Abstract

The Cenozoic strata of western Nebraska are an extensive sequence of continental deposits that extend eastward from the Hartville, Laramie, and Front Range uplifts and southeast from the Black Hills. The oldest Cenozoic sediments (Chadron Formation, White River Group) are Early Oligocene alluvial valley fills. Subsequent to filling of these drainages and continuing for about the next 7 m.y., landscape development in western Nebraska was dominated by eolian deposition of tremendous volumes of rhyolitic volcanic ash derived from western eruptions. A plain of low relief, with only an occasional narrow drainage heading in the western highlands, was maintained during most of this period. Uplift in the Rocky Mountains and Great Plains (pre-Gering Formation, Arikaree Group) caused erosion and brought epiclastic detritus into the area about 28 m.y. ago. Eolian sediment consisting mostly of pyroclastic detritus continued building the plains during and after Gering alluvial deposition until about 19 m.y. ago when Arikaree deposition ceased. About this time, western volcanic activity declined for several million years and was followed by a marked decrease in the volume of rhyolitic volcanism for the remainder of the Cenozoic. At the end of Arikaree deposition in western Nebraska, a major pulse of erosion (pre-Runningwater Formation, Ogallala Group) was followed by a fundamental change in depositional style and landscape evolution, characterized by a heterogeneous mixture of epiclastic valley fills. Sands and gravels from Rocky Mountain sources were first deposited in a major valley in the northern half of the area and later in valleys to the south. Episodic regional and local structural movements influenced the size and position of many Ogallala valleys. For the past 5 m.y. degradation, in response to major regional uplift, has greatly exceeded aggradation as the dominant factor affecting landscape evolution in western Nebraska.