Abstract

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40Ar/39Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies.

Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma.

We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

INTRODUCTION AND PREVIOUS STUDIES

Tephra layers (pyroclastic volcanic ash beds or tuffs) are volcanic units composed of material erupted and emplaced during a geologically short period of time—generally within a few hours or days. Ash from large-volume explosive eruptions may be distributed by wind over large areas. Such units thus represent virtual time horizons that can be identified and traced over a large area, provided that the tephra layers can be identified and distinguished from each other and their ages determined. Such layers thus provide correlation and age control to associated deposits, and can also provide information on rates of stratigraphic and tectonic processes, such as deposition, tectonic uplift, and fault offset.

We report on the age, stratigraphy, correlation, and areal distribution of four lower Pliocene pyroclastic units (pumice-and-ash-flow and pumice-and-ash-fall beds) that are present in stratigraphic superposition within the Sonoma Volcanics in the northern part of the San Francisco Bay area and on correlative ash-flow and ash-fall beds found in outlying areas within the San Francisco Bay area (Fig. 1). These tuffs also provide information on the sense of direction and rates of tectonic movements, including fault displacement on the eastern San Andreas fault system in this area. Three of these tephra layers are also found outside the San Francisco Bay area, broadly distributed throughout California and Nevada (Fig. 2). The four units studied are, from stratigraphically lowest to highest: the Lawlor Tuff, the Huichica tuff (of Sarna-Wojcicki and Davis, 1991), the informally named tuff of Napa (previously named Healdsburg tuff), and the locally distributed, informally named tuff of Monticello Road, previously named the St. Helena Rhyolite (Weaver, 1949) and rhyolite of Mount George (Fox, 1983). The stratigraphic relations of all four tuffs are not visible at any one site, and are pieced together from several exposures within the northern San Francisco Bay area.

The Lawlor Tuff, the most widespread of the four, is a compound, pyroclastic stratigraphic unit of variable thickness consisting of two or more beds of tephra, first mapped in the Los Medanos Hills north of Mount Diablo and south of Suisun Bay. The name of this unit is derived from one of its outcrop localities at Lawlor Ravine in these hills (Figs. 1, 3, and 4). Vitt (1936) cites several early geologic mapping and paleontologic studies in this region, and of this pyroclastic unit in particular. Vitt correlated this unit on the basis of physical characteristics and mineralogy with the Pinole tuff, another compound pyroclastic unit that is present further to the west, near the towns of Pinole and Rodeo, south of San Pablo Bay (Fig. 5). Based on this correlation, Vitt (1936) and Patten (1947) abandoned the name Lawlor Tuff, and used the name Pinole tuff instead. Weaver (1949), however, reverted to the name Lawlor Tuff, although he too suspected that the Lawlor Tuff “… May, in part, be equivalent to the Pinole tuff” (Weaver, 1949, p. 61). Evernden et al. (1964) obtained a potassium-argon (K-Ar) age of 5.2 Ma on the Pinole tuff, and Sarna-Wojcicki (1971, 1976) cited a K-Ar age of 3.96 ± 0.16 Ma for the Lawlor Tuff (G.H. Curtis, University of California, Berkeley, 1969, personal commun.). This age was later revised to 4.1 Ma following revision of the K40 decay constant (Steiger and Jager, 1977). Sarna-Wojcicki (1971) showed that the glasses of the Pinole and Lawlor Tuffs differed significantly in chemical composition, and that the Lawlor Tuff coarsened and thickened in the direction of the Sonoma Volcanics, to the northwest. Moreover, Sarna-Wojcicki (1976) showed that the Lawlor Tuff is present within the Sonoma Volcanics (Fig. 1), where it stratigraphically overlies the Pinole tuff. Differences in chemical composition as well as stratigraphic position and isotopic ages indicated that the Pinole and Lawlor Tuffs are two different volcanic units erupted at different times, although from the same volcanic field, the Sonoma Volcanics.

At its type locality, the Lawlor Tuff consists of a basal, light-gray plinian pumice-fall unit ∼2 m thick, and an upper, unwelded, pink to light-brown, pumice-ash–flow unit 3 m thick (Fig. 4). Because of its light-gray to light-brownish orange color and relative cohesion, the tuff stands out and is well exposed in these hills, underlain by darker sedimentary deposits that are generally overgrown by grass (Fig. 6). The tuff can be traced almost continuously in surface outcrops for ∼15 km, on the west side of the Los Medanos Hills (Figs. 1, 3, and 5). The Lawlor Tuff is as much as 15 m thick at its westernmost exposures near Port Chicago, and generally thins to the east.

Near Lawlor Ravine, the Lawlor Tuff lies unconformably over the upper Miocene Neroly Formation of the San Pablo Group, and to the east over unnamed younger sediments, and disconformably underlies the Los Medanos Formation of Clark (1943, p. 189). These gravels are also referred to by Weaver (1949) as the Wolfskill Formation; Sims and Sarna-Wojcicki (1975) subsequently correlated these gravels with the Tehama Formation based on the presence of the ∼3.3-Ma Putah Tuff (Miller, 1966) near the base. The latter tuff is exposed for several tens of kilometers to the north of Suisun Bay, along the west side of Sacramento Valley, within the lower part of the Tehama Formation, as well as near the base of the Los Medanos Formation that unconformably overlie the Lawlor Tuff near its type area.

Coarsest and thickest tephra correlated to the Lawlor Tuff is exposed in the southeastern part of the Sonoma Volcanics field west of Suisun Bay and east of the Green Valley fault (Fig. 1), indicating that the tephra composing this tuff was erupted from this volcanic field (Sarna-Wojcicki, 1971, 1976). This observation was further confirmed when coarse-pumice tephra of the Lawlor Tuff was found interbedded with pyroclastic deposits and volcanic-flow rocks of the Sonoma Volcanics west of the Green Valley fault, an additional ∼24 km farther to the north of the above-mentioned locality, along Monticello Road in the Maacamas Range (Sarna-Wojcicki, 1976) (Fig. 1).

Sarna-Wojcicki (1971, 1976), Isaacson (1990), and Walker et al. (1996) extended the correlation of the Lawlor Tuff to additional sites within the area east of San Francisco Bay, in Contra Costa and Alameda Counties south of Mount Diablo: to the upper part of the nonmarine Tassajara Formation, and farther south, to the Livermore Gravels of Clark (1930), south of Livermore, at the north end of the Diablo Range (Fig. l). The lower Livermore Gravels, containing the Lawlor Tuff and the overlying Huichica tuff, were subsequently renamed as the Contra Costa Formation by Wagner et al. (1991), and the Sycamore Canyon Formation by Graymer et al. (2002). Parts of what were originally mapped as the Orinda Formation in the eastern San Francisco Bay area were also renamed the Contra Costa Group by Creely et al. (1982). Wagner (1978) described the Contra Costa Group in detail and extended the name to include the lower part of the Livermore Gravels of Clark (1930).

The Lawlor Tuff is found at additional sites beyond the San Francisco Bay area, in unnamed, deformed upper Tertiary alluvium on the west side of the Great Valley–eastern foothills of the Diablo Range (Richesin et al., 1994) (Fig. 1; easternmost sample locality). The Lawlor Tuff is also present in the uppermost part of the marine Etchegoin Formation, in the Kettleman Hills, at the southwestern margin of San Joaquin Valley (Sarna-Wojcicki et al., 1979) (Fig. 2).

The informally named Huichica tuff (Sarna-Wojcicki et al., 1991), previously referred to as the upper tuff in the Livermore Gravels of Clark (1930) (Sarna-Wojcicki, 1971, 1976; Sarna-Wojcicki et al., 1979), is found near the base of the Huichica Formation of Weaver (1949), southwest of Napa (Sarna-Wojcicki et al., 1991) (Fig. 1). The Huichica tuff is also present near the base of the Paso Robles Formation, in the Gabilan Range, near Topo Valley, southeast of the San Francisco Bay area, as documented here (Fig. 2).

The informally named tuff of Napa has been previously referred to as the (informal) Healdsburg tuff and as the pumice breccia near Monticello Road, by Sarna-Wojcicki (1976). The tuff of Napa is interbedded with volcanic flow and pyroclastic rocks in the eastern part of the Sonoma Volcanics east of Napa, and is well exposed in old quarry pits along Monticello Road (Fig. 1). This tuff is also found at several localities within the Sonoma Volcanics, and within tuffaceous alluvium at Willow Wash, in east-central California, east of the Sierra Nevada (Reheis et al., 1991) (Fig. 2). A distal locality of this tuff was found within Pliocene alluvium within the Panaca Formation in eastern Nevada (Joel Pederson, Utah State University, Logan, 1998, written commun.; Lindsay et al., 2002) (Fig. 2).

The tuff of Monticello Road is a partly welded ash-flow tuff exposed at the top of the sequence of pyroclastic and volcanic-flow rocks near Monticello Road in the Maacamas Range (Fig. 1). This tuff was previously referred to as part of the St. Helena Rhyolite Member of the Sonoma Volcanics by Weaver (1949), and Sarna-Wojcicki (1976); it was subsequently referred to by Fox (1983) as “The 3.8-m.y.-old rhyolite at Mount George,” and assigned by him to the lower member of the Sonoma Volcanics. Because of its pyroclastic texture and its unique position at the top of the volcanic sequence in this southeastern area of the Sonoma Volcanics, we here informally refer to this unit as tuff of Monticello Road.

