Thin fallout tuffs in the Duchesne River Formation in the Uinta Basin, Utah are evidence that volcanism was active in northern Nevada and Utah in the late Eocene. The Uinta Basin is a sedimentary basin that formed during the Laramide orogeny. Ponded lakes of various salinity filled and emptied and during the late Eocene the northern rim was dominated by a wetland/floodplain depositional setting. Most of the tuffs have rhyolitic mineral assemblages including quartz, biotite, sanidine, and allanite. Rhyolitic glass shards were also found in one of the ash beds. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of biotite compositions suggest 3 or 4 volcanic events. Sanidine compositions from five samples grouped at Or73 and Or79. Only one sample had plagioclase with compositions ranging between An22 - An49. Some beds also contained accessory phases of titanite, apatite, and zircon. Whole rock compositions of the altered volcanic ash beds indicate these tuffs underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. Immobile element ratios and abundances, such as Zr/Nb and Y are typical of a subduction zone tectonic setting and rhyolitic composition. 40Ar/39Ar ages constrain the timing of volcanism. One plagioclase and one sanidine separate from two different tuff beds yielded ages of 39.47 ± 0.16 Ma and 39.36± 0.15 Ma respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field. These new ages, along with previously published ages in the Bishop Conglomerate which unconformably overlies the Duchesne River Formation, constrain the timing of two uplift periods of the Uinta Mountains at 39 Ma and 34 Ma. Finally, the ages also date the fauna of the Duchesnean Land Mammal Age to be about 39.4 Ma as opposed to less precise earlier estimates that placed it between 42 and 33 Ma.



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Physical and Mathematical Sciences; Geological Sciences



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fallout tuffs, 40Ar/39Ar ages, laramide orogeny, vernal northwest quadrangle, northeast Nevada volcanic field, Duchesnean Land Mammal Age