North Bay themed issue Geology and geochemistry of volcanic centers within the eastern half of the Sonoma volcanic field, northern San Francisco Bay region, California Donald S. Sweetkind1, James J. Rytuba2, Victoria E. Langenheim3, and Robert J. Fleck4 1U.S. Geological Survey, Denver Federal Center, Mail Stop 973, Denver, Colorado 80225, USA 2U.S. Geological Survey, 345 Middlefield Road, Mail Stop 901, Menlo Park, California 94025, USA 3U.S. Geological Survey, 345 Middlefield Road, Mail Stop 989, Menlo Park, California 94025, USA 4U.S. Geological Survey, 345 Middlefield Road, Mail Stop 937, Menlo Park, California 94025, USA ABSTRACT INTRODUCTION centers and to estimates of offset along the West Napa­Carneros fault zones. Volcanic rocks in the Sonoma volcanic field The rocks of the Sonoma volcanic field (Fig. 1) This study defines minor­ and major­element in the northern California Coast Ranges con- are part of a linear belt of exposures of Ceno­ geochemical trends within each volcanic tain heterogeneous assemblages of a variety zoic volcanic rocks that progressively young to center, building upon previous analyses of a of compositionally diverse volcanic rocks. We the northwest (Fox et al., 1985a) and have been relatively small number of samples from the have used field mapping, new and existing disrupted by dextral faults of the San Andreas Sonoma volcanic field (Johnson and O’Neil, age determinations, and 343 new major and fault system from their original depositional 1984; Whitlock, 2002). We define geochemi­ trace element analyses of whole-rock samples locations (Fox, 1983; McLaughlin et al., 1996; cal trends of intracaldera and outflow tuffs and from lavas and tuff to define for the first time Wakabayashi, 1999; Graymer et al., 2002a, distal fall equivalents from individual volcanic volcanic source areas for many parts of the 2002b). In the San Francisco Bay region, these centers. We use major­, minor­, and trace­ele­ Sonoma volcanic field. Geophysical data and volcanic rocks include the Quien Sabe Vol­ ment geochemical data to assist in correlation models have helped to define the thickness of canics, volcanic rocks in the Berkeley Hills, of volcanic units and to define caldera­related the volcanic pile and the location of caldera the Sonoma Volcanics, the Tolay Volcanics, the sources for regionally important tuffs, such structures. Volcanic rocks of the Sonoma Burdell Mountain Volcanics, and the Clear Lake as the Pinole and Lawlor Tuffs and the tuff of volcanic field show a broad range in erup- Volcanics, using the nomenclature of Fox et al. Napa, whose source areas have previously been tive style that is spatially variable and spe- (1985a) and Graymer et al. (2002b) (Fig. 1). only generally outlined. cific to an individual eruptive center. Major, Like other exposures of volcanic rocks in the The volcanic fields in the California Coast minor, and trace-element geochemical data northern California Coast Ranges, the Sonoma Ranges north of San Francisco Bay are tempo­ for intra caldera and outflow tuffs and their volcanic field contains heterogeneous assem­ rally and spatially associated with the northward distal fall equivalents suggest caldera-related blages of compositionally diverse lava flows, migration of the Mendocino triple junction sources for the Pinole and Lawlor Tuffs pyroclastic deposits, and local ash­flow tuffs (Dickinson and Snyder, 1979; Furlong, 1984; in southern Napa Valley and for the tuff of (Fig. 2) (Fox, 1983; Graymer et al., 2002a). Fox et al., 1985a; Dickinson, 1997). The north­ Franz Valley in northern Napa Valley. Strati- Although the Sonoma volcanic field is the ward younging of volcanism has been attrib­ graphic correlations based on similarity in largest of the volcanic fields in the northern uted to the transition from subduction and asso­ eruptive sequence and style coupled with California Coast Ranges, developing an under­ ciated arc volcanism to a slab window tectonic geochemical data allow an estimate of 30 km standing of the stratigraphy, volcanology, and environment (or “slabless window,” using the of right-lateral offset across the West Napa- geochemical evolution of this complex has terminology of Liu and Furlong, 1992) along Carneros fault zones since ~5 Ma. been difficult because of the limited erupted the western margin of the North American plate The volcanic fields in the California Coast volumes from individual volcanic centers and (Dickinson and Snyder, 1979; Johnson and Ranges north of San Francisco Bay are tem- consequent lack of lithologic continuity of O’Neil, 1984; Fox et al., 1985a). Recent work porally and spatially associated with the many of the units. Extensive structural disrup­ (Cousens et al., 2008) has defined a Miocene– northward migration of the Mendocino triple tion throughout the western half of the volcanic Pliocene Ancestral Cascades arc that was active junction and the transition from subduction field also contributes to the complexity of the at about the same time and roughly the same and associated arc