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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...
We reevaluate the Eocene geomagnetic polarity time scale on the basis of single-crystal 40Ar/39Ar ages for air-fall tuffs from the Wilkins Peak Member of the Green River Formation of Wyoming. Tuff 6 is dated as 49.1 � 0.2 Ma, and tuff 3 is dated as 50.4 � 0.3 Ma (maximum estimate). When combined with published magnetostratigraphic constraints, these age determinations suggest that the currently accepted age of chron C22r is 1.5?2.5 m.y. too old, which supports a significantly longer duration for the early Eocene, for the early Eocene climatic optimum, and the Wasatchian North American Land Mammal Age. Published in Geology, volume 32, issue 2, on pages 137 - 137, in 2004.


    map background search result map search result map Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States