Robert F. Diffendal

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

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.

Armored Mud Balls and Friable Sand Megaclasts from a Complex Early Pleistocene Alluvial Fill, Southwestern Morrill County, Nebraska

Armored mud balls and friable sand megaclasts occur in exposures of a complex Early Pleistocene sand and gravel alluvial fill which caps a strath terrace in southwestern Morrill County, Nebraska. The mud balls are limited to a tributary arroyo. The sand megaclasts occur in sediments deposited along a trunk stream. It is possible that the sand megaclasts were transported to the site in a frozen condition.

Geomorphic and Environmental Change Around a Large, Aging Reservoir: Lake C. W. McConaughy, Western Nebraska, USA

Lake C. W. McConaughy, a 63-year-old manmade reservoir in the North Platte Valley of western Nebraska, is the largest standing body of water in the state. From the time that Kingsley Dam was completed in 1941 until the present day (2004), many geomorphic and environmental changes have occurred along the shores and within the North Platte Valley. Erosion on the southeastern shoreline of the lake had been perceived as a problem for landowners and managers for at least three decades, but the full scale of erosion was revealed only after a cumulative 18-m drawdown in lake level: bedrock platforms, extending from cliffs on headlands, as well as minor caves, alcoves, potholes, and beach erosion scarps were revealed. Actual headland retreat since 1941 has probably been on a scale of magnitude of tens of meters. Erosional platforms, however, are as much as 266 m in length, indicating that they are, collectively, the results of a combination of pre-reservoir geomorphic conditions, erosion during reservoir filling, and wave erosion after filling. Serial observations of the shoreline made in the period 1999–2002 demonstrated that shoreline erosion continues. Depositional features such as pocket beaches and beach ridges have formed de novo in bays between headlands. Also, a delta that prograded at least 4 km from the North Platte River into the lake between 1952 and 1993 continues to grow, and major morphometric changes have occurred on the North Platte River immediately upstream since 1941 in response to the elevation of the local base level by the lake; most of the change in channel patterns in the river upstream from the delta took place in a mere 8 years from 1952 to 1960. Since 1999, some emergent beaches and areas of exposed lake floor have developed sand dunes; eolian erosion and re-deposition is widespread elsewhere in these areas during periods of high winds, which are frequent in western Nebraska. The water table beneath lands adjacent to the reservoir generally rose until 1953 as the lake was filling. The far-flung irrigation system of which Lake McConaughy is the key element has elevated water tables as far as 250 km from the lake. Since at least 2000, soil salinization (episodic thenardite accumulation) has developed in exposed lake sediments at the western end of the lake. On a smaller scale, groundwater has been discharging lakeward along shoreline beach scarps through springs and seeps while lake levels have been low. Slightly lowering the operating level of Lake McConaughy could slow some of the more dramatic effects of shoreline erosion when the lake returns to its pre-drought volume, but erosion will continue, particularly at the ends of headlands on the southeastern shore, which are exposed to strong waves driven by northerly to northwesterly winds. Eolian erosion and re-deposition will, on the other hand, continue as long as lake level remains particularly low as a result of drought and the removal of irrigation waters. Salinization can be considered an ephemeral phenomenon, dependent on future management of the lake, but it indirectly represents the potential for larger-scale changes in hydrogeologic systems. The longer-term effects of lake-related water-table changes, both near the lake and downstream in irrigated lands, as well as the ability of the lake to supply irrigation water if drought conditions continue, remain to be seen.

Asymmetrical distribution of Quaternary alluvial fills, Pumpkin Creek drainage basin, western Nebraska

A remnant alluvial fill of early Pleistocene age exposed in Pumpkin Creek Valley, Banner and Morrill counties, Nebraska, has yielded fossils of Mammuthus meridionalis (Nesti) and Equus sp. cf. E. scotti Gidley. Younger fill remnants of trunk and tributary streams allow a refinement of earlier views on the development of the Pumpkin Creek drainage basin during the Quaternary Period. Ancestral Pumpkin Creek both shifted to the north and entrenched its valley several times during the Quaternary Period leaving alluvial fills at three levels or more south of the present creek. Piracy of the headwaters of ancestral Pumpkin Creek took place after the last of these erosional events, probably in middle or late Pleistocene time. Asymmetrical distribution of alluvial fills occurs in Pumpkin Creek Valley, along the north side of parts of the North Platte River in western Nebraska, and along streams east of the Black Hills. The asymmetrical distribution of these deposits may have been due to structural warping, which could have caused these streams to shift their courses laterally and to entrench their valleys repeatedly.