QUADRANGLE-SCALE MAPPING OF QUATERNARY DEPOSITS IN NORTHEASTERN ILLINOIS

Detailed geological mapping at 1:24,000 scale has been initiated in a six-quadrangle area of Lake and McHenry Counties as part of ISGS-USGS sponsored programs (STATEMAP, Central Great Lakes Geologic Mapping Coalition, EDMAP). Conflicts over land use are becoming more frequent in these counties, two of the most rapidly developing in Illinois. These conflicts often occur in areas that are environmentally sensitive and where geological information is deficient. Various public agencies have requested more comprehensive and complex geological databases but want them to be flexible and user-friendly.

A regional three-dimensional model is being developed to provide a framework for mapping in northeastern Illinois. Concurrent studies at the quadrangle-scale involving the retrieval and compilation of existing geological data, and completion of intensive new subsurface drilling and geophysical fieldwork will be used to refine the regional model. Three dimensional modeling of the Quaternary sediments in these quadrangles will be used to refine the regional model. New mapping in Wadsworth, the northeastern-most of the quads, indicates that bedrock is covered by about 150 to 250 feet of glacial sediment, which thickens to the west. A silty clay diamicton (till) overlies two other diamictons, but their thicknesses and spatial continuity are variable. Near the western boundary of the quad, a proglacial sand and gravel unit underlies the upper diamicton. This sand and gravel thickens westward where it constitutes a major aquifer; it appears to pinch out to the east. The uppermost diamicton (Wadsworth Fm) exhibits considerable textural variability and is interpreted to represent till, redeposited supraglacial sediments, and subaqueous flow deposits.

The mapping will provide a large-scale, detailed database for modeling the complex geology and allow more extensive modeling of aquifers in this region of high population growth. As additional quads are mapped a more comprehensive model should emerge to assist in dealing with the land use and other important issues.