SLOPE PROCESSES AND SHALLOW BEDROCK DEFORMATION NEAR THE GLACIAL LIMIT IN SOUTHEASTERN NEBRASKA

The western limit of Late Cenozoic glaciation in Jefferson and eastern Thayer counties in Nebraska contains important information about the evolution of landscapes near the eastern margin of the Great Plains. The distributions of erratics and putative tills indicate that the westernmost extent of glaciation in this area was the Little Blue River (LBR). Bedrock-dominated terrains with thin (0-5 m) regolith (weathered bedrock, Sangamon Geosol or SG, Gilman Canyon Formation or GCF, and Peoria Loess or PL) dominate the area south and west of the LBR, whereas flatter terrain with much thicker (as much as 40 m) regolith, including tills, dominates to the north and east.

The Rose Creek Escarpment (RCE) is a prominent bedrock upland south of the LBR that represents the northernmost extension of the Smoky Hills physiographic province. It contains some of the most intriguing geomorphic features of the study area. Higher slopes on the RCE that are underlain by the lower part (Lincoln-Hartland members) of the Late Cretaceous Greenhorn Limestone (Kgh) show widespread evidence for extensive mass movement by creep, as well as later gully erosion and filling. The Kgh directly underneath these slopes exhibits downslope folding, thrusting, shearing, brecciation, and fracturing in soft, thinly-laminated shaly limestones within 4 m of the land surface. Folds range from gentle to tight, and some are even recumbent. At one site, a large volume of the lower Kgh is no less than a chaotic breccia of 2-20 cm fragments of shaly limestone. Thrusts typically involve tabular masses of Kgh several meters long that appear to have detached or moved along basal bentonitic shale seams. Masses of translocated strata usually dip gently (<15°), but a few dip as steeply as 47°.

Major slope deformation on the RCE does not appear to be an ongoing process. Rather, it is likely to have taken place under wetter conditions and possibly in association with freeze-thaw and solifluction. The SG is developed directly atop weathered and fractured Kgh on flat summits near the crest of the RCE. Therefore, slope deformation may date to Illinoian or even pre-Illinoian times.

This research was supported by a U.S.G.S. STATEMAP grant to the Conservation and Survey Division, University of Nebraska-Lincoln.