermediate-grained 3003; Neogene volcanics; fine-grained

1900000 3004; Neogene volcanics; tuff 3005; Neogene volcanics; basalt 1875000 6325000 6350000 6375000 6400000 4000; Petaluma Formation; LAYER 15 undifferentiated -2000’ to -2500’ 1975000 4001; Petaluma Formation; below land surface coarse-grained 4002; Petaluma Formation;

1950000 intermediate-grained 4003; Petaluma Formation; fine-grained 4004; Petaluma Formation;

1925000 tuff 4005; Petaluma Formation; basalt

1900000 5000; Undifferentiated basement Simplified trace of major faults

1875000 used in 3D lithologic and 6325000 6350000 6375000 6400000 stratigraphic models LAYER 16 -2500’ to -3000’ 1975000 Drill hole that penetrates below land surface the layer top 1950000 1925000 1900000 1875000 25,000-foot grid based on Calfornia State Plane system; Model grid, shown as faint horizontal and vertical lines, consists Lambert Conformal Conic projection, North American of 168 rows and 157 columns of cells 660 feet on a side datum 1983.

Plate 3. Maps showing discretized results from three-dimensional (3D) stratigraphy model, solid volume 3D texture class model, and attribute strat_text (see text) for layers 11–16. Thin horizontal and vertical lines portray the grid cell discretization, which consists of 168 rows and 157 columns of square cells 660 ft (~198 m) on a side. The locations of drill holes that are deep enough to penetrate the upper surface of each layer, and thus serve as a point of geologic information for that layer, are shown on the stratigraphy maps. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://dx.doi.org/10.1130/GES00513.SP3 or the full-text article on www.gsapubs.org to view the large-format ﬁ le of Plate 3.

260 Geosphere, June 2010

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1 Ma. Paleofl ow from northwestern Sonoma 1983; Clahan et al., 2003) for this study. Lim- 1460 ft and 2075 ft (~438 m, ~622 m), respec- Valley was instead diverted southwestward, to ited high-quality drill-hole information (Pow- tively, of sedimentary rocks without hitting the Cotati Basin side of the Trenton Ridge. ell et al., 2006) assisted in interpretation of this volcanic rocks (California Department of Con- Seismic refl ection profi les across the east- area. Although facies changes within the Wil- servation, Division of Oil, Gas, and Geothermal ern side of the Windsor Basin (Williams et al., son Grove Formation southeast of Sebastopol Resources, www.conservation.ca.gov/dog [July 2008) show a progressive increase in refl ec- made stratigraphic interpretation diffi cult, the 2008]). The only oil exploration well within the tor dip beneath ~100–200 m, indicating active combined attribute strat_text (Plates 1, 2, and Santa Rosa Plain to intercept volcanic rocks was growth of the ridge within relatively young sedi- 3) clearly highlights the location of the transi- at the south end of the Cotati Basin. This well ments. The ridge appears to affect sedimentation from the dominantly marine sands of the penetrated the entire thickness of Petaluma For- tion patterns, resulting in differing stratigraphic west to more heterogeneous, poorly sorted mation, which was interbedded with Tolay vol- packages within the Windsor and Cotati Basins, continental sediments. This transition consists canic rocks before intersecting Mesozoic rocks the Windsor Basin being dominated at shallow of an irregular northwest-trending boundary, at a depth of 5520 ft ~1656 m) (Wright, 1992). depths (Plate 1) by the Glen Ellen Formation, regardless of the formation in which the inter- The relative absence of volcanic rocks is consis- and the Cotati Basin being dominated by the val was interpreted. tent with the relatively quiet aeromagnetic sig- Petaluma Formation. The stratigraphic-texture maps between 150 nature from shallow sources over much of the and 500 ft (~45–150 m) below land surface basin (Langenheim et al., 2010). The relative Identifying the Marine-Continental (Plates 1 and 2) contain suffi cient drill-hole data absence of volcanic rocks within the basin may Transition to highlight the lithologic variability within the be due to the relatively localized nature of vol- Petaluma Formation and its overall heteroge- canism, where volcanic rocks are dominated by Interfi ngering of marine sandstone of the neous nature. Although individual drill holes fl ows of generally limited spatial extent. Wilson Grove Formation with transitional are often dominated by clay-rich sediment, the In the far southwest part of the study area, marine and nonmarine deposits of the Peta- 3D distribution of data shows lenticular pack- volcanic rocks described as basalts were luma Formation is inferred to occur beneath ages of spatially restricted sand and gravel reported near the base of the penetrated sec- the Santa Rosa Plain’s irregular northwest- deposits that may represent channels depos- tion in several wells. Neogene volcanic rocks trending boundary (Allen, 2003; Powell et ited in an overall estuarine environment. Allen are known to underlie the Wilson Grove For- al., 2004). Where the two formations inter- (2003) cited ostracode and diatom data associ- mation to the southeast and southwest of the fi nger, they represent an oscillating Miocene– ated with the mudstones and diatomites as sug- Petaluma Valley (Bezore et al., 2002, 2003) and Pliocene shoreline (Powell et al., 2004). The gestive of deposition in fresh to brackish water have been intercepted in the subsurface in the 3D stratigraphic model (Fig. 11; see stratig- (lagoonal?) settings. Local lenses of lignite are Petaluma Valley in oil and gas wells (Wagner raphy column in Plates 1, 2, and 3) tends to associated with the lagoonal to estuarine strata, et al., 2005). These rocks are probably related overrepresent the extent of the Petaluma For- suggesting deposition in a large river delta, to the Tolay Volcanics or possibly the Burdell mation because the marine-continental facies embayment, or lagoon (Allen, 2003). Grav- Mountain Volcanics. Volcanic rocks are not transition zone between the Wilson Grove and els of the Petaluma Formation are, in places, known to be present beneath the Wilson Grove Petaluma Formations was always coded as Pet- dominated by clasts derived from Franciscan Formation farther to the north near Sebasto- aluma Formation when picking stratigraphic Complex, Coast Range Ophiolite, and Great pol. The presence of volcanic rocks at the base tops from the well data. The strat_text maps Valley Sequence sources. More commonly, of the marine section in the southwest part of in Plates 1, 2, and 3 give a more realistic dis- 30%–50% of gravel clast suites are derived the study area suggests that west-northwest– play of the subsurface extent of the Petaluma from Tertiary volcanic sources to the east and/ striking faults such as the Bloomfi eld fault Formation and the location of the transition. In or the southeast, consistent with paleofl ow data may have a component of dextral offset such these maps, units classifi ed as coarse-grained that suggest that nonmarine Petaluma gravels that these older volcanic rocks are offset to the Petaluma Formation may be seen in places to were deposited in a west-northwest–fl owing northwest from localities farther to the south- be continuous with interpreted Wilson Grove fl uvial system (Allen, 2003). east (e.g., McLaughlin et al., 1996). Formation. However, the texture class fi ne- grained Petaluma Formation is more conserva- Relative Absence of Volcanic Rocks Generalized Depositional History of tive and still portrays the Petaluma as occupy- the Santa Rosa Plain ing much of the subsurface. The relative absence of volcanic rocks is a Using drillers’ logs presented diffi culty striking aspect of the subsurface maps of the In contrast to previous stratigraphic interpre- in defi ning the marine-continental transition Santa Rosa Plain, considering the area’s prox- tations (Cardwell, 1958; California Department where the Wilson Grove Formation is deeply imity to the Sonoma volcanic fi eld to the east. of Water Resources, 1975, 1982), the drill-hole buried and transitional in lithologic character. Although many of the water wells are too shal- lithology data and resulting 3D lithologic model Southeast of the town of Sebastopol, the upper low to penetrate a volcanic section at depth, emphasize the lateral extent of clay-rich Peta- part of the Wilson Grove Formation progres- even the rare deeper holes within the basin luma Formation throughout the deeper parts of sively becomes gravel rich southeastward, and penetrate relatively few volcanic rocks. Two the basins that underlie the Santa Rosa Plain the Petaluma Formation is also distinctly grav- wells situated on the west side of the Santa Rosa (Figs. 8 and 11). Previous studies have gener- elly (Fox, 1983; Clahan et al., 2003; Powell et Plain within the Cotati Basin penetrated 840 ft ally interpreted Wilson Grove Formation and al., 2004). Many of the gravelly facies in this and 1070 ft (~252 m, ~321 m), respectively, Neogene volcanic rocks to underlie the Santa area were assigned to the Petaluma Formation. of a 5.8 Ma to ca. 4.5 Ma sedimentary section Rosa Plain. The interpreted subsurface extent of The Wilson Grove Formation was interpreted without intercepting volcanic rocks (Powell et the Petaluma Formation is surprising given that, in the subsurface only north of the outcrops of al., 2006). Two oil exploration wells situated in contrast to Petaluma Valley to the south, out- the so-called “sand and gravel of Cotati” (Fox, in the same part of the Cotati Basin penetrated crops of the Petaluma Formation are uncommon

