Type area, reference section, and use of name: The name Petersburg, taken from Petersburg, Pike County, Ind., by Fuller and Ashley (1902, p. 2), was used for a coal in that area as well as for a formation consisting of the rocks between the bases of their Petersburg and Millersburg Coals. This name was extended by Cumings (1922, p. 529) to include "the interval from the disconformity over Coal lV to the disconformity over Coal VII," but the formation was restricted by Wier (1950, 1952, 1961, and 1965), so that the Petersburg now consists only of the rocks between the top of the Survant Coal Member of the Linton Formation and the top of the Springfield Coal Member of the Petersburg Formation. In some places in the Warrick County area where the Folsomville Member is present but the upper split of the Springfield coal is absent, the Folsomville marks the top of the Petersburg (Eggert, 1982). A reference section for this formation is part of the core from Indiana Geological Survey drill hole 81 that was drilled 4 miles (6.5 km) northeast of Petersburg in the NE¼NE¼NE¼ sec. 7, T. 1 N., R. 7 W.
Description: Four named members of the Petersburg, the Houchin Creek Coal, Stendal Limestone, Folsomville, and Springfield Coal Members, and unnamed beds of shale, siltstone, sandstone, and underclay make up the Petersburg Formation. Although the thickness of the Petersburg has been reported to range from 70 to 190 feet (21 to 58 m) (Wier, 1965), the interval between the Springfield and Houchin Creek coals ranges from less than 40 feet (12 m) to more than 120 feet (37 m) in thickness and consists of prodelta shales, coarsening-upward bay-fill sequences of shale and fine-grained sandstone, and fining-upward fluvial sandstone (Eggert, 1983 and 1984 Eggert and Adams, 1985). Thin rock intervals between the Springfield and Houchin Creek coals tend to parallel channels contemporaneous with the Springfield coal and tend to underlie thick Springfield coal.
Correlation: The Petersburg Formation is correlative with the middle part of the Carbondale Formation of Illinois and Kentucky .