RECONSTRUCTING RIVER MOBILITY FROM ANCIENT DEPOSITS: AN EXAMPLE FROM THE WILLWOOD FORMATION (BIGHORN BASIN, WY)
The lateral migration and avulsion of river channels are fundamental parts of river systems. While we can measure the rates of channel migration and witness singular avulsion events in modern rivers, these studies fail to capture the long-term relationships between these processes and the preservation of sediment over geologic timescales. Here, I leveraged ancient fluvial deposits to reconstruct paleo-channel mobility, began to test the theory regarding the long-term behavior of river migration and avulsion and explored the sensitivity of river mobility to past cases of extreme climate change. I reconstructed the paleo-channel mobility from the fluvial deposits of the Willwood Formation (Bighorn Basin, Wyoming) and tested hypotheses regarding potential changes in channel mobility in response to the Paleocene-Eocene Thermal Maximum (PETM) global warming event. I identified avulsion-generated channel-belt deposits and made detailed maps of intra-channel-belt facies and architecture with a particular focus on constraining the scale, stacking, lateral continuity, and preservation of bar deposits. I also calculated hydraulic parameters such as paleo-discharge, paleo-slope, Parker stability criterion, and channel mobility number. My results suggest that the Willwood rivers were extremely laterally mobile meandering rivers, confirming previous interpretations, and that their lateral mobility remained insensitive to the PETM climate change. However, an apparent increase in avulsion reoccupation at the channel-belt scale through the peak of the PETM suggests either a change in floodplain accumulation rates, a change in avulsion dynamics or the presence of channel-belt clustering within the Bighorn Basin. This study introduces a framework that allows ancient rivers to be directly compared to modern river and can be used to make predictions about which river systems are most susceptible to future climate change.