FLUVIAL RESPONSE TO CLIMATE CHANGE AT THE PALEOCENE-EOCENE BOUNDARY IN THE CLARKS FORK BASIN, NW WYOMING, U.S.A

The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event occurring ~55 million years ago, with far-reaching consequence for Earth's ecosystems. Yet many questions remain regarding the impact this event had on the hydrologic cycle. This study presents an assessment of the fluvial response to PETM-induced climate change within the Clarks Fork Basin of northwestern Wyoming by comparing characteristics of fluvial sand-bodies before, during, and after the PETM. Initial data suggests sand-body thicknesses increased by an order of magnitude during the event, and were predominantly tabular in geometry rather than lenticular. Bar clinoform heights, an estimate of bankfull flow depth, temporarily decreased on average by ~0.2 m, a ~25% change. Paleocurrents indicate a slight shift in paleodrainage during the event; flowing to the north-northwest before and after the PETM, but to the north-northeast during the PETM. Furthermore, the mean maximum grain size in channel lags more than doubled (from 2.1 to 4.6 cm), and pedogenic carbonate nodules compose nearly 100% of the lag clast compositions during the PETM. Overall, we conclude these results were caused by a temporary increase in reworking of floodplain deposits, potentially related to a decrease in bank stability. Chronologic studies indicate that these changes in alluvial architecture developed with no attendant change in long-term sedimentation rates. Hence, increased avulsion or meandering rates are required to obtain the thicker sand-bodies during the PETM. This fluvial response is consistent with increased variation in precipitation amounts and paleofloral compositions during the PETM, documented by previous studies, which likely altered hydrographs and influenced bank strength.