Mississippian System

Type locality and use of name in Indiana: The name Somerset Shale Member was given by Butts i 1922, p. 89, 104-107) to an upper part of the Warsaw Formation in Kentucky. The name was derived from Somerset in Pulaski County, although Butts indicated that the unit was better exposed in glades about 2 miles (3.2 km) southwest of Colesburg. In 1939 Stockdale (p. 226, pls. 6, 25) traced the Somerset from its type area to southern Monroe County, Ind., and interpreted it "as a basal argillaceous phase of the Salem limestone." This interpretation has been followed by most geologists, including Nicoll and Rexroad (1975), but the section at Somerset may be stratigraphically higher than the one at Colesburg from which the stated stratigraphic position has been accepted.

Description: On outcrop in southern Indiana and adjacent Kentucky, the Somerset Shale Member consists of as much as 16 feet (5 m) of dolomitic silty shale commonly containing gypsum- or quartz-filled geodes. It thins abruptly to the north and ranges from a few inches to little more than a foot (0.3 m) at its northern limits in Monroe County. Its subsurface extent has not been determined. In Kentucky similar shales with which the Somerset might be confused are present higher in the Salem.

Correlation: Current studies by Howard Feldman (written communication, April 17, 1984) show that the bulk of fossils found in the Somerset Shale Member, including Globoendothyra baileyi, are also common to the limestone facies of the Salem. The presence of several species of the crinoid Batocrinus that he recorded from the Somerset and the rest of the Salem sets these units apart from the underlying Harrodsburg Limestone, from which that genus is absent. Cyathocrinites and Barycrinus are found in the Somerset but not in the overlying Salem beds (Howard Feldman, written communication, April 17, 1984). The latest record of these two genera is in rocks of Keokuk age (middle Valmeyeran), and so their presence in the Somerset probably represents the last remnant of the well-known crinoid fauna of Keokuk age. Although part of the restriction of the Somerset crinoids may be ecologic, the overlap of the genera mentioned above characterizes a very limited stratigraphic interval.