SEQUENCE STRATIGRAPHY OF A SILICICLASTIC-DOMINATED ARID COASTLINE UNDER A REGIME OF HIGH-FREQUENCY SEA-LEVEL FLUCTUATIONS: THE PENNSYLVANIAN TENSLEEP FORMATION, NORTHERN WYOMING, USA

The cyclic character of the Pennsylvanian Tensleep Formation has long been recognized, and is generally assumed to reflect orbitally-driven changes in the extent of Gondwanan ice sheets. Previous work on the Tensleep Formation has been strongly steered toward the eolian facies. Relatively little attention has been paid to the significance of the marine carbonates and shales. With the aim of providing a sequence stratigraphic framework encompassing the full range of lithologies, we make use of abundant exposures and subsurface data available to provide detailed correlations across the Bighorn Basin of Northern Wyoming, USA. Thin section analysis and cathodoluminescence imaging allow for detailed characterization of the carbonate facies. The most common expression of the Tensleep Formation comprises thick eolian sandstones capped by dolomite beds. Dolomitic facies range in composition from almost pure carbonate to considerably more silty and shaley lithologies. Toward the central and southern parts of the basin, thick eolian sandstones are separated by supersurfaces and intervals dominated by flat-stratified sandstones. In the northern parts of the basin, the lower part of the Tensleep Formation dominantly consist of purple shales, with some consistently spaced, thin beds of dolomite and sandstone. Sandstone bodies, whether eolian or tidal in origin, are interpreted to have been deposited during periods of sea-level lowstand. Highstand system tracts are either shales or silty dolomites deposited in a marine environment, or are not preserved. In the latter situation, major deflationary surfaces separating eolian storeys are interpreted to reflect a rise in the groundwater table that would have accompanied a concomitant relative rise in sea-level. Transgressive system tracts are also commonly preserved, as indicated by the presence of intervals rich in desiccation cracks between eolian and marine deposits, interpreted to have been deposited in a frequently inundated, coastal environment. These results, and their comparison to time equivalent deposits in surrounding regions, aid in the development of a more detailed understanding of the response of late Paleozoic arid environments to changes in glacial regimes on Gondwana.