GEOPHYSICAL MAPPING OF THE TEAYS-MAHOMET BEDROCK VALLEY REVEALS POSSIBLE HYDRAULIC WINDOW

Research funded by the Illinois Board of Higher Education (IBHE)

Among sedimentary environments, glacially related sedimentary 3-D architectures are the most difficult ones to describe. The sedimentary models must integrate features related to ice as well as meltwater with various physical conditions within an ice sheet and account for modifications resulting from multiple glaciations. The till-sheets and loess of central Illinois form the most productive agricultural soils in America and their very low permeability confines and protects deeper groundwater aquifers from human-generated contaminants. Coarse, meltwater-related sediments act as aquifers and provide aggregate material.

Within the buried Mahomet Valley in central Illinois, the glacial sediments are characterized as multiple aquifers and aquitards. The valley fill of the buried fluvial valley (Mahomet sands) is the most important aquifer in central Illinois. Subsequent glacial events have deposited two major groups of till units, the Glasford Formation (Illinois Episode) and the Wedron Group (Wisconsin Episode). Hydro-chemical studies suggest that the unusually young water in the Mahomet sand aquifer may be connected to overlying aquifers.

In collaboration with Illinois State University students, Bloomington, IL, we have applied geophysical techniques to improve the understanding of the 3-D sedimentary architecture for a mapping project. Our high-resolution geophysical imaging included 20 km of P-Wave (compression), 7 km of SH-Wave (horizontal shear) seismic reflection, and 1.7 km of resistivity profiles. VSP (Vertical Seismic Profiles) and gamma logs conducted in 6 boreholes provided calibration. The most surprising results were not associated with the Mahomet sands, but with the aquitards overlying sediments. A 1 km wide and 50 m thick Illinois Episode subglacial tunnel-valley eroded through the Glasford till then was filled with sand. Similar features can be observed at the surface on the Illinoian till plain, 40 km further south. The modern Sangamon River flows in a 30 m thick coarse-sediment channel that cuts through the Wedron till. We suggest that where these two channel features cross, a hydraulic window connects shallow aquifers, and possibly the surface river water, with the lowermost aquifer.