Tephrochronology of the Miocene Monterey and Modelo Formations, California

The lithology, stratigraphy, and tectonics of the Miocene Monterey Formation, and the temporally equivalent Modelo Formation, have been extensively studied because of their importance as source rocks and reservoirs of petroleum (see Behl, 1999, and references therein). The Monterey Formation consists of siliceous shales deposited in shelf edge to basin plain environments of lower to middle bathyal depths (Behl, 1999). In contrast, the Modelo Formation consists of turbidites deposited as submarine-fan deposits at middle to upper bathyal depths (Rumelhart and Ingersoll, 1997).

In this paper, we provide the initial tephrochronology on the long-recognized volcanic tuffs, ash beds, and tuff beds found in the Monterey and Modelo Formations throughout California (Fig. 1). Kew (1924, p. 56) mentioned tuffs in the Modelo Formation. Kleinpell (1938) stated that tuffs are not uncommon in Miocene sediments of California and, specifically, are found in middle Miocene (Luisian Stage) sediments in southern California (Palos Verdes), the central coast (near Santa Barbara), and inland (near Bakersfield). Bramlette (1946) described numerous vitric ash and tuff beds along with altered tuffs within the Monterey Formation. In his comprehensive study of the Monterey Formation, Bramlette (1946, p. 22) issued a challenge that these tephra layers “should help in the making of more accurate stratigraphic correlations.”

Historically, volcanic material in the Monterey and Modelo Formations was called “tuff.” In this study, where we want to maintain the historical context, we italicize the word tuff. Because this study deals with multiple sites, the induration and particle size may vary from site to site. For simplicity and consistency, unless a tephra was previously and formally named as a tuff (i.e., Tuff of McMullen Creek), we use tephra layer or tephra bed to describe both indurated and poorly cemented pyroclastic material of all sizes.

Age control of the Monterey and Modelo Formations is predominantly by lithostratigraphy and paleontology. Kleinpell (1938) used benthic foraminifera to establish the foraminiferal stages of the Monterey Formation. Bramlette (1946) combined these foraminiferal stages with lithostratigraphy to extend the correlative age control of the Monterey Formation sequences in California. Subsequently, foraminiferal stages were supplemented by the diatom biostratigraphy (e.g., Barron, 1981, 1986, this volume; Barron and Isaacs, 2001).

In general, micropaleontology has established the base of the Monterey Formation at ca. 16 Ma; however, in some sections, the base is as old as ca. 18 Ma (Barron, this volume; Blake, this volume). The top of the Monterey Formation is generally considered to be ca. 7–6 Ma (Barron, 1986, this volume). In contrast, the base and top of the Modelo Formation range from ca. 13 Ma to 5.5 Ma (Barron, this volume). In essence, the micropaleontology shows that the Monterey and Modelo Formations encompass the early to late Miocene from ca. 18 Ma to ca. 5 Ma (e.g., Bramlette, 1946; Obradovich and Naeser, 1981; Barron, 1986, this volume).

Aside from paleontology, numerical and correlative dating of the Monterey and Modelo Formations is rare. Obradovich et al. (1978) determined zircon fission-track dates of 8.0 ± 0.09 Ma and 11.4 ± 1.1 Ma on tephra beds within the Modelo Formation (Table 1). Obradovich et al. (1978) also determined zircon fission-track dates of 3.4 ± 0.3 Ma and 1.2 ± 0.2 Ma on the overlying Fernando (Repetto) Formation at Malaga Cove and Pico Formation at Balcom Canyon, respectively (Table 1). The glass shard composition identified the Fernando Formation tephra layer as the 3.3 Ma Nomlaki Tuff (Sarna-Wojcicki et al., 1979), whereas the Pico Formation tephra layer was the 1.2 Ma Bailey ash bed (Sarna-Wojcicki et al., 1984). Obradovich and Naeser (1981) determined zircon fission-track dates of 11.3 Ma to 6.9 Ma for Monterey Formation samples and 11.5 Ma to 8.3 Ma for Modelo Formation samples (Table 1). These zircon fission-track dates were consistent with the foraminiferal stages of Kleinpell (1938); however, these dates had relatively low accuracy, with some 2σ errors exceeding 1 Ma.

Hornafius (1994) made an initial attempt at Monterey Formation tephrochronology in combination with paleomagnetics. Working at the Naples Beach and Gaviota Beach sections of the Monterey Formation in Santa Barbara County, California, Hornafius (1994) collected and analyzed whole-rock tuffs, ash, and bentonite samples for chemical composition using X-ray fluorescence (XRF). Hornafius (1994) limited his interpretation of the geochemical data to incompatible and rare earth elements because, as he discussed, the whole-rock samples included minerals, and the geochemical data were therefore “not as precise” as studies using only volcanic glass. The lack of precision of the whole-rock analyses means that the data Hornafius collected are not comparable to the glass shard analyses of this study.

Miocene tephrochronology that used only volcanic glass separates has been described most extensively in the Basin and Range of Nevada, Idaho, Utah, and California by Michael Perkins, Frank Brown, Barbara Nash, and colleagues (e.g., Perkins et al., 1995, 1998; Perkins and Nash, 2002; Nash and Perkins, 2012; Camilleri et al., 2017). These studies compiled representative major- and minor-element data on volcanic glass shards from 69 different tephra layers, with many beds sampled at multiple locations. Perkins et al. (1995, 1998) included a suite of trace-element concentrations measured by XRF for many of the tephra layers. In addition, they presented ⁴⁰Ar/³⁹Ar dates for a number of tephra beds. Tephra beds from these studies ranged in age from 16.3 Ma to 6 Ma and erupted from the Snake River Plain and the Southern Nevada volcanic field (Fig. 1). Several of the Southern Nevada volcanic field tephra beds geochemically characterized by Perkins, Nash, and colleagues were dated by the ⁴⁰Ar/³⁹Ar method by Sawyer et al. (1994) as well. The most southerly sedimentary sequence studied by Perkins et al. (1995, 1998) was located in the El Paso Basin of the northern Mojave Desert/southern Basin and Range (Fig. 1). The most northerly sites were in Oregon and Washington (Nash and Perkins, 2012; Camilleri et al., 2017).

Two tephrochronology studies in the Española Basin of north-central New Mexico identified a number of Miocene tephra beds, particularly from the Southern Nevada volcanic field (Koning et al., 2013; Slate et al., 2013). The Española Basin studies characterized tephra beds by major- and minor-element composition with age control by ⁴⁰Ar/³⁹Ar dating from other studies or paleontology (e.g., McIntosh and Quade, 1995; Izett and Obradovich, 2001; Koning et al., 2013).

Sarna-Wojcicki et al. (2021) compiled tephrochronology of the Mount Diablo region east of the San Francisco Bay that included a number of Miocene tephra layers from the Snake River Plain. Sarna-Wojcicki et al. (2021, personal commun.) has compiled data for the 12.08 Ma Ibex Hollow Tuff and related tephra layers that include major-element compositions of the glass shards from numerous locations along with trace-element data.

In this study, we present correlations of 38 tephra beds in the Monterey and Modelo Formations by tephrochronology and diatom biostratigraphy. These data provide numerical ages for sequences of both the Monterey and Modelo Formations by correlation to well-dated Miocene tephra layers found at various locations in western North America (Sawyer et al., 1994; Perkins et al., 1995, 1998; Perkins and Nash, 2002; Nash and Perkins, 2012; Koning et al., 2013; Slate et al., 2013; Sarna-Wojcicki et al., 2021). As predicted by Bramlette (1946), our results provide more accurate correlation of Monterey and Modelo Formation sedimentary sequences along with more precise numerical ages for the Monterey and Modelo Formations, and temporal correlation of marine sequences of the Monterey and Modelo Formations to nonmarine sequences in western North America. Our correlations improve the Miocene tephrostratigraphy in western North America and increase the known areal extent of air-fall tephra from silicic Miocene volcanic eruptions.

This study benefited from a number of geologists who collected tephra samples over a span of 50 yr (Supplemental Material Table S1¹). Tephrochronology samples were prepared using the methods described in Sarna-Wojcicki et al. (2005). Major-element (Si, Al, Fe, Ca, Na, and K) and minor-element (Mg, Mn, Ti) concentrations were determined by electron microprobe analysis at both the U.S. Geological Survey (USGS) and New Mexico Bureau of Geology and Mineral Resources. All results are reported as weight percent oxides with the FeO concentration converted to Fe₂O₃ (Table 2).

In addition to major- and minor-element concentrations, trace elements were measured by instrumental neutron activation analysis (INAA) and solution inductively coupled plasma mass spectrometry (S-ICP-MS). Samples analyzed by INAA followed the methods described in Sarna-Wojcicki et al. (1979, 2005). Pure volcanic glass samples from 200 mg to 500 mg in size were analyzed by three different INAA laboratories over a 20 yr period and are reported as values in parts per million (Table S2 [see ^{footnote 1}]).

