|St. Peter Sandstone, Illinois Basin
Young (1992a) identified the Upper Ordovician Maquoketa Shale as the primary confining unit of the Cambrian-Ordovician aquifer in the northern Midwest. The dolomite and shales of the Galena, Decorah, Platteville, and Glenwood Formations, which occur between the St. Peter Sandstone and the Maquoketa Shale, also serve as confining layers (Young, 1992a). The Maquoketa Shale is regionally continuous and lithologically homogeneous, and therefore provides an effective aquiclude across the central Illinois Basin. Young (1992a), Collinson and others (1988) and Kolata and Noger (1990) provided thickness-distribution maps of the Maquoketa Shale; we chose to grid the map of Collinson and others (1988) in the GIS because it covers the entire Illinois Basin (c7stpeterg).
Collinson, C., Sargent, M. L., and Jennings, J. R., 1988, Chapter 14: Illinois Basin region, in Sloss, L. L., ed., The Geology of North America, v. D-2, Sedimentary Cover-North American Craton: U.S.: Decade of North American Geology: Geological Society of America, p. 383-426.
Kolata, D. R., and Noger, M. C., 1990, Tippecanoe I sequence Middle and Upper Ordovician Series, in Leighton, M. W., Kolata, D. R., Oltz, D. F., and Eidel, J. J., eds., Interior cratonic basins: American Association of Petroleum Geologists Memoir 51, p. 89-99.
Young, H. L., 1992a, Summary of ground water hydrology of the Cambrian-Ordovician aquifer system in the northern Midwest, United States: U.S. Geological Survey, Professional Paper 1405-A, 55 p.