In this report, we present new 40Ar/39Ar age data for the four tuff units, document identification of three of the tuffs at new sites within the San Francisco Bay area (Fig. 1) and beyond based on the chemical composition of their glasses, and for the Lawlor Tuff, at multiple sites within the Sonoma Volcanics and at five widely separated distal sites in California: (1) in unnamed upper Tertiary alluvium of Crowder Flat Road, west of Alturas, in northeastern California; (2) in unnamed Tertiary alluvium northeast of Mono Lake, in Mono basin (Reheis et al., 2002); (3) in the Pliocene Horned Toad Hills Formation in the Horned Toad Hills (A.M. Sarna-Wojcicki, written communication to Steven May, 2007), in unnamed deposits in Afton Canyon (East Manix Basin), and near Amboy, all three latter sites in the Mojave Desert; (4) in a fault-bound sliver within the Red Mountain thrust-fault complex near Ventura, and (5) near the top of the marine Malaga Formation, in shoreline cliffs at Malaga Cove, Los Angeles (Fig. 2). In addition, the tuff was also recently identified by one of us (M.P.) in the Bouse Formation of southeastern California, an identification we confirm here (Fig. 2). We also provide new data on the paleomagnetic orientation of the Lawlor and Huichica tuffs.

METHODS

Incremental-Heating 40Ar/39Ar Analyses: Berkeley Geochronology Center

Samples collected for 40Ar/39Ar geochronology were prepared by crushing and sieving to 30–40 mesh followed by magnetic separation of plagioclase phenocrysts, ultrasonically cleaned in distilled water, and handpicked to remove any crystals that contained melt inclusions. These samples did not contain even trace amounts of K-feldspar. Completed separates were loaded into wells in an Al disk, interspersed with neutron fluence monitors (Fish Canyon sanidine, reference age 28.02 Ma; Renne et al., 1998), and irradiated in the TRIGA reactor at Oregon State University in the Cd-lined in-core irradiation tube (CLICIT) facility for 0.5 h.

Irradiated samples were analyzed at the Berkeley Geochronology Center (BGC). Multi-grain samples (20–40 mg) were degassed in UHV using a 50W CO2 laser equipped with a 6-mm integrator lens. Extracted gasses were cleaned of reactive species (CO, NO) using SAES getters and analyzed with a Mass Analyzer Products (MAP) 215-50 mass spectrometer for ∼20–30 min. Further details of 40Ar/39Ar dating procedures at BGC are available in Deino and Potts (1990), Deino et al. (1990), Best et al. (1995), and Sharp and Deino (1996).

40Ar/39Ar Analyses: U.S. Geological Survey, Menlo Park, California

Samples for 40Ar/39Ar analysis at the U.S. Geological Survey (USGS) in Menlo Park were evaluated petrographically in thin section for appropriate mineralogy, evidence of alteration, and signs of detrital contamination before preparation. Samples selected for analysis were crushed with a jaw crusher, ground in a roller mill to avoid undue heating associated with disk grinders, sieved to appropriate size ranges, and washed to remove adhering fine powder. Plagioclase separates were prepared by removing ferromagnetic material with Carpco and Frantz magnetic separators, followed by removal of glass with heavy liquids or quartz by diamagnetic separation where necessary. Plagioclase concentrates were leached with 8% HF solution to remove adhering glass, groundmass, and atmospheric argon from their surfaces prior to rinsing three times in deionized water in an ultrasonic cleaner. Where matrix grain size was sufficiently coarse and interstitial glass was minimal, unaltered basalt was analyzed as groundmass separates, avoiding early-formed phenocrysts and xenocrysts, as well as containing the highest potassium-bearing phases. Groundmass separates were prepared by crushing to 60–80 mesh (180–250 um), removing magnetite and some pyroxene in the most magnetic fraction with a Frantz magnetic separator, and finally removing olivine and plagioclase in the less magnetic fractions. The sample is then agitated aggressively in an ultrasonic cleaner until no additional fine-grained material is being removed. Final splits for analysis are resized and handpicked to remove any remaining unwanted grains.

Samples for 40Ar/39Ar analysis are irradiated in the central thimble of the USGS TRIGA reactor (GSTR) facility in Denver, Colorado, following procedures similar to those described by Dalrymple et al. (1981). TCR-2 sanidine from the Taylor Creek rhyolite is used as the flux monitor with an age of 27.87 Ma. Data are also reported with monitors recalculated to an age of 28.02 Ma for Fish Canyon tuff sanidine, which corresponds to an age of 28.20 Ma for TCR-2. Samples irradiated for periods less than ∼4 h are shielded with cadmium to reduce the thermal neutron flux and rotated during irradiation to reduce lateral flux variations. 40Ar/39Ar incremental-heating experiments utilize a temperature-controlled, tantalum-resistance furnace and molybdenum crucible with heating schedules designed to release argon from the sample in roughly equal fractions of the total 39Ar after eliminating as much atmospheric argon as possible in early degassing steps. Mass spectrometry was done using a MAP 216 mass spectrometer operated in the static mode. Potassium and argon isotopic abundances and the decay constants for 40K used are those recommended by Steiger and Jager (1977). Corrections for neutron-induced interferences were made using correction factors determined by analyzing argon from irradiated fluorite and potassium glass. Plateau ages of Ar-Ar age spectra are defined as the weighted mean ages of contiguous gas fractions representing more than 50% of the 39Ar released for which no difference can be detected between the ages of any two fractions at the 95% level of confidence (Fleck et al., 1977).

Laboratory Preparation of Tephra Samples (Modified from Sarna-Wojcicki et al., 2005, p. 8–12)

In the laboratory, we disaggregated or crushed and wet-sieved tephra samples in plastic sieves using 100, 200, and 325 nylon mesh screens (opening diameters of 150, 80, and 45 μm, respectively). Plastic and nylon were used to avoid contamination with metals. Sized fractions were then cleaned chemically, first with 10% HCl, then very briefly with 8% HF, and in an ultrasonic bath in deionized water, then separated using magnetic and heavy liquid methods into component parts—volcanic glass shards, magnetic minerals, nonmagnetic minerals, and lithic grains. Plagioclase grains were separated from the tephra samples for laser-fusion 40Ar/39Ar analysis.

Petrographic Analysis of Tephra Samples

Splits of heavy-mineral separates were placed in optic oils on glass slides and examined under a petrographic microscope. Line counts were made of the minerals present using a mechanical stage, and tabulated to determine quantitatively the relative abundances of the minerals in the tephra samples (Sarna-Wojcicki, 1971).

Chemical Analyses of the Volcanic Glasses of Tephra Samples (Modified from Sarna-Wojcicki et al., 2005)

Volcanic glasses separated from all tephra samples were analyzed by electron-microprobe analysis (EMA). Glasses from selected samples were also analyzed by energy-dispersive and wavelength-dispersive X-ray fluorescence (XRF) and instrumental neutron activation analyses (INAA). Ages, stratigraphic and petrographic data, and chemical compositions of the tephra samples were compared with those of other tephra layers from the western United States, and correlations were noted if the requisite criteria for correlation were considered to be fully met (see Methods of Data Evaluation section).

Electron-Microprobe (EM) Analysis

Methods used in EM analysis of glass shards are described in Sarna-Wojcicki et al. (1984, 2005). The current database of the U.S. Geological Survey's Tephrochronology Laboratory (Menlo Park) contains ∼5500 EM analyses of tephra samples from the conterminous western United States that can be used for comparison and correlation. The EM analyses were determined over a period of ∼30 yr on three different instruments: ARL, SEQM, and JEOL 8900 microprobes. With the JEOL 8900 microprobe, we use a 10-μm beam diameter, 10.05-nanoamp, 15-kV beam current. Acquisition time is 10 seconds for Na, and 20 seconds for the remaining oxides: SiO2, Al2O3, FeO, MgO, MnO, CaO, TiO2, and K2O. We use GSC and An40 as standards, and RLS 132, a homogeneous obsidian from La Puebla, Mexico, as an internal standard. The ZAF data reduction program is used to obtain oxide concentrations. Internal, polished surfaces of ∼15–20 individual glass shards were analyzed for each sample. The means of the individual shard analyses are averaged for the oxides analyzed to obtain an overall composition for each sample.

