Geologic Atlas of Cook County for Planning Purposes
The effects of rapid urban expansion on the environment and on society's ability to extract essential mineral resources require increasingly more detailed information about the bedrock geology in order to make informed planning decisions. In Cook County, the bedrock is a major source of water for residential, municipal, and industrial use. Consequently, key issues facing Cook County include the quality, quantity, distribution, and accessibility of bedrock groundwater resources. Other important bedrock issues include underground construction as well as mineral resource assessment and management.
Cook County is situated on the eastern flank of the southward-plunging Wisconsin Arch. Silurian rocks thicken eastward into the Michigan Basin, and the underlying Cambrian and Ordovician strata thicken southward into the Illinois Basin. The bedrock in Cook County is covered by up to 300 feet of unlithified surficial materials consisting of clay, silt, and sand and gravel deposited primarily by glacial processes. Silurian dolomite is present at the bedrock surface nearly countywide. These rocks are absent in small areas in the northwestern part of the county and range in thickness countywide from less than 1 foot to more than 300 feet on the far eastern side along the lake shore.
Silurian dolomite forms the uppermost bedrock aquifer in Cook County. The upper part of the dolomite has numerous fractures, crevices, and solution cavities that can yield moderate amounts of water to a well. Higher yields are obtained from the more deeply buried Ordovician St. Peter Sandstone, the Cambrian Ironton-Galesville Sandstone, and the upper part of the Mt. Simon Sandstone, which is also Cambrian.
The principal objective of this mapping effort was to compile a database that can be used to depict in three dimensions the thickness, distribution, lithologic character, and structure of the major bedrock units in Cook County. The database can be used to produce three-dimensional maps and cross sections of the complete succession of Paleozoic rocks down to the top of the underlying Precambrian crystalline rocks. It is anticipated that the database and the maps and models produced from the database will provide important in sight to subsurface conditions in Cook County.
Formation tops were determined for approximately 5,900 drill hole records on file at the Illinois State Geological Survey. The information was entered into a digital database and used to compile countywide structure and thickness maps, cross sections, three-dimensional block diagrams, and a stratigraphic column. The data are displayed in the Lambert Conformable Projection.
Curtis Abert, Ardith Hansel and Barbara Stiff helped to develop this series of maps. Joel A. Steinfeldt assisted with editing and Web design. Part of the mapping was done using Landmark Graphics software donated to the University of Illinois at Urbana-Champaign as part of the Landmark Graphics university grant program. The manuscript benefited from comprehensive review by Robert Finley and Jonathan Goodwin.
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Buschbach, T.C., and G.E.Heim, 1972, Preliminary geologic investigations of rock tunnel sites for flood and pollution control in the greater Chicago area: Illinois State Geological Survey, Environmental Geology Notes 52, 35p.
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Kolata, D.R., and A.M. Graese, 1983, Lithostratigraphy and depositional environments of the Maquoketa Group (Ordovician) in Northern Illinois: Illinois State Geological Survey, Circular 528,49 p.
Suter, M., R.E. Bergstrom, H.F. Smith, G.H. Emrich, W.C. Walton, and T.E. Larson, 1959, Preliminary report on ground-water resources of the Chicago region, Illinois: Illinois Water Survey and Illinois Geological Survey, Cooperative Ground-Water Report 1, 89 p.
Willman, H.B., 1971, Summary of the geology of the Chicago area: Illinois Geological Survey, Circular 460, 77 p.
This series of geologic maps was prepared for the purposes of resource evaluation and regional planning. The maps are based on interpretations of available data that were obtained from a variety of sources. Locations of data points have not been verified in the field nor have the data been rigorously reviewed for accuracy. The Illinois State Geological Survey does not guarantee the accuracy of the unverified data nor the interpretations based upon them.
Hannes E. Leetaru, Michael L. Sargent, and Dennis R. Kolata
Updated 04/28/2011 SLD