Mississippian System

Type locality, use of name, and description: Proposed (without documentation) in 1920 by Malott and Thompson (p. 522), the term Indian Springs Shale has since been honored more in the breach than in the observance; only one published use is known. In 1931 Malott (p. 224 and fig. 2) described the unit as "always present," illustrated it as 20 feet (6 m) of shale, and noted that it was "called the Indian Springs shale after the outcrop one-fourth of one mile northwest of the village" of that name in Martin County, Ind., but he failed to list it in his table of correlations (p. 222), and his intentions regarding its use are unclear. In some later usage the "20 feet of olive shale" (Malott, Esarey, and Bieberman, 1948, p. 24) was arbitrary included in the Golconda (now Haney) Limestone, but to clarify the nomenclature of the Golconda Limestone, Gray, Jenkins, and Weidman (1960, p. 39-41) placed the shale in the Big Clifty Formation, where it still resides.

There remains a need to recognize formally this shale unit that is widely present on the outcrop in Indiana and that has a distinctive and diverse fauna, and therefore the name Indian Springs Shale Member is here adopted for the upper part of the Big Clifty Formation. The member commonly consists of 3 to 14 feet (1 to 4 m) of gray fossiliferous shale and inter bedded thin limestone underlain by 3 to 10 feet (1 to 3 m) of varicolored shale and siltstone. Because no type section was originally presented, two reference sections are here designated: a railroad cut about 3 miles (4 km) east of Shoals, Martin County, in which the member is 22 feet (7 m) thick (Gray and others, 1957, p. 16), and Indiana Geological Survey drill hole 48 in Orange County (Gray, Jenkins, and Weidman, 1960, p. 75-77), in which the member is an exceptional 32 feet (10 m) thick. (For precise locations, see under "Big Clifty Formation." These sections also serve as reference sections for that unit.)

The Indian Springs Shale Member can be recognized almost continuously along the outcrop from Perry County (Gray and Powell, 1965, p. 18) to Greene County (Malott, 1952, p. 66). Although the shale is soft and poorly exposed except in relatively fresh cuts, it serves, as Malott (1931, p. 224) put it, as a "foil," and its place can commonly be noted by such features as a break in slope above the prominent sandstone of the lower Big Clifty and beneath the ledges of the Haney Limestone. In most places the member is 11 to 25 feet (3 to 8 m) thick. It is not distinguished in the subsurface, where the Big Clifty Formation is mainly shale.

The fauna of the Indian Springs member has been rather thoroughly studied, but little has yet been published. Horowitz (1979) presented from a single site a faunal list of more than 50 species, including notably bryozoans, brachiopods, and crinoids from the same site Kelly (1984), in an unpublished study, has tabulated an even more diverse fauna, including vertebrate remains, as a basis for a paleoecologic evaluation of the member. The fauna is consistent with that reported for the upper part of the Fraileys Shale and for the Haney Limestone elsewhere in the Illinois Basin.