LATE EOCENE AND EARLY OLIGOCENE TOPOGRAPHY OF THE WESTERN GREAT PLAINS

The topography of the western Great Plains alternated between incised valley-systems to relatively featureless fluvial and loessic plains during the deposition of the late Eocene to early Oligocene White River sequence. The White River rests on an erosional unconformity with as much as 250 m of relief along the Rocky Mountain front, but decreasing to 50 m away from the mountains. This erosional surface is locally covered by an unusual gravel consisting of siliceous clasts, primarily quartz, chert, quartzites, and silica-cemented sandstones. These gravels derived from Precambrian and Paleozoic rocks of the Rocky Mountain front and Black Hills but were highly weathered leaving only siliceous clasts. As erosion cut the Mesozoic and older Paleogene rocks, the gravels were draped over the hills and valley sides at the start of White River deposition. The late Eocene rocks of the Chadron Formation include fluvial complexes in the paleovalleys, and highly weathered loessic deposits in the interfluves. The Chadron was eroded in western South Dakota and northeastern Colorado before deposition of the early Oligocene Brule Formation. Hills of Chadron rocks were buried first by materials eroded from the Chadron highs mixed with volcaniclastic dust that was weathered into mudstones and later by widespread fluvial deposits carrying volcaniclastic dust, ending in widespread plains with little local relief. This fluvial plain became buried by volcaniclastic loesses flattening what little topography existed. Episodically these loess deposits were cut by river valleys that subsequently were filled and covered by more loess. These depositional features reflected major decrease in moisture from subhumid to semiarid conditions in the Great Plains during the Eocene-Oligocene climatic shift, with moist intervals of limited duration resulting in the paleovalley sequences within the upper loessic deposits during the early Oligocene.