RIDGETOP LOESS PEDOSTRATIGRAPHY: A KEY TO UNDERSTANDING LANDSCAPE EVOLUTION IN EASTERN NEBRASKA

Broad ridgetops in eastern Nebraska accumulated multiple pedogenically-altered loesses over the past 500-600 kyr. Episodic loess deposition and net accumulation, fluctuating depositional rates, and subsequent pedogenic response constitute a discrete geomorphological system that distinguishes upward-accreting ridgetops within the context of regional landscape evolution. The nature of the loess succession strongly suggests a Middle to Late Pleistocene increase in the rate of accumulation, averaged over glacial-interglacial cycles, an interpretation consistent with existing geochronological controls. Pedologic evidence plays a crucial role in this interpretation. The basal part of the sequence (Middle Pleistocene) is a stack of thin loess units with pervasive, strong soil structure and illuvial clay. The pervasiveness of these features indicates a slow rate of accumulation, in effect allowing formation of a single polygenetic soil profile as much as 7 m thick. The overlying Late Middle Pleistocene Loveland Loess (11-12 m) shows comparatively weaker evidence for pedogenesis, except for the well-developed Sangamon Geosol in the upper 2-3 m of the unit. Below the Sangamon Geosol, the Loveland Loess contains multiple fine-grained horizons with weak to moderate soil structure, evidencing episodes of slower loess accumulation. Radiocarbon dating indicates that the thin (< 1 m) Late Pleistocene Gilman Canyon Formation accumulated slowly atop the Sangamon Geosol. The Gilman Canyon Formation has strong granular to very fine blocky structure and abundant evidence for soil faunal activity. The overlying Late Pleistocene Peoria Loess, however, displays very little evidence for pedogenesis throughout its thickness (as much as 20 m).