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

摘要

A part of a filled Ogallala Group (upper Tertiary) paleovalley system exhumed by recent stream erosion in southwestern Morrill County, Nebraska, and adjacent areas contains steep gradient gullies filled with locally derived sediments. Younger channels filled with granitic gravels derived by stream erosion of the Rocky Mountains to the west and southwest cut across and through these older filled gullies, indicating that there were at least two periods of downcutting followed by periods of filling during the development of this system. Individual clasts from younger Ogallala gravels are often as large as or larger than clasts from Quaternary gravels transported comparable distances by streams. This fact supports the idea that streams of Ogallala age were often at least as competent as their Quaternary counterparts. In a tributary to Greenwood Creek, six major volcanic ash lentils occur in superposition. These ash deposits range in geometry from tabular to channel-shaped and were deposited on land surfaces or in ponds and gullies at different times during the deposition of the Ogallala Group. Some of the lentils grade laterally into diatomites. Caliches are common throughout the Ogallala Group. Major caliches are developed in older rocks directly beneath the Ogalalla and in sediments beneath the eroded upper surface of the group. Caliche horizons occur at many horizons within the group and have "tepee," honeycomb, and other typical structures. The preceding observations are in part directly opposed to observations made by earlier workers for the Ogallala in general or specifically for the study area. The Ogallala Group should be re-examined to see whether the earlier models or those offered here better explain its geologic history.