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Whither the PETM? Testing Predictive Models for Climatically-Triggered Fluvial Sheet Sand Progradation in a Rapidly Subsiding Basin


The Paleocene-Eocene Thermal Maximum (PETM) has been linked to generation of basin-wide fluvial sheet sand deposits in the Piceance, Uinta, Bighorn, and South-Pyrenean basins. Extrapolating to other basins with siliciclastic fluvial input, it could reasonably be expected that decreased precipitation and increased seasonality would lead to the development of similarly anomalous, sheetlike fluvial sandstone. In order to test this predictive model, strata of the Paleocene-Eocene Hanna Formation, in south-central Wyoming's Hanna Basin, were analyzed by integrating isotopic, palynostratigraphic, lithologic, and ichnologic data. Although the Hanna Formation has a well-documented biostratigraphically and isotopically-defined PETM section, a detailed lithostratigraphic section hasn't been published. Our sedimentological and stratigraphic logging of the PETM in the Hanna Formation has revealed that: 1) a series of stacked lacustrine and lake plain parasequences straddles the isotopically and palynologically defined PETM, 2) there is no fluvial sheet sand present, 3) crayfish burrows in lake plain deposits are smaller in the PETM section than in underlying Paleocene strata, 4) the coarsest-grained deposits in the Hanna Formation (cobble conglomerates with clasts up to 40 cm diameter) are present farther down in the Paleocene section and resulted from tectonic activity of surrounding Laramide uplifts, and 5) onset of lacustrine deposition began prior to the PETM and continued into the Eocene, despite postulated decreases in precipitation, most likely in response to tectonic rather than climatic forcing. The Hanna Basin is therefore an exception to the rule and indicates that caution should be exercised when predicting fluvial architecture from paleoclimatic data alone.