Silurian System

Type area and use of name: The Moccasin Springs Formation was named by Lowenstam (1949, p. 16) for exposures of multicolored silty argillaceous limestones and shales that overlie the St. Clair Limestone in the Silurian outcrop area on either side of the Mississippi River in Cape Girardeau and Ste. Genevieve Counties, Mo., and in Alexander and Union Counties, Ill. His wording, however, was "all the Niagaran strata overlying the St. Clair formation" in that area. At That time the stratigraphically overlying Bailey Limestone, whose type area is one county north of Cape Girardeau County, was considered to be Devonian in age, and no Upper Silurian rocks were identified. Therefore, Lowenstam did not exclude part of what is called the Moccasin Springs here, even though a part of it may be post-Niagaran in age.

The term Moccasin Springs was introduced to southwestern Indiana subsurface use by Becker (1974, p. 27), and further definition in that area of eight-county size was given by Becker and Droste (1978, p. 2-3) and Droste and Shaver (1980, and in preparation).

Description: The Moccasin Springs is striking for its coloration in pastel shades of pink, green, yellow, tan, and gray, partly in mottlings, and more intense dark-gray and red to purple colors. It consists mostly of dense to fine-grained somewhat argillaceous limestones that are interbedded as variably colored units. About the top 20 feet (6 m) of the formation in most places consists of dark-gray to black dolomitic shale interbedded with dark-greenish-gray very fine-grained argillaceous limestone. Dark-red carbonate rocks are particularly characteristic of the lower part of the formation.

The Moccasin Springs has a reef facies consisting of relatively pure carbonate rocks partly in reefs that appear to have begun growth during early Moccasin Springs deposition and partly in reefs that continued growth upward from the St. Clair Limestone. Such reefs and related rocks help to make up the feature called the Terre Haute Bank that on its basin flank extends along the line of somewhat arbitrary vertical cutoff between rocks of the Bainbridge Group (including the Moccasin Springs) and other rocks ranging stratigraphically upward from the Salamonie Dolomite through the Wabash Formation inclusively. (See maps in Becker, 1974, fig. 8; Becker and Droste, 1978, fig. 1; and Droste and Shaver, 1980, fig. 7.)

The Moccasin Springs is underlain by the St. Clair Limestone, and the boundary is especially marked by the change between impure carbonate rocks above and pure carbonate rocks below. It is overlain by the Bailey Limestone, and the contact is placed between darker and impurer carbonate rocks below and lighter and more neutrally colored and purer (except for chert) carbonate rocks above. Both contacts are believed to be conformable .

The nonreef Moccasin Springs in Indiana ranges from 60 to 140 feet (18 to 43 m) in thickness.

Correlation: Because diachroneity in a basin-to-platform sense may apply to both the top and the bottom Moccasin Springs contacts (as suggested by Droste and Shaver, 1980, fig. 4 and p. 569), simple, precise statements of correlation cannot be made. The Moccasin Springs of Indiana extends to units of the same name in southern Illinois and western Kentucky. In Kentucky areas distal from the Illinois Basin, equivalent rocks include the Dixon Limestone and the Brownsport Formation. In southwestern Indiana, lowest Moccasin Springs and highest St. Clair rocks may have some age equivalency, and northeastward partly or wholly age-equivalent rocks may include the Louisville Limestone and certainly the Mississinewa Shale Member of the Wabash Formation. Middle Racine rocks of northern Illinois are also correlative.

The conodont Spathognathodus eosteinhornensis has been found in the upper dark Moccasin Springs shale (Becker and Droste, 1978, p. 3). This stratigraphic position suggests an age from late Ludlovian into Pridolian.