Trace-element concentrations in glass shards were also measured by S-ICP-MS using 150–200 mg of pure, cleaned glass shards. Samples were submitted to the Peter Hooper GeoAnalytical Laboratory at Washington State University. Samples were dissolved in acid and analyzed with an Agilent 7700 ICP-MS for 14 rare earth elements as well as Ba, Th, Nb, Y, Hf, Ta, U, Pb, Rb, Cs, Sr, Sc, and Zr. Concentrations are reported in parts per million with analytical precision and accuracy often in the parts per billion range (Table S3 [see ^{footnote 1}]).

The reported electron microprobe data for major- and minor-element oxides were normalized to 100% and compared to over 7000 other analyses in the USGS tephrochronology database using the similarity coefficient of Borchardt et al. (1972). Similarity coefficients were calculated using SiO₂, Al₂O₃, Fe₂O₃, CaO, and TiO₂ concentrations (Table 2). The concentrations of 15 trace elements (Rb, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, Lu, Hf, Tb, and Gd) measured by both S-ICP-MS and INAA were compared using the similarity coefficient. Previous studies have shown that the S-ICP-MS and INAA data on volcanic glass samples are comparable (Knott et al., 2007). This comparability is demonstrated again herein by the close similarity (0.9667) of the GREFCO-1 samples analyzed by both INAA and S-ICP-MS (Tables S2 and S3). We used a minimum similarity coefficient of 0.93 as a guide for identifying correlative tephra layers after Sarna-Wojcicki et al. (1984).

Legacy ⁴⁰Ar/³⁹Ar ages were recalculated using the updated reference age of 28.20 ± 0.046 Ma for the Fish Canyon Tuff (Kuiper et al., 2008) and decay constants of Min et al. (2000). In some instances, Perkins et al. (1998) estimated the age of a tephra layer by a constant sedimentation rate between two other tephra layers dated by ⁴⁰Ar/³⁹Ar methods. We annotate sedimentation rate ages with “ca.” to denote that the age was estimated. Where the age of a tephra layer was determined by diatom biostratigraphy, we list the age range for the particular diatom zone or subzone from the North Pacific Diatom Zone framework (see Barron, this volume).

Samples analyzed for diatoms were processed using H₂O₂, HCl, and HNO₃ and mounted onto standard microscope slides. A minimum of 100 diatoms were identified on each slide. The diatom chronology is based on Barron (this volume) and Barron and Isaacs (2001).

The tephrochronology of the Monterey and Modelo Formations is presented stratigraphically (Fig. 2). The tephrochronology encompasses a time range from ca. 16 Ma to 7 Ma, which is most of the time range of the Monterey and Modelo Formations. Whenever possible, we correlated tephra layers from the Monterey and Modelo Formations to the Miocene tephrostratigraphy established elsewhere in the United States (Perkins et al., 1995, 1998; Perkins and Nash, 2002; Nash and Perkins, 2012; Slate et al., 2013; Koning et al., 2013; Camilleri et al., 2017; Sarna-Wojcicki et al., 2021). We used existing informal names for the tephra layers; however, for newly described, unnamed tephra layers, we used sample numbers to avoid introducing additional names (Sarna-Wojcicki, 2000). Our correlations were based on a combination of geochemical data, stratigraphic position relative to other tephra layers in the stratigraphic sequence, and diatom biostratigraphy.

In addition to the tephra layers from the Monterey and Modelo Formations, we also present geochemical data for a number of tephra layers from reference stratigraphic sequences found in other areas of western North America. For example, we present new trace-element INAA data for many of the samples that Perkins et al. (1995, 1998) analyzed using XRF. The INAA data provide concentrations for a number of additional, precisely measured elements to augment the original XRF data.

Two sequences of tephra layers were sampled in 1987 from the Monterey Formation (Morton et al., 1974) in the Laguna Hills area, San Juan Capistrano 7.5′ quadrangle, Orange County, California (Fig. 3). Although widely exposed throughout Orange County, study of the Monterey Formation in Orange County has focused on the section exposed along the bluffs of Newport Bay (e.g., Ingle and Barron, 1978). Our results include tephra layers from Newport Beach, Laguna Hills, and San Juan Capistrano regions.

The AV50.2 sequence in the Laguna Hills consists of five tephra layers. Samples AV50.2-TB (oldest) through TE (youngest) are from a 33-m-thick (110-ft-thick) section (Fig. 4). Sample AV50.2-TA was collected south of the measured section. Collected nearly 30 yr later, sample LY-OC-121615 was collected near sample AV50.2-TA (Fig. 4).

The AV12.1 section is 1.3 km to the northeast of the AV50.2 section. The 10-m-thick (35-ft-thick) AV12.1 section consists of three tephra layers (where AV12.1 is the lowest [oldest], and AV12.3 is the uppermost [youngest]). These tephra beds overlie diatomite beds containing a “nearly complete cranium of a walrus” (Hugh Wagner, 1987, written commun. to Sarna-Wojcicki). Samples from both sections were collected during construction, and the outcrops are now buried beneath engineered fill.

Diatoms from the Monterey Formation in the Laguna Hills area (Fig. 4) fall within the 15.7–14.9 Ma Denticulopsis lauta (D. lauta) subzone. Diatoms from sample LY-OC-121615 are very rare, making a biostratigraphic correlation impossible.

The major-element composition of sample AV50.2-TB, specifically the Fe₂O₃ and CaO concentrations (Table 2), does not match tephra erupted from either the Southern Nevada volcanic field or Snake River Plain (Fig. 5). Unfortunately, the composition of AV50.2-TB does not match with any of the 7000 analyses in the USGS tephrochronology database. The glass shard composition is similar to tephra produced by the Quien Sabe volcanic field of central California (Fig. 1); however, the Quien Sabe source did not produce large eruptions (Sarna-Wojcicki et al., 2021), so air-fall deposition where the Monterey basin was located in the early Miocene is unlikely. Another hypothesis is that the tephra erupted from a volcanic center in Mexico, where the Transverse Ranges were located during the Miocene (McQuarrie and Wernicke, 2005). On the basis of stratigraphic relations, AV50.2-TB is older than 15.65 Ma, but the source of the eruption that produced this tephra bed is unknown.

Based on the major-element composition (Table 2), the volcanic glass shards of AV50.2-TC are similar to tephra erupted from the Snake River Plain (Fig. 1). Unfortunately, the composition of AV50.2-TC does not match with any samples in the USGS tephrochronology database. Based on stratigraphic relations (Fig. 4), sample AV50.2-TC is older than 15.65 Ma, and the Owyhee-Humboldt volcanic field of the Snake River Plain is the likely source (Fig. 1).

Sample AV50.2-TD stratigraphically overlies AV50.2-TC (Fig. 4). The major- and minor-element glass shard composition of AV50.2-TD (Table 2) is similar to samples OC-227-271006-FGA, EB-ASH-WA4, and RDO-091106c from northern New Mexico (Koning et al., 2013; Slate et al., 2013). These tephra layers are all samples of the 15.65 ± 0.06 Ma #4 white ash bed (McIntosh and Quade, 1995; Koning et al., 2013). The 15.65 Ma date of the #4 white ash bed is consistent with the 15.7–14.9 Ma D. lauta subzone as well. Slate et al. (2013) inferred from the low Fe₂O₃ concentration that the #4 white ash bed erupted from the Southern Nevada volcanic field (Fig. 1) of Sawyer et al. (1994).

Based on the major- and minor-element composition (Table 2), the volcanic glass shards of sample AV50.2-TA are similar to the Paradise Valley ash bed of Perkins et al. (1998, sample buf94-618). Sample LY-OC-121615, collected 0.5 km to the south (Fig. 4), also correlates with the Paradise Valley ash bed. The trace-element concentrations confirm that samples LY-OC-121615 and AV50.2-TA are the same tephra layer (Tables S2 and S3 [see ^{footnote 1}]).

The Fe₂O₃ and CaO concentrations of the Paradise Valley ash bed are similar to tephra erupted from the Owyhee-Humboldt volcanic field of the Snake River Plain (Figs. 1 and 5). In addition, the rare earth element concentrations of the Paradise Valley ash bed (LY-OC-121515) are less than those in the older Huntington Creek 2 ash bed (RJR-SJV-1) and greater than those in the younger Roadcut ash bed (TD-SJH-0713), which shows the evolution of the Owhyee-Humboldt volcanic field magma over time (Figs. 6A).

