RECENT INSIGHTS INTO QUATERNARY STRATIGRAPHY IN SOUTHERN LANCASTER COUNTY NEBRASKA AND ENVIRONS

Coring of Quaternary regolith around Lincoln, Nebraska establishes sediment-landform associations and the history of landscape development. Peoria Loess (PL) is thickest (5 m) on flat hill summits where little erosion has occurred since 12 ka. Four optical (OSL) age estimates produced from sand within PL near Roca, Nebraska range 22-15 ka and record local eolian mobilization of sediment from floodplains or colluvial aprons. Where PL directly overlies pre-Illinoian tills (PIT) its lower third to half is strongly reduced (deoxidized) due to the retarded infiltration of percolating water. Gilman Canyon Formation (GCF) loess, <1 m thick, is usually present under PL and has strong granular to subangular blocky soil structure. Loveland Loess (LL) is rarely preserved under PL and GCF, and then only under high, broad summits, particularly major drainage divides or dissected spurs thereof. Even under a single, broad summit, LL is discontinuous, indicating major early to middle Wisconsinan erosion. The Sangamon Geosol is prominent atop remnant LL in the study area, and similar weathering and soil development is also apparent atop PIT where tills are overlain directly by PL. In the valley of Salt Creek, PL, GCF, and probable LL-derived alluvial fan sediments, mantle old alluvium in "pseudoterraces" with muted relief. Preliminary OSL age estimates from this old alluvium range from >50 ka to >80 ka for two sites near Jamaica, Nebraska.

PIT reaches thicknesses of 30-35 m and the upper 10-20 m of PIT are always heavily oxidized and fractured. Unoxidized, grayish till, which never appears in natural exposures, locally contains fractures lined with gypsum resulting from the translocation of sulfates after oxidation higher in the section. The number of separate tills in the area is difficult to ascertain. At Roca, Nebraska, two tills can be identified, and in some test holes, sands 3-4 m thick appear within till successions, but most boreholes show no conclusive physical evidence for multiple tills. Laminated to rippled very fine sands and silts, interpreted as lacustrine deposits, underlie tills across the area. Coarser sub-till paleovalley-fill sands represent pre- or interglacial streams.

This research emerged from the STATEMAP geologic mapping program and the Eastern Nebraska Water Resources Assessment (ENWRA).