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Geochemical and Mineralogical Analysis of Tatman Formation, Big Horn Basin, Wyoming

Jeffrey E. Filkins, Jon D. Olander, Richard A. Beck

The Tatman Formation of north-central Wyoming is a lower to middle Eocene fluvial and lacustrine deposit. It occurs as isolated erosional remnants in the Big Horn basin and consists of sandstones, mudstones, reworked volcanic ash, coal, and kerogen-rich shale. The remnants are part of a once much larger Tatman deposit. Extensive carbonaceous shales and claystones in the lower Tatman represent a decrease in gradient relative to the underlying fluvial sandstone and mudstone deposits of the Willwood Formation.

Tatman sandstones are dominantly very fine to fine-grained quartzarenites with lithic wackes occurring in places. Beach-derived sandstones are common in the upper Tatman at Squaw Buttes, whereas sandstones at Tatman Mountain are fluvial. Commonly, biotite books occur within the sandstone and are undergoing alteration. Other heavy minerals are of extremely low concentration. Calcite is the dominant cementing agent, whereas, iron oxide plays a lesser role. X-ray diffraction studies of the fine-grained sediments revealed that greenish, smectite-rich claystones were present in the lower Tatman, and that thin, areally extensive, silicified strata in the lower Tatman represented reworked volcanic ash.

An organic geochemical analysis of six coal horizons involved removing extractable organics and identifying compounds using gas chromatography-mass spectroscopy. This analysis isolated and identified biomarkers, principally steranes, from the extractable organics. The relative abundance of C-27, C-28, and C-29 steranes was then established, and the steranes were correlated to their sterol precursors and plotted onto a ternary diagram for use in paleoenvironment determination.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.