EVIDENCE FOR COMPLEX FLOODPLAIN MORPHODYNAMICS IN THE STRATIGRAPHIC RECORD: CHARACTERIZING FLOODPLAIN CHANNELS IN THE WASATCH FORMATION (PALEOCENE-EOCENE, COLORADO, USA)

Floodplain channels are flow and sediment conduits disconnected from an active channel belt, which are important for flow routing during major flood events in modern rivers. Controls on the spatial distribution, extent, and formation of floodplain channels remain unclear, and few ancient examples of floodplain channels have been described. Studying paleo-floodplain channels is important for reconstructing paleo-floodplain hydraulics and understanding their response to climate and tectonic forcings. We analyze floodplain channels and other overbank deposits in the Paleocene-Eocene Wasatch Formation in western Colorado, including three sites in the lower Atwell Gulch Member and two sites in the upper Shire Member. Using field mapping, high resolution imagery, and particle size analysis, we described and characterized the sedimentology, lateral continuity, and scour surfaces in these floodplain deposits. We investigate both members to document floodplain deposit variability, and because these members bracket the Paleocene-Eocene Thermal Maximum (PETM), a globally significant and abrupt climate change event.

Preliminary results show that the Atwell Gulch floodplains contain well-developed paleosols with variable lateral continuity (less than ten meters to hundreds of meters wide). Paleosol horizons are frequently scoured into by incisions that range from 0.3-1.0m deep, and are filled with silt to fine-lower sand. We interpret these as candidate floodplain channels, because they cannot be traced laterally to parent channels, and they have distinctly finer grain sizes than Atwell Gulch splays, which are more tabular and can sometimes be mapped to a parent channel. These results suggest complex floodplain morphodynamics in the Atwell Gulch Member, with abundant scour-and-fill in the distal floodplain. In contrast, the Shire member has laterally continuous well-developed paleosol horizons, tabular splays, and rare, small-scale scours with less than 0.5m of incision. These results suggest that detailed lateral mapping and particle size analysis of floodplain deposits can be used to interpret complex floodplain morphodynamic processes, including possible ancient floodplain channels. These results also show that floodplain morphodynamics changed before and after the PETM in western Colorado.