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APPENDIX 1 around the margins of the Santa Rosa Plain. The in order to prevent marine transgression into formation crops out to the east of the Santa Rosa the Petaluma depocenter. Consistent with this 3D Lithologic and Stratigraphic Model Results Plain, where it is involved within the Rodgers interpretation, the thickness and facies patterns Compared to Published Cross Sections Creek fault zone, and is exposed in the core of within the Wilson Grove Formation have been fault-bounded Meacham Hill anticline at the interpreted to suggest that the marine sands A groundwater resources evaluation of Sonoma southern boundary of the Santa Rosa Plain; no gradually lapped onto and buried preexisting County (California Department of Water Resources, Petaluma Formation crops out to the west or paleotopographic highs of Franciscan basement 1975, 1982) included 12 geologic cross sections depicting inferred geology beneath the Santa Rosa northwest of the Santa Rosa Plain where the (Powell et al., 2004). Plain. Eight of the cross-section lines crossed the Wilson Grove Formation unconformably over- entire county (California Department of Water lies pre-Cenozoic rocks (Fig. 1). CONCLUSIONS Resources, 1975); the remaining four sections were Both the Cotati and Windsor Basins appear confi ned to the southern half of the Santa Rosa Plain to contain Petaluma sediments (Plate 2) at This study relies heavily on lithologic infor- (California Department of Water Resources, 1982). It is clear that these sections were constructed on the depth, although the two basins are now sepa- mation from water well data, usually assumed basis of geologic map and well data, but the specifi c rated by the Trenton Ridge. The Petaluma to be poor sources of geologic and lithologic data sources used for each section are unknown. In Formation is inferred to have been deposited information. However, our analysis and result- order to make comparisons with these earlier inter- within a single large basin that was subse- ing 3D modeling show that drillers’ logs can pretations and to directly compare the results of the 3D subsurface lithologic and stratigraphic models, we quently segmented by the Trenton Ridge. The provide valid geologic information if the logs cut vertical profi les through our solid models of lithol- Petaluma Formation was deposited over a pro- are carefully classifi ed and screened. Although ogy and stratigraphy along the same lines of section tracted time period during which strike-slip lacking time control, the 3D lithologic inter- as seven of the previously published cross sections displacement propagated northward associated polation and identifi cation of stratigraphic (Fig. A1-1). with northwestward movement of the Men- units work reasonably well where the units are End points and section bends from the published cross sections were located in a GIS by georeferenc- docino triple junction and the development relatively homogeneous and drill-hole data are ing the index maps from the published reports. Cross of the San Andreas and related fault systems abundant. Construction of 3D lithologic, strati- sections were constructed through both the 3D solid (Allen, 2003). Strike-slip offset in the North graphic, and textural models of the Santa Rosa volume lithologic and stratigraphic models along Bay is interpreted to have undergone a major Plain has resulted in several new interpretations these same lines of section (part B in Figs. A1-2– A1-8). The original published cross sections were reorganization ca. 9 Ma whereby slip was regarding the thickness, extent, and 3D distri- digitized (part C in Figs. A1-2–A1-8) and then scaled transferred from a proto-Hayward fault south- bution of the important geologic units in the to match the cross sections from our models. Each west of the Santa Rosa Plain, to faults on the Windsor and Cotati Basins. cross section illustration also includes lithologic logs east side of the Santa Rosa Plain (McLaughlin Interpretation of numerous drillers’ logs from from all drill holes within 500 m on either side of the et al., 1996). This displacement transfer was the Santa Rosa Plain has allowed for the delin- section line (part A in Figs. A1-2–A1-8); drill holes were projected onto the section perpendicular to the accomplished across a major right step in the eation of the principal stratigraphic units, each trend of the section line. The drill-hole data provide strike-slip system, which resulted in the exten- of which had a reasonably distinct mappable a convenient display of the density of the subsurface sional opening of the early Santa Rosa Plain character in the subsurface. Drillers’ descrip- data and the strength (and limitations) of the numeri- basin. In this scenario, there was initially a tions tend to use unique, albeit simple, litho- cal interpolation. The southernmost cross section, A–A′′′ (Fig. A1-2) single basin fi lled largely by deposition from logic nomenclature to describe each downhole crosses the south end of the study area in Petaluma a west-northwest–fl owing, Miocene–late Plio- interval, and do not tend to repeat these descrip- Valley, south of the Santa Rosa Plain. This section cene fl uvial to marine depositional system and tions in a single hole. Careful classifi cation and is south of the region where stratigraphy was inter- interbedded volcanic rocks. Deposition in this screening of these data produced a surprisingly preted (Fig. A1-1C), so modeled stratigraphy is not initial basin was controlled by transtension clean lithologic data set. Although lacking shown on the section. However the sandy and sand and clay lithologic units on the west half of the sec- from ca. 7 to 5 Ma, followed by transpression time control, the 3D lithologic interpolation tion (Figs. A1-2A, A1-2B) are typical of the Wilson that formed the Trenton Ridge and separated and identifi cation of stratigraphic units work Grove Formation, and the eastern half of the section is the Windsor and Cotati Basins (McLaughlin et reasonably well where the units are relatively dominated by fi ne-grained units characteristic of the al., 1996; Langenheim et al., 2006, 2010). homogeneous and drill-hole data are abundant. Petaluma Formation. In general pattern, the modeled lithology is very similar to the previously published The gravity inversion (Langenheim et al., The models break down in the vicinity of rapid interpretation (Fig. A1-2C). The nature of the contact 2006, 2010) of the Cotati Basin, south of the facies transitions, where there is signifi cant dip between the two major units cannot be determined Trenton Ridge, suggests a 2–3-km-thick accu- to stratigraphic units, and deep in the section from the lithologic model alone, but it is likely fault mulation of Wilson Grove and Petaluma Forma- where drill-hole data are scarce. related. No gravelly lithologic units characteristic tion sediments beneath the southern part of the The compilation of drill-hole data and resul- of the Glen Ellen Formation are evident in the shallow subsurface. Volcanic units appear at depth in the Santa Rosa Plain. In this location, however, both tant 3D framework modeling allowed us to map lithology model on the east side of the section, where of these units indicate shallow-water deposition. the marine-continental transition in some detail they are interbedded within the Petaluma Forma- Facies considerations suggest that the great sed- and also demonstrated the dominance of the tion. Sonoma Volcanics were interpreted to occupy imentary thickness cannot be accounted for by Petaluma Formation within most of the basins the deepest portion of the basin in the previously published interpretation (Fig. A1-2C), based on the fi lling a preexisting structural depression. In this that underlie the Santa Rosa Plain. Structural existence of outcrops of volcanic rocks to the south area, basin subsidence must have kept pace with control on depositional patterns is evident in of Petaluma Valley; the lithologic models suggest that sediment deposition during basin formation. the distribution of the Glen Ellen Formation to Neogene volcanic rocks do not project as far north as The marine-continental facies transition the north of the Trenton Ridge. Complexity of this section line. ′′′ is located very close to the western structural the stratigraphic relations at the south end of Cross-section line B–B traverses the southern end of the Santa Rosa Plain, extending through the margin of the basin as defi ned by the gravity the Cotati Basin, previously inferred based on town of Rohnert Park in a southwest-northeast trend inversion. It appears that the western margin of geologic mapping (Fox, 1983), is confi rmed by (Fig. A1-1A). On the west (Fig. A1-3A), a relatively the basin must have remained relatively high subsurface mapping of lithology. thin section of sandy and sand and clay lithologic

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 3D geologic modeling—Santa Rosa Plain A Index geologic map showing cross section locations Map units Easting (UTM) (Geology after Saucedo and others, 2000) 530,000 520,000 510,000 Quaternary alluvium Figure A1-1. (A–C) Index geologic Quaternary and Pliocene gravels (Glen Ellen Formation) map showing the trace of previously H‘’‘’’’’’ Petaluma Formation D‘D published geologic cross sections and 4,270,000 Wilson Grove Formation perspective views of vertical sections H‘’H ’ C‘C Neogene volcanic rocks cut through three-dimensional (3D)

Northing (UTM) Mesozoic rocks solid volume lithology and stratig- 4,260,000 H‘’H Great Valley sequence and Coast Range ophiolite raphy models along the same lines BB‘‘’’ D A2‘AA2 B‘’B Franciscan Complex of section. All three images have the

4,250,000 Ultramafic rocks same viewpoint from the south (185°) D2D H‘ looking to the north from an eleva- D2‘D2 Simplified trace of major faults used in 3D lithologic and stratigraphic models tion of 35° above the horizon. Colors 4,240,000 A22 C H appear variable due to the effects of Line of section B‘B‘ A‘’A B A‘A illumination in the 3D views. UTM— A Universal Transverse Mercator. Vertical exaggeration is 1.5x.

B Perspective 3D view of vertical sections cut through 3D lithology model

Easting (UTM) 530,000 510,000 520,000

H‘’‘’‘ ’’ Gravel Clay and trace gravel Sand and gravel Clay and sand D‘ 4,270,000 Sandstone and gravel Clay and sandstone Sand Clay H‘’H ’ C‘ Sandstone Conglomerate Sand and clay Volcanic conglomerate Northing (UTM) 4,260,000 H‘’HH‘ Sandstone and clay Basalt B‘’B B‘’BB‘ ’ D Clay, sand, and gravel Ash and (or) tuff Clay, sand, and trace gravel Undifferentiated basement 4,250,000 A2‘A Clay and gravel No data H‘H

4,240,000 C D22 D2‘D2D ‘ 3D representation of vertical fault used in 3D H 400 ERS lithologic and stratigraphic models A‘’A‘A B B‘ A22 A‘A 0 A -400

Vertical exaggeration is 8x. ELEVATION, IN MET

All three images have the same viewpoint from the south (185 degrees) looking to the north from an Perspective 3D view of vertical sections cut through 3D stratigraphy model elevation of 35 degrees above the horizon. C Colors appear variable due to the effects Easting (UTM) of illumination in the 3D views. 530,000 510,000 520,000

4,270,000 Glen Ellen Formation H‘’’’’’ D‘ Wilson Grove Formation 4,260,000 H‘’’ C‘ Neogene volcanics

H‘’ Petaluma Formation Northing (UTM) Undifferentiated 4,250,000 B‘’B basement D B‘’’ A2‘A 2,000 3D representation of vertical fault used in 3D lithologic and stratigraphic models 4,240,000

HH‘ RS

D2 The 3D stratigraphic model does not extend as far south as C H D2‘2‘2 0 the lithologic model. As a result, section A–A’ is not shown

A2 and only the northern part of section B–B’ is shown. ELEVATION, IN METE -2,000

Vertical exaggeration is 3x.

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A Drill-hole lithologic data and lithology model A A' A'' Index Map METERS METERS 400 400 section

Petaluma Bend in 200 Valley 200 N 0 0 A‘’ A A‘ -200 -200 Lithology model trimmed at base with depth-to-basement model -400 -400 Vertical Exaggeration ×10

Explanation of symbols B Lithology model Gravel A A' A'' Sand and gravel METERS METERS Sandstone and gravel 400 Petaluma 400 Valley Sand section Bend in 200 Bloomfield fault Sebastopol fault 200 Sandstone Sand and clay 0 0 Sandstone and clay Clay, sand, and gravel -200 -200 Clay, sand, and trace gravel Lithology model trimmed at base with depth-to-basement model Clay and gravel -400 -400 Vertical Exaggeration ×10 Clay and trace gravel Clay and sand Clay and sandstone C Published interpretation Clay A A' A'' Conglomerate METERS METERS 400 400 Volcanic conglomerate Rodgers Petaluma Creek Basalt section Valley Bend in fault 200 Adobe Creek 200 Ash and (or) tuff Wilson Grove anticline Formation Alluvium Undifferentiated basement 0 ? Sonoma 0 ? No data ? Petaluma Volcanics Neogene Formation Franciscan ? -200 Formation Volcanics ? -200 Franciscan Tolay fault Formation -400 -400 Vertical Exaggeration ×10 After California Department of Water Resources (1975). All annotation as in original publication except Merced Formation is shown as Wilson Grove Formation. Figure A1-2. (A) Drill-hole lithologic data. (B) Three-dimensional (3D) solid volume lithology model results. (C) Comparison to previously published geologic cross-section A–A′′. Lithologic logs are shown for all drill holes within 500 m on either side of the section line; drill-hole data projected onto the section may not necessarily match the land surface elevation and/or the lithologic modeling results portrayed along the line of section.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 3D geologic modeling—Santa Rosa Plain . Lithologic logs ′′ are shown for all drill holes shown for are side within 500 m on either of the section line; drill-hole onto the sec- data projected tion may not necessarily match the land surface ele- the lithologic vation and/or portrayed modeling results along the line of section. Figure A1-3. (A) Drill-hole Figure Three- lithologic data. (B) dimensional (3D) solid volume lithology model results. (C) Comparison to previously published geologic cross- section B–B B‘’’ B‘’ B‘ B Index Map Gravel Sand and gravel Sandstone and gravel Sand Sandstone Sand and clay Sandstone and clay sand, and gravel Clay, sand, and trace gravel Clay, Clay and gravel Clay and trace gravel Clay and sand Clay and sandstone Clay Conglomerate conglomerate Volcanic Basalt Ash and (or) tuff Undifferentiated basement No data Explanation of symbols Bsmt, Undifferentiated basement GE, Glen Ellen Formation Sonoma Volcanics SV, N 400 200 0 -200 -400 400 200 0 -200 -400 B''' METERS 400 200 0 -200 -400 METERS METERS B''' B'''

section

section section

Bend in Bend

Bend in Bend Bend in Bend B'' B'' Sonoma Volcanics Neogene volcanics volcanics fault Valley Valley Bennett Petaluma Formation Formation Bsmt fault Creek Neogene Neogene volcanics volcanics Rodgers Base of lithology model (-400 m) SV fault Creek Rodgers Petaluma Formation Petaluma Formation SV D2–D2’ Alluvium Formation Glen Ellen A2–A2’ Glen Ellen Formation Formation SV H–H’’’’ fault Wilson Grove Depth-to-basement model suggests to -1,600 m basin extends Sebastopol SV

Petaluma Formation Hill fault Tolay After California Department of Water Resources (1975). All annotation as in original Resources (1975). DepartmentWater After California of Formation. Wilson Grove as is shown Merced Formation except publication Neogene volcanics fault anticline Tolay Tolay Meacham 10 10 × 10 × × SV Formation Wilson Grove Bloom- field fault Formation Formation

Wilson Grove Grove Wilson Grove

section

section section

B'in Bend B'' Bend in Bend Bend in Bend Franciscan Formation Franciscan Undifferentiated basement Lithology model trimmed at base with depth-to-basement model Vertical Exaggeration Exaggeration Vertical Vertical Exaggeration Exaggeration Vertical Vertical Exaggeration Exaggeration Vertical BB' Drill-hole lithologic data and lithology model Lithology and stratigraphy models Published interpretation BB' B 0 0 0 400 200 400 200 400 200 -200 -400 -200 -400 -200 -400 A B C METERS METERS METERS

