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ARTICLE IN PRESS Quaternary International 178 (2008) 246–260 Geochemical correlation and 40Ar/39Ar dating of the Kern River ash bed and related tephra layers: Implications for the stratigraphy of petroleum-bearing formations in the San Joaquin Valley, California Dirk Barona,Ã, Robert M. Negrinia, Elizabeth M. Goloba, Don Millerb, Andrei Sarna-Wojcickic, Robert J. Fleckc, Bradley Hackerd, Alex Erendie aDepartment of Physics and Geology, California State University, Bakersfield, 9001 Stockdale Highway, Bakersfield, CA 93311, USA bBankers Petroleum, 601 East Daily Drive, Camarillo, CA 93010, USA cU.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA dDepartment of Earth Science, University of California, Santa Barbara, CA 93106, USA eChevron Corporation, Bakersfield, CA 93311, USA Available online 27 March 2007 Abstract The Kern River ash (KRA) bed is a prominent tephra layer separating the K and G sands in the upper part of the Kern River Formation, a major petroleum-bearing formation in the southern San Joaquin Valley (SSJV) of California. The minimum age of the Kern River Formation was based on the tentative major-element correlation with the Bishop Tuff, a 0.75970.002 Ma volcanic tephra layer erupted from the Long Valley Caldera. We report a 6.1270.05 Ma 40Ar/39Ar date for the KRA, updated major-element correlations, trace-element correlations of the KRA and geochemically similar tephra, and a 6.070.2 Ma 40Ar/39Ar age for a tephra layer from the Volcano Hills/Silver Peak eruptive center in Nevada. Both major and trace-element correlations show that despite the similarity to the Bishop Tuff, the KRA correlates most closely with tephra from the Volcano Hills/Silver Peak eruptive center. This geochemical correlation is supported by the radiometric dates which are consistent with a correlation of the KRA to the Volcano Hills/Silver Peak center but not to the Bishop Tuff. The 6.1270.05 Ma age for the KRA and the 6.070.2 Ma age for the tephra layer from the Volcano Hills/Silver Peak eruptive center suggest that the upper age of the Kern River Formation is over 5 Ma older than previously thought. Re-interpreted stratigraphy of the SSJV based on the new, significantly older age for the Kern River Formation opens up new opportunities for petroleum exploration in the SSJV and places better constraints on the tectonostratigraphic development of the SSJV. r 2007 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction For example, in 2006 the field produced over 31 million barrels of oil. The Kern River Formation crops out as uplifted and The maximum age of the Kern River Formation tilted Neogene sediments that are dissected by the Kern (8 Ma) is based on the presence of early Hemphillian River between the southern Sierra Nevada mountains of mammalian fossils near the base of the formation (Savage California and the southern San Joaquin Valley (SSJV) et al., 1954; Lindsay et al., 1987; Tedford et al., 1987). The (Fig. 1). The Kern River Formation is the reservoir rock minimum age of the Kern River Formation is based on a for the Kern River oil field, one of the largest oil fields in tentative geochemical correlation of the Kern River ash California. The Kern River oil field has produced almost 2 (KRA) bed, which is found between the K and G sands billion barrels of oil since its discovery in 1899 and (Fig. 2), with the Bishop Tuff (Fig. 1) (Sarna-Wojcicki, continues to produce over 30 million barrels per year. written communication to M. McGuire, 1988; McGuire et al., 1989). The Bishop Tuff is a volcanic tephra ÃCorresponding author. Tel.: +1 661 654 3044. layer erupted from the Long Valley Caldera (Fig. 1) E-mail address: [email protected] (D. Baron). at 0.75970.002 Ma (Izett and Obradovich, 1994; 1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2007.03.011 ARTICLE IN PRESS D. Baron et al. / Quaternary International 178 (2008) 246–260 247 119ºW 118º Long Valley 6 38ºN Caldera 7 Volcano To S 5 Hills N White M Silver I Peak Range ountains E 1 Bi E 95 V Inyo Mountains 37º 6 37º R A 395 3 Deat 4 R NV Southern h San J CA D oaquin Valley 99 Valley A 36º A 36º Bakersfield Arch 5 Crestline (approx.) N 025 mi 2 Ba km 0 40 35º Mo 35º Kern River 58 Formation 119º 118º 117º Fig. 1. Location of tephra samples, Long Valley Caldera, Volcano Hills/Silver Peak Range, and the Kern River Formation. The black lines on the map indicate roads and the major cities are also noted (Ba ¼ Bakersfield, Mo ¼ Mojave, Bi ¼ Bishop, To ¼ Tonopah). This map shows the numbered location of samples used in this study. (1) Bishop Tuff sample BT-11-D1; (2) Kern River ash samples Luck 218 696.50/6980 (shown at the end of the arrow next to 2); (3) Lava Creek B tephra samples JRK-DV5/JRK-DV57, (4) Lava Creek B sample TECO-30A; (5) Fish Lake Valley tephra sample FLV-SP-3; and (6) Friant