PALEOHYDRAULIC RECONSTRUCTIONS OF EPHEMERAL OLIGOCENE RIVERS AT THE ONSET OF THE NORTH AMERICAN MONSOON

The Eocene-Oligocene greenhouse to icehouse climate transition is associated with an approximate 6°C drop in global sea surface temperature and highly variable responses in terrestrial climate. The concomitant increase in monsoon intensity in North America is thought to have influenced continental precipitation patterns and aridity, thereby changing fluvial water and sediment discharges. However, the characteristics of river landscapes that existed after this change in global climate are poorly constrained. The Eocene-Oligocene deposits of the White River Group (WRG), exposed in Toadstool Geologic Park, Nebraska, provide an excellent opportunity to characterize the river dynamics at this time in detail.

We mapped bar dimensions, sedimentary structures, and particle size of the exhumed river and floodplain deposits of the Oligocene Orella Member. Flow depths, estimated using the thickness of fully preserved bar deposits, were 2 - 4m and the resulting fluvial sand bodies are 3 - 5m thick. Channel deposits are dominated by plane-laminated, trough-cross-stratified and climbing ripple-laminated, fine- to coarse-grained, poorly-sorted sandstones. Floodplain strata exhibit weakly developed paleosols and contain abundant thin to medium bedded tabular sandstones that are interpreted as deposits from frequent overbank floods. Exposure surfaces on channel beds, marked by well-preserved mammal tracks and weak mud cracks, suggest that these channels were intermittently dry. Paleohydraulic reconstructions from thick deposits of very fine-grained, sub- to super-critically climbing ripple laminated strata (0.2 - 3m), with abundant soft-sediment deformation, suggest that floods with durations of 20-120 hours, and high suspended sediment volumes frequently traveled through these channels.

These data indicate that early Oligocene Orella Member strata were created by intermittently dry, ephemeral rivers with significant discharge variability; high discharge events caused frequent floodplain inundation, and rapid in-channel deposition. This work highlights the connection between global climate, changing precipitation patterns, and fluvial morphodynamics, and informs our comprehension of river response to climatic change.