Evaluating Floodplain Controls on Paleo-channel Avulsion and Migration: Wasatch Formation (Paleocene/Eocene, Piceance Basin, CO, USA)

Abstract:

Comparing ancient channel and floodplain deposits provides insight into long-term channel and floodplain dynamics in alluvial basins. Channel lateral migration and avulsion behavior may be affected by floodplain characteristics, including sediment cohesion, vegetation cover, and floodplain-drainage conditions. Channel-deposit architecture reflects paleo-channel dynamics; for example, channel form, migration, and reworking can be inferred from bar deposits, while multi-storied sand bodies can indicate a complex history of channel avulsion, reoccupation and migration. Furthermore, associated floodplain deposits can be used to reconstruct some aspects of floodplain sedimentation patterns, drainage, and soil development. In order to better understand how channel mobility may be linked to floodplain conditions, we compare the three members of the Paleocene-Eocene Wasatch Formation, which were deposited under broadly similar basin subsidence conditions, and straddle the Paleocene-Eocene Thermal Maximum (PETM) climate-change event. These ancient deposits show varying styles of channel and floodplain deposits. The Atwell Gulch (pre-PETM) and Shire (post-PETM) members of the Wasatch Formation are both mud-stone dominated, with abundant, well-developed paleosol deposits. In contrast, the middle Molina Member is sand-rich and contains more weakly developed paleosols. Using terrestrial lidar scans and high-resolution photo panels of outcrops, along with detailed field measurements, we identify key surfaces (including bar-accretion, and story-bounding surfaces) and facies within channel deposits, and describe floodplain deposits within each member. Concurrent changes in paleo-channel architecture and floodplain deposits observed within the succession suggest that floodplain conditions are important controls on channel mobility in Wasatch rivers.