Replicate analyses of internal and external standards over the past 30 yr indicate that analyses are mutually compatible, despite differences in instrumentation. Precision and accuracy of the analyses for each oxide are indicated by replicate analysis of RLS 132, a homogeneous obsidian from near La Puebla, Mexico; GSC, a synthetic glass standard made for the U.S. Geological Survey by Corning Glass Company (Myers et al., 1976); other chemical standards, all having comparative values derived from wet-chemical analysis; as well as replicate analyses of specific type samples of tephra conducted periodically over a number of years with the same and different types of instrumentation. Data for F, P, S, Cl, and Ba are available for some samples but are generally not helpful in chemical characterization of the silicic tephra layers in this study using EM analysis. Similarly, MgO, MnO, and TiO2 are not consistently useful in characterization of silicic tephra by this method owing to low concentrations in some of the volcanic glasses, and consequently relatively large scatter in analyzed values (Sarna-Wojcicki and Davis, 1991). Glass of tephra erupted from the Sonoma Volcanics, however, is sufficiently high for these minor oxides so that they are useful in distinguishing among different eruptive units from this source. Despite some drawbacks, EM analysis makes it possible to determine the chemical composition of individual glass shards for the above major and minor oxides, and thus to determine the homogeneity of tephra samples and detect presence of multiple compositional modes, as well as to identify individual beds in most cases (Sarna-Wojcicki and Davis, 1991).

The alkalis within tephra of the units we report on, and in general within the Sonoma Volcanics, are quite variable. Sodium values in EMA are particularly scattered, from one analyzed point to the next, and one sample of the same tephra to another. The alkalis, sodium in particular, may be partially leached out of the glass, and in some cases enriched, as appears to be the case with potassium. Such differences are associated with differences in hydration of the glass, as determined by a deficit from the sum of 100 wt% of the oxides on analysis. Sodium analyses are particularly problematical, and differences on reanalysis of sodium in the same sample at different times are frequently observed. For this reason, our data evaluation procedures involve steps that both include and exclude the alkalis in the comparisons. This is not only because of the specific problems associated with analysis of sodium, but also because a tephra layer that is hydrated, leached, weathered, or altered under the same depositional and natural “storage” conditions will tend to have the same alkali concentrations for samples collected at multiple locations, whereas a tephra layer subjected to different conditions at different localities will tend to yield different alkali concentrations, even though the other oxides may remain essentially the same.

To reduce scatter caused by differential hydration of the glass, we generally recalculate the totals to a 100% fluid-free basis. The deficit of the oxide and element total from 100% ranges from about 1% to as much as 10%. Most of this deficit is water of hydration, but may include other fluids, such as CO2, as well as small amounts of other minor oxides and elements that have not been analyzed by EMA, such as BaO, ZrO2, S, P, Cl, and F. The latter oxides and elements summed together generally do not exceed ∼0.5–1 wt% of the total in silicic volcanic glasses.

When EM analysis alone is not sufficiently diagnostic for characterizing some tephra layers, or when other criteria for correlation such as numerical age, stratigraphic position, or petrographic characteristics are lacking or insufficient to obtain a reliable identification of a tephra layer, analyses using X-ray fluorescence or instrumental neutron activation analysis provide additional data to discriminate among tephra layers.

Wavelength-Dispersive X-Ray Fluorescence (WDXRF) Analysis

About 2 g of pure volcanic glass, separated and cleaned in the laboratory, was powdered and analyzed, together with previously analyzed standards, using a Norelco universal vacuum spectrometer. Analytical methods used are described in Campbell and Thatcher (1962), Jack and Carmichael (1968), and Sarna-Wojcicki (1971). Analyses included Fe, Ti, Ba, Mn, Zr, Rb, Sr, Zn, Y, Ga, Nb, Cu, and Ni. Chemical reference standards analyzed by wet-chemical methods were used to determine the elemental concentrations in the volcanic glass of the tephra samples. The main reference standards used were U.S. Geological Survey granites G-1 and G-2, and diabase W-1. In this type of analysis, cross-checks with results obtained by EM analysis can be made for Fe, generally more precise concentrations are obtained for Mn and Ti than in EM analysis, and the other minor and trace elements provide additional critical variables to help discriminate among tephra layers. Wavelength-dispersive X-ray fluorescence analyses were used previously for characterizing some of the tephra layers in this study, and are summarized here for the sake of completeness and comparison with the other analytical methods (Sarna-Wojcicki, 1971; Sarna-Wojcicki et al., 1984).

Instrumental Neutron Activation (INAA) Analysis

Between 0.2 and 0.5 g of pure volcanic glass was powdered and analyzed using techniques described in Bowman et al. (1973), Sarna-Wojcicki et al. (1979), and Baedecker and McKown (1987). Currently, there are ∼500 INAA analyses of tephra samples in the database of the Tephrochronology Laboratory (Menlo Park) that can be used for comparison. These samples were analyzed over a period of ∼20 yr in three different laboratories: Lawrence Berkeley Laboratory of the University of California, Berkeley, and the Radiochemistry Laboratories of the U.S. Geological Survey in Reston, Virginia, and in Denver, Colorado. Of the three methods, INAA analysis is by far the most precise for a number of minor and trace elements. Analyses were made for 24 or more elements; between 15 and 20 of these generally proved useful in characterizing silicic tephra samples (Sarna-Wojcicki and Davis, 1991): Sc, Mn, Fe, Zn, Rb, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Dy, Yt, Lu, Hf, Ta, Th, and U. Small but systematic differences in analytical results were observed among laboratories and between successive sets of analyses, as determined on replicate analyses of sample splits submitted with each set. Corrections for these differences were made using regression analyses of replicate sample sets.

Methods of Data Evaluation (Modified from Sarna-Wojcicki et al., 2005)

Analytical data on the chemical composition of volcanic glasses of tephra layers that we obtained in this study were first evaluated using the similarity coefficient of Borchardt et al. (1972) and Borchardt (1974). The similarity coefficient is used as a guide to select a pool of candidate samples that are further evaluated in terms of the chemical and geological criteria for the closeness of a match. We used a computer program that compares any single sample with all previously analyzed samples, for those elements that are considered the most reliable in chemical identification of tephra layers, and that allow us to distinguish most clearly between tephra layers of similar composition (Sarna-Wojcicki et al., 1984; Sarna-Wojcicki and Davis, 1991). The program output lists the tephra samples that match most closely to the sample that is being evaluated for the specified set of elements, and these samples are ranked in order of the value of the similarity coefficient. The highest ranking samples represent a pool of candidates for further evaluation for correlation. Selected samples and sample groups were also compared using cluster analysis (R-mode and Q-mode), with dendrograms (Parks, 1970, and XLSTAT software program; Sarna-Wojcicki, 1976).

The best matches from the initial comparison run are run again, using other element combinations, to evaluate the effects of possible postdepositional cation exchange and alteration. For example, one run would include the alkalis, while a second would exclude them. Minor elements, such as Mg, Mn, and Ti, may be included in subsequent runs in various combinations, if present in sufficient concentrations, to narrow the field of potentially correlative samples. The closest candidates are also run through the program to see if the rankings hold up in reverse comparisons (“knowns” against “unknowns”). The remaining pool of candidate samples is evaluated with regard to other criteria: independent age data, stratigraphic data, stratigraphic sequence relative to other tephra layers, petrographic characteristics, and paleomagnetic data, if available. For a more complete and detailed discussion of field criteria, petrographic characteristics, mineralogy, chemical analysis of glass, and data evaluation methods used in this report, see Sarna-Wojcicki (1971), Sarna-Wojcicki et al. (1984), Sarna-Wojcicki and Davis (1991), Sarna-Wojcicki (2000), and Sarna-Wojcicki et al. (2005).

PETROGRAPHIC CHARACTERISTICS OF THE LAWLOR TUFF

At proximal sites, the Lawlor Tuff is composed of variable amounts of pumice clasts, volcanic glass shards, lithic fragments, and minor amounts of mineral grains. The pumice clasts, lithics, and mineral grains generally decrease in abundance in the air-fall material away from the putative source area (the Sonoma Volcanics east and southeast of Santa Rosa; Fig. 1), while the lighter glass shards are progressively enriched with distance from the eruptive source. Minerals present in the Lawlor Tuff are predominantly plagioclase feldspar, with smaller amounts of dark-brown amphibole, clinopyroxene, and orthopyroxene. Also present are sparse, dark-green amphibole, ilmenite, magnetite, and zircon (Sarna-Wojcicki, 1971). The relative abundances of the mafic minerals, based on line counts of grains mounted in optic oils under a petrographic microscope, are given in Table 1.

The relative abundances of minerals in the Lawlor Tuff vary widely, based on the distance of the samples from the source area as well as the mode of emplacement of the tephra (ash flow, pumice or ash fall, water deposited, or water reworked). Some samples of the Lawlor Tuff contained few or no mineral grains, and may represent the elutriated fines from the ash flows of this unit. The variations in components of this tephra unit from site to site are a consequence of distance from eruptive source and eolian and hydraulic sorting, among other factors. For this reason, the mineralogy of the Lawlor Tuff is not as reliable a criterion for correlation as the composition of the glass, which is essentially unchanged regardless of the distance from the eruptive source area.

