Blue River Group,
Type area and use of name: The name St. Louis Limestone was first used by Engelmann (1847, p. 119-120) with reference to extensive exposures of limestone near St. Louis, Mo. Present use stems from E. O. Ulrich's suggestion (Buckley and Buehler, 1904, p. 109-110) of restricting the St. Louis Limestone to the limestone above the Spergen Limestone (now called the Salem Limestone) and below the Ste. Genevieve Limestone.
Early stratigraphic nomenclature in Indiana included in the Mitchell Limestone rocks that are now assigned to the St. Louis, Ste. Genevieve, and Paoli Limestones. Beede and others (1915, p. 207) noted that the St. Louis and the Ste. Genevieve could be differentiated, and most authors since that time have attempted to recognize a distinction between those formations. Few vertically extensive exposures and no reference sections exist in the state, however, and the St. Louis-Ste. Genevieve boundary has been a particularly difficult one to establish. (See the discussion of this boundary problem under "Ste. Genevieve Limestone.")
Description: The St. Louis Limestone in Indiana can be divided into two parts on the basis of lithology (Pinsak, 1957, p. 23-24). The upper St. Louis, which is the upper one-eighth to one-third of the formation, consists mainly of thin beds of medium- to dark-gray-brown micritic, pelletal, and skeletal limestone and very thin beds of medium-gray shale (Carr, Leininger, and Golde, 1978, p. 79). Nodules and thin discontinuous beds of mottled light- and dark-gray dense chert are generally abundant in the upper 25 to 90 feet (8 to 27 m) of the unit; thin-bedded silty dolomite is as much as 20 percent of the total in places.
The lower St. Louis on outcrop consists mainly of pellet-micritic limestone, calcareous shale, and silty dolomite. In the subsurface in a large belt across southern Illinois, southwestern Indiana, and north-central Kentucky, these are interbedded with anhydrite and gypsum to a total thickness exceeding 160 feet (49 m) (MeGregor, 1954, pl. 2 Jorgensen and Carr, 1973, p. 46). Evaporite deposition was cyclical and produced as many as 18 distinct repetitions of lithologies. In the subsurface of southwestern Indiana, a distinct lower St. Louis facies consisting of calcarenite interbedded with variable lithologies was designated the Sisson Member by Keller and Becker (1980, p. 11-15).
The St. Louis is conformably underlain by the Salem Limestone and is conformably overlain by the Ste. Genevieve Limestone. It crops out from Harrison County on the Ohio River to northeastern Parke County, where it is overlapped by Pennsylvanian rocks. Seventy feet (21 m) thick in Putnam County, it thickens irregularly southward to about 150 feet (46 m) in Washington County (Sunder-man, 1968, p. 26). In a quarry in southernmost Crawford County it is 300 feet (91 m) thick (Carr, Leininger, and Golde, 1978, p. 18). Its maximum subsurface thickness is about 400 feet (122 m) in western Posey County. Most of the thickening and thinning appears to take place in the lower part of the formation (Pinsak, 1957, p. 23).
Correlation: By means of the corals Lithostrotion proliferum, Lithostrotionella castelnaui, and L. hemisphaerica, the St. Louis Limestone of Indiana is correlated with the type St. Louis. (See, however, the discussion of the St. Louis-Ste. Genevieve boundary under "Ste. Genevieve Limestone.") The upper part of the formation belongs in the Apatognathus scalenus-Cavusgnathus Assemblage Zone (conodonts), to which the upper part of the type St. Louis has also been assigned (Collinson, Rexroad, and Thompson, 1971).