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Numerous 40Ar/39Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and Uinta Basins of Wyoming, Colorado, and Utah constrain ~8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, 40Ar loss, and 39Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a significantly increased level of precision. To compare our results...
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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...
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U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from 40Ar/39Ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean 238U-206Pb age of 51.66 {+/-} 0.20 Ma (full external 2{sigma} uncertainty), indistinguishable from the recalibrated 40Ar/39Ar age of 51.40 {+/-} 0.25 Ma (full external 2{sigma} uncertainty), which is adjusted to eliminate samples with low radiogenic 40Ar*. Significant 238U-206Pb age scatter likely reflects...
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    map background search result map search result map Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming Eocene clocks agree: Coeval 40Ar/39Ar, U-Pb, and astronomical ages from the Green River Formation 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming Eocene clocks agree: Coeval 40Ar/39Ar, U-Pb, and astronomical ages from the Green River Formation Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States