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units characteristic of the Wilson Grove Formation is Formation must climb high in the section to overlie clay-dominated lithologic units are much more typical truncated against the structurally complex Meacham the Petaluma Formation. However, the lithology of the Petaluma Formation. It is possible that some of Hill anticline (Clahan et al., 2003), as suggested by model clearly shows that the Wilson Grove Formation the Glen Ellen Formation in central Windsor Basin, to the irregular Petaluma Formation–Neogene volcanics extends some distance eastward into the basin to the the north of the Trenton Ridge, is clayey and possibly contact in the stratigraphic model (Fig. A1-3B), but transition between the two formations. lacustrine in nature, and thus would have been classi- not captured by the horizontal interpolation of the 3D The fi ne-grained units typical of the Petaluma For- fi ed as Petaluma Formation. In contrast to the previ- lithologic model except as an abrupt transition to clay- mation fi ll most of the center of the basin (Fig. A1-4B), ously published section (Fig. A1-6C), we do not inter- rich Petaluma Formation. East of the Sebastopol fault, an interpretation not consistent with that of older cross pret Wilson Grove Formation in the basin as far east the published section (Fig. A1-3C) interprets a dipping sections (Fig. A1-4C). Intervals interpreted as Peta- as this cross section. At the south end of the section, section that includes the Wilson Grove and Glen Ellen luma Formation include locally thick but discontinu- transitional sand- and gravel-dominated units were Formations. In this location both the Wilson Grove and ous gravel packages. To the northeast of the Trenton assigned to the Petaluma Formation. At the north end the Petaluma Formations become gravelly, represent- Ridge, the Glen Ellen Formation shows a distinct of the section, the Neogene volcanics are interpreted ing a transition to a more continental facies (Powell et thickening and the underlying Petaluma Formation within the basin fi ll to the north of the Sebastopol fault al., 2004), mapped as gravel of the Cotati Formation alternates with intervals of Neogene volcanics. Volca- (Fig. A1-6B). (Fox, 1983; Clahan et al., 2003). In the stratigraphic nic units predominate east of the Rodgers Creek fault, We also show vertical profi les through the 3D solid model these sandy and gravelly facies were assigned along with a thick interval of Glen Ellen Formation models of lithology and stratigraphy along two shorter to the Petaluma Formation (Fig. A1-3B), although in Rincon Valley to the east of the city of Santa Rosa, cross sections, A2–A2′ and D2–D2′ (Fig. A1-1) across they are known to be transitional. On the east side of near the trace of the Bennett Valley fault. the southern half of the Santa Rosa Plain (California the sections both the Rodgers Creek and Bennett Val- Section line D–D′ crosses the north-central part Department of Water Resources, 1982). The advan- ley faults are expressed as abrupt change in lithologic of the Santa Rosa Plain on a southwest-northeast tage of showing these shorter lines is that they can be packages and the appearance of the Neogene volca- trend, nearly parallel to section C–C′ (Fig. A1-1A). shown at a smaller scale, so that it is easier to see how nics. Again the 3D lithologic model does not incor- On the west end of the section, a relatively thin sec- the lithology model is interpolating between borehole porate the structural complexity associated with these tion of sandy and sand and clay lithologic units char- data and how the stratigraphic units were selected faults and implied by the irregular top of the Petaluma acteristic of the Wilson Grove Formation overlies from the lithology packages. Formation (Fig. A1-3B), except as abruptly terminat- the basement rocks (Fig. A1-5A). This depositional Cross-section line A2–A2′ extends from the south- ing local packets of lithology that cannot be extrapo- unit projects east beyond the Sebastopol fault and is ern end of the Santa Rosa Plain to the southeast of the lated very far. truncated at the northwest end of the Trenton Ridge, town of Rohnert Park (Fig. A1-1A). Drill-hole data and Section C–C′ is >38 km long and crosses the where to the northeast the basin fi ll is primarily clay- the resultant 3D lithologic model (Fig. A1-7A) por- central part of the Santa Rosa Plain in a southwest- dominated units of the Petaluma Formation capped by tray a clay-rich section with discrete lenses and sand northeast–trending direction through the city of Santa relatively thick, gravel-dominated Glen Ellen Forma- and gravel. The published cross section (Fig. A1-7C) Rosa (Fig. A1-1A). The west end of the section is tion (Fig. A1-5B). This section transects the Wind- infers that the Wilson Grove Formation is present at dominated by sandy and sand and clay lithologic units sor Basin, where the Glen Ellen Formation has been depth along the entire section line; the 3D stratigraphic (Fig. A1-4A) characteristic of the Wilson Grove For- shown to be as thick as 160 m in certain drill holes. It model includes the discontinuous sandy facies within mation (Fig. A1-4B). Volcanic units appear at the base is likely that the stratigraphic model, based on numer- the Petaluma Formation (Fig. A1-7B), with only thin of the Wilson Grove section on the west, where they ous shallow water wells, underestimates the thickness intervals of overlying Glen Ellen and Wilson Grove probably represent buried Tolay or Burdell Mountain of the Glen Ellen Formation here. Neogene volcanics Formation sediments. The section trends northwest Volcanics, and near the top of the section, where they dominate to the east of the Rodgers Creek fault, and along the west side of the basin, parallel to the trend represent ashy, nonwelded tuff including outcrop of continue eastward to the Maacama fault, where they of the facies transition between the Wilson Grove Roblar tuff (Sarna-Wojcicki, 1992). are faulted against basement rocks (McLaughlin et Formation and the Petaluma Formation. It is likely The marine-continental facies transition between al., 2004). The overall geometry is similar between that the lenses of sand and gravel material may repre- Wilson Grove Formation and Petaluma Formation the sections cut through the 3D models and the present interfi ngering of the two facies; such transitional is evident from the lithologic model (Fig. A1-4A) viously published section (Fig. A1-5C); however, the intervals were routinely coded as Petaluma Formation beneath the tiepoint with section A2-A2′ (Fig. A1-4B), published section describes the basin-fi lling clay sec- when picking stratigraphic tops from the well data. to the east of the Sebastopol fault. The top of the Wil- tion as Glen Ellen Formation, rather than Petaluma The published cross section (Fig. A1-7C) suggests son Grove Formation is offset in an east-side-down Formation, and the published section infers that the that the Sebastopol fault is crossed at the north end of manner by the Sebastopol fault. The steeply dipping Wilson Grove Formation might extend beneath the the section, an interpretation that does not appear to be black line that transects the sandy lithology of the entire basin, an interpretation that does not appear to supported by the available lithologic data. Wilson Grove Formation near the Sebastopol fault be supported by the subsurface data and models. Cross-section line D2–D2′ trends generally west (Fig. A1-4B) is the Wilson Grove–Petaluma contact Cross section H–H′′′′ extends from the southern to east across the southernmost part of the basin, as derived from the 3D stratigraphic model. The loca- end of the Santa Rosa Plain west of the town of Cotati through the town of Rohnert Park (Fig. A1-1A). The tion and orientation of this contact between the two and projects north along the west side of the plain to line intersects the Rodgers Creek fault on the east side formations are obtained via the method by which the town of Healdsburg (Fig. A1-6A). The section (Fig. A1-8). West of the fault, the Glen Ellen Forma- the base of the Wilson Grove Formation was forced crosses the curving trace of the Sebastopol fault at tion is interpreted to overlie a thick section of the to honor the depth-to-basement surface beneath out- the south and north ends of the cross-section line; the Petaluma Formation (Fig. A1-8B). The discontinuous crops of the formation. Basinward, where the Wilson section crosses the basement high associated with the gravels within the Petaluma probably represent a part Grove Formation is covered, the base of the formation Trenton fault near the center of the basin (Fig. A1-6B). of the continental-marine transitional facies known to climbs steeply because the transition zone between The Glen Ellen Formation occurs as a relatively thin occur at the south end of the basin. In the published the Wilson Grove Formation and Petaluma Forma- mantle to the south of the Trenton Ridge, but nearly section Fig. A1-8C), the Wilson Grove Formation is tions was always coded as Petaluma Formation when doubles in thickness and becomes much more projected at depth in the center of the basin, offset by picking stratigraphic tops from the well data. For the gravel rich north of the fault. The published section the Sebastopol fault; this interpretation is not apparent stratigraphic model, where units must maintain a pre- (Fig. A1-6C) portrays a very thick section of Glen from the available lithologic data or the results of the scribed stacking order, the base of the Wilson Grove Ellen Formation throughout the basin, although the 3D modeling.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 3D geologic modeling—Santa Rosa Plain C‘ Index Map Gravel Sand and gravel Sandstone and gravel Sand Sandstone Sand and clay Sandstone and clay sand, and gravel Clay, sand, and trace gravel Clay, Clay and gravel Clay and trace gravel Clay and sand Clay and sandstone Clay Conglomerate conglomerate Volcanic Basalt Ash and (or) tuff Undifferentiated basement No data C Explanation of symbols SV, Sonoma Volcanics SV, Petaluma Formation PF, Bsmt, Undifferentiated basement N ogic 200 0 -400 400 -200 200 -400 400 0 -200 METERS 400 200 0 -200 -400 METERS METERS C' C' C' Sonoma Volcanics Neogene Neogene volcanics volcanics Bsmt basement Undifferentiated Bsmt Sonoma Volcanics SV fault Neogene Valley Valley volcanics volcanics SV Bennett PF PF Formation Glen Ellen Glen Ellen Formation SV fault Creek Rodgers Sonoma Volcanics Ridge Trenton Trenton Glen Ellen Formation Petaluma Formation Petaluma Formation Formation Glen Ellen H–H’’’’ ? A2–A2’ . Lithologic logs are shown for all drill holes within 500 m on either side of the all drill holes within 500 m on either shown for . Lithologic logs are ′ Depth-to-basement model suggests to -3,000 m basin extends fault ? After California Department of Water Resources (1975). All annotation as in original Resources (1975). DepartmentWater After California of Formation. Wilson Grove as is shown Merced Formation except publication Sebastopol Formation Formation Formation Wilson Grove Wilson Grove Wilson Grove Wilson Grove 10 10 10 × × × basement Bloom- Undifferentiated field fault Franciscan Formation Franciscan Formation Formation Formation Wilson Grove Wilson Grove Wilson Grove Lithology model trimmed at base with depth-to-basement model Wilson Grove Drill-hole lithologic data and lithology model Lithology and stratigraphy models Published interpretation Vertical Exaggeration Exaggeration Vertical Vertical Exaggeration Exaggeration Vertical C Vertical Exaggeration Exaggeration Vertical C C 0 0 0 400 200 400 200 400 200 -200 -400 -200 -400 -200 -400 A METERS B C METERS METERS Figure A1-4. (A) Drill-hole lithologic data. (B) Three-dimensional (3D) solid volume lithology model results. (C) Comparison to (3D) solid volume lithology model results. Three-dimensional A1-4. (A) Drill-hole lithologic data. (B) Figure C–C published geologic cross-section previously the lithol onto the section may not necessarily match land surface elevation and/or section line; drill-hole data projected modeling results portrayed along the line of section. modeling results

