Type locality and use of name: The name Salem Limestone was proposed by Cumings (1901) to replace the preoccupied name Bedford Limestone for rocks lying between the Harrodsburg Limestone (below) and the Mitchell Limestone (above). The abandoned term Mitchell corresponded closely to the present Blue River Group, of which the St. Louis Limestone is the bottom formation. A type section was not designated, but Cumings quoted a section described by Gorby (1886, p. 143). Although that section has been quarried away, the abandoned part of the Hoosier Stone and Concrete Corp. quarry in the NE¼ sec. 24, T. 2 N., R. 3 E., on the west edge of Salem, Washington County, Ind., is in nearly the same place and was designated by Smith (1970, p. 152) as the principal reference section. The Somerset Shale Member (Butts, 1922) was later assigned by Nicoll and Rexroad (1975, p. 9-10) to the Salem as its basal member rather than to the Harrodsburg, which was Smith's (1960) assignment.
History of nomenclature: From the first quarry record of the Salem Limestone in 1827 until the late 1800's, quarrymen did not realize the lateral continuity of the formation, so that the building stone was sold under many local names, such as Bedford stone, Bloomington stone, Ellettsville stone, Salem stone, and White River stone (capitalization here as in the original sources). Still another and widely used quarry term is Bastard stone, which refers to an upper impure and normally discarded part of the Salem. Most of these names appear in various annual reports of what is now the Indiana Geological Survey (Siebenthal, 1901a). Variations of the local term Spergen Hill or Spergen were also in use, primarily because of the renown of that locality in Washington County, Ind., as a fossil-collecting site (for example, Spergen fossil bed of Hall, 1864), and Lyon (1860) correlated strata in Kentucky with the rocks that he called the Spergen's Hill bed at Spergen Hill. With such a plethora of names that had been used in a wide variety of ways, the difficulty in establishing a single formal stratigraphic name for the unit is not surprising. Because the name Bedford stone had come to dominate the building-stone industry, the Salem was first formally named the Bedford Oolitic Limestone (Hopkins and Siebenthal, 1897). The formation was renamed the Salem Limestone by Cumings in 1901 because the name Bedford was preoccupied by the Bedford Shale of Ohio (Newberry, 1871). Nevertheless, E. O. Ulrich (as published by Buckley and Buehler, 1904, p. 110) adopted the term Spergen Hill Limestone, which evolved to the Spergen Limestone (Wilmarth, 1938). Although the Indiana and Illinois Geological Surveys consistently used the name Salem, it was not until the 1950's that the U.S. Geological Survey changed its use from "Spergen Limestone" to "Salem Limestone." (See Smith, 1970, p. 152-153.) The Salem Limestone is now generally known in the building-stone trade as the Indiana Limestone.
Description: Crossbedded calcarenite that is medium to coarse grained, tan, gray tan, and light gray, porous, and fairly well sorted and that occurs in exceptionally thick beds is the most widely known rock type of the Salem Limestone and is the internationally known budding-stone facies. Individual grains are mostly microfossils (including especially the foraminiferid Globoendothyra baileyi), macrofossil fragments, and whole diminutive forms of macrofossils. Coated grains are also common. Other lithologies, besides the shale of the Somerset, include much finer and coarser calcarenites, biocalcirudites, very fine grained argillaceous dolomite commonly containing wavy black carbonaceous laminae, very fine grained to dense limestone in places including oolites, and dense argillaceous dark-gray to dark-brown limestone (Pinsak, 1957).
Where the Somerset Shale Member is absent, recognition of the conformable Harrodsburg-Salem boundary is difficult. The more obvious lithologic changes at the boundary represent shifting ecologic conditions rather than a hiatus. Similarly, vertical transition or even lateral gradation between lithologies representative of the Salem and the overlying St. Louis Limestone is the general situation in the subsurface of the Illinois Basin (Lineback, 1972), but in part of the Indiana outcrop belt the evidence of continuity of deposition between the Salem and the St. Louis is not strong. In places along its northern limits, the Salem is unconformably overlain by the Pennsylvanian Mansfield Formation.
The Salem crops out along an irregular arcuate line extending from northwestern Putnam County to the Ohio River at the east edge of Harrison County. Commonly it is 60 to 100 feet (18 to 30 m) thick along the central and southern parts of the outcrop belt; it is thinner to the north. The northern limit is difficult to place because it is near a depositional pinchout of the Salem and was also affected by pre-Pennsylvanian erosion and, therefore, is very irregular. The Salem is everywhere present in the subsurface south and west of the outcrop-pinchout belt. Subsurface thickness of the Salem is variable, but in much of southwestern Indiana it averages about 175 feet (52 m). The Salem reaches a maximum thickness of about 280 feet (85 m) in northwestern Posey County and southeastern Gibson County (Keller and Becker, 1980, p. 15-16).
Correlation: The Salem Limestone is identified throughout the Illinois Basin in Illinois and Kentucky and maintains about the same stratigraphic relationships throughout. It is somewhat younger in Indiana than in western Illinois, however, and from Madison County northward in Illinois the upper part of the Salem grades laterally into the St. Louis Limestone (Lineback, 1972). The Salem is within the Taphrognathus varians-Apatognathus Assemblage Zone of conodonts (Collinson, Rexroad, and Thompson, 1971). It correlates with rocks within the TC Zone of Neves and others (1971), which is based on spores, and approximately with rocks within North American foraminiferal Zones 10, 11, and 12 of Mamet and Skipp (1971). In European terms it is early Visean in age.