Natural Resources, School of
Date of this Version
10-1993
Document Type
Article
Abstract
Evidence of the climatic conditions in the mountain sediment source areas for the late Tertiary Ash Hollow Formation of the Ogallala Group in western Nebraska has been sought in an SEM (scanning electron microscope) study of surface features on detrital quartz grains from these deposits. The following lines of indirect evidence raised the possibility of glacial conditions: uplift of up to 3500 m in the source areas, paleohydrologic parameters indicative of a transport system analogous to the modern North Saskatchewan River, and possible cold climate surface features on cobbles from the deposits. SEM analysis shows that five of the nine samples from the deposits have similar distributions of surface features, indicating that they have been influenced by similar environmental processes. On angular grains with high to medium relief, large breakage blocks, conchoidal fractures, straight and arcuate steps, and fractured plates or planes are abundant. Striations, straight, and curved grooves or scratches, small breakage blocks, mechanical V-shaped pits, and meandering ridges are also present. Quartz grains in the remaining samples are so extensively altered by dissolution and precipitation of silica that their environmental histories cannot be determined. The surface feature distributions of the five similar samples were compared with distributions from glacial, colluvial, source rock weathering, and glacio-fluvial environments reported previously from other geological settings. The Ash Hollow distributions corresponded closely only to the glacial and glacio-fluvial distribution in the range and abundance of mechanical surface features and in grain morphology. These results, considered with the other evidence, suggest a probable glacial history for at least part of the Ash Hollow deposits in western Nebraska.
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Comments
Open access material