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/237/3339091/237.pdf by guest on 30 September 2021 Sweetkind et al. . Lithologic logs ′ Figure A1-5. (A) Drill-hole Figure Three- lithologic data. (B) dimensional (3D) solid volume lithology model results. (C) Comparison to previously published geologic cross- section D–D all drill holes shown for are side within 500 m on either of the section line; drill-hole onto the sec- data projected tion may not necessarily match the land surface ele- the lithologic vation and/or portrayed modeling results along the line of section. D‘ Index Map Clay Conglomerate conglomerate Volcanic Basalt Ash and (or) tuff Undifferentiated basement No data Gravel Sand and gravel Sandstone and gravel Sand Sandstone Sand and clay Sandstone and clay sand, and gravel Clay, sand, and trace gravel Clay, Clay and gravel Clay and trace gravel Clay and sand Clay and sandstone D N Explanation of symbols SV, Sonoma Volcanics SV, Bsmt, Undifferentiated basement 0 400 200 -200 -400 -600 200 -400 400 0 -200 METERS 400 200 0 -200 -400 D' METERS D' METERS D' Neogene Neogene volcanics volcanics SV Depth-to-basement model suggests basin extends to -2,200 m Bsmt Glen Ellen Formation basement Sonoma Volcanics Undifferentiated SV fault Maacama SV Neogene volcanics volcanics basement Undifferentiated Glen Ellen Formation Sonoma Volcanics ? fault Glen Ellen Formation Creek Rodgers Alluvium (-400 m) Wilson Grove Formation Wilson Grove Petaluma Formation Petaluma Formation Glen Ellen Formation Glen Ellen Formation Depth-to-basement model suggests to -1,800 m basin extends Base of lithology model After California Department of Water Resources (1975). All annotation as in original Resources (1975). DepartmentWater After California of Formation. Wilson Grove as is shown Merced Formation except publication H–H’’’’ 10 10 10 × × × Ridge Trenton Trenton Formation Wilson Grove fault Sebastopol basement SV Undifferentiated Formation Formation Wilson Grove Franciscan Formation Franciscan Wilson Grove Lithology model trimmed at base Vertical Exaggeration Exaggeration Vertical Vertical Exaggeration Exaggeration Vertical Vertical Exaggeration Exaggeration Vertical Drill-hole lithologic data and lithology model Lithology and stratigraphy models Published interpretation D D D 0 0 0 400 200 400 200 400 200 -200 -400 -200 -400 -200 -400 -600 METERS A B C METERS METERS ment

268 Geosphere, June 2010

Alluvium

fault

section section section

Sebastopol

Lithology model trimmed at base with depth-to-basement model Bend in Bend Bend in Bend Bend in Bend

section section section

Bend in Bend ? Bend in Bend Bend in Bend ? H'' H''' H'''' H'' H''' H'''' Wilson Grove Formation Wilson Grove Depth-to-basement model to suggests basin extends -1,100 m D–D’ Bsmt Ridge Trenton Trenton Glen Ellen Formation Glen Ellen Formation After California Department of Water Resources (1975). All annotation as in original Resources (1975). DepartmentWater After California of Formation. Wilson Grove as is shown Merced Formation except publication C–C’ Depth-to-basement model to suggests basin extends -3,000 m 10 10 10 × × ×

Petaluma Formation Petaluma Formation

section section section (-400 m)

Bend in Bend Bend in Bend Bend in Bend . Lithologic logs are shown for all drill holes within 500 m on either side of the section line; drill-hole data projected onto side of the section line; drill-hole data projected all drill holes within 500 m on either shown for . Lithologic logs are A2–A2’ Base of lithology model ′′′′ Sonoma Volcanics Alluvium D2–D2’ H–H’’’’ Formation Wilson Grove Vertical Exaggeration Exaggeration Vertical Published interpretation Vertical Exaggeration Exaggeration Vertical Vertical Exaggeration Exaggeration Vertical Drill-hole lithologic data and lithology model Lithology and stratigraphy models HH' HH' H H' H'' H''' H'''' 0 0 0 200 200 200 -200 -400 -600 -800 -200 -400 -200 -400 METERS METERS A METERS B C necessarily match the land surface elevation and/or the lithologic modeling results portrayed along the line of section. the lithologic modeling results necessarily match the land surface elevation and/or cross-section H–H cross-section Figure A1-6. (A) Drill-hole lithologic data. (B) Three-dimensional (3D) solid volume lithology model results. (C) Comparison to (3D) solid volume lithology model results. Three-dimensional A1-6. (A) Drill-hole lithologic data. (B) Figure

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A Drill-hole lithologic data and lithology model A2 A2' METERS METERS 200 200 Index Map 0 0

-200 -200 A2‘

-400 -400 N Base of lithology model (-400 m)

-600 -600

Explanation of symbols B Lithology and stratigraphy models Gravel Sand and gravel D2–D2’ C–C’ A2' A2 B–B’’’ H–H’’’’ Sandstone and gravel METERS METERS 200 200 Sand Wilson Grove Glen Ellen Formation Formation Sandstone 0 0 Sand and clay Sandstone and clay

-200 Petaluma -200 Clay, sand, and gravel Formation Clay, sand, and trace gravel -400 -400 Clay and gravel Base of lithology model (-400 m) Clay and trace gravel -600 -600 Clay and sand Bsmt Bsmt Clay and sandstone -800 -800 Clay Depth-to-basement model suggests basin extends to -1,100 m Conglomerate Volcanic conglomerate Basalt C Published interpretation A2 A2' Ash and (or) tuff METERS METERS Undifferentiated basement 200 200 No data Sonoma Volcanics Basin deposits Alluvial fan deposits 0 0 Bsmt, Undifferentiated basement

Wilson Grove Formation ? -200 ? -200 Petaluma Formation

-400 -400 Sebastopol Franciscan Formation fault

-600 -600

Figure A1-7. (A) Drill-hole lithologic data. (B) Three-dimensional (3D) solid volume lithology model results. (C) Comparison to previously published geologic cross-section A2–A2′. Lithologic logs are shown for all drill holes within 500 m on either side of the section line; drill-hole data projected onto the section may not necessarily match the land surface elevation and/or the lithologic modeling results portrayed along the line of section.