The Paradise Valley ash bed is from the Buffalo Canyon family of tuffs found in the Lower Buffalo Canyon and Stewart Valley sections north of Aldrich Station, Nevada (Fig. 1; Perkins et al., 1998). Perkins et al. (1998) determined ⁴⁰Ar/³⁹Ar dates of 15.367 ± 0.032 Ma and 15.541 ± 0.050 Ma for the Paradise Valley ash. Perkins et al. (1998) preferred the 15.367 Ma date; however, sample AV50-TA was stratigraphically below both the 15.45 Ma Tuff of Redrock Valley and 15.34 Ma Virgin Valley 12 ash bed in the Laguna Hills. Based on these stratigraphic relations, we prefer the 15.541 ± 0.05 Ma date for the Paradise Valley ash bed.

Sample AV12.1-T1 is the stratigraphically lowest tephra layer in the AV12.1 section at Laguna Hills (Fig. 4). There is no glass shard composition comparable to sample AV12.1-T1 in the USGS tephrochronology database (Table 2). Based on the stratigraphic position of sample AV12.1-T1 below the 15.34 Ma Virgin Valley 12 tephra layer (Fig. 4) and the low Fe₂O₃ concentration of the glass shards (Table 2), we tentatively correlate sample AV12.1-T1 to the 15.45 ± 0.06 Ma Tuff of Redrock Valley from Sawyer et al. (1994), which was a major eruption from the Southern Nevada volcanic field (Fig. 1).

Samples AV50.2-TE and AV12.1-T2 from the Monterey Formation in the Laguna Hills (Fig. 3) correlate with each other and to the 15.34 ± 0.03 Ma Virgin Valley 12 ash bed (sample vvy93-12) of Perkins et al. (1998) (Table 2). The glass shard composition of these ash beds also correlates with sample 1-36-39J from the nonmarine upper Middlegate Formation of central Nevada (Stewart et al., 1999a). The Virgin Valley 12 ash bed is similar in composition to the Lower Buffalo Canyon family of tephra layers (Perkins et al., 1998) that likely erupted from the Owyhee-Humboldt volcanic field on the Snake River Plain (Fig. 1; Perkins and Nash, 2002).

Sample AV12.1-T3 is the stratigraphically highest tephra bed in the Laguna Hills sections (Fig. 4). Based on the major-element composition (Table 2), the volcanic glass shards of AV12.1-T3 are similar to tephra erupted from the Snake River Plain (Figs. 1 and 5). Unfortunately, the composition of AV12.1-T3 is not similar to any specific sample in the USGS tephrochronology database. The best age estimate is that sample AV12.1-T3 is younger than the underlying 15.34 Ma Virgin Valley 12 ash bed (Fig. 2).

A set of samples (TD-OC-2015, TD-SJH-071315-1, TD-SJH-071415-1) collected from the Monterey Formation (Morton and Miller, 2006) at a construction site in the San Joaquin Hills of Newport Beach, California (Figs. 3 and 6E), was collected from the same tephra bed, and these are hereafter are called the TD-OC samples. Diatoms observed in the TD-OC samples are from the 14.8–13.2 Ma Denticulopsis hyalina (D. hyalina) zone. The absence of Actinoptychus minutus and Thalassiosira grunowii indicates that the TD-OC samples are most likely older than 13.6 Ma.

Within this 14.8–13.6 Ma diatom age range, the glass shard composition of the gray, fine-grained TD-OC tephra beds (Figs. 7A) is most similar to the ca. 13.90 ± 0.25 Ma Roadcut ash bed (sample htc93-447; Table 2) of Perkins et al. (1998). This correlation is relatively poor, due to the differences in TiO₂ (Table 2), but still within acceptable range overall. The trace-element data show that the TD-SJH-0713 and TD-SJH-0714 tephra beds (Table S3) are distinct from other ash beds in the Monterey Formation of Orange County (Figs. 6A).

The TD-OC samples correlate well with sample Jacomita-1 of Slate et al. (2013) from the Española Basin of northern New Mexico (Fig. 1; Table 2). The Jacomita-1 tephra bed is stratigraphically below the 13.7 Ma Pojoaque TW ash of Izett and Obradovich (2001) and has a glass shard composition consistent with Snake River Plain eruptions. Thus, based on the diatom assemblage and glass shard composition, we correlate the TD-OC samples and Jacomita-1 to the 13.90 Ma Roadcut ash bed of Perkins et al. (1998).

Based on the major- and minor-element composition of the glass shards (Table 2), sample DPB-10 (for nearby Dos Pueblos Beach) in the Naples Beach section of the Monterey Formation, Santa Barbara County, California (Fig. 1), is correlative with sample Jacomita-9 from Slate et al. (2013). The diatom biostratigraphy places DPB-10 in the 14.8–13.2 Ma D. hyalina zone (Fig. 8). Based on the relatively low Fe₂O₃ and CaO concentrations, Slate et al. (2013) inferred that Jacomita-9 erupted from the Southern Nevada volcanic field (Fig. 4).

There are no radiometric dates on the Jacomita-9 tuff (Slate et al., 2013), but the regional vertebrate paleontology provides an estimated age of ca. 13–12 Ma (D. Koning, 2020, personal commun.). In the Española basin, New Mexico, Jacomita-9 overlies the 13.7 Ma Pojoaque TW ash of Izett and Obradovich (2001). Based on the diatom stratigraphy, we suggest an age of ca. 13.3 Ma for the Jacomita-9 tuff. Our ca. 13.3 Ma age for Jacomita-9 and the glass shard composition are consistent with the age of the Crater Flat Group tuffs, which were major eruptions from the Southern Nevada volcanic field (Sawyer et al., 1994).

Based on major- and minor-element composition of the glass shards (Table 2), sample DPB-11 in the Naples Beach section of the Monterey Formation, Santa Barbara County, California (Fig. 1), is correlative with the Jacomita-12 tuff of Slate et al. (2013) in the Española basin, New Mexico. The diatom biostratigraphy places DPB-11 in the 14.8–13.2 Ma D. hyalina zone (Fig. 8). The Jacomita-12 tuff overlies the Jacomita-9 tuff, so Jacomita-12 is also younger than the 13.7 Ma Pojoaque TW ash of Izett and Obradovich (2001). There are no radiometric dates on the Jacomita-12 tuff; however, we prefer an age of ca. 13.2 Ma based on the diatom biostratigraphy and stratigraphic position relative to the Jacomita-9 tuff. From the glass shard composition, Slate et al. (2013) inferred that Jacomita-12 erupted from the Southern Nevada volcanic field as part of the (13.25 Ma) Crater Flat Group tuffs, which were erupted at about that time.

Sample DPB-12 from the Monterey Formation at Naples Beach (Table 2), Santa Barbara County, California (Fig. 8), is correlated with the 12.08 ± 0.03 Ma Ibex Hollow Tuff of Perkins et al. (1998, sample TC89-21A) and sample BE-16 of Eastwood (1969; BE is for Bill Eastwood). The diatom biostratigraphy at Naples Beach places DPB-12 in the 12.3–9.3 Ma Denticulopsis hustedtii–D. lauta subzone c/d. The Ibex Hollow Tuff is a key marker bed within the Naples Beach section (Figs. 7D).

Based on glass shard major- and minor-element composition (Table 2), sample DPB-13 from the Naples Beach section of the Monterey Formation in Santa Barbara County, California (Figs. 7D), is correlated with the 11.95 ± 0.04 Ma Ibex Peak 8 tephra bed of Perkins et al. (1998, sample TC89-24) and sample JI-SM-1. The diatom biostratigraphy at Naples Beach places sample DPB-13 in the 12.3–9.3 Ma D. hustedtii–D. lauta subzone c/d. Perkins et al. (1998) identified the Ibex Peak 8 ash in the El Paso Basin, northern Mojave Desert, California. The likely source of the Ibex Peak 8 tephra is the Bruneau-Jarbidge volcanic center on the Snake River Plain (Fig. 1).

Samples TC89-24 and JI-SM-1 correlate well with each other based on trace-element composition as well (Table S2). Sample JI-SM-1 is from the Santa Margarita Formation, which is a shallow-marine deposit that conformably overlies the Monterey Formation in the La Panza Range of central California (Fig. 1; Addicott et al., 1978).

Although mapped as a lithologically separate unit, the benthic foraminifers in the Santa Margarita Formation are consistent with the middle to late Miocene Mohnian Stage (Kleinpell, 1938; Addicott et al., 1978). Many Monterey Formation sequences, such as that at Naples Beach, are Mohnian Stage (Bramlette, 1946). Thus, although mapped as a separate lithologic unit, the Santa Margarita Formation of the La Panza Range is temporally equivalent to the Monterey Formation, as demonstrated by the presence of the Ibex Peak 8 tephra layer in both units (Fig. 2).

