(JBD & JBP)
Type locality and use of name: The term Prairie du Chien Formation was introduced by Bain (1906, p. 18) to replace "Lower Magnesian" of earlier reports that referred to exposures near Prairie du Chien, Crawford County, Wis., where this unit consists of as much as 300 feet (91 m) of dolomite and sandstone. The Prairie du Chien has been intensely studied in the subsurface of Illinois, where it consists in ascending order of the Gunter Sandstone, the Oneota Dolomite, the New Richmond Sandstone, and the Shakopee Dolomite (Willman and Templeton, 1951). In Illinois it is accorded group status as it is in most other places of recognition. The Illinois understanding of the group was extended to Indiana by Droste and Patton (1985), but with this modification: Neither the Gunter Sandstone nor the New Richmond Sandstone is recognized in Indiana, so that the Prairie du Chien Group consists of the Oneota Dolomite below and the Shakopee Dolomite above.
Description: The Prairie du Chien Group is recognized throughout the subsurface of Indiana except for the area of one well in northwesternmost Indiana where pre-Middle Ordovician erosion removed the entire group. Faulted blocks of the Shakopee Dolomite in the Kentland structure, Newton County, Ind. (Gutshick, 1983), are the only Prairie du Chien exposures in the state. The group is overlain unconformably by rocks of the Ancell Group in most of Indiana, by the Everton Dolomite in southwestern Indiana, and by the Black River Group in isolated parts of eastern Indiana where Ancell rocks are absent because of nondeposition. The Prairie du Chien increases in thickness from its eroded limit in northwestern Indiana to more than 2,000 feet (610 m) in southwestern Indiana.
Correlation: The Prairie du Chien of Indiana is represented by this same name in Illinois, Wisconsin, and Michigan, and it is equivalent to the upper part of the undifferentiated Knox Dolomite of Ohio and to the Beekmantown Dolomite, the Gunter Sandstone, the Gasconade Dolomite, the Roubidoux Formation, the Jefferson City Dolomite, and the Cotter Dolomite of Kentucky. (See Droste and Shaver, 1983, and Shaver and others, 1985.)