LATE CENOZOIC PEDOGENESIS AND LANDSCAPE EVOLUTION AROUND THE GLACIAL LIMIT IN SOUTHEASTERN NEBRASKA

Jefferson County, Nebraska is a diverse area in terms of Late Cenozoic landscape evolution. South of the Little Blue River, the apparently unglaciated Rose Creek Escarpment has thin, relatively weakly-developed modern soils on slopes underlain by Cretaceous bedrock. Some of these soils are nonetheless distinguished by stone zones in colluvium derived from the Greenhorn Limestone (Kgh). Others, developed on the upper Graneros Shale (Kg) and Dakota Formation (Kd), show acid weathering and secondary gypsum and jarosite; exotic sulfates (e.g., copiapite, alunogen) are produced locally by stronger acid weathering in fresh lower Kg and upper Kd bedrock. On ridgetops and the escarpment's crest, well-developed modern soils on thin Peoria Loess overlie thin Gilman Canyon Formation and prominent, clayey Sangamon Geosol profiles with strong soil structure and thick argillans. The Sangamon Geosol directly overlies heavily-weathered Kgh or thin colluvium derived from it. The lower, shaly Kgh has undergone widespread weathering by oxidation and extensive deformation and brecciation by creep, probably under periglacial conditions. Clasts of Kgh in sub-Sangamon colluvium show circumgranular cracking, neomorphic microspar, and ferruginization. At one ridge-crest site, intense weathering has produced hard, porous, goethitic ironstone within joints in the Kgh, just below the modern solum. The glaciated area north of the Little Blue River, in contrast, is underlain by a thick sequence of Loveland Loess, pre-Loveland silts (probably in part loessic), western-source sands and gravels, and glacial till. This contrast indicates major differences in landscape evolution, along a transect of less than 10 km, since the initial Pliocene glaciation.