EARLY PALEOGENE SHIFTS IN FLUVIAL DEPOSITION WITHIN THE HUERFANO BASIN, COLORADO, USA: EVALUATION OF TECTONIC AND CLIMATIC CONTROL

There is mounting evidence linking abrupt geomorphic changes to hyperthermal events during the early Paleogene. In terrestrial basins, these geomorphic responses are observed as changes in the stacking patterns and grain size distribution of fluvial sand-bodies, as well as variations in coeval floodplain successions. In western North America, the early Paleogene was also a period of extensive tectonic activity associated with the Laramide Orogeny and resulting basin strata preserve a complex history of both allogenic forcings.

This study attempts to deconstruct these effects in the Huerfano Basin. Paleocene–Eocene alluvial deposits of the Poison Canyon and Cuchara formations are exposed along the western basin margin. The Poison Canyon–Cuchara transition is marked by an abrupt shift in fluvial deposition characterized by changes in lithofacies abundance, increased sand-body stacking patterns and channel dimensions, and coarser grain size. Fine-grained overbank facies indicate wetter Paleocene conditions shifting to drier conditions during the Eocene. Our provenance analysis using U–Pb detrital zircon geochronology and sandstone petrography indicates a uniform sediment source from the Wet Mountains for the lower Paleogene succession. A new bulk organic carbon isotope record from this section shows substantial short-term variability throughout the section, but a negative shift of ~2‰ that occurs near the Poison Canyon–Cuchara contact. Samples were also analyzed for magnetic susceptibility as well as hysteresis parameters. There is marked change at the Poison Canyon–Cuchara transition in the high and low field magnetic susceptibility as well as magnetic coercively and remanence. Precise biostratigraphic information is lacking in this portion of the section, but grossly constrains the time period as late Paleocene to early Eocene.

These datasets show a progression similar to other Laramide basins where patterns are attributed to hydrologic changes during the Paleocene–Eocene Thermal Maximum. In the Huerfano Basin, we hypothesize that climate was a major contributor to the observed stratigraphic patterns. The carbon isotope excursion is broadly consistent with the PETM, but the signal is less distinct and is of a lesser magnitude compared to isotope records in other Laramide basins.