The volcanic glass of the Lawlor Tuff is clear when observed in plane-polarized light with a petrographic microscope, and has a refractive index of 1.505 ± 0.002 (Sarna-Wojcicki, 1971). The refractive index (R.I.) of the volcanic glass is a measure of the velocity with which light passes through the glass as a ratio to its velocity in air. More hydrated shards have slightly lower indices of refraction, whereas less hydrated shards have slightly higher indices. The R.I. is primarily a function of the density of the glass, which in turn is an effect of its chemical composition. The R.I. is a single parameter that has been used in the past by others to help identify individual tephra layers (for example, Wilcox, 1965), but it is not as useful a criterion as the spectrum of individual concentrations of major, minor, and trace elements that are present in the glass, and which can be measured with considerable precision and accuracy to obtain a characteristic, multi-parametric, chemical “fingerprint” for volcanic glasses. The chemical fingerprint provides the main basis for tephra identification, and is used here to identify and correlate the Lawlor Tuff and the other tephra units among sites between which tephra layers are not continuously exposed. In addition to the chemical characteristics of glass of the tephra layers, the presence and absence of cogenetic minerals, stratigraphic position (particularly relative to other tephra layers), magnetostratigraphy, and radiometric ages are used to derive a composite stratigraphy and correlations.

PETROGRAPHIC CHARACTERISTICS OF THE HUICHICA TUFF, THE TUFF OF NAPA, AND THE TUFF OF MONTICELLO ROAD

The Huichica tuff contains an assemblage of phenocrystic minerals similar to that of the Lawlor Tuff. Plagioclase feldspar is the predominant phenocrystic mineral in both. Of the mafic minerals, dark-brown hornblende and clinopyroxene are sparser and orthopyroxene is more abundant in the Huichica tuff compared to the Lawlor Tuff (Table 1).

Field and laboratory analyses in this study indicate that the tuff of Napa is aphyric, consisting entirely of pumice near its inferred eruptive source area, near the town of Napa, where it is coarsest, and of glass shards at distal sites. Because comagmatic phenocrystic minerals have not been found in this tephra layer, direct radiometric ages could not be obtained. Near its eruptive source east of the town of Napa (Fig. 1), however, a dacite flow directly underlies coarse pumice breccia of the tuff of Napa, and angular blocks of the dacite derived from the front of the flow are suspended in the pumice matrix of the overlying tuff of Napa, suggesting that the emplacement of the two units was close in time. We consider the Ar/Ar ages obtained here on plagioclase from the chilled margin of blocks, and of the underlying dacite flow, to be close approximations of the age of the tuff of Napa (see below). The ages obtained for the dacite are concordant with other ages obtained for underlying and overlying pyroclastic units.

The tuff of Monticello Road is a poorly welded ash-flow tuff containing mostly vitric glass and pumice with some plagioclase and iron oxides that did not exhibit any characteristic crystal shapes. No other minerals were found in magnetic and heavy-liquid separates from this unit.

GEOCHRONOLOGY OF THE TEPHRA UNITS

One set of incremental-heating 40Ar/39Ar release experiments was performed on plagioclase separates at the Berkeley Geochronology Laboratory (A.L.D.), on samples of the Lawlor and Huichica tuffs obtained at their type localities. In Lawlor Ravine along Bailey Road south of Pittsburg (Weaver, 1949), we sampled the pumiceous, ∼5-m-thick Plinian air fall (LAWL-2; Table 2), as well as the overlying, unwelded, pumiceous ash-flow tuff (LAWL-1; Table 2) of the Lawlor Tuff. At the Huichica tuff type locality (on the Chandon winery south of Carneros Highway, southwest of Napa), only the Huichica tuff itself was exposed and sampled (CHANDON-1; Table 2). Note that the above samples, LAWL-2 (fall pumice) and LAWL-1 (ash flow), correspond to the chemically analyzed samples LAWLOR-1 and LAWLOR-2, respectively (see Tables 57). For the Huichica tuff, sample CHANDON-1 corresponds to the chemically analyzed samples H-1 and H-2 (Tables 57).

The analytical data resulting from the incremental heating experiments are provided in the Supplemental Table 11 and summarized in Table 2. Conventional incremental % 39Ar release plots are shown in Figure 7. Apparent-age plateaus contained in these spectra are used to identify portions of the experiment that may identify geologically accurate ages. A plateau age is defined by at least three contiguous steps, constituting >50% of the 39Ar released, that have a MSWD (mean square of weighted deviates) that corresponds to 95% probability that the scatter in the data can be produced by analytical error. Note that these plots and associated calculations do not include steps that yielded less than 2% of the 39Ar released, because these runs may be analytically difficult to measure and may not be geologically meaningful.

Four of the five incremental heating experiments yielded extended plateaus. The spectrum obtained from plagioclase of sample LAWL-1 (the ash flow) was highly discordant and did not yield a plateau (Fig. 7). Two aliquots of a plagioclase separate from sample LAWL-2 (the pumice fall), however, demonstrated plateaus encompassing 66% and 100% of the total 39Ar released with very similar apparent ages of 4.843 ± 0.016 Ma and 4.833 ± 0.013 Ma (1σ error, incorporating uncertainty in J, the neutron fluence parameter). The “integrated ages,” calculated from isotopic recombination of all incremental heating steps (Table 2 and Fig. 7), are statistically indistinguishable between aliquots of this sample and from the plateau ages at 2σ. Similarly, two aliquots of a plagioclase separate from CHANDON-1 (Huichica tuff; equivalent to the chemically analyzed H-1 and H-2) yielded apparent-age plateaus encompassing 97%–100% of the 39Ar released. The plateau ages for aliquots 20450-01 and 20451-01 of 4.76 ± 0.02 and 4.74 ± 0.02 Ma, respectively, are concordant with the integrated ages.

“Inverse” isochrons (isotope correlation diagrams showing 36Ar/40Ar versus 39Ar/40Ar) for the two aliquots from sample LAWL-2 and CHANDON-1 are shown in Figure 8. Isochron ages for the two aliquots are indistinguishable from each other and from the plateau ages. The isochron ages are preferred to the plateau ages because the former are independent of assumptions regarding the (40Ar/36Ar)tr (“trapped”) component of the material, whereas the plateau ages assume (40Ar/36Ar)tr = 295.5, the standard atmospheric air composition. The weighted mean of the isochron ages for the Lawlor and Huichica tuffs is 4.834 ± 0.011 and 4.76 ± 0.03 Ma (1σ), respectively; these values are taken as the reference ages for these tuffs resulting from this study. The two mean ages are statistically distinguishable at the 95% confidence level. All errors associated with radiometric ages cited above and farther below are given as 1 sigma.

At the U.S. Geological Survey Isotope Laboratory at Menlo Park, California, we also dated a basalt that overlies the Lawlor Tuff near Leslie Road in the central part of the Sonoma Volcanics northeast of Monticello Road, and obtained a 40Ar/39Ar age of 4.85 ± 0.03 and 4.91 ± 0.03 Ma, respectively, for the two values of the monitor, for this unit, slightly older than the 4.834 ± 0.011 Ma that we obtained for the Lawlor Tuff at its type locality (Table 3). The tuff at Leslie Road and the overlying basalt are essentially the same age within analytical error. We also obtained a 40Ar/39Ar date of 4.67 ± 0.03 and 4.73 ± 0.03 (respectively, for the two values of the monitor), for the welded top of the Lawlor Tuff east of the Green Valley fault, west of Suisun Bay (Fig. 1 and Table 3). Here, the Lawlor Tuff is overlain by an andesitic basalt, and the welded top of the tuff is a deep brick-red color. The somewhat younger age of the Lawlor Tuff obtained at this location may be explained by loss of argon owing to welding, possibly caused by later emplacement of the overlying andesitic basalt.

Incremental heating experiments to determine the age of the tuff of Napa and the tuff of Monticello Road were determined at the U.S. Geological Survey Isotope Laboratory at Menlo Park, California (R.J.F.), and are presented in Table 3. Because the tuff of Napa was aphyric, we could not date it directly by the 40Ar/39Ar technique. Instead, we determined the age of a dacite lava flow exposed along Monticello Road in the eastern part of the Sonoma Volcanics that directly underlies the tuff of Napa as described above (Fig. 1). This dacite flow contains plagioclase, with large blocks of the dacite present at the margin of the flow, suspended in a matrix of coarse pumice of the tuff of Napa. There is no soil or weathered surface at the top or side of the flow, or in the blocks of dacite that have fallen from the front of the flow. These observations strongly suggest that the dacite and the tuff of Napa were erupted in close succession, and the age of the dacite closely approximates that of the tuff of Napa. Two ages determined on plagioclase from the dacite were 4.65 ± 0.03 and 4.67 ± 0.03 Ma (using a monitor value for the Taylor Creek rhyolite of 27.87 Ma), or 4.70 ± 0.03 and 4.71 ± 0.03 Ma (using the currently more widely used value of 28.20 Ma for the same monitor). Analytical errors for all ages presented below are 1σ. The U.S. Geological Survey Isotope Laboratory in Menlo Park uses the younger age of 27.69 Ma for the Fish Canyon Tuff as a monitor in the age determinations cited here, while the Berkeley Geochronology Center uses the older age of 28.02 Ma for this monitor. To provide a uniform basis of comparison, we have recalculated the Menlo Park ages to the older monitor age (Table 3), but also present the original values to preserve the integrity of the experiments and facilitate data tracking related to these should that become necessary. Many isotopic ages have gone through multiple revisions and recalculations, and it is important to record and retain the original values, so that the history of recalculation can be verified.

