Historical:St. Clair Limestone
H. B. Willman and Elwood Atherton
The St. Clair Limestone (Penrose, 1891a, p. 102-103) (fig. S-6) is named for St. Clair Springs, 8 miles northeast of Batesville, Independence County, Arkansas.
The St. Clair Limestone is well exposed in the Mississippi River bluffs north of the mouth of Dongola Hollow, in Alexander County (SW SE NW 12, 14S-3W).
The St. Clair Limestone is thought to be equivalent to the Joliet Formation and the lower part of the Racine Formation in northern Illinois. The Brandon Bridge Member of the Joliet, like the St. Clair, contains a few thin beds of pink crinoidal dolomite and abundant Foraminifera. Pink mottling also occurs in the Romeo Member of the Joliet and locally also in the Racine. The St. Clair is correlated with the Osgood, Laurel, Waldron, and Louisville Formations in Indiana.
Extent and Thickness
Although only 10-20 feet thick where exposed in Alexander County, the St. Clair Limestone thickens northward to 80-100 feet in the deeper part of the Illinois Basin, and it is as much as 150 feet thick at its northern margin, where it grades into the Joliet and Racine Formations.
The St. Clair Formation is the basal formation of the Niagaran Series in southern Illinois. The strata now included in the St. Clair were originally in the upper part of the Sexton Creek (Savage, 1909), but Weller later (1940) included them in the lower part of the Bainbridge Formation (fig. S-10). Lowenstam (1949), noting that the pink crinoidal limestone of the lower Bainbridge is almost identical to that of the St. Clair in Arkansas, extended the St. Clair to Illinois and introduced Moccasin Springs for the upper, red, shaly part of the Bainbridge and elevated Bainbridge to a group.
The St. Clair consists of coarsely crystalline calcite grains and abundant pink crinoidal remains in a fine-grained limestone matrix. The amount of crinoidal debris decreases upward. The matrix contains a small amount of silt, clay, and some very fine sand. The limestone occurs in medium to thick beds and is generally free of chert.
The St. Clair contains an abundance of arenaceous Foraminifera.
LOWENSTAM, H. A., 1949, Niagaran reefs in Illinois and their relation to oil accumulation: Illinois State Geological Survey Report of Investigations 145, 36 p.
PENROSE, R. A. F., JR., 1891a, Batesville region of Arkansas: Arkansas Geological Survey Annual Report 1890, v. 1, p. 102-174.
SAVAGE, T. E., 1909, Ordovician and Silurian formations in Alexander County, Illinois: American Journal of Science, v. 28, p. 509-519.
WELLER, J. M., 1940, Geology and oil possibilities of extreme southern Illinois: Illinois State Geological Survey Report of Investigations 71, 71 p.
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