Paleohydrology of Some Ogallala (Neogene) Streams in the Southern Panhandle of Nebraska

Stratification and estimated paleoflow conditions for valley-fill deposits suggest that Ogallala Group streams in western Nebraska were similar to modem streams of south-central Alberta. Ogallala stratification includes medium-scale (0.5 to 2. O m thick) trough crossbedded sand and gravel, tabular indistinctly horizontally bedded and imbricated gravel, and horizontally bedded sand and pebbly sand. Valley fills are 15 to 55 m thick and 800 to 1800 m wide at the top. Some are in bedrock-floored channels resembling the “inner channels” of Shepherd and Schumm (1974). Gradients for three well exposed paleovalley floors range from 0.0014 to 0.0020 (m/m) after tectonic correction. This compares with 0.00135 for the modem North Platte River Valley in Nebraska. The average intermediate diameters of the 10 largest clasts from tabular gravel beds found at 17 sites varied between 0.077 and 0.15 m. The average median intermediate diameter for gravel from four well exposed tabular gravel beds is 0.024 m. Consistent paleodepth estimates of about 2 m correlate well with the scale of cross-stratification observed in the valley fills. Paleovelocities are estimated at about 2 m/sec, and Froude numbers of about 0.4 are consistent with a lower flow regime in the stability field of dunes. Two-dimensional specific in-channel paleodischarges were 3 to 4 m2/sec. Total paleodischarge estimates based on slopedischarge relationships for gravel-bed rivers range from 340 to 1240 m3/sec and are comparable to average annual peak discharges on the North Platte River reported 80 to 90 years ago. Ogallala streams were probably dominated by macroforms similar to the “crescent-shaped bars” of the North Saskatchewan River. Deposition also took place on longitudinal bars in deeper channels. Shallow upper-flow regime transport and deposition is recorded by horizontally bedded sand and pebbly sand. INTRODUCTION Historical reviews have been prepared by Schultz (1977) The Ogallala Group (Neogene) is a continental deposit of markedly variable thickness that covers most of the southern third of the Nebraska panhandle (Fig. 1). The sediments and sedimentary rocks of the group are dominantly epiclastic and alluvial. Airfall volcanic ash, lacustrine diatomaceous beds, and pedogenic calcareous and siliceous horizons occur within the sequence. Source areas for the epiclastic component were primarily in the Southern Rocky Mountains of southern Wyoming and northern Colorado, but some sediments were derived locally in Nebraska from stream erosion of older Tertiary deposits. The purposes of this paper are to review briefly the stratigraphic and sedimentologic work done on the Ogallala Group in western Nebraska since 1970, to provide site-specific data on the sediments of individual valley fills and the paleovalleys they occupy where possible, and to provide insight into the nature and behavior of the fluvial systems that produced the group.