M. E. Hopkins and J. A. Simon
The Kewanee Group (Kosanke et al., 1960, p. 31) is named for Kewanee, Henry County, in western Illinois in the outcrop belt of its two formations - Spoon and Carbondale (fig. P-2).
It includes strata previously classified as the Carbondale Formation or Group plus older and younger strata (fig. P-5). The Kewanee Group is continuously recognized in all of the Illinois Basin within the confines of its outcrop limits. It normally lies above the Abbott Formation, but in northern and northeastern Illinois where the Abbott is missing it lies on rocks ranging in age from Valmeyeran (middle Mississippian) to Champlainian (middle Ordovician). It is overlain by the McLeansboro Group.
The Kewanee Group contains the best developed cyclothems and more than 99 percent of the mapped coal reserves of the state (Cady et al., 1952). Lateral continuity of many of the lithologic units, especially the marine limestones, black fissile shales, coals, and underclays is remarkably extensive. The limestones, generally less than 5 feet thick, were deposited during marine transgressions from the west and southwest. Along with marine shales they alternate with fluvial and deltaic sediments that include siltstones, silty shales, elongate "channel" sandstones, and coals. The limestones are usually gray and argillaceous, and they contain a diverse marine fauna dominated by brachiopods, gastropods, crinoids, pelecypods, and Foraminifera (including fusulinids). The sandstones are texturally and mineralogically less mature than those of the McCormick Group in that they contain much more clay matrix, mica flakes, and feldspar grains, and their sand grains are less rounded and not as well sorted.
CADY, G. H., et al., 1952, Minable coal reserves of Illinois: Illinois State Geological Survey Bulletin 78, 138 p.
KOSANKE, R. M., J. A. SIMON, H. R. WANLESS, and H. B. WILLMAN, 1960, Classification of the Pennsylvanian strata of Illinois: Illinois State Geological Survey Report of Investigations 214, 84 p.
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