EVALUATING CLIMATIC AND TECTONIC CONTROLS ON FLUVIAL DEPOSITION SPANNING THE PALEOCENE-EOCENE BOUNDARY IN THE PICEANCE CREEK BASIN (WESTERN COLORADO, USA)

The early Paleogene-aged Wasatch Formation of western Colorado records alluvial deposition within an actively subsiding intermontane basin. The formation is subdivided into three members based upon the overall sandstone-to-mudstone ratio. The lowermost and uppermost members are overwhelming composed of overbank fine-grained facies (the Atwell Gulch and Shire members, respectively), whereas the intervening member is dominated by laterally continuous fluvial sand-bodies (the Molina Member). Here, we evaluate the potential climatic and tectonic causes for this dramatic stratigraphic shift using a combination of techniques. A tectonic subsidence analysis of the basin indicates major uplift of surrounding ranges and loading of the lithosphere occurred ~70 Ma, with largely constant subsidence rates from 60 to 50 Ma during Wasatch deposition. U/Pb age spectra from detrital zircons and paleocurrent directions are similar in all three members, and indicate rivers systems derived sediment the same source terrane and drained to the north-northwest throughout deposition. Thus, evidence for a tectonic driver on the observed stratigraphic change is unlikely. In contrast, there is substantial evidence for a climatic driver, namely an increase in seasonal precipitation associated with the Paleocene-Eocene Thermal Maximum (PETM). We document an ~4‰ negative carbon isotope excursion in dispersed organic matter coincident with the onset of Molina Member deposition. Flow depths, reconstructed from the relief on bar clinoforms, increase by 50% and display twice the range. Sand-bodies become more vertically and laterally amalgamated and are dominated by upper flow regime lithofacies including upper plane bed laminations as well as climbing ripple and dune cross-stratification. Furthermore, crevasse splay deposits are ubiquitous within the Molina Member, but absent in the other members, which instead display well-developed levee complexes. Overall, these features are consistent with higher variability in flow conditions, more frequent flooding events, greater channel mobility, and weakened bank strengths, potentially caused by previously hypothesized increases in seasonality and vegetation overturn within the mid-latitudes during the PETM.