The diatom biostratigraphy at Naples Beach places sample DPB-14 in the 12.3–9.3 Ma D. hustedtii–D. lauta subzone c/d. The major- and minor-element composition (Table 2) of sample DPB-14 is similar to sample i88-48 (Ibapah Badlands section, Nevada), which Perkins et al. (1998) inferred was the 11.87 ± 0.05 Ma Rainier Mesa Tuff from the Southern Nevada volcanic field (Sawyer et al., 1994) based on the relatively low concentrations of Fe₂O₃ (0.57 wt%) and CaO (0.35 wt%). The stratigraphic position of sample DPB-14 above both the Ibex Hollow Tuff and Ibex Peak 8 tephra layer supports the correlation to the Rainier Mesa Tuff (Figs. 7D).

Eastwood (1969) described sample BE-62 at Aldrich Station, Nevada (Fig. 1), as a 7.6-m-thick, white tuff with abundant biotite overlying the 12.08 Ma Ibex Hollow Tuff (Fig. 9). Based on the major-, minor-, and trace-element composition of the volcanic glass shards (Tables 2 and S2) along with the stratigraphic position relative to the Ibex Hollow Tuff, we correlate sample BE-16 and sample DPB-14 with the Rainier Mesa Tuff.

The Grefco diatomaceous earth quarry, or Palos Colorados mine (Jenkins, 1982), is located 11 km southwest of Lompoc, Santa Barbara County, California (Fig. 1). The quarry exposes the upper Monterey and lower Sisquoc Formations, where, in 1985, samples of diatomite and tephra layers were collected from the Monterey Formation. Sample GREFCO-1 is the stratigraphically lowest tephra layer collected at the Grefco quarry (Fig. 2).

Using the major- and minor-element composition, Sarna-Wojcicki et al. (2021) correlated sample GREFCO-1 with the ca. 11.31 ± 0.10 Ma Cougar Point Tuff XI (sample TC89-27c) from Trapper Creek, Idaho, sample BE-26 from Aldrich Station, Nevada, and sample ALVES-1 from the San Francisco Bay area, California.

The diatoms at the Grefco quarry are from the 12.3–10 Ma upper D. hustedtii–D. lauta subzone c. The major- and minor-element composition shows that glass shard samples GREFCO-1, ALVES-1, BE-26, and TC89–027 are of similar composition (Table 2). Here, we correlate sample GREFCO-1 to samples JRK-SB-1 and LY-OC-091316 from the Monterey Formation, Orange County, California (Fig. 3). Moreover, we also show by INAA that the trace-element compositions of samples TC89-27, GREFCO-1, BE-26, and ALVES-1 are similar (Tables S2 and S3). Trace-element compositions of samples GREFCO-1, JRK-SB-1, and LY-OC-091316 are also similar, according to S-ICP-MS data. These data show that TC89–27, GREFCO-1, BE-26, ALVES-1, JRK-SB-1, and LY-OC-091316 are all correlative with ca. 11.31 ± 0.10 Ma Cougar Point Tuff XI. Cougar Point Tuff XI is a supereruption from the Bruneau-Jarbidge volcanic field of the Snake River Plain (Perkins and Nash, 2002; Knott et al., 2020), an assignment confirmed by its rare earth element composition (Figs. 5B).

Sample GREFCO-2 stratigraphically overlies GREFCO-1 at the Grefco quarry (Fig. 2). The glass shard composition of GREFCO-2 is the same within error of analysis as the ca. 11.19 ± 0.10 Ma Cougar Point Tuff XII (sample TC89-28a; Table 2; Tables S2 and S3) from Trapper Creek, Idaho (Perkins et al., 1995). This correlation is consistent with the diatom data, which places sample GREFCO-2 within the 12.3–10 Ma D. hustedtii–D. lauta subzone c. The major-, minor-, and trace-element composition of GREFCO-2 is also consistent with sample TORO-1 (Fig. 2) from the Monterey Formation along Toro Road near the Monterey Formation type locality in Monterey County, California (Fig. 1). Cougar Point Tuff XII erupted from the Bruneau-Jarbidge volcanic field of the Snake River Plain (Perkins and Nash, 2002).

Sample GREFCO-3 stratigraphically overlies GREFCO-2 in the Grefco quarry (Fig. 2). Based on the glass shard composition, we assign GREFCO-3 to the 11.02 ± 0.03 Ma Cougar Point Tuff XIII (sample tc89-31a; Table 2; Table S2) from the Trapper Creek, Idaho, section (Perkins et al., 1995, 1998). Cougar Point Tuff XIII is a supereruption from the Bruneau-Jarbidge volcanic field of the Snake River Plain (Perkins and Nash, 2002; Knott et al., 2020). Correlation of GREFCO-3 to Cougar Point Tuff XIII is consistent with the 12.3–10 Ma D. hustedtii–D. lauta subzone c determined from the diatom assemblage.

We correlate GREFCO-3 (Cougar Point Tuff XIII) by major-, minor-, and trace-element composition of the glass shards to several other samples in the USGS tephrochronology database (Fig. 1). In California, GREFCO-3 is similar to samples TORO-2 (Figs. 7A) and TORO-3 from the Toro Road sequence, Monterey County, California (Figs. 1 and 2; Tables 2 and Table S2). Samples TORO-2 and TORO-3 are from the same tephra bed, but at different levels above the base. Sample M93FI132, from Fort Irwin, Mojave Desert, California (Fig. 1), is also similar to GREFCO-3. In Nevada, GREFCO-3 is similar to samples BE-210, BE-268, and JS-92-64 (Figs. 2 and 5). Samples BE-210 and BE-268 are from the nonmarine, clastic deposits of the Aldrich Station and Coal Valley Formations at Aldrich Station (Fig. 9; Axelrod, 1956; Eastwood, 1969; Gilbert and Reynolds, 1973). Sample JS-92-64 is from poorly consolidated, fluvial, nonmarine, Miocene deposits of Cobble Cuesta, Nevada (Stewart et al., 1999b).

Sample GREFCO-4 stratigraphically overlies GREFCO-3 in the Grefco quarry (Fig. 1). The glass shard composition of GREFCO-4 (Table 2; Table S2) matches the ca. 10.36 ± 0.10 Ma unnamed ash bed (sample TC90-17) from the Trapper Creek, Idaho, section (Perkins et al., 1995). Sample TC90-17 overlies (younger than) the 10.94 Ma Cougar Point Tuff XI and underlies the (older than) 10.26 Ma tuff of Wooden Shoe Butte in the Trapper Creek section (Perkins et al., 1995). The diatom biostratigraphy from the Grefco quarry places sample GREFCO-4 within the upper part of the 12.3–10 Ma D. hustedtii–D. lauta subzone c, which is consistent with the Trapper Creek tephrostratigraphy. The likely source of the GREFCO-4 ash bed is the Twin Falls volcanic field of the Snake River Plain, which was active from 10.5 Ma to ca. 10 Ma (Perkins and Nash, 2002).

The glass shard composition shows that GREFCO-4 also correlates with sample BE-95 (Table 2; Table S2) from Aldrich Station (Fig. 9) and sample DPB-15 from the Naples Beach section (Fig. 8). Based on the combination of diatom assemblage (D. hustedtii–D. lauta subzone c), and major- and minor-element composition (Table 2), we also correlate sample JB-PD-23 from the Monterey Formation near Point Dume, Malibu, California (Fig. 1), to the GREFCO-4 ash bed.

Sample DPB-16 from the Naples Beach section (Fig. 8) of the Monterey Formation, Santa Barbara County, California (Fig. 1), correlates by major- and minor-element composition (Table 2) with the 10.26 ± 0.03 Ma tuff of Wooden Shoe Butte (sample TC90-20 of Perkins et al., 1995) in the Trapper Creek section, Idaho (Fig. 1). The diatom biostratigraphy places sample DBP-16 within the upper part of the 12.3–10 Ma D. hustedtii–D. lauta subzone c. The composition of the DPB-16 ash bed is consistent with tephra erupted from the Twin Falls volcanic field on the Snake River Plain (Fig. 5; Perkins and Nash, 2002).

Sarna-Wojcicki et al. (2021) used the major- and minor-element composition of glass shards to correlate sample MOD-6 from the Modelo Formation, Balcom Canyon (Figs. 10 and 11), Ventura County, California (Fig. 1), with the McGuire Peak upper ash bed (sample MGP-16) in the Briones Formation and an ash bed in the Purisima Formation (sample PU-17). Sarna-Wojcicki et al. (2021) stated that the McGuire Peak upper ash bed erupted from either the Twin Falls (12–6.5 Ma) or Picabo (10.3 Ma) calderas of the Snake River Plain (Perkins and Nash, 2002).

Here, we present the trace-element composition of glass shards (Table S2) from the McGuire Peak upper ash (sample MOD-6). Sample MOD-6 is near the hinge of the South Mountain anticline (Fig. 9) and within the 9.3–8.6 Ma D. hustedtii diatom subzone (Fig. 11). At Balcom Canyon, the McGuire Peak upper ash (sample MOD-6) is the oldest tephra layer on the south limb of the asymmetric South Mountain anticline (Fig. 10), and it underlies the 9.2 Ma Section 26 ash bed (Fig. 11). Based on the stratigraphic position below the 9.2 Ma Section 26 ash bed and the diatom biostratigraphy, we estimate the McGuire Peak upper ash bed is ca. 9.3 Ma.

