Type area and use of name: The St. Clair Limestone was named by Penrose (1891, p. 102-103) for exposures at St. Clair Springs 8 miles (12.9 km) northeast of Batesville, Independence County, Ark. Lowenstam (1949, p. 13) applied the name to the crinoidal limestones of early Niagaran age that extend discontinuously from the type area through western Tennessee and into the Illinois Basin parts of Kentucky and Illinois. Becker (1974, fig. 8 and p. 16) adopted the name for use in the subsurface and in an area of eight-county size in southwestern Indiana. Here the term has had further definition through the work of Becker and Droste (1978) and Droste and Shaver (1980 and in preparation), including what amounts to a vertical cutoff boundary with rocks of the Salamonie Dolomite and the Louisville Limestone.
Description: In southwestern Indiana the St. Clair Limestone is a grayish-red to intense ferruginous-red and yellowish-gray fine- to medium-grained limestone. The basic matrix color is mostly whitish to yellowish, but the other colors are expressed partly as beds and partly as splotches and mottlings down to the size of individual carbonate grains.
The St. Clair rests unconformably on the brown cherty rocks of the Sexton Creek Limestone, whereas the upper contact with the Moccasin Springs Formation generally involves an upward transition through several feet of interbedded pure limestones and argillaceous limestones. The formational contact is usually placed at the base of the transitional zone.
The St. Clair ranges from 30 to 90 feet (9 to 27 m) in thickness and averages about 60 feet (18 m) in thickness in the subsurface of southwestern Indiana.
Correlation: As proposed by Droste and Shaver (1980, p. 569, and 1985, fig. 1), the upper part of the St. Clair may be diachronous between its basin position and farthest shelfward position. Therefore, parts of the St. Clair may correlate variably with other formations, the Salamonie Dolomite, the Waldron Shale, the Louisville Limestone, and the lower parts of the Moccasin Springs and Pleasant Mills Formations of Indiana; with most of the Joliet Formation (except the lower part of the Brandon Bridge Member), the Sugar Run Formation, and the lower part of the Racine Formation in northeastern Illinois: and with the Osgood Formation upward through the Lego Limestone of western Kentucky.
Much of this correlation is based on physical and geophysical tracing, as direct faunal ages for the St. Clair of Indiana are lacking, but ages from early into late Niagaran and late Llandoverian into early Ludlovian in the American and global standards respectively have been assigned (Shaver and others, 1985).