--> Osmium Isotope Stratigraphy and Radiogenic Age Determinations of the Green River Formation in Utah, U.S.A.

AAPG ACE 2018

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Osmium Isotope Stratigraphy and Radiogenic Age Determinations of the Green River Formation in Utah, U.S.A.

Abstract

The focus of this study is to investigate the Re-Os isotopic system in oil shales of the Eocene Green River Formation to constrain its utility as a chronostratigraphic tool and its geochronometric uncertainty in lacustrine basins. The Green River Formation, one of the most heavily studied pre-Quaternary lacustrine deposits in the world, is an ideal laboratory to calibrate the Re-Os isotopic system. In sedimentary rocks hydrogenous Re and Os are chelated by organic matter providing an isotopic system that can be used to determine depositional age and the 187Os/188Os ratio of the water at that time. In thick lacustrine shale deposits with endemic faunas, barren zones, and a lack of interbedded volcanic ash, Re-Os isochron ages may provide the only absolute age control. Additionally, since the 187Os/188Os ratio of lake water is a mixture of Os from bedrock weathering in the drainage basin, groundwater discharge, volcanic ash, windblown dust, and extraterrestrial material it may provide insight into the evolution of the drainage basin, volcanic activity, or changing wind patterns.

Samples of the same oil shale bed collected from two cores in the eastern Uinta Basin separated by 16 km were analyzed. A Model 3 isochron age determination using all samples yields a depositional age of 48.3 ±2.8 Ma. This is indistinguishable from the 49.58 ±0.32 Ma 40Ar/39Ar age determination of the Skyline ash located 1-2 m below this bed (Smith and Carroll, 2015). However, a high MSWD (8.2) and use of a Model 3 age suggests that there is geologic variability in initial 187Os/188Os ratios between samples. Although this limits the resolution of age determinations, it supports the notion that variability in initial 187Os/188Os ratios is geologically significant. Stratigraphic comparisons between the two cores indicate that initial 187Os/188Os ratios follow similar trends and appear to correlate with changes in lithology. Even though the variability is low compared to the 2s uncertainties, the similarity of the stratigraphic trends is compelling, indicating that initial 187Os/188Os ratios may be a sensitive indicator of changing lake water chemistry. Combined with Re-Os analyses from three additional oil shales in the basin (Cumming et al., 2012), a longer-term trend of initial 187Os/188Os ratios that spans changes in lake basin type can be investigated. Together, this represents the largest reported lacustrine Os isotopic dataset in the world.