Perkins et al. (1998) presented the major-, minor-, and trace-element (XRF) composition of glass shards from the ca. 9.70 ± 0.20 Ma Section 26 tephra bed (sample TC90-22). Here, we show that sample MOD-11 (MOD-27 is a replicate sample) from the Modelo Formation at Balcom Canyon in Ventura County, California (Fig. 1), is correlative by geochemical composition of the glass shards with the Section 26 tephra layer of Perkins et al. (1998) (Table 2). The major- and minor-element correlation is marginal due to differences in Fe₂O₃ and TiO₂ concentrations (Table 2); however, the trace-element correlation is strong (Table S2). The glass shard composition of the Section 26 tephra layer is consistent with an eruption from the Picabo volcanic center of the Snake River Plain (Fig. 1). Sample MOD-11 is within the 9.3–8.6 Ma D. hustedtii subzone.

MOD-11 is on the south limb, near the hinge, of the South Mountain anticline (Fig. 10), but stratigraphically above (younger than) the ca. 9.3 Ma McGuire Peak upper ash bed (Fig. 11). Perkins et al. (1998) estimated an age of ca. 9.70 ± 0.20 Ma for the Section 26 ash based on a constant sedimentation rate between a 9.79 Ma ash bed and an 8.84 Ma ash-flow tuff. The 9.3–8.6 Ma D. hustedtii subzone establishes a maximum age of 9.3 Ma for the Section 26 ash and shows that the ca. 9.7 Ma age estimated by Perkins et al. (1998) is too old. At this time, we suggest an age of ca. 9.2 Ma for the Section 26 tephra layer. A ca. 9.2 Ma age is consistent with the stratigraphic position of the Section 26 ash at both Trapper Creek and Balcom Canyon as well as with the diatom biostratigraphy.

The glass shard composition of sample DPB-22 from the Monterey Formation at Naples Beach (Fig. 8), Santa Barbara County, California (Fig. 1), is correlative with MOD-2 (MOD-21 is a replicate sample.) from the Modelo Formation at Balcom Canyon, Ventura County, California (Table 2; Tables S2 and S3 [see ^{footnote 1}]). Because the samples at Naples Beach and Balcom Canyon were collected over 30 years ago, another sample was collected from Balcom Canyon (sample PRM-BC-1; Figs. 7B), which confirmed the earlier data (Tables 2 and S3).

The diatom assemblage for sample MOD-2 is within the 9.3–8.6 Ma D. hustedtii subzone (Figs. 2 and 11). There are no major- and minor-element data for the glass shards from the Tuff of McMullen Creek from the Trapper Creek, Idaho, section. There are, however, trace-element data for the glass shards of the Tuff of McMullen Creek (sample TC90-24). Using the trace-element composition of the glass shards (Tables S2 and S3), DBP-22, MOD-2, and PRM-BC-1 correlate with the Tuff of McMullen Creek (sample TC90-24). The rare earth element composition of the Tuff of McMullen Creek (Figs. 5B) is consistent with a supereruption from the Twin Falls volcanic center of the Snake River Plain (Fig. 1). Knott et al. (2020) determined a zircon ²⁰⁶Pb/²³⁸U date of 8.989 ± 0.84 Ma for the Tuff of McMullen Creek, which is consistent with the diatom assemblage.

Sample MOD-3 (MOD-19 is a replicate sample) from the Modelo Formation at Balcom Canyon (Fig. 10) in Ventura County, California (Fig. 1), is correlative with samples KF-OC-1, KF-OC-2 (Figs. 7C), ART-OC-1, and ART-OC-2 from the Monterey Formation in Orange County, California (Fig. 3; Tables 2 and S3). Samples KF-OC-1 and KF-OC-2 are white tephra layers interbedded with brown to green mudstone and sandstone of the Monterey Formation along the north side of San Juan Creek, San Juan Capistrano, California (Figs. 7B). Diatoms found in sample KF-OC-2 are from the 9.3–8.6 Ma D. hustedtii subzone. The age of MOD-3/19 is narrowed to 8.989–8.6 Ma because MOD-3/19 overlies (younger than) the 8.989 Ma Tuff of McMullen Creek (sample MOD-2 above) at Balcom Canyon.

The Fe₂O₃ (1.22–1.31 wt%) and CaO (0.73–0.81 wt%) concentrations (Table 2) for the MOD-3/19 tephra layers are similar to the range of Fe₂O₃ and CaO for eruptions from the Southern Nevada volcanic field. However, the TiO₂ concentration of MOD-3/19 tephra layers (0.20–0.28 wt%) is twice the TiO₂ concentration of other Southern Nevada volcanic field tephra layers (0.08–0.14 wt%). In addition, the rare earth element composition has a shallower Eu anomaly, which is inconsistent with tephra layers from either the Snake River Plain or Southern Nevada volcanic field (Figs. 6B; Tables S2 and S3). The geochemistry of the MOD-3/19 tephra layers is comparable to tephra beds erupted from either the Cascade Range or Sonoma volcanic field (Sarna-Wojcicki et al., 1979). However, tephra layers older than 6 Ma are known only from the Sonoma volcanic field (McLaughlin et al., 2012). Another possible source of the MOD-3/19 tephra layers is the silicic proto-Cascade volcanics associated with the Mehrten volcanic rocks that erupted near the central-northern Sierra Nevada crest (Busby et al., 2013). Another hypothesis is that the source of the MOD-3/19 tephra layers was in Mexico, where the Transverse Ranges were during the Miocene. At this time, we cannot determine which of these possible sources produced the MOD-3/19 tephra layers, so we must list the source as unknown.

Sample MOD-1 (MOD-22 is a replicate sample) from the Modelo Formation at Balcom Canyon, Ventura County, California (Fig. 1), is correlative with sample DPB-24 from the Naples Beach section of the Monterey Formation, Santa Barbara County, California (Fig. 2). This correlation is based on major-, minor- (Table 2), and trace-element concentrations (Table S2). Because the samples at Naples Beach and Balcom Canyon were collected over 30 years ago, another sample was collected from Balcom Canyon (sample PRM-BC-2; Figs. 7F), which confirmed the earlier data (Table 2).

Diatom biostratigraphy indicates that both MOD-1 and DPB-24 are within the 9.3–8.6 Ma D. hustedtii subzone (Fig. 2). Although the Fe₂O₃ concentration (2.98–3.13 wt%) in the glass shards is relatively high, the trace-element concentrations (Table S3) and rare earth element pattern for MOD-1/22 (Figs. 6C) are consistent with tephra layers that erupted from the Picabo volcanic center of the Snake River Plain (Fig. 1).

Sample MOD-24 from the Modelo Formation at Balcom Canyon, Ventura County, California (Fig. 1), on the north limb of the South Mountain anticline, is correlative with sample MOD-13 from the south limb of the South Mountain anticline (Figs. 10 and 11). This is the only tephra layer found on both limbs of the South Mountain anticline.

The MOD-24 tephra layer is stratigraphically below (older than) the ca. 7.9 Ma Rush Valley ash bed and stratigraphically above the 8.989 Ma Tuff of McMullen Creek (Fig. 11). On the south limb of the South Mountain anticline, MOD-13 is stratigraphically above (younger than) the ca. 9.2 Ma Section 26 ash bed (sample MOD-11 above). No diatom data are available for the MOD-24 tephra layer, and so the age of the MOD-24 tephra layer is ca. 8.9–7.9 Ma.

The MOD-24 tephra layer is an andesite (Table 2) with unusually high Fe₂O₃ (6.76 wt%), CaO (5.14 wt%), and TiO₂ (1.28 wt%) compared to other samples in this study (not plotted on Fig. 5). The sample MOD-24 tephra layer does not correlate well with any other tephra layers in the USGS tephrochronology database, and the eruptive source is unknown.

The major- and minor-element composition of glass shards from sample MOD-25 from the Modelo Formation at Balcom Canyon, California (Fig. 1), is consistent with the ca. 7.90 ± 0.50 Ma Rush Valley ash bed (rv88–12) of Perkins et al. (1998) from Rush Valley, Utah (Fig. 1; Table 2). Samples MOD-25 and rv88-12 also match sample RJB-IQ-2 collected from the Monterey Formation at the Imerys Quarry (diatomaceous earth) near Lompoc, California (Figs. 2 and 5). For the purposes of this study, the Imerys Quarry is in the same location as the Grefco quarry (Fig. 1). Also, we tentatively correlate MOD-25 with sample EL-59-FV of Reheis et al. (2002) from Drumm Summit, Fairview Valley, west-central Nevada; those authors speculated that sample EL-59-FV was Pliocene(?) in age. Without better age control on EL-59-FV, our correlation is tentative.

