Type and reference sections and description: The Tobinsport Formation was named by Gray (1978, p. 10-11) for the settlement by that name at the southernmost point in Perry County, Ind. A section near Tobinsport earlier described by Malott (1925, p, 117-118) was designated as the type, and Indiana Geological Survey drill hole 132 near Oriole in northeastern Perry County was designated as a reference section. The Tobinsport Formation is dominantly shale and mudstone but includes four widespread members, the Bristow and Mount Pleasant Sandstone Members and the Siberia and Negli Creek Limestone Members.
The name Tobinsport Formation is restricted to surface use. The base of the formation is the top of the Leopold Limestone Member of the Branchville Formation. This contact is conformable. The formation is disconformably overlain by the Mansfield Formation (Morrowan), and so its thickness is variable. Where substantially all members of the formation are present, it is 90 to 125 feet (27 to 38 m) thick, but it is truncated northward by the disconformity, so that it extends only a short distance north of the north boundary of Perry County.
Correlation: The Tobinsport Formation correlates with rocks within the Namurian Series (probably Zone E2) of European usage. Conodont faunas of rocks now a part of the Tobinsport Formation were studied by Collinson, Rexroad, and Thompson (1971), who assigned these rocks to the Kadognathus-Cavusgnathus naviculus Assemblage Zone of standard North American usage.