Champlainian Series

Ordovician System

Type area and use of name: The name Black River Limestone was proposed by Vanuxem (1842, p. 38-45) for rocks exposed in cliffs along the Black River in Oneida and Lewis Counties, N.Y. The early history of use of the name Black River, including its use as a group in New York, was given by Wilmarth (1938, p. 205). The original name, Black River Limestone, was used for many years in Indiana (Gutstadt, 1958a; Gray, 1970), but in 1982 (Droste, Abdulkareem, and Patton) the Black River rocks were assigned group status and divided into two parts, the Pecatonica Formation below and the Plattin Formation above.

Description: The Black River Group in Indiana is known only in the subsurface except for faulted blocks of the Pecatonica and the Plattin in the quarried Kentland structure, Newton County, Ind. (Gutschick, 1983). The group consists predominantly of light- to dark-gray and brown lithographic to very finely crystalline limestones that are so common to rocks of Blackriverian age in the eastern United States. In northwestern Indiana the Black River rocks are dolomite and very dolomitic limestone. The group ranges in thickness from slightly less than 100 feet (30 m) in northwestern Indiana to more than 500 feet (152 m) in southwestern Indiana.

The Black River Group generally overlies the Joachim Dolomite, possibly with minor unconformity, and is overlain, possibly with some unconformity, by the Trenton Limestone throughout most of the state. In that part of Indiana where the Trenton Limestone is not recognized, its lateral equivalents, the Kope Formation and the Lexington Limestone, variably overlie the Black River. In a few isolated places in eastern Indiana where Angel rocks are missing because of non deposition, the Black River Group overlies the Prairie du Chien Group (Ordovician) or the Potosi Dolomite (Cambrian) with major unconformity (Droste and Shaver, 1983, fig. 2).

Correlation: The Black River Group of Indiana is equivalent to the Platteville Group of Illinois, the Black River Group of Michigan, most of the Black River Limestone of Ohio, and the upper part of the High Bridge Group of Kentucky. According to Votaw (1978), conodonts from the upper few feet of the Black River (Platting Formation) taken from a cored well in White County, Ind., appear to indicate fauna B of Sweet, Ethington, and Barnes (1971) conodonts from the rest of the Plattin Limestone and the Pecatonica Formation represent fauna 7 (Votaw, 1978). This includes a sandy interval at the base of the Black River that is equivalent to the Hennepin Member (Pecatonica Formation) of Illinois but that Votaw referred to the St. Peter Sandstone.