The age of the tuff of Monticello Road (previously known as the St. Helena Rhyolite), which lies at the top of the section stratigraphically above the tuff of Napa and the Lawlor Tuff, north of Monticello Road in the eastern Sonoma Volcanics (Fig. 1), is 4.45 ± 0.02 and 4.50 ± 0.02, respectively, for the two different values of the monitor (Table 3).

PALEOMAGNETISM OF THE LAWLOR AND HUICHICA TUFFS

We collected oriented, one-inch-diameter cores from the Lawlor Tuff near its type locality at Lawlor Ravine, where the beds dip at a moderate angle to the north. We also sampled the Lawlor Tuff and the closely overlying Huichica tuff in a gently dipping section exposed in a road cut near Del Valle Reservoir, south of Livermore Valley (Fig. 1). At each site, azimuthal control of the core orientations was obtained with a solar compass. Standard demagnetization methods were employed to isolate stable remanent magnetizations through the application of alternating-field and thermal methods. Alternating-field treatments were made in a shielded coil that imparted a 400-Hz magnetic field up to a maximum strength of 100 mT. Thermal demagnetization was performed by progressively heating specimens in a shielded furnace to 600 °C. All measurements of remanent magnetization were made with a superconducting rock magnetometer in a shielded room (<500 nT). Principal component analysis of the demagnetization data, which followed the least-squares line–fitting method of Kirschvink (1980), was used to determine the primary component of magnetization of each specimen. After inspection of the primary components, we determined that alternating-field treatment gave the least directional scatter and best representation of the primary remanence for the Lawlor Tuff. In contrast, thermal treatment gave the best result for the Huichica tuff. Results from five of the 43 specimens were rejected due to excessive scatter from the main body of data for each site. The rejected results were from two specimens collected at Lawlor Ravine and three specimens from the Huichica tuff at Del Valle Reservoir. Site-mean directions of magnetization and 95% confidence limits (Table 4) were then calculated by the conventional method of Fisher (1953).

Comparing the two site magnetizations for the Lawlor Tuff after application of the tilt corrections shows that the unit has normal magnetic polarity. The tilt corrections reduce separation of the site-mean directions, but the inclinations remain significantly different. Inclination measured at the Lawlor Ravine locality is ∼9° shallower than the mean inclination determined for the Lawlor Tuff at Del Valle Reservoir. This discordance indicates that either: (1) the magnetizations were not acquired simultaneously at the two sites, and secular variation explains the difference, or (2) inclination bias due to compaction or unremoved secondary magnetization has affected the results. We interpret the reduced separation of site means after the tilt corrections as evidence that the bulk of the magnetization was acquired before tilting occurred, and the polarity of magnetization reflects the ambient field state during deposition of the Lawlor Tuff. The Huichica tuff also shows normal magnetic polarity after application of the tilt correction. See Table 4.

Given its normal magnetic polarity and isotopic age of 4.84 ± 0.01 Ma, the Lawlor Tuff should fall well within the C3n.3n, Sidufjall Normal Polarity Subchron of the Gilbert Reversed Polarity Chron, which has a calibrated age of 4.799–4.896 Ma (Gradstein et al., 2004). The overlying Huichica tuff has a normal magnetic polarity as well, but its isotopic age of 4.76 ± 0.03 Ma should place it within the reversed C3n.3r interval of the Gilbert Chron, with a calibrated range of 4.631 to 4.799. Within its one-sigma analytical error of 0.03 Ma, however, it could easily fall into the older Sidufjall Polarity Subchron, but not into the overlying normal Nunivak Polarity Subchron of the Gilbert Chron, which has a calibrated age of 4.493–4.631 Ma (Gradstein et al., 2004).

CORRELATION OF THE LAWLOR TUFF ON THE BASIS OF ITS GLASS COMPOSITION AND COMPARISON TO OTHER TEPHRA LAYERS

Chemical data for the Lawlor Tuff and the three younger units are presented in Table 5 (electron-microprobe analysis, EMA); Table 6 (instrumental neutron activation analysis, INAA); and Table 7 (wavelength-dispersive X-ray fluorescence analysis, XRF). Locations of all samples analyzed in the present study are presented in Appendix Table 1. The glass composition of the Lawlor Tuff is very similar to that of the closely overlying Huichica tuff, and except for CaO, can be distinguished from it only with difficulty using EMA. Although the means of most of the oxides of these two units overlap within one or two standard deviations in EMA, the CaO contents are separated by more than three standard deviations (Table 5), clearly distinguishing between the two populations using this oxide and EMA. Statistically, two populations are different at a confidence level of 95% when their means are separated by two standard deviations each from their means. Two populations that are separated by three or more standard deviations from each of their means are considered to be different at a confidence level close to 100%.

Results of wavelength X-ray fluorescence analysis (XRFA, Table 7) indicate higher concentrations of Mn and Sr, and lower concentrations of Ba and Rb, in the Huichica compared to the Lawlor. However, these two units cannot be distinguished at a confidence level of 95% (two standard deviations) by XRFA alone.

Data from INAA (Table 6), provide a number of numerical parameters (i.e., concentrations of elements) by which these two units can be readily distinguished. For example, large differences in concentrations of rare-earth elements (REEs) are observed between glasses of the Huichica and Lawlor tuffs. Concentrations of Ce, Sm, and La for the two units are separated by 7, 5, and 4 standard deviations, respectively. Other REEs that show high compositional contrast are Dy and Tb, with 3 and 2 standard deviations separating the means, respectively. Other elements that show high compositional contrasts are Cs, Ba, and Hf, the populations of which are separated by ∼2 standard deviations each (Table 6). Because only four samples of the Huichica tuff were analyzed by INAA, the standard deviations of these elements may increase as additional samples are analyzed. Even with the less diagnostic XRF data, however, multivariate factor analysis can distinguish between the Lawlor and Huichica glass compositions with a high degree of confidence. For example, we have previously used the Student's t-test and Parks’ cluster analysis (1970) to successfully discriminate among similar glasses from tephra layers of different age, for which individual oxides or elements are separated by less than two standard deviations of the means, but for which groups of oxides or elements are systematically higher or lower in one group than another, such as the siderophile versus the lithophile groups (Sarna-Wojcicki, 1976; Sarna-Wojcicki et al., 1984). The Lawlor Tuff is more evolved (with higher concentrations of incompatible elements in the glass), whereas the Huichica tuff is less evolved (with higher concentrations of siderophile elements in the glass).

The tuff of Napa, stratigraphically above the Lawlor Tuff in the Sonoma volcanic section and in outlying areas, can be easily distinguished from both the Lawlor and the Huichica tuffs by its higher Fe content in EMA (more than six standard deviations separate its mean from that of the latter two tuffs; Table 5). It has higher concentrations of siderophile elements in the glass than either the Lawlor of the Huichica tuffs. The tuff of Napa contains more Mn and Ti than the Huichica and Lawlor tuffs, but the means are separated by less than two standard deviations (i.e., the confidence level is less than 95%). We do not have INAA analysis of the tuff of Napa.

The tuff of Monticello Road, capping the Sonoma Volcanics section east of Napa, is more evolved than the three lower tephra units and can be readily distinguished from the other three by its silica and potassium, and lower alumina, iron, magnesia, calcium, titanium, and sodium concentrations (Table 5). Neither this tuff, nor any of its Plinian or reworked phases, has been identified anywhere else within the Sonoma Volcanics or outlying areas. Consequently, this pyroclastic unit probably did not have a widespread plinian or coignimbritic component, and is restricted to the area within the southeastern Sonoma Volcanics. Though limited in extent, this tuff provides additional age control to the sequence of tephra layers present in the Monticello Road section of the Sonoma Volcanics, and to the chronostratigraphy of the Sonoma Volcanics in general.

The tuff of Monticello Road can also be readily distinguished from younger tephra layers erupted from the Mount St. Helena caldera, in the northeastern part of the Sonoma Volcanics. The rhyolitic Putah Tuff, 3.34 Ma K-Ar age (Miller, 1966), and 3.27–3.35 Ma Ar/Ar age (Alan Deino, 1996, written commun.) contains higher concentrations of iron and calcium (Sarna-Wojcicki, 1971, 1976). Other, younger tephra units erupted from this source area, ranging in age from ∼3.2 to 2.5 Ma, are dacitic in composition, and are readily distinguishable from the lower tephra units within the Sonoma Volcanics (McLaughlin et al., 2005). Figure 9 is a summary of the current state of knowledge regarding the stratigraphic sequence and isotopic chronology of the Sonoma Volcanics compiled from several sources, including this study, and this chronology and sequence can be correlated to outlying sedimentary sections by means of the more widespread tephra layers.