volcanism to a slab win- regional correlation of units (Fox, 1983). This latitude as the volcanic centers of the Sonoma dow tectonic environment. Our geochemi- study delineates for the first time volcanic cen­ volcanic field. In this paper, we discuss the geo­ cal analyses from the Sonoma volcanic field ters primarily within the eastern, less deformed chemical and tectonic setting of volcanic rocks highlight the geochemical diversity of these half of the Sonoma volcanic field (Fig. 3) of the Sonoma volcanic field in the context volcanic rocks, allowing us to clearly distin- based on a combination of geologic mapping, of the northward migration of the Mendocino guish these volcanic rocks from those of the geophysical signature, and geochemical and Triple Junction (Johnson and O’Neil, 1984; roughly coeval ancestral Cascades magmatic petrographic criteria to tie eruptive products Dickinson, 1997) and also compare these rocks arc to the west, and also to compare rocks of to specific source areas. These combined tech­ to the generally contemporaneous rocks of the Sonoma volcanic field to rocks from other niques have led to our recognition of distinct the Ancestral Cascades volcanic arc (Cousens slab window settings. eruptive styles that vary between the different et al., 2008). Geosphere; June 2011; v. 7; no. 3; p. 629–657; doi:10.1130/GES00625.1; 18 figures; 3 tables; 1 supplemental table. For permission to copy, contact [email protected] 629 © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/629/3339746/629.pdf by guest on 26 September 2021 Sweetkind et al. 123°W 122°30′W 122°W Clear WA Lake 39°N OR ID Clear Lake CSZ Maacama faul volcanic field MFZ t MTJ NV Pacific Sonoma Ocean QSV CA SA volcanic field Area shown F on map 38°30′N Napa Sonoma V alle y RCF San Valle TF Area shown y Andreas fault on Figs. 2–7 Pacific Ocean TV BMV San Pablo V Bay WP R 38°N EXPLANATION HaywardP faul Quaternary sedimentary deposits San Fr Cenozoic sedimentary rocks BHV ancisco Bay Quaternary volcanic rocks t CF Neogene volcanic rocks pre-Cenozoic rocks 036121824k0 m Figure 1. Simplified geology of the northern San Francisco Bay area showing the Sonoma and Clear Lake volcanic fields (modified from Saucedo et al., 2002; Ludington et al., 2006). Black lines are faults, sense of displacement not shown (modified from Bryant, 2005; Graymer et al., 2006b); CF—Calaveras fault; RCF—Rodgers Creek fault; TF—Tolay fault. BHV—volcanic rocks in the Berkeley Hills; BMV—Burdell Mountain Volcanics; TV—Tolay Volcanics; P—Pinole; R—Rodeo; V—Vallejo; WP—Wilson Point. Inset tectonic map shows the Cascadia sub- duction zone (CSZ), Mendocino fracture zone (MFZ), Mendocino Triple Junction (MTJ), and San Andreas fault (SAF). Extent of modern Cascades arc volcanics shown in gray pattern (from Luedke and Smith, 1981); QSV— Quien Sabe Volcanics. GEOLOGIC, GEOCHEMICAL, AND fieldwork supplemented and augmented geo­ paleomagnetic (Mankinen, 1972), geochrono­ GEOPHYSICAL METHODS logic mapping being conducted concurrently by logic (Fox et al., 1985a; McLaughlin et al., 2004, the California Geological Survey (Bezore et al., 2008), and tephrochronologic (Sarna­Wojcicki, The regional volcanic stratigraphy, identifica­ 2004, 2005; Clahan et al., 2004, 2005; Wagner 1976; Sarna­Wojcicki et al., 1979, 1984) data tion of the location of volcanic centers (Fig. 3), et al., 2003, 2004, 2006) and by other investi­ to synthesize the regional volcanic stratigraphy. and delineation of the spatial distributions of indi­ gators at the U.S. Geological Survey (Graymer Stratigraphic correlation of volcanic units was vidual volcanic units were accomplished through et al., 2002a, 2007; McLaughlin et al., 2004, accomplished through comparison of phenocryst extensive field observation by the authors. Our 2008). Where possible, we used new and existing assemblage and mineralogy , pumice and lithic 630 Geosphere, June 2011 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/629/3339746/629.pdf by guest on 26 September 2021 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/629/3339746/629.pdf 122°30′W 122°15′W 122°30′W 122°15′W Middletown " EXPLANATION 38°45 N Mayacamas EXPLANATION ′ Quaternary deposits 38°45′N Mountains Cenozoic sedimentary rocks Volcanic feature Diatomite k Dome Basalt and andesite flows X Rhyolite and dacite flows Vent YJFZ MSH Tuff Caldera P Welded tuff X X CENOZOIC (NEOGENE) MSH X Intracaldera megabreccia X X Volcanic center Intrusive rocks k WL Calistoga Angwin Francsican Complex Geologic units as shown FV " " k on Figure 2. KC Great Valley Sequence k Sonomavolcanicfield,California andgeochemistry, Geology k Serpentinite CD DM MESOZOIC k k GM water k " 38°30′N St. Helena 38°30′N Napa Geosphere, June2011 MH RR AD AC k " XX Santa SL k k Rosa BM K Yountville k TM " k SG SL We Rodgers Creek faul k Santa Ros st Napa fault SC BM k Va MV kk V ll Sonoma all Xk ey a X MG k ey MG X k Sonoma BK k k SONX k Carneros fault t Napa CS X SH " " LV k k Sonoma AM X k Va AH CS Mountain CV 38°15′N PV ll 38°15′N ey " 0246810 km GV 0246810km s Figure 2.