Type locality and use of name in Indiana: The name Carbondale Formation was first used by Shaw and Savage (1912, p. 6) in a description of rocks exposed near Carbondale in Jackson County, Ill. A type section consisting of three outcrops was established in Fulton County, western Illinois, by Kosanke and others (1960, p. 34, 46), who described the formation as consisting of rocks "between the base of the Colchester (No. 2) Coal and the top of the Danville (No. 7) Coal." The term Carbondale Group is used in Indiana for the rocks between the top of the Seelyville Coal Member of the Staunton Formation and the top of the Danville Coal Member of the Dugger Formation (Wier and Gray, 1961: Wier, 1961, 1965), and therefore the Carbondale in Indiana includes rocks between the Colchester and Seelyville coals that are not included in the Carbondale Formation of Illinois. Before Wier's classification, the Carbondale rocks of Indiana were assigned by Fuller and Ashley (1902) to the lower part of the Millersburg Formation, to the Petersburg Formation, and to the upper part of the Brazil Formation and by Cumings (1922, p. 525-529) to the Petersburg Group and the upper part of the Staunton Group.
Description: In Indiana the Carbondale Group ranges from 260 to 470 feet (79 to 143 m) in thickness but averages slightly more than 300 feet (91 m). It is thickest in central Posey County and generally thins to the north and the east. The outcrop belt in Indiana extends from Warrick County northward to Vermillion County. The Carbondale Group includes some laterally persistent limestones and four of Indiana's commercially important coals. Persistent shales and underclays are associated with several of these coals. Most of the thickness of the Carbondale Group consists of variable shales and sandstones. Three formations (the Linton, Petersburg, and Dugger Formations) make up the Carbondale Group, which is overlain by the McLeansboro Group and underlain by the Raccoon Creek Group.
Correlation: The Carbondale Group in Indiana is essentially equivalent to the Carbondale Formation in the upper part of the Kewanee Group in Illinois. It correlates with much of the Carbondale Formation and the lower part of the Sturgis Formation in western Kentucky.