FLUVIAL RESPONSE TO THE EOCENE OLIGOCENE CLIMATE TRANSITION IN NORTH AMERICA

The Eocene-Oligocene transition (EOT), occurring at ~33.5 Ma, was one of the largest climate shifts in Earth’s history; it was marked by a global shift from greenhouse to icehouse conditions and an estimated drop in global sea surface temperatures of 6°C. We investigated the alluvial sedimentary record across this period to characterize the impact of contemporaneous changes in precipitation magnitude and frequency on landscape dynamics. We used sedimentary structures, flow depth proxies and particle size distributions to characterize flow and sediment transport within the rivers of the White River Group (WRG) in Nebraska.

Channels of the Late Eocene Chamberlain Pass Formation had flow depths of 3 - 5 m and estimated flow widths of 10 - 16m; they produced channel belts roughly 11 m thick, and 2 - 3 times flow depth. Channel sand bodies contain bar deposits with coarsely stratified lower plane bed cobble conglomerates and trough cross-stratified, gravel-rich coarse- to fine-grained sandstones. Well-developed paleosols suggest warm, humid environmental conditions with intense weathering and floodplains that remained stable for thousands of years.

Oligocene channels of the Brule Formation contain bars dominated by trough cross-stratification, thick intervals (<3m) of climbing-ripple laminated fine- to medium-grained sandstones and soft sediment deformation. Channel beds display exposure surfaces with mammal tracks and mud-cracks. These features indicate that these streams were ephemeral and sometimes conduits for flows with high suspended sediment concentrations. Flow depths were 2 - 4 m and flow widths were 8 - 15 m. Channel sand bodies are 3 - 5 m thick and 1 - 1.5 times the flow depth. Floodplains contain abundant tabular, thin- to medium-bedded very fine grained, plane-laminated, ripple-laminated or structureless sandstones; paleosols are weakly developed, suggestive of a cooler, drier climate and/or dynamic floodplains.

The data suggest that, across the EOT, rivers changed from perennial streams with low avulsion frequency and stable floodplains to shallower, ephemeral streams with significant discharge variability, high avulsion frequency, and more dynamic floodplains. These results highlight the sensitive coupling between global climate change and the dynamics of rivers and floodplains.