RECONSTRUCTING THE LATE MIOCENE DEPOSITIONAL ENVIRONMENT OF THE OGALLALA FORMATION IN THE CENTRAL HIGH PLAINS, USA

The late Miocene Ogallala Formation underlies most of the North American High Plains, though is poorly studied in the central High Plains region where only a small fraction of its total thickness is exposed. One exception is in western Kansas where up to 35 m of the Ogallala Formation crop out along the bluffs of Ladder Creek Canyon. These deposits consist of stacked and laterally extensive, multi-story channel bodies dominated by sandy bedforms with little or no intervening floodplain mudstones. We interpret each ~2–4 m thick story as a sandstone bed formed during one depositional episode and infilling a broad and relatively shallow braided river channel. Each story preserves elements suggesting successive, likely seasonal, high and low discharge conditions, or the complete abandonment of the river channel after active streamflow has either migrated away from the study area or ceased regionally for a relatively long period of time. High discharge conditions are characterized by channel-filling, current-formed transverse bars with downstream and lateral accretionary bounding surfaces, laminated sand sheets, and gravel-rich longitudinal bars deposited while the channel was largely flooded and nearly the entire river bed mobilized. Low discharge conditions are indicated by channel-belt-wide subaerial exposure and pedogenesis of stabilized bar surfaces for a period of time sufficient to promote colonization by soil-burrowing organisms and moderate pedogenic modification. The remaining flow was confined to narrow and low-sinuosity braided channels flowing between or incised into exposed bar units. Abandonment of the broader channel belt is suggested by advanced calcretization within a story and the presence of aerially restricted and fossiliferous mud-filled channel pools where the final repositories of surface water attracted local paleofauna before in-filling or drying completely. The Ogallala Formation in the study area is most similar to the modern Platte River, a relatively shallow, sand-dominated braided and low-sinuosity river system. Comparisons with previously studied localities in the northern and southern High Plains reveal differences in fluvial style, sediment source areas, cut-and-fill geometries, and eolian input.