Epeirogenic and Climatic Controls of Early Pleistocene Fluvial Sediment Dispersal in Nebraska

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.

Regional implications of the geology of the Ogallala Group (upper Tertiary) of southwestern Morrill County, Nebraska, and adjacent areas

“…Measurements made on the ten largest clasts by Breyer (1974Breyer ( , 1975 and by Swinehart (1979) generally are lower than those reported earlier by Stanley and Wayne (1972) at the same sample sites. A field check at the sites supported the values cited in the later reports.…”

“…Distinctive Gravel Sizes and Mineralogies Stanley (1971) and Stanley and Wayne (1972) believed that Ogallala gravels could be separated from post-Ogallala gravels in western Nebraska, and they made a number of assertions related to the general topics of sediment dispersal, source areas, and stream competence for Ogallala and early Pleistocene gravels. Stanley and Wayne (1972) stated that Ogallala gravels in western Nebraska .are generally free of anorthosite, that with a few exceptions the presence of anorthosite and larger clast size may be used to separate early Pleistocene from Ogallala gravels, and that the main early Pleistocene drainage way in the Nebraska Panhandle was through the ancestral Pumpkin Creek Valley. They also concluded that Pleistocene streams were more competent than Ogallala streams in western Nebraska.…”

Hydrostratigraphic interpretation of test-hole and geophysical data, Upper Loup River Basin, Nebraska, 2008-10

“…These sediments, from sources in central Wyoming and northern Colorado ( fig. 1) (Stanley and Wayne, 1972) are unevenly deposited and preserved but contain coarse sand and gravel separated by finer-grained deposits covering the Ogallala in much of the upper Loup study area. Pliocene and Pleistocene fluvial deposits average 50 ft (15 m) in thickness, but can be as thick as 300 ft (91 m) (Swinehart and Diffendal, 1989), and are in hydrologic connection with the underlying Ogallala.…”

Architecture, heterogeneity, and origin of late Miocene fluvial deposits hosting the most important aquifer in the Great Plains, USA

“…Abbreviations of USA states are: Colorado (CO), Iowa (IA), Kansas (KS), Nebraska (NE), New Mexico (NM), Oklahoma (OK), South Dakota (SD), Texas (TX), Wyoming (WY). Stanley and Wayne, 1972;Witzke and Ludvigson, 1990) suggest that late Miocene drainages with paleo-headwaters in the Rockies continued from present-day Nebraska across Iowa and into the ancestral Mississippi River (Figure 2). …”