Natural Resources, School of
Date of this Version
1995
Document Type
Article
Citation
United States Geological Survey Information Services
Miscellaneous Investigations Series Map I-2496
Abstract
Morrill County, Nebraska, is perhaps the most geologically interesting county in the panhandle of Nebraska. Surficial deposits of alluvium, colluvium, loess, and eolian sand of Quaternary age cover large parts of the county. Holocene and latest Pleistocene(?) eolian sand as much as 150 It thick covers much of the area north of North Platte Valley and mantles Quaternary alluvial deposits to the south. A thin veneer of loess overlies bedrock in the uplands of the southern part of the county. Alluvium underlies the valley floors and caps river and stream terraces. Two units of older Quaternary alluvium are exposed in the North Platte River and Pumpkin Creek drainages. Alluvium and lake deposits of the Broadwater Formation (Pliocene) are exposed along the north side of the North Platte River valley. Although only remnants of the formation are preserved, they define the general shape of the bedrock valley in which the formation was deposited. The south side of the ancient valley is close to the south side of the present North Platte Valley. In the northwestern part of the county, the north side .of the paleovalley has been eroded away, so its former position is unknown. From Broadwater eastward, however, the general valley shape can be inferred from outcrop and drill-hole data. The valley was at least 450 ft deep and as much as 5 mi wide in the southeastern part of the county (Swinehart and Diffendal, 1987). Loose gravel and sand, conglomerate, pebbly sandstone, sandstone, siltstone, diatomite, volcanic ash, calcrete, and silcrete of the Ogallala Group (upper and middle Miocene) crop out along the north side of the North Platte Valley from the Morrill-Garden County line westward and are mostly buried beneath the Sand Hills to the north. Remnants of the Ogallala Group rocks cap the highest parts of Wildcat Ridge in the west-central part of the county. Ogallala rocks also filled a paleovalley system in the southern part of the county, and deposits of high-gradient streams that were tributary to the main valleys are well exposed there (Diffendal, 1982; Swinehart and Diffendal, 1987). Eolian and fluvial volcaniclastic deposits of the Arikaree Group (lower Miocene and upper Oligocene) crop out along the north side of the North Platte Valley and underlie the tableland and Sand Hills area to the north. They crop out on the flanks of Wildcat Ridge and underlie the isolated erosion remnants, Jail and Courthouse Rocks, to the east. Conglomerate containing pumice clasts is present locally in the basal part of the Gering Formation (upper Oligocene). Volcaniclastic eolian sedimentary rocks of the White River Group (lower Oligocene and upper Eocene) crop out along the north side of the North Platte River valley both east and west of Broadwater, and along the south side from Bridgeport westward. Siltstone of the Brule Formation (lower Oligocene) of the White River Group also crops out along both sides of Pumpkin Creek Valley and its tributaries in the southwestern part of the county. A variety of geomorphic features are represented in the county. Several forms of sand dunes are present in the northern part of the county and also overlie Quaternary terraces along the North Platte River and Pumpkin Creek (Swinehart, 1990). Strath terraces, fill terraces, and the axial deposits of tributary and trunk streams are present in the river and creek valleys (Diffendal and Corner, 1983; Swinehart and Diffendal, 1987). Evidence for Quaternary stream piracy within the county has been documented by Dilfendal (1984). Folding has affected the rocks (both at the surface and in the subsurface) in the eastern part of the county (Diffendal, 1980). Folds developed and were reactivated periodically throughout the Phanerozoic (Sonnenberg and Weimer, 1981; Swinehart and others, 1985). This map is necessarily generalized. For more detailed information, consult the files of the Conservation and Survey Division, University of Nebraska. Previously published smaller scale geologic maps that cover Morrill County include those by Darton (1903a,b), Lugn (1939), and Weeks and Gutentag (1981). We thank V.L. Souders (Conservation and Survey Division), J.J. Gottula (Nebraska Department of Environmental Quality), R.G. Goodwin (Woodward-Clyde), and Conservation and Survey Division field assistants S.S. Goodwin, Jr. , M.E. Rebone, and K.K. Kolster for their help in the field. We thank especially the landowners in Morrill County who gave us access to their properties.