AIRBORNE GEOPHYSICS RESOLVES STRATIGRAPHIC AND HYDROGEOLOGIC COMPLEXITY AT THE EASTERN BOUNDARY OF THE GREAT PLAINS IN NEBRASKA

Northeastern Nebraska is the only place in the Midcontinent in which upper Neogene clastic sediments of the High Plains aquifer are juxtaposed against dominantly fine-grained Laurentide glacial deposits. Abrupt contrasts in topography, geomorphology, and hydrogeology are common in the area. Resolving stratigraphic and hydrogeologic relationships is critically important in developing a systematic understanding of the High Plains aquifer setting. We present new interpretations of airborne geophysical survey data that offer novel insights into these relationships. Three-dimensional geologic models constructed with integrated geophysical and borehole data reveal the geometries of sediment bodies and their stratigraphic architecture, cross-cutting relationships of fluvial channel fills, and the depositional topography of glacial landforms. Buried valleys, interpreted to have formed at the ice margin, are mapped in unsurpassed detail. Valley-fills are as much as 10 km wide and 50 m thick. East-west trending valley-fills are cut by northwest-southeast trending valley-fills, and both groups of fills are truncated by Laurentide glacial deposits to the east. Both non-glacial and glacial deposits in the study area show evidence for the inversion of topography, suggesting a complex history of incision, filling of accommodation, and landscape evolution at the glacial margin. Such details provide a fresh look at the geologic controls on groundwater flow, the genesis of aquifers and confining units, and the presence of hydraulic barriers and windows. Our research significantly illuminates the geologic history of the eastern margin of the Great Plains and promotes the protection and management of groundwater supplies.