Ordovician System
Type area, representative sections, and former names: The name Dillsboro Formation was proposed by Brown and Lineback (1966, p. 1020-1021) for "the sequence of highly fossiliferous argillaceous limestones and cat careens shales that lie between the shale of the Kope Formation and the dolomitic limestone of the Saluda Formation [now a member]." The designated type area is in southwestern Dearborn County and east-central Ripley County near Dillsboro, Ind. Two sections showing relationships with adjacent formations are, for the lower contact, along UPS. Highway 50, 1.5 miles (2 km) west of Aurora in southeastern Dearborn County (NW¼ sec. 6, T. 4 N., R. 1 W.) and, for the upper contact, along UPS. Highway 50 in the N2SE¼ sec. 12, T. 7 N., R. 11 E., Ripley County.
The name Dillsboro Formation replaces a number of names formerly in use by the Indiana Geological Survey: in descending order, Liberty, Waynesville, Arnheim, Mount Auburn, Corryville, Bellevue, Fairmount, and Mount Hope Formations (Patton, Perry, and Wayne, 1953). The first two of these formations were assigned to the Richmond Group, and the others were assigned to the Maysville Group. Most of these names as applied denoted faunal zones rather than lithostratigraphic units. At least two, however, may be sufficiently distinct lithologically to reenter the rock-stratigraphic nomenclature when they have been properly redescribed. These are the Waynesville, which is principally shale, and the Bellevue, a distinctive rubbly limestone.
Description: In its type area the Dillsboro Formation consists in about equal parts of argillaceous limestone and calcareous shale. Northward more shale is present, and southward more limestone (Gray, 1972b). A distinctive rubbly limestone that contains the brachiopods Platystrophia and Rafinesquina, the Bellevue Limestone of some authors, is recognizable some 100 feet (30 m) above the base of the formation. Overall, the Dillsboro contains about 30 percent limestone (Brown and Lineback, 1966, p.1020).
The Dillsboro Formation is about 400 feet (120 m) thick throughout much of the area in which it is recognized. It is conformably overlain by the Whitewater Formation (Cincinnatian) and is underlain by the Kope Formation (Cincinnatian). In some places the Kope-Dillsboro contact must be arbitrarily picked in a gradational sequence, but in most places a sharp upward increase in limestone content marks this contact (Brown and Lineback, 1966, p. 1021).
The Dillsboro Formation is recognized only in southeastern Indiana, primarily along the outcrop belt of Ordovician rocks and in drill holes close to the outcrop area. This is not entirely because the basal contact is "too subtle" for subsurface use, as stated by Gray (1972b, p. 21), but more importantly it is because the northward increase in shale content ultimately makes invalid the criteria by which the Dillsboro is recognized. Gray (1972b, fig. 5) implied that a step wise shift in the basal contact might be applicable Hay (1981 and Hay, Pope, and Frey, 1981) erected a largely new nomenclature and recognized several formations and members in place of the Dillsboro, but she still encountered much difficulty in applying these names and concepts in the near subsurface.
The Dillsboro Formation was designed and is useful for regional mapping and probably should be regarded as only a step toward establishment of a true lithostratigraphic scheme of nomenclature in rocks that had long been subdivided by faunal content only. Most of Hay's (1981) nomenclature is readily applied in much of the outcrop area and will probably find acceptance when it is formally proposed, but difficulties remain in correlating Hay's units into the near subsurface.
Correlation: The Dillsboro Formation is Maysvillian and early Richmondian in age and is correlative with the Fort Atkinson Limestone and at least the upper part of the underlying Scales Shale, which are widely recognized in Illinois and in the subsurface of western and northern Indiana.