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The Illinois Basin is a spoon-shaped structure located mostly in central and southern Illinois, southwestern Indiana, and western Kentucky (Figs. 1 and 2). Its long dimension extends northwest-southeast for 600 km, and its maximum width exceeds 320 km. The greatest thickness of sedimentary fill is in southern Illinois and western Kentucky (in the Rough Creek Graben), where a maximum of approximately 7,000 m has been reported (Anderson, 1984; Bertagne and others, 1986; Hester, 1984; Schwalb, 1983). This section thins both depositionally and erosionally to less than 450 m on arches and domes that surround the basin. The Paleozoic sedimentary fill ranges in age from Early or Middle Cambrian to very Early Permian. Permian rock, however, is present only in a small fault block in the subsurface of western Kentucky. Pennsylvanian bedrock is present directly below surficial material over most of the basin (Fig. 2) and is surrounded by broad outcrop belts of Mississippian strata on the western, eastern, and southern sides. Silurian, Ordovician, and Cambrian rocks dominate the northern slopes. Devonian and Silurian rocks form broad outcrop belts in northeastern Illinois, as well as northern and eastern Indiana. Relatively narrow outcrop belts of lower and middle Paleozoic rocks flank the Ozark Uplift to the southwest.

Various boundaries have been suggested to separate the Illinois Basin (in the restricted sense) from the surrounding arches (Fig. 2). If the generally used -500-ft (-150-m) contour on top of the Ordovician Ottawa (top of Trenton and equivalents) Mega-group (megagroups are formally called supergroups in

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