Reconstructing late Miocene depositional environments in the central High Plains, USA: Lithofacies and architectural elements of the Ogallala Formation

Abstract

The late Miocene Ogallala Formation hosts much of the High Plains aquifer of North America, though it 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 40 m of the Ogallala Formation crop out along the bluffs of Ladder Creek Canyon. These deposits consist of stacked and laterally extensive, multistory channel bodies dominated by sandy bedforms with little or no intervening floodplain mudstone. We interpret each ~2–4 m-thick story as a sandstone bed that formed during a single depositional episode and infilled a broad and relatively shallow braided river channel. Each story preserves elements suggesting high discharge conditions, followed by low discharge or the complete abandonment of the river channel after active streamflow 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, gravel-rich longitudinal bars, and sand sheets deposited while the channel was largely flooded and nearly the entire riverbed mobilized. Low discharge conditions are indicated by lens-shaped, trough cross-stratified channels scoured into previously deposited bars. Channel-fill deposits were formed when the remaining flow was confined to one or more narrow streams that braided across the exposed channel belt. Persistent low discharge conditions are suggested by channel-belt-wide subaerial exposure of bar surfaces for a period of time sufficient to promote colonization by soil-burrowing organisms and moderate soil development. Abandonment of the broader channel belt is indicated 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 infilling or drying completely. The depositional environment of the Ogallala Formation in the study area is most similar to “Platte type” fluvial systems characterized as shallow, perennial, and sand-dominated braided rivers. 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.