Sample MOD-25 is from the north limb of the South Mountain anticline (Fig. 10). The diatom assemblages in sample RJB-IQ-2 are consistent with the 7.9–7.8 Ma uppermost subzone b of the Thalassiosira antiqua zone (Fig. 11). The glass shard and rare earth element composition (e.g., high Ba and La) of the Rush Valley ash bed is consistent with tephra erupted from the Picabo volcanic center on the Snake River Plain (Figs. 6C; Table S3).

The glass shard composition of sample MOD-5 (MOD-26 is a replicate sample) from the Modelo Formation at Balcom Canyon, Ventura County, California (Fig. 1), is correlated with sample DPB-25 from the Naples Beach section of the Monterey Formation in Santa Barbara County, California (Table 2; Table S2). Samples MOD-5 and DPB-25 are also similar to sample RJB-IQ-1 from the Monterey Formation at the Imerys Quarry, near the Grefco quarry, Lompoc, California (Fig. 2). These correlations are based on major- and minor-element concentrations (Table 2) as well as INAA (Table S2) and S-ICP-MS (Table S3) data.

The diatom assemblage for RJB-IQ-1 is consistent with the 7.6–7.0 Ma Nitzschia reinholdii zone. The glass shard composition of the MOD-26, DPB-25, and RJB-IQ-1 tephra layer (Figs. 6C; Tables S2 and S3) is consistent with tephra erupted from the Picabo volcanic center on the Snake River Plain (Fig. 1).

Geochemical data for eight additional tephra beds from the Naples Beach section of the Monterey Formation (Table S4 [see ^{footnote 1}]), Santa Barbara County, California, and 11 additional tephra layers from the Balcom Canyon section of the Modelo Formation (Table S5), Ventura County, California, are presented in Table 2. Although presently we are unable to correlate these tephra layers to any other tephra layers in the USGS tephrochronology database, these data are included here to assist future studies.

In this section, we present the geochemical data and stratigraphic context for tephra layers not found in the Monterey or Modelo Formations, but present in Miocene-age marine and nonmarine sequences in California and Nevada. These tephra layers are, nevertheless, important in the development of a regional tephrochronostratigraphic framework and may prove valuable to future tephrochronology studies of the Monterey Formation and other stratigraphic units.

Sample RJR-SJV-1 is from the Round Mountain Silt, ~14 km ESE from Bakersfield, California (Table S1; LSA Associates, 2009), The Round Mountain Silt is stratigraphically equivalent to the lower Monterey Formation (Bartow, 1981, 1991). Sample RJR-SJB-1 was collected near the base of the Round Mountain Silt, stratigraphically below the well-known Sharktooth Hill bone bed (LSA Associates, 2009; Pyenson et al., 2009). The diatom assemblage from the Round Mountain Silt is from the 16–15 Ma D. lauta subzone a (Barron, 1986).

The glass shard composition of sample RJR-SJV-1 tephra (Tables 2 and S3; Figs. 6A) is consistent with eruptions from the Owyhee-Humboldt volcanic center of the Snake River Plain (Fig. 1; Perkins and Nash, 2002). Based on the major- and minor-element composition, sample RJR-SJV-1 correlates with the Huntington Creek 2 ash bed (sample buf94-617) from the Lower Buffalo Canyon family of tuffs (Perkins et al., 1998) near Aldrich Station (Fig. 1). In addition, we correlate sample RJR-SJV-1 with sample OC-167-280904-djk from the Española basin (Table 2; Koning et al., 2013; Slate et al., 2013). Stewart et al. (1981) correlated samples 1-12-5j and 1-12-9j intercalated with nonmarine sediments to the Huntington Creek 2 ash bed as well.

There are no direct dates on the Huntington Creek 2 ash bed. In the Española basin, sample OC-167-280904-djk is stratigraphically between the 15.7 Ma White Ash #3 and 15.8 Ma White Ash #2 in the basal Rio Del Oso section (McIntosh and Quade, 1995; Izett and Obradovich, 2001; Koning et al., 2013). Using a constant sedimentation rate, the estimated age of OC-167-280904-djk and the Huntington Creek 2 ash bed is ca. 15.77 Ma.

Previous trace-element concentrations from the Huntington Creek 2 ash bed were limited to XRF (Perkins et al., 1998). Our S-ICP-MS data provide a more comprehensive and accurate measure of the trace-element composition of the Huntington Creek 2 ash bed (Table S3; Figs. 5A).

Based on the major- and minor-element composition of the glass shards and stratigraphic position below the 12.08 Ma Ibex Hollow Tuff, sample BE-15 (Table 2) from the Aldrich Station Formation (Fig. 9) is correlated with the ca. 12.20 ± 0.1 Ma Aldrich Hill 2 ash bed (sample i88-88 of Perkins et al., 1998). The Aldrich Hill 2 tephra bed is 15 m thick at Aldrich Station, Nevada (Eastwood, 1969), and the Fe₂O₃ and CaO concentrations (Table 2; Fig. 4) are consistent with eruptions from the 12.5–11.5 Ma Timber Hill–Oasis caldera complex of the Southern Nevada volcanic field, 250 km to the southeast (Sawyer et al., 1994). Sarna-Wojcicki et al. (2021) correlated an ash bed in the Cierbo Formation (sample MRT-3A), east of San Francisco, with the Aldrich Hill 2 tephra layer. We provide the trace-element concentrations from the glass shards of the Aldrich Hill 2 tephra layer (Table S2), a widespread stratigraphic unit.

Based on glass shard major- and trace-element composition (Table 2), sample BE-23 from the Aldrich Station Formation (Fig. 9) is correlated with the ca. 11.80 ± 0.10 Ma Cougar Point Tuff IX (sample TC89-25; Perkins et al., 1998). The trace-element composition (Table S2) of Cougar Point Tuff IX is consistent with eruptions from the Bruneau-Jarbidge volcanic center on the Snake River Plain (Perkins and Nash, 2002). Although not found in the Monterey or Modelo Formations, Perkins et al. (1998) found the Cougar Point Tuff IX across Nevada and as far southwest as the El Paso Basin of the Mojave Desert, California (Fig. 1).

Based on major- and minor-element glass shard composition (Table 2) and stratigraphic position above the Cougar Point Tuff IX, we correlate sample BE-24 from the Aldrich Station Formation with the 11.72 ± 0.05 Ma Ammonia Tanks Tuff (sample sv-93-282 of Perkins et al., 1998; Sawyer et al., 1994). The glass shard composition is consistent with eruption from Southern Nevada volcanic field (Table 2; Table S2; Fig. 5). Based on ignimbrite volume in the Southern Nevada volcanic field, Mason et al. (2004) classified the Ammonia Tanks Tuff as a supereruption. Perkins et al. (1998) found the Ammonia Tanks Tuff across Nevada and as far south as the El Paso Basin, California (Fig. 1).

Based on glass shard composition (Tables 2 and S2), sample BE-92 from the Coal Valley Formation, Nevada (Fig. 9), is correlated with sample A-7 of Huber (1981). The glass shard compositions of samples BE-92 and A-7 are trachydacite (Table 2; Fig. 6), which differs from the rhyolite composition of many of the other tephra beds in this study. Sample A-7 is a pumice pebble from Miocene channel deposits of the San Joaquin River beneath the 10.4–9.5 Ma Trachyandesite of Kennedy Table near Friant Dam in the western Sierra Nevada (Fig. 1; Huber, 1981).

Huber (1981) proposed that the A-7 pumice was a distal clast from the 12.2–11.4 Ma welded tuff described by Gilbert et al. (1968) east of Mono Lake. Huber (1981) determined a K/Ar date of 11.3 ± 0.2 Ma on plagioclase from the A-7 pumice, but suggested that 11.3 Ma was a minimum age. Based on the tephrostratigraphy of Eastwood (1969), sample BE-92 is between 11.51 Ma and 11.31 Ma, which is consistent with the ages determined by Gilbert et al. (1968) and Huber (1981). For the present, we use an age of ca. 11.3 Ma for BE-92 and A-7.

Based on glass shard major- and minor-element composition (Table 2) and stratigraphic position between the 11.72 Ma Ammonia Tanks Tuff and 11.08 Ma Cougar Point Tuff XIII, sample BE-25 from the Aldrich Station Formation (Fig. 9) is correlated with the ca. 11.51 ± 0.10 Ma Coal Valley ash bed (sample sv-93-282 of Perkins et al., 1998). In this study, we provide the trace-element data for the Coal Valley ash glass shards (Table S2). The glass shard composition (Fig. 5) and age are consistent with eruptions from the Bruneau-Jarbidge volcanic center on the Snake River Plain.

