40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming

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The deposits of Eocene Lake Gosiute that constitute the Green River Formation of Wyoming contain numerous tuff beds that represent isochronous, correlatable stratigraphic markers. Tuff beds selected for 40Ar/39Ar analysis occur within laminated mudstone, are matrix supported, and lack evidence of reworking. These tuffs contain 2%–15% euhedral phenocrysts of quartz, plagioclase, sanidine, biotite, and minor amphibole, pyroxene, and zircon, encased in a matrix of altered glassy ash. Air abrasion and handpicking under refractive- index oils were required to obtain clean, unaltered phenocrysts of sanidine. 40Ar/39Ar age determinations from single-crystal and <1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination. Laser-fusion experiments on phenocrysts from the Rife, Firehole, C Bed, Grey, Main, Sixth, and Analcite tuff beds from the Tipton, Wilkins Peak, and Laney Members yielded weighted-mean ages (±2σ analytical uncertainties) of 51.25 ± 0.31 Ma, 50.70 ± 0.14 Ma, 50.56 ± 0.26 Ma, 50.39 ± 0.13 Ma, 49.96 ± 0.08 Ma, 49.70 ± 0.10 Ma, and 48.94 ± 0.12 Ma, respectively. Ages for sanidine and biotite from the Main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation. Moreover, the 40Ar/39Ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the Absaroka and Challis volcanic fields. Our 40Ar/39Ar-based age model indicates that sediment accumulated three times more rapidly (327 ± 86 μm/yr) during the evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 ± 34 μm/yr) and Laney (104 ± 18 μm/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for <1-mm-thick laminae and precessional forcing of 1–3-m-thick depositional cycles in these units. However, previously described cycles in the Wilkins Peak Member have average durations that are significantly shorter than the 19–23 k.y. precessional modes. The Green River Formation encompasses an ∼5 m.y. period between ca. 53.5 and 48.5 Ma, spanning magnetic chrons 24n through 21r. The Green River Formation was therefore deposited during the warmest period of the Cenozoic corresponding to the early Eocene climatic optimum as defined by the global marine O isotope record. Deposition of bedded evaporites (trona) of the Wilkins Peak Member began at ca. 51 Ma, or ∼2 m.y. after the onset of the highest inferred temperatures of the climatic optimum. The Bridgerian–Wasatchian faunal turnover occurred subsequently during Wilkins Peak time, at ca. 50.6 Ma. Thus, our 40Ar/39Ar ages strongly suggest that Wilkins Peak evaporite deposition and the turnover from Wasatchian to Bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions.