DEVELOPMENT OF A 3-DIMENSIONAL GEOLOGICAL MODEL OF BURIED VALLEYS IN EAST-CENTRAL ILLINOIS

In Midwestern United States, deep valleys formed in the glacial sediment or bedrock are concealed by thick sedimentary sequences formed during multiple Quaternary glaciations. These valleys are infilled with sediments holding significant supplies of groundwater, aggregate material, and shallow gas. In Illinois, buried valleys commonly contain glacial aquifers that are a primary source of water for multiple jurisdictions. One of the most productive of these aquifers, the Mahomet aquifer, is situated in east-central Illinois over the Mahomet Bedrock Valley, and extends across fourteen counties.

For a recent hydrogeological study of buried valleys in east-central Illinois, a multi-scalar approach was employed to analyze and interpret geological and geophysical data. Multiple routines developed by the authors and their collaborators were run to standardize data in a large database (over 43,500 records). The methodologies used enforced the consistent characterization of materials or lithologies and unit assignments, and retained the original descriptive information. A regional-scale geological model was developed from land surface to bedrock from a smaller subset of this database incorporating the main lithological units. In the process, some geologic/lithologic units were combined (based upon their lateral extent, texture, and stratigraphic position) for input into a hydrostratigraphic model. For example, the Mahomet aquifer includes at least three units of sand and gravel deposited during multiple glacial and interglacial periods.

Geologic/lithologic units overlying the Mahomet aquifer are also important from a hydrogeological perspective. This aquifer is overlain by an extensive aquitard, which in turn is overlain by a more complex unit which contains discontinuous sand-gravel layers forming small aquifer zones, interstratified with layers of finer-grained sediment. A geological framework model was developed to analyze sediment landform architecture and heterogeneity of this unit at the facies assemblage scale. At a regional scale, the unit is a “hybrid” hydrostratigraphic unit that cannot easily be separated into the traditional aquifers and aquitards. Hydrostratigraphic units like this present a major challenge for hydrogeological investigations and groundwater flow modeling.