Based on glass shard composition (Tables 2 and S2), BE-28 from the Coal Valley Formation, Nevada (Eastwood, 1969), likely erupted from the Southern Nevada volcanic field (Fig. 5). Based on the stratigraphy of Eastwood (1969), sample BE-28 is between 11.31 Ma and 10.94 Ma (Fig. 9), which coincides with the Fortymile Canyon assemblage of Sawyer et al. (1994) at the Southern Nevada volcanic field. There are no good correlations for sample BE-28 in the USGS tephrochronology database, and BE-28 is not similar to any tephra beds characterized by Perkins et al. (1995, 1998).

The glass shard composition (Tables 2 and S2) provides no good correlations for sample BE-96 from the Coal Valley Formation, Nevada (Fig. 9), in the USGS tephrochronology database, nor is sample BE-96 similar in composition to any tephra beds characterized by Perkins et al. (1995, 1998). Based on the stratigraphy of Eastwood (1969), sample BE-96 is between 11.31 Ma and 10.94 Ma. The glass shard composition of sample BE-96 is dacite with relatively high Fe₂O₃, CaO, and TiO₂ concentrations (Fig. 5), and the source of sample BE-96 is unknown. This glass shard composition is consistent with tephra from the Quien Sabe volcanics of central California (Fig. 1); however, the Quien Sabe volcanic center did not produce large eruptions (Sarna-Wojcicki et al., 2021). Thus, the source of the BE-96 tephra layer is unknown.

There are no good matches to sample BE-222 from the Coal Valley Formation, Nevada (Fig. 9), in the USGS tephrochronology database (Tables 2 and S2). The Fe₂O₃ and CaO concentrations are similar to eruptions from the Southern Nevada volcanic field (Fig. 5). Based on the stratigraphy of Eastwood (1969), sample BE-222 is between 10.94 Ma and 10.36 Ma (Fig. 9), which coincides with small-volume eruptions at the Southern Nevada volcanic field (Sawyer, et al., 1994).

The trace-element composition of sample BE-18 (Table S2) is correlated with sample TC90-19 (from Trapper Creek, 1D). Here, we present major- and minor-element concentrations for glass shards from BE-18; however, there are no major- or minor-element data for TC90-19. Sample TC90-19 is immediately below (older than) the 10.26 Ma tuff of Wooden Shoe Butte (sample TC90-19), which provides a minimum age for the BE-18 tephra layer. The glass shard composition shows that sample BE-18 is not the tuff of Wooden Shoe Butte. At Aldrich Station, BE-18 is younger than the 10.34 Ma GREFCO-4 tephra bed (Fig. 9). Thus, the BE-18 tephra layer is between 10.34 Ma and 10.26 Ma. The glass shard composition of sample BE-18 is consistent with eruption from the Twin Falls volcanic center of the Snake River Plain (Fig. 1).

Based on the major-element glass shard composition (Table 2), sample BE-252 from the Morgan Ranch Formation, Nevada (Fig. 8), is correlated with the ca. 9.73 ± 0.20 Ma Quarry G9 ash bed of Perkins et al. (1998) at Hazen, Nevada (Fig. 1). The Fe₂O₃ and CaO concentrations are similar to those for eruptions from the Southern Nevada volcanic field (Fig. 5); however, the 9.73 Ma age for the Quarry G9 ash bed does not correspond with a major eruption at the Southern Nevada volcanic field (Sawyer et al., 1994).

This study is the initial phase in correlating Monterey and Modelo Formation sequences to each other by the combination of tephrochronology and diatom biostratigraphy. We say initial phase because we have not collected tephra layers from many localities where they are known to exist (e.g., Kew, 1924; Bramlette, 1946). In addition, we suspect that because geologists are now aware of the utility of the Monterey and Modelo Formation tephra layers, more tephrochronology will be done. Development of a tephrostratigraphy may provide more precisely dated micropaleontologic zonation as well as assisting with correlating depositional facies within the Monterey and Modelo Formations.

A clear example of the correlation of different Monterey and Modelo Formation sequences is the correlation of the 7.6–7.0 Ma MOD-5/26 tephra layer from the Modelo Formation at Balcom Canyon to tephra beds found at the Naples Beach and Grefco sections. The geochemical data provide strong evidence that the same tephra beds exist at all three locations, and the correlations are supported with the age control obtained from the diatom assemblage. The MOD-5/26 tephra layer provides a precise time-stratigraphic marker bed between the upper bathyal (400–600 m water depth) Modelo Formation submarine-fan environment at Balcom Canyon (Rumelhart and Ingersoll, 1997) and the lower-middle bathyal (>1500 m water depth) shelf to basin environments at Naples Beach and Grefco (Isaacs, 2001). A similar paleoenvironmental connection is possible between 9 Ma and 8.5 Ma (Fig. 2) with correlation of several tephra layers at Balcom Canyon (MOD-1, MOD-3, and MOD-2) to both the Naples Beach section (DPB-24 and DPB-22) and deep-water sediments in Orange County (e.g., KF-OC-2).

The Monterey Formation tephrochronology also connects marine and nonmarine sequences. For example, the 7.90 Ma Rush Valley ash of Perkins et al. (1998) provides a time-stratigraphic connection between nonmarine sediments in western Utah and the marine environments of the Modelo Formation at Balcom Canyon and Monterey Formation near Lompoc. Discrete temporal links between nonmarine and marine depositional environments are possible with a number of tephra beds, including the GREFCO-4 ash, Cougar Point Tuff XII, Cougar Point Tuff XI, and Rainier Mesa Tuff. Revisiting these depositional systems with improved stratigraphic connections may provide greater insight into paleogeography and paleoclimate studies.

The 14 Ma to 5 Ma Monterey Formation exposed in the eastern bluffs of Newport Bay, Orange County (Fig. 3), have been extensively studied (Ingle and Barron, 1978). Prior to this study, age control on the widespread Monterey Formation deposits east of Newport Bay was lacking (Morton et al., 1974). In this study, we correlated a tephra layer with the 13.90 Ma Roadcut ash bed at a construction site less than 3 km east of the Newport Bay bluffs. In the Laguna Hills, the tephra beds and diatom assemblage establish the Monterey Formation is 16–15 Ma in this area. Farther east, our correlation of a number of tephra beds shows that the Monterey Formation ranges from 11.31 Ma to ca. 8.7 Ma.

Our correlation of the tephra layers near the type locality of the Monterey Formation at Toro Road with the ca. 11.19 Ma Cougar Point Tuff XII and 11.08 Ma Cougar Point Tuff XIII resolves a problem in ages reported by Obradovich and Naeser (1981) (Table 1). These authors obtained zircon fission-track dates of 11.3 Ma (lower) and 8.4 Ma (upper) for the tephra layers along Toro Road. They suspected that the 8.4 Ma age for the upper tephra bed was too young because it was inconsistent with the age of the foraminiferal Delmontian Stage (Barron, 1976; Barron and Isaacs, 2001). We correlate the lower tephra layer at Toro Road (TORO-1) with the 11.19 Ma Cougar Point Tuff XII and the upper tephra layer (TORO-2 and TORO-3) with the 11.08 Ma Cougar Point Tuff XIII. The strong correlation of samples TORO-2 and TORO-3 to Cougar Point Tuff XIII leaves no doubt in our minds that the upper tephra layer at Toro Road is the 11.08 Ma Cougar Point Tuff XII, and this reaffirms Obradovich and Naeser’s suspicion that the 8.4 Ma fission-track date was spurious.

The correlation of the upper tephra layer at Toro Road highlights the benefit of tephrochronology over mineral-specific dating methods in the Monterey and Modelo Formations, where mineral grains may have been reworked and redeposited in these sedimentary units. The multishard analyses used in tephrochronology reduce potential reworking issues by utilizing multiple shards to obtain major- and minor-element concentrations and thousands of shards for trace-element concentrations. This reduces the dependency on single-crystal analysis. Tephrochronology provides identification of specific tephra layers, the ages of which are, or may be, determined at those locations (proximal sites) where the best material (coarse mineral grains) for dating may be obtained and detrital or accidental (xenocrystic) contamination may be eliminated.

Volcanic eruptions with a volcanic explosivity index greater than eight, i.e., supereruptions, result in deposition of ash and tephra over millions of square kilometers (Pyle, 2000; Mason et al., 2004). Supereruption designation is based on the volume or mass of erupted material (Mason et al., 2004). Volume or mass estimates are determined based on near-field ignimbrites because estimating the volume and mass of distal deposits is difficult and imprecise due to variable and indeterminant thickness and areal distribution of air-fall deposits, as well as fluvial and eolian reworking. Nevertheless, distal fall tephra from volcanic eruptions is a potential hazard (Mason et al., 2004). Determination of the air-fall distribution of tephra provides a more accurate estimate of the total volume or mass of an eruption, as well as the extent of its impact.