The Pinole tuff, a compound unit of andesitic to dacitic ash flows, pumice fall, ash fall, and reworked tephra, stratigraphically underlies the Lawlor Tuff within the Monticello Road section of the Sonoma Volcanics, and elsewhere in outlying areas to the southeast. Tephra of the Pinole tuff can be easily distinguished from the overlying, more evolved dacitic and rhyolitic tuffs analyzed in this study, on the basis of glass composition (Sarna-Wojcicki, 1971, 1976), as well as from the Roblar tuff, which underlies the Pinole tuff. A K-Ar age of 5.2 Ma was obtained on the Pinole tuff south of San Pablo Bay (Evernden et al., 1964), and a K-Ar age of 5.4 Ma was obtained on tephra chemically correlative with the Pinole, at the base of the Monticello Road section (G.H. Curtis, University of California, Berkeley, cited in Sarna-Wojcicki, 1971, 1976). No 40Ar/39Ar ages have been obtained on the Pinole tuff to date.

The tuff of Monticello Road, however, is compositionally similar to the (informal) Roblar tuff (previously referred to as the tuff in the Merced Formation of Sonoma County, Sarna-Wojcicki, 1971), a tephra unit present below the Pinole tuff within the southwestern and western parts of the Sonoma Volcanics. The two units could thus be confused based on their chemical composition. The Roblar tuff, dated 6.26 Ma by the 40Ar/39Ar technique, is widespread to the northwest and to the southeast as air fall and reworked tephra, both within the Sonoma Volcanics and in coeval outlying sedimentary sections (Sarna-Wojcicki, 1992). The Roblar tuff underlies the Lawlor and Pinole tuffs with considerable stratigraphic separation within the Tassajara Hills south of Mount Diablo in the southeastern San Francisco Bay area. The tuff of Monticello Road can be distinguished from the Roblar tuff, however, by its higher iron content. Mineral abundances within the tuff of Monticello Road and the Roblar tuff are also different, the former lacking any ferromagnesian silicates (see above), whereas the latter contains variable amounts of dark-brown amphibole, orthopyroxene, and clinopyroxene (Sarna-Wojcicki, 1971).

In most instances, the entire suite of andesitic to rhyolitic tephra layers erupted from the Sonoma Volcanics, from ∼8 to 2.5 million years ago, is easily distinguished on the basis of glass chemistry from late Cenozoic tephra suites erupted from other volcanic sources—for example, those from (1) the southern Nevada volcanic field, the Silver Peak Range, the Mono Glass Mountain, Long Valley, and Inyo and Mono craters, east of the central and southern Sierra Nevada; (2) the Snake River Plain–Yellowstone west-to-east spatial and temporal sequence of rhyolitic calderas in northern Nevada, Idaho, and Wyoming; and (3) the Jemez Mountains in northwestern New Mexico (Sarna-Wojcicki and Davis, 1991; Sarna-Wojcicki, 2000). Tephra derived from the high Cascade Range is also easily distinguishable from that of the Sonoma Volcanics. However, some tephra erupted from several Quaternary sources to the east of the Cascade Range in northeastern California and in central Oregon is remarkably similar in major-element glass chemistry to that of some tephra layers erupted from the Sonoma Volcanics, but the former are considerably younger. For example, Holocene and uppermost Pleistocene tephra erupted from near the Bend area in Oregon, from Newberry volcano, and from Medicine Lake, California, resemble those erupted from the Sonoma Volcanics (Tephrochronology Laboratory database, U.S. Geological Survey, Menlo Park, California). Specifically, Holocene pumice erupted from Newberry volcano ∼1600 yr B.P. cannot be distinguished from the Lawlor Tuff on the basis of major element glass chemistry (SiO2, Al2O3, Fe2O3, CaO, Na2O, and K2O). Fortunately, the glasses of this and other tephra layers from these two very different sources (both with regard to geographic location and age) differ significantly in minor- and trace-element contents, so that they can be distinguished and correctly correlated. In addition, as one reviewer correctly pointed out, the much younger units are generally less hydrated than the older ones, and the alkalis tend to be less perturbed or scattered in the former than in the latter, providing an additional criterion for discrimination between the younger and older sets of tephra layers. The degree of hydration of the glass, however, is also dependent on the environment of deposition and burial, including climate, microclimate, position relative to water table, and fluctuations in these conditions. Similar “cognate” or “faux amis” suites are observed between upper Pliocene tephra derived from the southern Cascades, and uppermost Pleistocene and Holocene tephra derived from the northern Cascades. We do not understand the cause for the formation of such diachronous cognates. One possibility is that they may result from oblique subduction of the same part of a tectonic plate or plates at different times under the North American plate, from south to north, with ensuing similar differentiation paths of the parent magma bodies, and/or to the replication of identical physical and chemical conditions during partial fusion and differentiation of parent rocks of similar composition.

STRATIGRAPHIC AND TECTONIC RELATIONSHIPS OF THE LAWLOR TUFF AND RELATED VOLCANIC AND SEDIMENTARY UNITS IN THE NORTHERN SAN FRANCISCO BAY AREA

Within the San Francisco Bay region, the three widespread, chemically identifiable tephra units, the Lawlor Tuff (4.84 Ma), Huichica tuff (4.76 Ma), and the tuff of Napa (∼4.70 Ma), are found at numerous sites (Fig. 1) interbedded either with volcanic rocks of the Sonoma Volcanics in the northern part of the area, or with nonmarine sedimentary units in the eastern and southern part. In the Bay area, none of these tephra units has been found west of the main trace of the San Andreas fault. An exposure of ash-flow Lawlor Tuff is found within a complex faulted zone north of Santa Rosa, west of the main trace of the Rodgers Creek fault (McLaughlin et al., this volume), and an outcrop of the Huichica tuff is present in the southern part of the area, along the Silver Creek fault, west of a fault complex that represents the splaying of the Hayward fault away from the Calaveras fault. Presumably, these three tephra units could be present in the subsurface considerably north of the Bay area, translated to the northwest by dextral offset of the western blocks of the San Andreas fault system—the Rodgers Creek–Healdsburg fault and the main trace of the San Andreas fault (Fig. 1).

Within the San Francisco Bay area, exposures of the Lawlor Tuff are found progressively farther to the northwest within structural blocks bounded by strands of the eastern San Andreas fault system (ESAFS) (Figs. 1 and 10). As expected, this relationship is most clearly expressed by the distribution of the ash-flow phase of the Lawlor Tuff, because the latter was confined to a relatively small area, whereas its air-fall component was more broadly distributed, and was dispersed in different directions by wind (Fig. 2). The Lawlor Tuff is distributed north and south of Mount Diablo in the easternmost block, east of the Green Valley–Calaveras and Eastern Berkeley Hills fault system, but is present farther to the northwest, near Napa, in the block bounded by the Green Valley fault on the east and the West Napa, Bennett Valley, Carneros, and Maacama faults to the west and northwest. It is exposed farther northwest in the block bounded on the east by the latter fault system, and on the west by the Rodgers Creek and San Andreas faults (McLaughlin et al., 2005, and this volume). These relationships indicate progressive right-lateral displacements of the ash-flow phase of the Lawlor Tuff along the faults of the ESAFS, and the net displacements may provide approximations of long-term rates of motion for these faults since the Lawlor Tuff was emplaced.

Assuming that the Lawlor Tuff localities within the northern San Francisco Bay area were contiguous and distributed approximately normal to the strands of the ESAFS, the displacements measured along the trends of each fault, are:

(1) Across the Clayton–Mount Diablo thrust–Greenville faults: ∼8 ± 2 km, with a long-term average displacement rate of ∼1.7 mm/yr.

(2) Across the Green Valley fault: ∼21 ± 3 km, with a long-term average displacement rate of ∼4.3 mm/yr.

(3) Across the West Napa–Bennett Valley–Carneros faults: ∼27 ± 5 km, with a long-term average displacement rate of ∼5.6 mm/yr.

Total apparent right-lateral displacement on the ESAFS, excluding the Rodgers Creek–Healdsburg fault, is thus 56 ± 6 km in 4.84 Ma, or ∼11.6 ± 1.2 mm/yr (with the total error of the estimate being the square root of the sum of the squares of the individual errors).

If the localities of the Lawlor Tuff were not distributed normally to the faults, but obliquely to them, different displacements result. If the localities were distributed along a N 69° W trend, such as the unfaulted trend of the Lawlor Tuff within the Los Medanos Hills in the eastern part of the region (Figs. 1 and 10), the displacements would be:

(1) and (2) (above) would be unchanged, but:

(3) Across the West Napa–Bennett Valley–Carneros faults: ∼7 ± 2 km, with a long-term average displacement rate of ∼1.4 ± 0.4 mm/yr.

Total apparent dextral slip on the ESAFS in this case would be less, ∼36 ± 4.1 km in 4.83 Ma, or a long-term slip rate of 7.4 ± 0.8 mm/yr, the difference being the amount of displacement on the West Napa, Carneros, and Bennett Valley faults in the two sets of calculations above.

