CROSS-CUTTING BURIED VALLEYS AT THE EASTERN MARGIN OF THE NORTHERN HIGH PLAINS AQUIFER: GENESIS, EVOLUTION, AND GEOHYDROLOGIC BEHAVIOR

Bedrock paleovalleys at the base of the Cenozoic succession under eastern Nebraska and northeastern Kansas, as well as their relationships with overlying strata, are very poorly understood. Improved understanding will be a necessary component in comprehensive groundwater characterization and management at the eastern margin of the High Plains aquifer (HPA). Recent investigations using airborne electromagnetic (AEM) surveys, coupled with borehole data and 3-D modeling, uniquely detail the complex stratigraphic architecture of paleovalley fills. We have documented the sedimentary architecture of cross-cutting buried valleys in a ~400 km² area in southeastern Nebraska near the western margin of the Laurentide glacial margin. We reconstruct a progenitor bedrock paleovalley succeeded by at least five generations of northward-younging, sub-glacial tunnel valleys. Tunnel valley infills are highly variable, apparently reflecting underfilled and overfilled conditions. Underfilled tunnel valleys are incompletely filled with sediment and contain predominantly silt, clay, and diamicton; further, they are expressed on the modern landscape as small drainages and valleys. They form hydraulic barriers that laterally disconnect the paleovalley aquifer from the HPA proper. Overfilled tunnel valleys, in contrast, consist mostly of sand and gravel of valley-fill and outwash origins; they form hydraulic windows between deep aquifer units and the land surface. These interpretations underscore the complex physical hydrology of the eastern margin of the northern HPA and also provide a fresh outlook on the relationships between depositional processes and glacial landforms in a dissected, pre-Illinoian till sheet.