RECONSTRUCTING PALEOCHANNEL MOBILITY CHANGES IN RESPONSE TO THE PALEOCENE EOCENE THERMAL MAXIMUM

Paul Heller had an interest in understanding whether facies models could be used to faithfully reconstruct river planform from ancient deposits. He proposed an unpublished approach to mapping bar migration distances from preserved channel deposits as a way of differentiating meandering and braided river deposits. Based on analyses of modern rivers and experimental deposits, Paul demonstrated that meandering river deposits would be more likely to contain bar packages showing persistent accretion relative to a characteristic bar width than other types of rivers. Recent work by others has shown that this type of bar-scale measurement may be useful for characterizing and comparing paleomorphodynamics of ancient meandering rivers. Here we use several approaches to constrain the degree of lateral migration relative to channel-belt avulsion in ancient fluvial deposits. We focus on deposits spanning the abrupt climate-warming event of the Paleocene Eocene Thermal Maximum (PETM), including the Willwood Formation (Bighorn Basin) and Wasatch Formation (Piceance Basin). Our results show evidence for modest increase in channel lateral migration during the peak of the PETM in the Willwood Formation, and evidence for a slight reduction in bar migration relative to avulsion in the Wasatch Formation in response to the PETM. Willwood Formation rivers remained firmly in a single-thread, strongly meandering channel configuration, with no apparent change in planform, avulsion pattern, or avulsion style in response to warming-induced changes in discharge, sediment supply, and floodplain conditions. In contrast Wasatch rivers changed planform, avulsion pattern, and avulsion style in response to the same climate forcing. These results suggest that, while Wasatch Rivers were sensitive to changes imposed during the PETM, Willwood rivers were relatively resilient, and the results raise questions about the importance of regional tectonic, land cover, and drainage-basin controls in determining fluvial landscape response to global climate change. Additionally, this approach, inspired by Paul’s ideas, provides a mechanism for extracting information about the vulnerability of different types of ancient rivers in a manner that may help inform Earthcasting and decision-making related to present-day river management.