The geometry of distribution of the ash-flow phase of the Lawlor Tuff suggests that the eruptions emanated from a volcanic source area in the vicinity, but some distance to the northwest, of Napa and that this source area may have been dissected by the ESAFS, with part of it translated to the northwest by the West Napa, Carneros, and Bennett Valley faults. The displacement of the Lawlor Tuff along these latter faults is not as clear as that seen along the Green Valley fault to the east, because the eastern locality of the Lawlor Tuff is far from the West Napa, Carneros, and Bennett Valley faults, and we are uncertain in what direction to project the Lawlor Tuff distribution to these faults. It is possible that volcanism and faulting were initially contemporaneous, with some component of east-west extension to arrive at the present configuration of tuff localities.

By far the greatest displacement of the Lawlor Tuff would be expected along the main active trace of the San Andreas fault, had the ash-flow tuff crossed west across the fault, but no such sites have been identified to date. At long-term rates of ∼20–25 mm/yr for Quaternary to Holocene displacement (Prentice et al., 1991), to 36 mm/yr for latest Miocene to early Pliocene displacement (Sarna-Wojcicki, 1992) on the main trace of the San Andreas fault, we would expect the Lawlor Tuff to be offset between ∼100 and 180 km farther to the northwest (of its most northwesterly exposure east of the main trace of the San Andreas fault), on the west side of the San Andreas fault, beneath the Pacific Ocean floor. The older, 6.26-Ma Roblar tuff, erupted in the southwestern part of the Sonoma Volcanics, has been identified in the Delgada submarine fan to the west of the main trace of the San Andreas fault, ∼228 km to the northwest of the nearest locality of this tuff southeast of the main trace of the San Andreas fault, yielding the latter long-term displacement rate of 36 mm/yr (Sarna-Wojcicki, 1992).

Within the eastern Sonoma Volcanics in the Monticello Road section, the Lawlor Tuff is present in intercalated pyroclastic and volcanic rocks that contain the tuff of Napa near the top of the section (Fig. 11). These units are unconformably overlain by the tuff of Monticello Road (the St. Helena Rhyolite Member of the Sonoma Volcanics of Weaver [1949], or the rhyolite of Mount George of Fox [1983]). The tephra correlations and new ages clarify the stratigraphic relationships between the Lawlor Tuff and the tuff of Napa to the west. The Lawlor Tuff probably lies below the unconformity that separates the lower part of the volcanic pile in the Mount George area, from the overlying tuff of Monticello Road. Thus, the stratigraphic sequence in the Monticello Road section in the eastern area of the Sonoma Volcanics, as derived from stratigraphic position and Ar/Ar ages, is as shown in Figure 11. The Huichica tuff is not exposed in this block, and its eruptive center must have been situated to the west of Napa, closer to Huichica, where the near-source, ash-flow phase of this tuff is exposed.

The Huichica tuff overlies the Lawlor Tuff with ∼8 m of separation, within the Contra Costa Formation (Livermore Gravels of Clark, 1930), south of the town of Livermore (Fig. 1). The Huichica tuff and tuff of Napa have not been found superposed at any one locality. The Huichica tuff appears to be distributed generally to the south and southeast of the town of Napa, and the tuff of Napa to the north, northeast, southeast, and in one instance, far to the southeast (Fig. 2). In the Los Medanos Hills, the tuff of Napa, present within the basal part of the Tahama Formation (Sims and Sarna-Wojcicki, 1975), overlies the Lawlor Tuff by as much as 27 m, but the contact between the top of the Lawlor and the base of the Tehama is unconformable, and the stratigraphic separation between the Lawlor and the tuff of Napa decreases to as little as 3 m, from east to west (Eriksson, 1998). Except where an unconformity is present, the tuff of Napa generally has greater stratigraphic separation from the Lawlor than does the Huichica tuff, and should thus be stratigraphically higher. The 40Ar/39Ar ages support this observation.

AREAL DISTRIBUTION OF THE LAWLOR TUFF BEYOND THE SAN FRANCISCO BAY REGION

The presently known areal distribution of the Lawlor Tuff is extensive, covering much of California and parts of western Nevada and probably extending into western Arizona (Figs. 2 and 12). The ash-flow phase of the Lawlor Tuff is mostly limited to the area near its eruptive source, the Sonoma Volcanics, and extends at most for a few tens of kilometers to the east and southeast of that volcanic field, in the Los Medanos Hills, and to the west, in fault blocks offset to the northwest from the inferred eruption center west and/or northwest of Napa. The Lawlor Tuff was encountered in a core near Collinsville, east of Suisun Bay, thus the ash-flow phase may extend farther to the east in the subsurface. The Lawlor Tuff provides a stratigraphic datum by means of which the stratigraphic and tectonic relationships of many pyroclastic, volcanic, and sedimentary units can be determined in the area covered by its ash flows and tephra fall.

REGIONAL CORRELATION OF THE LAWLOR AND HUICHICA TUFFS AND THE TUFF OF NAPA

Regional correlation of the Lawlor and Huichica tuffs and the tuff of Napa allow correlation of sediments and rocks containing these tephra units over a large area in California and Nevada (Figs. 1, 2, and 12). Using the Lawlor Tuff as a chronostratigraphic marker, we can show that the fluvial-lacustrine Alturas Formation of Dorf (1933) in northeastern California, near the town of Alturas2, is at least in part the same age as the volcanic and pyroclastic rocks of the Sonoma Volcanics, the fluvial Tehama Formation present along the western margin of the central Great Valley of California, and the fluvial Tassajara and Contra Costa Formations south of Mount Diablo, in the east-central Coast Ranges of California, eastern San Francisco Bay area. These formations, in turn, correlate with the upper part of the marine Etchegoin Formation in the Kettleman Hills, in southwestern San Joaquin Valley, and with the marine Malaga Cove Formation, in the western Los Angeles Basin, north of the Palos Verdes Peninsula. These units, in turn, correlate with fan alluvium and playa sediments of the Horned Toad Hills Formation, in the Horned Toad Hills of northwestern Mojave Desert, with fluvial deposits in the East Manix Basin and near Amboy, and with fluvial and lacustrine sediments of the Bouse Formation in southeastern California (Buzzard's Peak quadrangle) (Fig. 2).

The presence of the Lawlor Tuff within the fluvial and lacustrine Bouse Formation may provide an age for the inception of Colorado River drainage into the Salton Trough and northern Sea of Cortez (for example, Spencer et al., 2008), though the exact timing of this event, and our age for the Lawlor Tuff, appears to be at odds with the ∼5–5.5 Ma age proposed for inception of Colorado River drainage (Dorsey et al., 2007). Thus, the details of this particular story are still being investigated.

The Lawlor Tuff is also present in an unnamed sequence of diatomite, mudstone, siltstone, sandstone, and pebble conglomerate outcropping along the northeastern margin of Mono Lake basin, in eastern California. There, it underlies Pliocene, upper Pliocene, and Quaternary alluvium and associated volcanic rocks that define an early outlet of Mono Lake basin to the northeast, toward present Walker Lake (Reheis et al., 2002).

The Huichica tuff and tuff of Napa are more limited in areal distribution compared to the Lawlor Tuff. The Huichica tuff is present as a thick ash flow west of the Napa eruptive center, and within the Livermore Valley. There, it is present south of Mount Diablo, north of the city of Livermore, within the Tassajara Formation, and south of that city, north of the Diablo Range, within the Contra Costa Formation (formerly, the Livermore Gravels of Clark, 1930) where both the Lawlor Tuff and the Huichica tuff above it are water-deposited fall ash. At the latter locality, both the Lawlor and Huichica tuffs are exposed in a single continuous section, separated by 10 m of alluvium (Sarna-Wojcicki, 1971). The Huichica tuff is also exposed farther to the south, within alluvium of the Packwood Gravels southeast of San Jose, and farther south in the southern Coast Ranges northeast of Paso Robles, near the base of the alluvial Paso Robles Formation, as fall ash. Areal distribution of the sites indicates that the ash of the Huichica tuff was transported mainly to the south (Fig. 2).

The tuff of Napa is coarsest just to the northeast of Napa, where it is exposed in road cuts along Monticello Road and in abandoned pumice quarries in this area, in the southeastern part of the Sonoma Volcanics (Figs. 1, 10, and 12). Pumice clasts as much as 20 cm in diameter in this area indicate that the eruptive source of this unit was close by. Both the tuff of Napa and Huichica tuff are present in the southwestern and central parts of the Sonoma Volcanics, and thus their distributions may have overlapped at least in part in this area, but such a site has yet to be found. The tuff of Napa is present to the east and southeast of the Napa source area, both in the subsurface in the Sacramento–San Joaquin River delta country on the west side of the Montezuma Hills (Fig. 1), and in exposures in the Los Medanos Hills to the south of the latter locality, but is absent south of Mount Diablo. In outlying areas beyond the Sonoma Volcanics, the tuff of Napa has not been found in stratigraphic superposition with the Huichica tuff, thus their areas of fallout farther from the eruptive sources do not appear to overlap. The latter apparently had a more southerly distribution, the former, a more easterly one. Both tuffs are found above the Lawlor Tuff, the Huichica tuff generally stratigraphically closer than the tuff of Napa, a stratigraphic relationship in agreement with their radiometric ages.

