2014 Rocky Mountain Section AAPG Annual Meeting

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Core Facies and Geochemical Analysis of the Late Cretaceous Turner Sandstone, Powder River Basin, Wyoming


This paper presents the workflow and preliminary results of a core facies and geochemical analysis of three cores of the Late Cretaceous Turner Sandstone Member of the Carlile Shale in the southern Powder River Basin. Two of these cores were drilled two miles apart in the Brooks Draw Field area of northwestern Niobrara County. The third was drilled 20 miles to the northwest. The cored interval includes the Pool Creek, Turner Sandstone and Sage Breaks members of the Carlile Shale. These three cores provide an opportunity to examine lithofacies, ichnofacies, and continuity of stratigraphic sequences and test the usefulness of elemental geochemistry as a paleoenvironment and correlation tool. The Pool Creek consists of pervasively bioturbated mudstone and contains a high diversity, normal marine outer shelf trace fossil assemblage. The contact between the Pool Creek and the overlying Turner Sandstone is sharp and disconformable. The lower Turner Sandstone consists of streaky, lenticular, and wavy bedded heterolithic strata and contains a low diversity, stressed, restricted, or brackish water trace fossil assemblage. The upper Turner Sandstone consists of upward coarsening vertical successions of bioturbated mudstone, bioturbated shaly sandstone, and bioturbated sandstone and contains a high diversity, normal marine inner shelf trace fossil assemblage dominated by filter-feeding organisms. The Turner Sandstone is conformably overlain by the Sage Breaks. The change from low to high diversity ichnofacies is coincident with an interpreted flooding surface between the lower and upper Turner sandstones, suggesting that salinity and/or oxygen levels improved during a relative sea-level rise. Elemental data was measured with a hand-held XRF instrument In order to determine if this possible change in seaway conditions was captured in the sediments. Results will be presented along with cores, core descriptions, and wire line logs.