Abstract

The change from Pliocene to Pleistocene fluvial sedimentation in Nebraska is denoted by gravel with relative enrichment of mechanically weak rock species and a two-fold increase in largest clast size. These changes in fluvial sediments suggest modification in degradational energy affecting detritus apparently related to deterioration of climate in the early Pleistocene. Cooler Pleistocene climates with increased moisture resulted in greater discharge and carrying capacity for streams headed in the Rocky Mountains and flowing across Nebraska. These streams carried granitic detritus eastward toward the continental glacier margin in easternmost Nebraska. There, streams flowing off ice sheets carrying sedimentary and metamorphic detritus derived from the ice joined the east-flowing streams from the mountains. Detritus derived from continental glaciers in easternmost Nebraska, therefore, was not transported westward, but instead, was mixed with Rocky Mountain–derived detritus and transported southward along the ice-front margin.

Even though the drainage basin of the Platte River system came into existence in the Tertiary, the present course of the Platte River dates only from mid-Pleistocene time. Widespread occurrence of lower Pleistocene braided channel deposits east of the Chadron–Cambridge Arch that contain Laramie Range–derived anorthosite indicates repeated channel switching and meandering during times of aggradation over surfaces of minimal relief. Relations of these gravels to the Chadron–Cambridge and Siouxana Arches suggest that uplift on these structures was sufficient to deflect and control the course of streams headed in the Laramie Range and flowing across the plains. Activity on the Chadron–Cambridge Arch also is suggested by the distribution of earthquake epicenters, modern drainage patterns, and the relation of the pre-Pleistocene bedrock surface to the arch and the profile of the Platte River. The presence of knickpoints on rivers crossing the arch suggests that these rivers are maintaining a course antecedent to a spasmodically rising arch. Rivers are entrenched now, but during the early Pleistocene when streams carried a heavy load of sand and gravel, similar activity along the Chadron–Cambridge Arch would have been adequate to spill the aggrading stream over its fan and divert it southward where it could follow a new course. Eastward-flowing streams heading in the Rocky Mountains were controlled by changes in discharge of streams and movement on epeirogenic structures during the early Pleistocene.

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