270 Geosphere, June 2010

200 200 N D2 D2‘ 0 0

Explanation of symbols Gravel -200 -200 Sand and gravel Sandstone and gravel Sand -400 Base of lithology model (-400 m) -400 Vertical Exaggeration ×10 Sandstone B Lithology and stratigraphy models Sand and clay Sandstone and clay D2 D2' Clay, sand, and gravel METERSRodgers METERS 400 Creek 400 Clay, sand, and trace gravel fault A2–A2’ Clay and gravel H–H’’’’ B–B’’’ Clay and trace gravel 200 Glen Ellen 200 Formation Clay and sand Neogene Clay and sandstone volcanics 0 0 Clay Conglomerate

Petaluma Volcanic conglomerate Formation -200 -200 Basalt Ash and (or) tuff Bsmt Undifferentiated basement -400 Base of lithology model (-400 m) -400 Vertical Exaggeration ×10 Depth-to-basement model suggests No data basin extends to -1,800 m C Published interpretation SV, Sonoma Volcanics D2 D2' METERS METERS 400 400

200 200 Figure A1-8. (A) Drill-hole lithologic data. (B) Three-dimensional (3D) solid volume lithol- SV ogy model results. (C) Comparison to previ- 0 Basin deposits 0 ously published geologic cross-section D2–D2′. Sonoma Wilson Grove Alluvial fan deposits Volcanics Lithologic logs are shown for all drill holes Formation within 500 m on either side of the section line; -200 SV -200 drill-hole data projected onto the section may Petaluma Wilson Grove not necessarily match the land surface eleva- Formation Formation tion and/or the lithologic modeling results por- Petaluma Formation trayed along the line of section. -400 Sebastopol fault -400 Vertical Exaggeration ×10 After California Department of Water Resources (1982). All annotation as in original publication except Merced Formation is shown as Wilson Grove Formation.

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APPENDIX 2 3. When the download is complete, click the Run Recommended Procedure When Viewing Files button that your browser will display to start the instal- in Rockplot3D 3D Images of the Lithologic and Stratigraphic lation. Or, you can double-click on the downloaded When the supplemental 3D fi le opens, it will not Model Results Using the Rockworks 3D fi le name on your desktop. have the viewpoint or vertical exaggeration of the Viewer—Rockplot3D 4. Follow the recommended installation defaults. fi gure shown in the paper. Once opened, the user in encouraged to set the viewpoint using the command This appendix contains two supplemental fi les of Opening the Rockplot3D Viewer and View>Custom View and enter in the compass bear- dynamic three-dimensional images of certain static the Supplemental Figures ing and the angle from horizon as described in this two-dimensional views included as fi gures in the Once the RockPlot3D viewer is installed on the appendix for each fi gure. Enter the settings, click main paper. The diagrams were created in the Rock- user’s machine, and the fi les in the ZIP archives are on the Apply button, then click on the close button. Works 3D modeling software package (Rockware extracted, the RockPlot3D supplemental fi gures in Then use the command View>Dimensions to set the Earth science and GIS software: www.rockware.com) this appendix can be opened by double-clicking on the vertical exaggeration specifi ed for each fi gure in this and are viewable and able to be manipulated using R3DXML fi le name from Windows Explorer or My appendix; alternatively, you could also use the vertical the RockWorks 3D viewer, RockPlot3D (available Computer. The RockPlot3D viewer will automatically exaggeration button. At this point the 3D image will as a free software download at www.rockware.com/ open and display the requested fi le. Alternatively, the have a viewpoint and vertical exaggeration equal to downloads/trialware.php#R [February 2010]). RockPlot3D viewer can be launched using Start>All that shown in the static image in the body of the paper. RockPlot3D is a 3D display tool that is used for programs>RockWare>RockWorks14 and selecting Then use the zoom and pan tools to zoom in to the display of 3D objects, such as stratigraphic and solid the viewer; the viewer will open as a blank window. diagram and view different parts. models. RockPlot3D permits interactive movement The user can then use the commands on the toolbar at Once the initial viewpoint has been set as a refer- of the display (rotate, zoom, pan) and easy viewing the top of the screen to open a selected fi le, using the ence, feel free to change vertical exaggeration and/or of image objects. A number of interactive tools, i.e., command File>Open. use the rotate tool to view the diagram from differ- zoom, rotate, turn items on and/or off, are included, ent viewpoints. This tool is rather sensitive and takes as well as print and export features. The 3D viewer Understanding the Rockplot3D Viewer Interface a certain amount of practice. Be cognizant of the axis comes with extensive help documentation; the short The RockPlot3D interface has a window with three labels (up, down, north, south, east, west) in order to discussion here is meant to help the reader to quickly panes. The image that corresponds to a static view keep track of the view direction. One easy “fi x” is to begin using the viewer to access the supplemental fi g- shown in the body of the paper will be displayed in revert to a preset viewpoint using, for example, the ures included in this appendix, and is not a substitute the larger pane, a list of the components shown in the command “View>Above” to return to a viewpoint that for the help menu. fi gure (including reference items such as coordinate is from a known direction. In order to view Supplemental Figures 1 and 2 (see axes, legends, and data layers) is shown in another footnotes 1 and 2), users will have to perform the fol- pane, and a list of any linked fi les is in the third. Supplemental Figure 1: 3D Lithologic Model lowing general steps. At the top of the interface are two types of tool This 3D image corresponds to Figure 8B herein 1. Download and install the RockPlot3D Viewer, bars, one with a menu of word commands and one and presents in three dimensions vertical sections cut from the RockWare website (http://www.rockware with a series of graphic tool buttons. The menu of through the 3D solid lithologic model. .com/downloads/trialware.php#R [February 2010]). word commands, shown below, includes File, which To reproduce the view shown in Figure 8B, set 2. Unzip the supplemental 3D fi les and save them contains most fi le-management functions including the viewpoint using the command View>Custom to a folder on the user’s computer. open, close, save, print, and export commands; Edit, View and enter in the compass bearing as 245 and 3. Launch RockPlot3D and open the 3D fi les from which has some viewer-specifi c commands that are the angle from horizon as +45; then use the com- within the viewer. not needed for the fi rst-time user; a View command, mand View>Dimensions to set the vertical exag- which contains a menu of specifi c viewing functions geration to 6. System Requirements necessary to interact with the 3D graphic image; and In the left window, under the data list, the following Minimum system requirements include the fol- the Help command, which accesses the full help menu. data sets are part of this image. lowing: IBM-compatible computer running Windows Draped 100K topo: a 1:100,000-scale USGS topo- 2000, NT, XP, or Vista, 512 MB of RAM (1GB+ graphic map draped on a regional-scale DEM (Gra- recommended), Pentium III or newer CPU (1.4 GHz ham and Pike, 1998) (this layer is provided for loca- or faster recommended), 75 MB of free disk space tional reference and is not shown in Fig. 1). for program installation, and display set to >800 × Draped geology: the simplifi ed geologic map 600 pixels. Supports most Windows-supported shown in Figure 1 (see Fig. 1 for an explanation of peripherals. Neither Windows98 nor Windows ME colors; this layer is provided for locational reference are supported. The graphical buttons shown below repeat, to a cer- and is not shown in Fig. 1). tain extent, the menu words on the tool bar above. The Lithology fence: vertical sections cut through the Unzipping the Supplemental 3D Files graphical buttons include, from left to right, three fi le- 3D solid lithologic model, as shown in Figure 8B, ver- RockPlot3D cannot open a ZIP-format fi le. To management buttons (fi le open, fi le append, fi le save), tical sections are spaced 3000 m apart. access the contents of the ZIP fi les, you will need to four basic view controls (rotate, zoom in, zoom out, EV_faults_topodrape: the simplifi ed faults shown have a software program capable of extracting fi les and pan), three buttons to set diagram extents (vertical in Figure 1, draped on the regional-scale DEM such from the ZIP archive. Many Windows-based machines exaggeration, stretch to fi ll visible window, and view that the fault traces follow land surface. have the utility WinZip, which decompresses a fi le without stretch), a button to control lighting settings, a Perimeter cage: Rockworks-generated reference and places it in the folder of your choice. Right-click pull-down color palette for changing the background grid and labels. or double-click on each of the zipped archives to Open color of the diagram, three auto-rotate controls to EV_faults_lithomod: simplifi ed faults as shown in WinZip, select Extract from the Actions pull-down allow the diagram to rotate around each of the three Figure 1, arbitrarily inserted into the diagram at eleva- menu or click the Extract toolbar button; WinZip then major axes (these controls work like radio buttons: tions of 200 ft, 100 ft (~60 m, 33.3 m), 0 ft, −100 ft, lets you choose the folder where you’d like to place click on and off to start or stop the rotation), and a and −200 ft as a reference. the extracted fi les. Place all of the fi les in a directory pull-down menu for selecting preset zoom amounts The user may wish to explore this diagram by try- of your choice. for the view. ing the following.

Installing RockPlot3D 1. Visit http://www.rockware.com/downloads/ productUpdates.php#rockworks and click the small download link below the RockWorks14 RockPlot3D Viewer heading. 2. Choose Save, when prompted, and save the ﬁ le to your computer’s desktop.