The minimum areal distribution of fall tephra from supereruptions in North America is well documented for some eruptions, such as the 2.101 Ma Huckleberry Ridge and 0.628 Ma Lava Creek B supereruptions of the Snake River Plain (Izett and Wilcox, 1982; Sarna-Wojcicki and Davis, 1991; Sarna-Wojcicki, 2000; Sarna-Wojcicki et al., 2021). In the ca. 18–5 Ma time frame represented by the Monterey and Modelo Formations, there were 13 documented supereruptions in North America (Mason et al., 2004; Knott et al., 2020). Nine of these supereruptions were from the Snake River Plain, and four were from the Southern Nevada volcanic field. Two other Snake River Plain eruptions (Wooden Shoe Butte and Brown’s View) were just below the supereruption threshold as determined by proximal deposits (Knott et al., 2020).

The tephra deposits from four supereruptions listed by Mason et al. (2004) are found interbedded with Monterey and Modelo Formation sediments. Although many of these tephra layers are reworked, nevertheless, the drainage basins of these deposits must have been within the air-fall limits of the eruption. The 11.87 Ma Rainier Mesa Tuff is found at the Naples Beach section ~450 km south-southwest from the Southern Nevada volcanic field today. Tectonic reconstructions show that Naples Beach, as part of the Transverse Ranges, was 270 km farther south at 12 Ma (McQuarrie and Wernicke, 2005), ~670 km distance from the eruptive source of the Rainier Mesa Tuff. Tephra layers from the 11.31 Ma Cougar Point Tuff XI and 11.08 Ma Cougar Point Tuff XIII are found in the Grefco quarry in Lompoc, California, which is also part of the western Transverse Ranges. At present, Lompoc, California, is ~1000 km southwest of the Bruneau-Jarbidge volcanic center in the Snake River Plain (Fig. 12). Again, allowing for the tectonic displacement of the western Transverse Ranges since 12 Ma (McQuarrie and Wernicke, 2005), the air-fall radius for the Cougar Point Tuff XI and Cougar Point Tuff XIII is 1270 km southwest from the source. The air-fall distribution of both Cougar Point Tuff XI and Cougar Point Tuff XIII was 500 km to the northwest of the eruptive source in central Washington (Nash and Perkins, 2012; Camilleri et al., 2017), with Cougar Point Tuff XI also found on the Great Plains as well (Nash and Perkins, 2012). The youngest supereruption tephra layer in the Monterey and Modelo Formations is the Tuff of McMullen Creek, which is present in the Modelo Formation at Balcom Canyon (Fig. 11). Like Cougar Point Tuff XI and Cougar Point Tuff XIII, the air-fall from the Tuff of McMullen Creek was within the drainage basin limits when the western Transverse Ranges were still ~1270 km southwest of the Twin Falls volcanic center at 9 Ma. Distal air-fall tephra of the Tuff of McMullen Creek was also found in northern Nevada (Camilleri et al., 2017). The 10.26 Ma tuff of Wooden Shoe Butte was a large, but not super, eruption (Knott et al., 2020), and the ash from this eruption is in the Naples Beach section, which was 1270 km from the source at 10.26 Ma.

The presence of tephra from Snake River Plain supereruptions in the Monterey and Modelo Formations raises the question: What magnitudes of eruption do other Snake River Plain and Southern Nevada volcanic field tephra layers and tuffs, like GREFCO-4 (Fig. 12), found in the Monterey and Modelo Formation deposits represent? The GREFCO-4 tephra layer is an unnamed tephra layer in the Trapper Creek section (TC90-17), yet this tephra layer has the same southerly air-fall distribution of tephra as the Cougar Point Tuff XI (magnitude 8.4) and Cougar Point Tuff XIII (magnitude 8.5) supereruptions. The tuff of Wooden Shoe Butte is a large (magnitude 7.5), but not super, eruption (Knott et al., 2020), but it has a comparable distribution of air-fall tephra as the Cougar Point Tuff XI and Cougar Point Tuff XIII supereruptions, including deposition to the northwest in Washington (Nash and Perkins, 2012).

The distal fallout of other tephra layers present in the Monterey Formation is also large. The 13.90 Ma Roadcut ash is present in Orange County and northern New Mexico, which suggests a substantial distribution of tephra to the south and southeast from the Snake River Plain. Similarly, the #4 white ash, Jacomita-9, and Jacomita-12 tephra layers all were erupted from the Southern Nevada volcanic field and were dispersed from southern California to New Mexico. All of these tephra layers preceded supereruptions at the eruptive source (Perkins et al., 1995, 1998; Sawyer et al., 1994) that may obscure the proximal deposits used to measure eruptive magnitude.

Although not found in the Monterey or Modelo Formations and not recognized as a supereruption, the 12.20 Ma Aldrich Hill 2 ash from the Southern Nevada volcanic field is 15 m thick at Aldrich Station (sample BE-15) and found in the Española basin (sample Jacomita-14), Ibapah Badlands (Perkins et al., 1998), and east of San Francisco (Sarna-Wojcicki et al., 2021). The same may be said for the ca. 15.77 Ma Huntington Creek 2 tephra layer, which is found in the San Joaquin Valley of California (sample RJR-SJV-01) and the Española basin of New Mexico (sample OC-167 of Slate et al., 2013).

Knott et al. (2020) found that misidentification of ignimbrites in near-field areas prevented recognition of the Tuff of McMullen Creek as a supereruption. Possibly, the same fate has befallen other ignimbrite deposits in the near field where younger eruptions, such as the tuff of Wooden Shoe Butte, have possibly obscured or destroyed the evidence of older ignimbrites, like the GREFCO-4 tephra layer, in the vicinity of the eruptive source. Moreover, highly explosive eruptions may produce large volumes of Plinian fall, but limited associated ignimbrites, thus leading to an underestimation of the size of the eruption if magnitude is solely based on ignimbrite volume. This may leave distal tephra layers, like those found in the Monterey and Modelo Formations of California and other areas of North America, as the only record of these supereruptions. The correlation of these tephra layers and tuffs in the Monterey and Modelo Formations may stimulate more study of Snake River Plain and Southern Nevada volcanic field proximal deposits and greater consideration of distal volcanic deposits as a record of eruption magnitude.

In this study, we correlated 38 different tephra layers in several sedimentary sequences of the Monterey Formation ranging in age from ca. 16 Ma to ca. 7 Ma. We also correlated seven tephra layers in the Modelo Formation at Balcom Canyon. Our tephrochronology is complemented by diatom biostratigraphy correlated to the North Pacific Diatom Zones. We also provided geochemical data on a number of tephra layers for which we could not find a correlative in the USGS tephrochronology database. These data are presented in the hope that future researchers working on the Monterey and Modelo Formations, and elsewhere, will find correlative tephra layers.

Bramlette (1946) speculated that investigation of the Monterey Formation tuffs would assist with correlations of rock sequences. Our correlations provide a clear temporal link between the Monterey and Modelo Formations, along with linking Monterey Formation sequences in Monterey, Santa Barbara, Los Angeles, and Orange County, California. By correlating tephra layers in the Monterey and Modelo Formations to dated tephra layers in Nevada, Idaho, Utah, and New Mexico, we were able to provide precise ages for several Monterey Formation sedimentary sequences in addition to linking marine and nonmarine deposits.

The sources of Monterey and Modelo Formation tephra layers were predominantly from the Snake River Plain volcanic centers and the Southern Nevada volcanic field, with one from the Mono Lake area and several from unknown sources. All of the tephra layers in the Monterey and Modelo Formations are (and were) several hundred kilometers from the eruptive source and document far-field distribution of air-fall tephra from large eruptions, including supereruptions.

Data included in this study were collected by Andrei Sarna-Wojcicki and John Barron over ~50 years. Charley Meyer, Hugh Wagner, Brian Olson, Lynne Yost, Aron Taylor, Katie Farrington, the late Bob Reynolds, Jim Ingle, Dave Miller, and Rick Behl submitted samples to the U.S. Geological Survey (USGS) Tephrochronology Laboratory that became part of this study. We thank Jim Budahn of the USGS, who completed the instrumental neutron activation analysis (INAA) analyses; Laura Walkup of the USGS, who helped with sample data; the staff at the Peter Hooper GeoAnalytical Laboratory at Washington State University for the solution inductively coupled plasma–mass spectrometry (S-ICP-MS) analyses; Sarah Rieboldt of LSA Associates for sharing the paleontological information on the Round Mountain Silt; and Al Deino, Berkeley Geochronology Laboratory, for sharing the ⁴⁰Ar/³⁹Ar conversion spreadsheet. This manuscript greatly benefited from constructive reviews by David Miller, Robert Negrini, Jorge Vazquez, Richard G. Stanley, Ivano Aiello, and Daniel Koning. This study was supported by National Science Foundation (EAR-1516593) and California State University–Fullerton Junior/Senior grants (0359518) to Knott. A portion of this project constituted the undergraduate thesis of Priscilla Martinez at California State University–Fullerton. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.