Valmeyeran and Chesterian Series,
Type area: The Blue River Group was named Mississippian System by Gray, Jenkins, and Weidman (1960, p. 48) for the Blue River in Washington, Harrison, and Crawford Counties, Ind. Many excellent exposures (of nearly half the group in places) are in the valley walls of the Blue River from about 2.5 miles (4.0 km) northeast of Fredericksburg, Washington County, to the Ohio River.
Description: The Blue River Group is formed largely of carbonate rocks but has significant amounts of gypsum, anhydrite, shale, chert, and calcareous sandstone. The three component formations of the group in ascending order are the St. Louis, Ste. Genevieve, and Paoli Limestones. In the subsurface of southwestern Indiana the group includes the St. Louis and Ste. Genevieve Limestones and the Aux Vases and Renault Formations.
The Blue River Group crops out in an area extending from the Ohio River in Harrison County to Sugar Creek in northeaster Parke County and adjacent Montgomery County. The outcrop belt has approximately county width in Harrison County, narrows sharply northward in Orange and Lawrence Counties, and gradually diminishes northward to its termination, where it is overlapped by Pennsylvanian rocks. The combined thickness of the constituent formations is 540 feet (165 m) in southern Crawford and Harrison Counties (Carr and others, 1978, p. 12) about 400 feet (122 m) in northwestern Orange County (Gray, Jenkins, and Weidman, 1960, p. 48) and 240 feet (73 m) in southern Monroe County (Malott, 1952, p. 57). From well records the group is known to be 150 to 170 feet (46 to 52 m) thick in parts of Owen and Putnam Counties, and in other areas in the subsurface its thickness ranges from 625 to 450 feet (99 to 137 m) near the outcrop to a maximum of 650 feet (198 m) in Posey County (Pinsak, 1957, pl. 1).
The Blue River Group rests conformably on the Sanders Group and is overlain, generally conformably but with local disconformity, by rocks of the West Baden Group. North of Owen County the Mansfield Formation of Pennsylvanian age disconformably overlaps successively older Blue River rocks northward.
Correlation and the Mitchell problem: The group has no exact named equivalent in neighboring states, but it is equivalent to the section extending from the St. Louis Limestone through the Cedar Bluff Group of Illinois usage (Swann, 1963). It spans the Valmeyeran-Chesterian boundary as that boundary is generally recognized. The Blue River Group includes rocks that at different times were assigned to units with the now-obsolete names Mitchell Formation (Limestone, Group, of Hopkins and Siebenthal, 1897, p. 298-299; Elrod, 1899, p. 259; Ashley and Kindle, 1903, p. 73; Malott, 1919, p. 8-10, and 1921, p. 365; Logan, 1926, p. 343) and Lower Kaskaskia Limestone (Kindle, 1896, p. 331-332). The upper boundary of the Mitchell in these older uses was as low as the top of the St. Louis Limestone (Fired, 1899, p. 259) and as high as the top of the Beaver Bend Limestone (Malott, 1919, 1921; Logan, 1926). Because of these irregularities in earlier usage, Cumings (1922, p. 507) and Perry and Smith (1958, p. 19) recommended that the term Mitchell be abandoned, but it remained for Gray, Jenkins, and Weidman (1960) to describe and name the Blue River Group as an appropriate replacement term.