Fluvial response to the Paleocene-Eocene Thermal Maximum in northwest Wyoming and western Colorado, USA

The Willwood and Wasatch formations of northwest Wyoming and western Colorado record alluvial deposition within the intermontane Bighorn and Piceance Creek basins, respectively. Both display substantial shifts in the character of fluvial sand-bodies coincident with an abrupt negative carbon isotope excursion linked to the Paleocene Eocene Thermal Maximum (PETM) climate change event at ~55 Ma. In the northern Bighorn Basin, an anomalously thick and laterally persistent multi-story fluvial sand-body crops out within the main body of the PETM isotopic excursion. The internal architecture and lithofacies within the sand-body are similar to pre- and post-PETM sand-bodies, and mean paleo-flow depths do not appear to change substantially. The most significant change is the increase in vertical and lateral amalgamation within the PETM sand-body. Long-term basin sedimentation rates are constant spanning the event implying a transient increase in channel mobility via avulsion and meandering processes during the PETM, which preferentially evacuated fine-grained overbank material out of the basin to the north. Similarly, fluvial sand-bodies are more laterally and vertically amalgamated during the PETM in the Piceance Creek Basin. Yet here the sand-bodies are a recurrent phenomenon throughout the PETM, persist after the PETM, and show dramatic internal architectural changes. Flow depths increase by ~50% and are twice as variable during the PETM, lithofacies are dominated by upper flow regime structures, and crevasse splay deposits are ubiquitous in the associated floodplain strata. In both basins enhanced channel mobility was likely facilitated by a combination of vegetation overturn and alteration of precipitation patterns. Sediment stored higher in the catchment and on related hill-slopes was released, choked basin river systems, instigated greater in-channel deposition, and caused more rapid avulsions. Introduction of coarser sediment loads and vegetation change would have weakened bank strengths allowing more rapid meandering by river systems. However, the differential response in the two basins suggests that vegetation overturn played a greater role in the Bighorn Basin as channel size, discharge, and flow conditions did not substantially change whereas increases in the seasonality of precipitation likely played a more dominant role in the Piceance Creek Basin where discharge and flow conditions were greatly altered during the event.