tephra sample Friant-5A. The sampling location for the fourth Lava Creek B tephra, ORNA-1, in northeastern Nevada is shown on the outline of the state of California in the upper right corner and numbered 7. The cross-section that appears in Fig. 2 is shown as the dotted, WSW–ENE trending line across the Kern River Formation. Sarna-Wojcicki et al., 2005). The initial correlation of the Formation. Improved age control of the Kern River KRA to the Bishop Tuff was based on only nine major Formation and its stratigraphic relation to other adjacent elements determined by electron microprobe analysis formations may generate new petroleum exploration (EMA). In the initial unpublished report (Sarna-Wojcicki, opportunities and a better understanding of the tectonos- written communication to M. McGuire, 1988), Sarna- tratigraphic development of the SSJV. Wojcicki cautioned that additional chemical analyses We report: (1) 40Ar/39Ar age (sanidine) of the KRA; would be needed for a definitive correlation. The similarity (2) new major-element microprobe analyses and 40Ar/39Ar of the Bishop Tuff to the 0.8–1.2 Ma Upper Glass age for a tephra layer from Fish Lake Valley, Nevada, Mountain ash beds prevents a definitive correlation based erupted from the Volcano Hills/Silver Peak range in on major-element compositions alone (Sarna-Wojcicki Nevada (Fig. 1); and (3) new trace-element analyses by et al., 1984). solution inductively coupled plasma mass spectrometry The objective of this study is to provide additional major (ICP/MS) of KRA, Bishop Tuff, and the Volcano Hills/ and trace-element geochemical data and radiometric ages Silver Peak tephra layer. We also measured the trace- of the KRA and related tephra for a more definitive element composition of four samples of the 0.639 Ma Lava correlation of the KRA, thereby providing a better Creek B ash bed (Lanphere et al., 2002) from the constraint for the minimum age of the Kern River Yellowstone Caldera complex in Wyoming in order to ARTICLE IN PRESS 248 D. Baron et al. / Quaternary International 178 (2008) 246–260 Fig. 2. Cross-section showing the major sand units of the Kern River Formation and deeper, older formations in the southern San Joaquin Valley. Cross- section along the dotted line shown in Fig. 1 and along dip. The Kern River ash is found between the K and G sands of the Kern River Formation. The lower age of the Kern River Formation of 8.2 Ma is indicated at the bottom of the R sands. After Miller et al. (1998). measure the variability of trace-element composition of been recognized and informally named (R2, R1, R, K2, samples from one eruption to help evaluate the significance K1, K, G, C1, C) within the producing interval of the Kern of trace-element correlations between samples from differ- River Formation (Nicholson, 1980)(Fig. 2). ent sources. Finally, we measured the trace-element In the SSJV, the Kern River Formation dips gently to composition of a tephra layer from Friant, California the west (o101) toward the depocenter. SeveralNNW or (Fig. 1), that is thought to have erupted from the Long WNW trending normal faults cut through these sedi- Valley Caldera. ments (Nicholson, 1980; Bartow, 1991). The Kern River Formation lies on the Bakersfield Arch (Fig. 1), a broad, 2. Background SSW-plunging anticlinal high, in part responsible for the uplift that led to outcrops which exposed the sediments 2.1. Kern river formation along the margin of the valley. The uplift associated with the Bakersfield Arch most likely commenced during the The Kern River Formation (Fig. 1) is discussed in early Eocene (Reid and Cox, 1989). Although probably not several reviews and studies including those by Hackel active in the Oligocene through middle Miocene, correla- (1965), Nicholson (1980), Bartow et al. (1983), Miller tions across the Bakersfield Arch indicate renewed activity (1986), Olson et al. (1986), Graham et al. (1988), Kuespert beginning in the early Miocene. This antiformal feature is (1990), Kuespert and Sanford (1990), and Bartow (1991). probably active today as evidenced by the topographic Most early studies interpreted the Kern River Formation separation of the Lake Tulare and Kern-Buena Vista Lake as a braided-stream deposit associated with a broad depocenters to the north and south of the Bakersfield Arch. alluvial fan or series of fans fed by Sierra Nevada streams. The maximum age for the Kern River Formation is well More recently, Graham et al. (1988) reinterpreted the established by the presence of Hemphillian (late Miocene- middle and upper part of the formation as a glacio-fluvial early Pliocene) fauna near the base of the formation environment based on the presence of sand grains modified (Savage et al., 1954; Lindsay et al., 1987; Tedford et al., by glacial transport and faceted cobbles.

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