DISCUSSION AND CONCLUSIONS

In the present study, we obtained new 40Ar/39Ar ages on the Lawlor Tuff and several stratigraphically overlying tephra layers that were erupted from the Sonoma Volcanic field in the central Coast Ranges north of San Francisco Bay, California. The 40Ar/39Ar ages are concordant with stratigraphic position (Table 8). All the new 40Ar/39Ar ages are significantly older than K-Ar ages determined previously on the same units in several different previous studies. Systematically older 40Ar/39Ar age determinations relative to K-Ar ages on the same units may result when sanidine feldspar is the mineral analyzed, owing to the high viscosity of its melts, and consequently the very high temperature required to extract all the radiogenic argon from the melts (Webb and McDougall, 1967; McDowell, 1983). In the latter case, the high viscosity of sanidine melts, which tend to retain radiogenic argon in K-Ar age determinations, are believed to be the cause for younger K-Ar ages. Plagioclase melts, however, are presumably less viscous than sanidine melts, and thus should not present as much of a problem in argon extraction. Because plagioclase was the mineral used in the 40Ar/39Ar age determinations in this study, the reason for the systematic age discrepancies is unknown.

Except for the tuff of Monticello Road, the tephra layers are widespread, particularly the Lawlor Tuff (Fig. 2), and thus provide chronostratigraphic horizons by which they and associated volcanic and sedimentary units can be correlated. Correlations of these layers can be accomplished by major-, minor-, and trace-element analysis of the volcanic glasses in the tephra layers. A composite chemical “fingerprint” for glass composition of the Lawlor Tuff based on EMA, XRF, and INAA (Figs. 13–15) provides a large number of quantitative parameters by which the tuff can be identified and distinguished from other tephra layers. Such chemical “fingerprints” can be enhanced by other analytical techniques such as inductively coupled plasma–mass spectrometry (ICP-MS).

The chemically most similar tephra unit to the Lawlor Tuff that we've identified to date is the Huichica tuff, which closely overlies the Lawlor, and is ∼80 ka younger than the latter, based on our age determinations. Chemical compositions of tephra layers such as the Lawlor and the Huichica tuffs, erupted from the same source within relatively short periods of time, tend to be compositionally similar, but similarity generally decreases with increasing age among tephra layers from the same source (Sarna-Wojcicki, 1971, 1976, 2000; Sarna-Wojcicki et al., 1984; Sarna-Wojcicki and Davis, 1991). Such temporal and/or compositional trends indicate that evolution of parent magmas over time is a major controlling factor in compositional differences among tephra layers. Moreover, differences among silicic tephra layers erupted from different sources tend to be greater than those among silicic tephra layers erupted from the same source (Sarna-Wojcicki, 1971). The tectonic setting of the volcanic source area appears to be a major factor in the degree of similarity or dissimilarity among silicic tephra layers (Sarna-Wojcicki and Davis, 1991; Sarna-Wojcicki, 2000). Tephra layers erupted from convergent margins, for example, tend to be more similar to each other than those from other tectonic settings such as divergent margins or intraplate sources such as hot spots. The greatest differences are observed among tephra layers when silicic tephra layers are compared with less evolved pyroclastic materials, such as intermediate or basaltic tephra (Sarna-Wojcicki, 1971, 2000; Sarna-Wojcicki and Davis, 1991). In our studies, we have focused on the silicic tephra layers, because these are produced by the more explosive eruptions, and thus generally provide the most widespread chronostratigraphic marker beds.

It is often difficult or impossible to distinguish among similar tephra layers erupted from the same source using electron-microprobe analysis alone; that is, by the major oxides (SiO2, Al2O3, Fe2O3, CaO, Na2O, and K2O) and a few, less precisely determined minor oxides or elements (MgO, MnO, TiO2, BaO, F, Cl, and P). A larger suite of trace and minor elements determined by a precise analytical technique such as INAA or ICP-MS, however, can resolve even the smallest differences among the most similar tephra layers that we have studied to date.

Identification of the Lawlor and Huichica tuffs and of the tuff of Napa at numerous sites in California and Nevada, combined with 40Ar/39Ar isotopic analysis and magnetostratigraphy, provide a spatial and temporal framework for the study of Pliocene deposits in this region, and form part of a broader framework, in time and space, when combined with other similar chronostratigraphic studies of other tephra layers, both older and younger than those of this study. Results of the present study are applicable to studies of regional stratigraphy, correlation of sediments in diverse depositional environments (for example, correlation of marine and terrestrial sediments and their corresponding paleofaunas and paleofloras, Fig. 12), and regional tectonics.

Apparent displacements of the ash-flow phase of the Lawlor Tuff in the northeastern San Francisco Bay area along several major faults comprising the eastern part of the San Andreas fault system in the northern San Francisco Bay area indicate a total apparent right-lateral displacement of between ∼36 and 56 km on the combined Clayton, Green Valley, Carneros, West Napa, and Bennett Valley faults over the 4.84 million years since the tuff was emplaced, yielding possible displacement rates ranging between 7.4 and 11.6 mm/yr. Greater displacement was obtained by assuming an initial fault-normal orientation of the Lawlor Tuff to the Carneros, West Napa, and Bennett Valley faults. A smaller displacement was obtained when a trend of N 69° W was used for measuring the combined displacement on these faults.

The ∼6.2-Ma Roblar tuff, erupted in the southwestern part of the Sonoma Volcanics, has been identified in the Delgada submarine fan to the west of the main trace of the San Andreas fault, ∼228 km to the northwest of the western part of the Wilson Grove Formation (Sarna-Wojcicki, 1992; Graymer et al., 2002; Wagner et al., 2011), yielding a long-term slip rate for the northern main trace of the San Andreas fault of 36.8 mm/yr. At this rate, the Lawlor Tuff west of the main trace of the San Andreas fault could be displaced as much as 178 km to the northwest of its currently northwesternmost locality on the west side of the Healdsburg–Rodgers Creek fault. The tuff could be present west of the present coastline in sediments on the Pacific Ocean floor; however, it has not been found there to date.

We obtain estimates of displacements on individual faults or groups of faults and the associated long-term displacement rates for these faults. Summing these, we derive a total displacement and a total displacement rate for the entire San Andreas fault system in this area (Table 9).

The total long-term rate of motion we estimate here for the San Andreas fault system in the northern San Francisco Bay area is between ∼48 and 57 mm/yr, over periods ranging from 3 to 7 Ma. Estimates derived from relative plate-motion reconstructions using other lines of evidence such as hot spots and modern geodetic observations range from ∼37 to 50 mm/yr. The two sets of estimates overlap, but our set ranges higher than the latter. This suggests that we have either overestimated displacement rates on some of the fault strands, or that the longer-term estimates incorporate faster rates of faulting in the past, or that there may be some trade-off in time and space in rates of motion between the several segments of the San Andreas fault system in this area, as proposed by Graymer et al. (2002), inasmuch as the rate calculations were made on the several fault strands at different locations in the San Francisco Bay area and for different intervals of time. A major source of error for some of the rate calculations may be that we are attempting to reconstruct motion from offset areas rather than piercing points; we do not know what the actual initial distributions of these ash-flow units were relative to the individual fault strands of the San Andreas fault system.

Many people have helped at different times over the ∼40–45 years during which ASW has been studying the Lawlor Tuff and the related tephra units. Some were analysts who determined the chemical composition of the volcanic glass, or provided advice and assistance in chemical analysis. Others provided samples from various localities. Yet others provided regional information or land access. Still others performed lab preparation on tephra samples, or performed isotopic age analyses. Others collaborated in studies that used the Lawlor and other tuffs as datums in resolution of regional and topical stratigraphic, paleontologic, and tectonic problems. We thank Robert Jack, Jochim Hempel, Frank H. Brown, Garniss Curtis, Ian Carmichael, Charles Meyer, Bi-Shia King, James Budahn, Harry Bowman, P.A. Baedecker, Frank Asaro, Helen Michael, John Dohrenwend, Clark Blake, John Obradovich, James Walker, Dennis Sorg, Paul Russel, Jose Rivera, Marta Woodward, John Diaz, Kate Lormand, Robert Fehr, Michael Haemer, Regina Broussard, Frank Arata, Marith Reheis, David Miller, Steven May, Charles Repenning, Michael Woodburne, Everett Lindsay, and Dwight Taylor. Edward Mankinen and Victoria Langenheim provided constructive reviews that improved the original manuscript. Jeffery Knott, an anonymous reviewer, and the editorial staff of Geosphere provided many additional useful comments that further improved the manuscript.

1Supplemental Table 1. Excel file. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00609.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 1.
2Note that although we have previously identified this 1- to 1.5-cm-thick, lenticular ash layer from a road cut within the Crowder Flat Road section of the Alturas Formation, our two subsequent attempts to find, sample, and analyze it again have failed. The presence of the Kilgore Tuff, erupted from the Heise volcanic field and dated at 4.45 Ma (Morgan and McIntosh, 2005), above the sampled interval in which the Lawlor was originally found supports identification of the Lawlor Tuff, but our inability to reoccupy the sampling site and resample this ash is disturbing and may indicate either a mistake in sample identification in the field or laboratory or removal or concealment of the ash by subsequent erosion along the road cut.

Supplementary data