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