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Jachens, R.C., and Moring, B.C., 1990, Maps of the thick- 1. Use the command View>Dimensions to set the REFERENCES CITED vertical exaggeration to 1.5 or 2; this will give a more ness of Cenozoic deposits and the isostatic residual natural look to the topographic and geologic maps, gravity over basement for Nevada: U.S. Geo- which are distorted when viewed with a vertical exag- Allen, J.R., 2003, Stratigraphy and tectonics of Neogene logical Survey Open-File Report 90–404, 15 p., scale 1:1,000,000. geration of 6. strata, northern San Francisco Bay area [M.S. thesis]: San Jose, California, San Jose State University, Langenheim, V.E., Roberts, C.W., McCabe, C.A., McPhee, 2. Turn off the perimeter cage and coordinate 183 p. D.K., Tilden, J.E., and Jachens, R.C., 2006, Prelim- axes by unchecking the boxes next to each item; this Bezore, S.P., Randolph-Loar, C.E., and Witter, R.C., 2002, inary isostatic gravity map of the Sonoma Volcanic reduces the clutter on the diagram. Geologic map of the Petaluma 7.5′ quadrangle, Field and vicinity, Sonoma and Napa Counties, Cal- 3. View the image from the southeast using a preset Sonoma and Marin counties, California: A digital ifornia: U.S. Geological Survey Open-File Report viewpoint. Use the command View>Above>South- database: California Geological Survey Prelimi- 2006–1056, scale 1:100,000, http://pubs.usgs.gov/ East and adjust the viewing area using the zoom and nary Geologic Map, scale 1:24,000, ftp://ftp.consrv of/2006/1056/. pan tools. This view is looking along the axis of the .ca.gov/pub/dmg/rgmp/Prelim_geo_pdf/petaluma_ Langenheim, V.E., Graymer, R.W., Jachens, R., McLaugh- lin, B., Wagner, D., Sweetkind, D.S., and Williams, Santa Rosa Plain where the continental-marine tran- layout_highres.pdf. Bezore, S.P., Koehler, R.D., and Witter, R.C., 2003, Geo- R., 2010, Geophysical framework of the northern sition is well expressed in the change from the blue logic map of the Two Rock 7.5′ quadrangle, San Francisco Bay region, northern California: colors of the estuarine clays to the yellows and light Sonoma County, California: A digital database: Geosphere, doi: 10.1130/GES00510.1 (in press). greens of the marine sandstones. California Geological Survey Preliminary Geologic Laudon, J., and Belitz, K., 1991, Texture and depositional Map, scale 1:24,000, ftp://ftp.consrv.ca.gov/pub/ history of late Pleistocene–Holocene alluvium in Supplemental Figure 2: 3D Stratigraphic Model dmg/rgmp/Prelim_geo_pdf/Two_Rock_prelim.pdf. the central part of the western San Joaquin Valley, California: Association of Engineering Geologists This 3D image corresponds to Figure 11 herein Blake, M.C., Jr., Howell, D.G., and Jayko, A.S., 1984, Tec- tonostratigraphic terranes of the San Francisco Bay Bulletin, v. 28, p. 73–88. and presents in three dimensions vertical sections cut region, in Blake, M.C., ed., 1984, Franciscan geol- McLaughlin, R.J., and Ohlin, H.N., 1984, Tectonostrati- through the 3D solid stratigraphic model. ogy of northern California: Pacifi c Section, Society graphic framework of the Geysers–Clear Lake To reproduce the view shown in Figure 11, set the of Economic Paleontologists and Mineralogists region, California, in Blake, M.C., Jr., ed., Francis- viewpoint using the command View>Custom View Special Publication 43, p. 5–22. can geology of northern California: Pacifi c Section, and to reproduce Figure 11A, enter the compass Blake, M.C., Jr., Graymer, R.W., and Stamski, R.E., 2002, Society of Economic Paleontologists and Mineralo- bearing as 140 and the angle from horizon as +30; Geologic map and map database of western gists Special Publication 43, p. 221–254. to reproduce Figure 11B, enter the compass bearing Sonoma, northernmost Marin, and southernmost McLaughlin, R.J., and Sarna-Wojcicki, A., 2003, Geology of the Right Stepover region between the Rodgers as 300 and the angle from horizon as +45. For either Mendocino Counties, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-2402, Creek, Healdsburg, and Maacama faults, north- viewpoint, use the command View>Dimensions to set 42 p., scale 1:100,000. ern San Francisco Bay region—A contribution to the vertical exaggeration to 4. California Department of Water Resources, 1975, Evaluation Northern California Geological Society Field Trip In the left window, under the data list, the following of ground water resources: Sonoma County: Volume Guide, June 6–8, 2003: U.S. Geological Survey data sets denoted by an abbreviated fi le name, are part 1, Geologic and hydrologic data: California Depart- Open-File Report 03–502, 23 p., http://pubs.usgs of this image. ment of Water Resources Bulletin 118–4, 177 p. .gov/of/2003/of03-502/. Topo: a 1:100,000-scale USGS topographic map California Department of Water Resources, 1982, Evalua- McLaughlin, R.J., Sliter, W.V., Sorg, D.H., Russell, P.C., and draped on a regional-scale DEM (Graham and Pike, tion of ground water resources: Sonoma County: Sarna-Wojcicki, A.M., 1996, Large-scale right-slip displacement on the East San Francisco Bay region 1998); this layer is provided for locational reference Volume 2, Santa Rosa Plain: California Department of Water Resources Bulletin 118–4, 107 p. fault system: Implications for the location of late and is not shown in Figure 1. Cardwell, G.T., 1958, Geology and ground water in the Miocene to Pliocene plate boundary: Tectonics, GeoMap: the simplifi ed geologic map shown in Santa Rosa and Petaluma Valley areas, Sonoma v. 15, p. 1–18, doi: 10.1029/95TC02347. Figure 1 (see Fig. 1 for an explanation of colors, this County, California: U.S. Geological Survey Water- McLaughlin, R.J., Sarna-Wojcicki, A.M., Fleck, R.J., layer is provided for locational reference and is not Supply Paper 1427, 273 p. Wright, W.H., Levin, V.R.G., and Valin, Z.C., 2004, shown in Fig. 1). Clahan, K.B., Bezore, S.P., Koehler, R.D., and Witter, R.C., Geology, tephrochronology, radiometric ages, and Stratigraphic fence: vertical sections cut through 2003, Geologic map of the Cotati 7.5′ quadrangle, cross sections of the Mark West Springs 7.5′ quad- the 3D solid stratigraphic model, as shown in Fig- Sonoma County, California: A digital database: rangle, Sonoma and Napa Counties, California: U.S. Geological Survey Scientifi c Investigations ure 11 (vertical sections are spaced 2500 m apart). California Geological Survey Preliminary Geologic Map, scale 1:24,000, ftp://ftp.consrv.ca.gov/pub/ Map 2858, scale 1:24,000. EV_faults_topodrape: the simplifi ed faults shown dmg/rgmp/Prelim_geo_pdf/cotati_prelim.pdf. McLaughlin, R.J., Sarna-Wojcicki, A.M., Fleck, R.J., Lan- in Figure 1, draped on the regional-scale DEM such Davies, E.A., 1986, Stratigraphic and structural relation- genheim, V.E., and Jachens, R.C., 2005, Frame- that the fault traces follow land surface. ships of the Miocene and Pliocene formations of work geology and structure of the Sonoma Volca- Perimeter cage: Rockworks-generated reference the Petaluma Valley area of California [M.S. thesis]: nics and associated sedimentary deposits, of the grid and labels. Berkeley, University of California, 96 p. right-stepped Rodgers Creek–Macama fault system The user may wish to explore this diagram by try- Faunt, C.C., Belitz, K., and Hansen, R.T., 2010, Develop- and concealed basins beneath Santa Rosa plain, ing the following. ment of a three-dimensional model of sedimentary in Stevens, C., and Cooper, J., eds., Late Neogene transition from transform to subduction margin 1. Turn off and on the GeoMap data set in the data texture in valley-fi ll deposits of Central Valley, Cali- fornia, USA: Hydrogeology Journal, doi: 10.1007/ east of the San Andreas fault in the wine country list by unchecking and checking the box next to the x10040-009-0539-7. of the northern San Francisco Bay area, Califor- data set name; in this way the user can evaluate the Fox, K.F., Jr., 1983, Tectonic setting of late Miocene, Plio- nia: Fieldtrip guidebook and volume prepared for correspondence between mapped surface geology and cene, and Pleistocene rocks in part of the Coast the joint meeting of the Cordilleran Section-GSA subsurface geology. Ranges north of San Francisco, California: U.S. and Pacifi c Section-AAPG, April 29–May 1, 2005, 2. Use the lighting tool. If colors in the vertical Geological Survey Professional Paper 1239, 33 p. San Jose, California, Fieldtrip 10: Pacifi c Section, slices appear too washed out, reduce the level of ambi- Graham, S.E., and Pike, R.J., 1998, Elevation maps of the SEPM (Society for Sedimentary Geology) Book ent lighting. San Francisco Bay region, California–A digital 98, p. 29–81. McLaughlin, R.J., Sarna-Wojcicki, A.M., Fleck, R.J., Lan- 3. From the data list, expand the entry for the data database: U.S. Geological Survey Open-File Report 2005–1318, p. 1–16 genheim, V.E., McCabe, C.A., and Wan, E., 2008, set Stratigraphic Fence by clicking on the plus symbol Graymer, R.W., Bryant, W., McCabe, C.A., Hecker, S., and Geologic framework of the Santa Rosa 7.5′ quad- next to the data set name. The fi ve data sets listed cor- Prentice, C.S., 2006, Map of Quaternary-active rangle, in McLaughlin, R.J., et al., eds., Geologic respond to the principal stratigraphic units: GE—Glen faults in the San Francisco Bay region: U.S. Geo- and geophysical framework of the Santa Rosa Ellen Formation; WG—Wilson Grove Formation; logical Survey Scientifi c Investigations Map 2919, 7.5′quadrangle, Sonoma County, California: U.S. SV—Neogene volcanic units; PF—Petaluma For- http://pubs.usgs.gov/sim/2006/2919. Geological Survey Open-File Report 2008–1009, mation; and BSMT—undifferentiated pre-Cenozoic Graymer, R.W., Brabb, E.E., Jones, D.J., Barnes, J., Nich- p. 7–33, http://pubs.usgs.gov/of/2008/1009/. basement. You may turn off different stratigraphic olson, R.S., and Stamski, R.E., 2007, Geologic McPhee, D.K., Langenheim, V.E., Jachens, R.C., McLaugh- lin, R.J., and Roberts, C.W., 2007, Basin structure units by unchecking the box next to each name. For map and map database of eastern Sonoma and western Napa Counties, California: U.S. Geo- beneath the Santa Rosa Plain, northern Califor- example, turn off the SV or PF to observe how these logical Survey Scientifi c Investigations Map 2956, nia: Implications for damage caused by the 1969 units fi ll the deepest parts of the Cenozoic basins. scale 1:100,000. Santa Rosa and 1906 San Francisco earthquakes:

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Seismological Society of America Bulletin, v. 97, File Report 2005–1318, 16 p., http://pubs.usgs.gov/ Williams, R.A., Langenheim, V.E., McLaughlin, R.J., p. 1449–1457, doi: 10.1785/0120060269. of/2005/1318. Odum, J.K., Worley, D.M., Stephenson, W.J., Morse, R.R., and Bailey, T.L., 1935, Geological observations Wagner, D.L., and Bortugno, E.J., 1982, Geologic map of Kent, R.L., McCullough, S.M., Knepprath, N.E., in the Petaluma District, California: Geological the Santa Rosa quadrangle: California Division of and Leslie, S.R., 2008, Seismic refl ection profi les Society of America Bulletin, v. 46, p. 1437–1456. Mines and Geology Regional Geologic Map 2a, image the Rodgers Creek fault and Trenton Ridge Powell, C.L., II, Allen, J.R., and Holland, P.J., 2004, Inverte- scale 1:250,000. beneath urban Santa Rosa, California: Seismologi- brate paleontology of the Wilson Grove Formation Wagner, D.L., Randolph-Loar, C.E., Witter, R.C., and Huff- cal Research Letters, v. 79, no. 2, p. 317. (late Miocene to late Pliocene), Sonoma and Marin man, M.E., 2003, Preliminary geologic map of Wright, T.L., ed., 1992, Field trip guide to late Cenozoic geol- Counties, California, with some observations on its the Glen Ellen 7.5′ Quadrangle: California Geo- ogy in the North Bay region: San Anselmo, Califor- stratigraphy, thickness, and structure: U.S. Geologi- logical Survey Preliminary Geologic Map, scale nia, Northern California Geological Society, 150 p. cal Survey Open-File Report 2004–1017, 106 p., 1:24,000, ftp://ftp.consrv.ca.gov/pub/dmg/rgmp/ Wright, T.L., and Smith, N., 1992, Right step from the Hay- http://pubs.usgs.gov/of/2004/1017. Prelim_geo_pdf/Glen_Ellen_prelim.pdf. ward fault to the Rodgers Creek fault beneath San Powell, C.L., II, McLaughlin, R.J., and Wan, E., 2006, Wagner, D.L., Fleck, R.J., Sarna-Wojcicki, A., and Deino, Pablo Bay, in Borchardt, G., et al., eds., Proceed- Biostratigraphic and lithologic correlations of two A., 2005, Golden Gate to southern Sonoma County, ings of the second conference on earthquake haz- Sonoma County Water Agency pilot wells with the Rodgers Creek fault, Burdell Mountain, Donnell ards in the eastern San Francisco Bay area: Califor- type Wilson Grove Formation, Sonoma County, Ranch, and southern Sonoma Volcanics, in Stevens, nia Department of Conservation, Division of Mines central California: U.S. Geological Survey Open- C., and Cooper, J., eds., Late Neogene transition and Geology Special Publication 113, p. 407–417. File Report 2006–1196, 37 p. from transform to subduction margin east of the San Zieglar, D.L., Wright, T.L., and Smith, N., 2005, Subsur- Sarna-Wojcicki, A.M., 1992, Long-term displacement Andreas fault in the wine country of the northern face geology in the northern San Francisco Bay rates of the San Andreas fault system in northern San Francisco Bay area, California: Fieldtrip guide- region, California: Geological Society of America California from the 6-Ma Roblar tuff [abs.], in Bor- book and volume prepared for the joint meeting of Abstracts with Programs, v. 37, no. 4, p. 69. chardt, G., et al., eds., Proceedings of the second the Cordilleran Section-GSA and Pacifi c Section- conference on earthquake hazards in the eastern San AAPG, April 29–May 1, 2005, San Jose, California, Francisco Bay area: California Department of Con- Fieldtrip 10: Pacifi c Section, SEPM (Society for servation, Division of Mines and Geology Special Sedimentary Geology) Book 98, p. 1–28. Publication 113, p. 29–30. Wagner, D.L., Gutierrez, C.I., and Clahan, K.B., 2006, Geo- Saucedo, G.J., Bedford, D.R., Raines, G.L., Miller, R.J., and logic map of the south half Napa 30′× 60′ Quad- Wentworth, C.M., 2000, GIS data for the geologic rangle, California: California Geological Survey map of California, version 2.0: California Depart- Preliminary Geologic Map, scale 1:100,000, ftp:// ment of Conservation, Division of Mines and Geol- ftp.consrv.ca.gov/pub/dmg/rgmp/Prelim_geo_pdf/ ogy, CD 2000–07. Napa100Ksouth_prelim.pdf. Sweetkind, D.S., and Drake, R.M., II, 2007, Geologic char- Weaver, C.E., 1949, Geology of the Coast Ranges immedi- acterization of young alluvial basin-fi ll deposits ately north of the San Francisco Bay region, Cali- from borehole data in Yucca Flat, Nye County, fornia: Geological Society of America Memoir 35, Nevada: U.S. Geological Survey Scientifi c Investi- 242 p. gations Series Report 2007–5062, 17 p., http://pubs Weissmann, G.S., Carle, S.F., and Fogg, G.E., 1999, Three- .usgs.gov/sir/2007/5062/. dimensional hydrofacies modeling based on soil Valin, Z.C., and McLaughlin, R.J., 2005, Locations and data surveys and transition probability geostatistics: MANUSCRIPT RECEIVED 18 MARCH 2009 for water wells of the Santa Rosa Valley, Sonoma Water Resources Research, v. 35, p. 1761–1770, REVISED MANUSCRIPT RECEIVED 06 JULY 2009 County, California: U.S. Geological Survey Open- doi: 10.1029/1999WR900048. MANUSCRIPT ACCEPTED 27 OCTOBER 2009

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