Three‐dimensional architecture and hydrostratigraphy of cross‐cutting buried valleys using airborne electromagnetics, glaciated Central Lowlands, Nebraska, USA

Abstract

Buried valleys are characteristic features of glaciated landscapes, and their deposits host important aquifers worldwide. Understanding the stratigraphic architecture of these deposits is essential for protecting groundwater and interpreting sedimentary processes in subglacial and ice-marginal environments. The relationships between depositional architecture, topography and hydrostratigraphy in dissected, pre-Illinoian till sheets is poorly understood. Boreholes alone are inadequate to characterize the complex geology of buried valleys, but airborne electromagnetic surveys have proven useful for this purpose. A key question is whether the sedimentary architecture of buried valleys can be interpreted from airborne electromagnetic profiles. This study employs airborne electromagnetic resistivity profiles to interpret the threedimensional sedimentary architecture of cross-cutting buried valleys in a ca 400 km 2 area along the western margin of Laurentide glaciation in North America. A progenitor bedrock valley is succeeded by at least five generations of tunnel valleys that become progressively younger northward. Tunnel-valley infills are highly variable, reflecting under-filled and over-filled conditions. Under-filled tunnel valleys are expressed on the modern landscape and contain fine sediments that act as hydraulic barriers. Over-filled tunnel valleys are not recognized in the modern landscape, but where they are present they form hydraulic windows between deep aquifer units and the land surface. The interpretation of tunnel-valley genesis herein provides evidence of the relationships between depositional processes and glacial landforms in a dissected, pre-Illinoian till sheet, and contributes to the understanding of the complex physical hydrology of glacial aquifers in general.

Combining Clustering Methods With MPS to Estimate Structural Uncertainty for Hydrological Models

“…Large spatial coverage can be obtained using airborne electromagnetic (AEM) methods. The SkyTEM system (Sørensen and Auken, 2004) is one of the available AEM methods that have been used extensively for groundwater mapping (Møller et al, 2009b;Pryet et al, 2012;Korus et al, 2017;Knight et al, 2018). By inverting the EM dataset a model of the electrical resistivity of the subsurface can be produced.…”

Combining Hydraulic Head Analysis with Airborne Electromagnetics to Detect and Map Impermeable Aquifer Boundaries

“…The wide spacing of the SkyTEM profiles (~5 km) prevented the use of geostatistical modeling because the spacing exceeded the spatial dimensions of the geologic features. With the RESOVLE survey, resistivity contrasts defining the bounding surfaces of the geological features of interest varied from place-to-place, so a cognitive approach, rather than a geostatistical one, was used to map the area [45].…”

“…The paleovalley fill contains abundant sand and gravel, forming an important aquifer in the region. The aquifer is locally truncated by younger deposits associated with subglacial tunnel valleys [45]. These valleys are narrow (1-3 km), steep-sided (10°-30°), and linear to slightly sinuous.…”

“…The depth of investigation varies between ~50-80 m, which is sufficient to penetrate the entire thickness of unconsolidated materials above bedrock in the studied area. A detailed, 3D geological model has been presented in previous work [45,46]. Resistivity-depth profiles show a zone of high resistivity (>20 ohm-m) in the southwest and a zone of low-resistivity (<20 ohm-m) in the northeast (Figure 9).…”

“…Site 3, near the Village of Firth, is located on the southern margin of a 12-km wide, west-east trending buried valley that extends at least 180 km eastward of the margin of the High Plains aquifer [45]. The paleovalley fill contains abundant sand and gravel, forming an important aquifer in the region.…”