Reconstruction of pre-Illinoian ice margins and glaciotectonic structures from airborne ElectroMagnetic (AEM) surveys at the western limit of Laurentide glaciation, Midcontinent U.S.A.

Early and early Middle Pleistocene glaciations in midcontinental USA are poorly understood relative to more recent Illinoian and Wisconsinan glaciations, largely because pre-Illinoian glacial landforms and deposits are eroded and buried. In this paper, we present a new interpretation of buried, pre-Illinoian glacial features along the Laurentide glacial margin in northeastern Nebraska using Airborne ElectroMagnetics (AEM) supplemented with borehole logs and 2 m LiDAR elevation data. We detect and map large-scale (101–102 km) geological features using contrasts in electrical resistivity. The Laurentide glacial limit is marked by a continuous (>120 km) contrast between conductive (<15 Ω-m), clayey tills and resistive (>40 Ω-m) sandy sediments. Several smaller (102 km2) till salients extend 10s of km westward of this margin. We recognize a lithologically heterogeneous zone characterized by variable resistivity and complex geophysical structures extending as much as 17 km west of the glacial limit. This zone is interpreted as a glaciotectonic thrust complex, and it is analogous to a similar thrust complex in Denmark where structural analysis of co-located seismic and AEM surveys provides a standard for comparison. Our study suggests that the maximum advancement of pre-Illinoian glacial ice into Nebraska involved extensive deformation of sedimentary strata, local overriding of these deformed strata by smaller ice tongues, and emplacement of tills as much as 30 km west of the principal Laurentide ice margin. These insights provide the first glimpse of the large-scale stratigraphic architecture of glacial sediments in Nebraska and point to future clarifications of the geology and geomorphology of the Laurentide glacial limit.