T244 Sources, Sinks, Transport, and Storage of Sediment in Fluvial Systems (Posters)

Wednesday, 18 October 2023
8:00 AM - 5:30 PM
Hall B

RECONSTRUCTIONS OF HYDRODYNAMICS AND SEDIMENT TRANSPORT THROUGH EPHEMERAL STREAMS AT THE ONSET OF THE OLIGOCENE IN NORTH AMERICA

Paper number: T244-11
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

The onset of the Oligocene at 33.9 Ma is associated with a global transition from greenhouse to icehouse conditions. It was characterized by short, intense fluctuations in temperature and moisture superimposed upon a long term trend of cooling and drying. In North America, the Eocene-Oligocene transition is characterized by increased aridity and intensification of the North American monsoon. Thus far, the impacts of altered temperature and moisture availability on the water and sediment discharge in Oligocene rivers have remained poorly constrained. To elucidate the impact of climatic variability on river dynamics and related natural hazards, we characterize river hydrodynamics and sediment transport from White River Group fluvial strata associated with a short and intense cooling and drying event at the onset of the Oligocene in Nebraska.

Using preserved bar geometries, bed thicknesses, sedimentary structures and grain-size, we estimated channel hydraulic geometry, scales of river bed topography, water and sediment fluxes, deposition rates and the magnitudes and durations of floods. Paleohydraulic estimates indicate that fluvial strata were deposited by meandering streams that were 2-4 m deep and 30-40 m wide at bankfull flow, and displayed approximate bankfull flood discharges of 100 m3/s and paleoslopes that were approximately 6 x 10-4. We estimate that bankfull floods lasted for hours, and were associated with high suspended sediment deposition rates. Estimates from the thicknesses of climbing ripple laminated and climbing dune stratified beds yield suspended sediment deposition rates of approximately 0.87 m/hour. Channel beds were characterized by dunes that were approximately 0.4 m high and bedload fluxes were 2 x 10-4 m3/s. These streams were ephemeral. Evidence of rapid deposition from waning floods are preserved in sub- to super-critically climbing ripples and dunes, and soft sediment deformation; mammal tracks and mud cracks on the channel bed indicate that channels were intermittently dry in periods between high flow.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-393329

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