Age:

Ordovician

Type designation:

Type area: The name "Dillsboro Formation" was proposed by Brown and Lineback (1966, p. 1,020-1,021) for "the sequence of highly fossiliferous argillaceous limestones and calcareous shales that lie between the shale of the Kope Formation and the dolomitic limestone of the Saluda Formation [now a member]" (Burger and Shaver, 1970; Gray, 1986). The designated type area is in southwestern Dearborn County and east-central Ripley County near Dillsboro, Indiana (Burger and Shaver, 1970; Gray, 1986).

Representative sections: Two sections showing relationships with adjacent formations are, for the lower contact, along U.S. Highway 50, 1.5 miles (2.4 km) west of Aurora in the NW¼ sec. 6, T. 4 N., R. 1 W. (Aurora quadrangle), southeastern Dearborn County and, for the upper contact, along U.S. Highway 50 in the N½SE¼ sec. 12, T. 7 N., R. 11 E. (Milan quadrangle), Ripley County (Burger and Shaver, 1970; Gray, 1986).

History of usage:

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; Gray, 1986). The first two of these formations were assigned to the Richmond Group, and the others were assigned to the Maysville Group (Gray, 1986). 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 (Gray, 1986). These are the Waynesville, which is principally shale, and the Bellevue, a distinctive rubbly limestone (Gray, 1986).

Description:

In its type area the Dillsboro Formation consists in about equal parts of argillaceous limestone and calcareous shale (Burger and Shaver, 1970; Gray, 1986). 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 ft (30 m) above the base of the formation (Burger and Shaver, 1970; Gray, 1986). Overall, the Dillsboro contains about 30 percent limestone (Brown and Lineback, 1966, p. 1,020; Burger and Shaver, 1970; Gray, 1986).

The Dillsboro Formation is about 400 ft (120 m) thick throughout much of the area in which it is recognized (Gray, 1986).

Miscellaneous Information: 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, 1986). Gray (1972b, fig. 5) implied that a stepwise 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 (Gray, 1986).

Distribution: 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 (Gray, 1986).

Boundaries:

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. 1,021; Burger and Shaver, 1970; Gray, 1986).

Correlations:

The Dillsboro Formation is Maysvillian and early Richmondian in age (Frey, 1995) 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 (Gray, 1986).

Regional Indiana usage:

Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: Maquoketa Group
Formation: Dillsboro Formation

Misc/Abandoned Names:

Maysville Group, Richmond Group, Waynesville Formation

Geologic Map Unit Designation:

Od

Note: Hansen (1991, p. 52) in Suggestions to authors of the reports of the United States Geological Survey noted that letter symbols for map units are considered to be unique to each geologic map and that adjacent maps do not necessarily need to use the same symbols for the same map unit. Therefore, map unit abbreviations in the Indiana Geologic Names Information System should be regarded simply as recommendations.

COSUNA areas and regional terminology

Names for geologic units vary across Indiana. The Midwestern Basin and Arches Region COSUNA chart (Shaver, 1984) was developed to strategically document such variations in terminology. The geologic map (below left) is derived from this chart and provides an index to the five defined COSUNA regions in Indiana. The regions are generally based on regional bedrock outcrop patterns and major structural features in Indiana. (Click the maps below to view more detailed maps of COSUNA regions and major structural features in Indiana.)

COSUNA areas and numbers that approximate regional bedrock outcrop patterns and major structural features in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

References:

Brown, G. D., Jr., and Lineback, J. A., 1966, Lithostratigraphy of Cincinnatian Series (Upper Ordovician) in southeastern Indiana: American Association of Petroleum Geologists Bulletin, v. 50, p. 1,018–1,023.

Burger, A. M., and Shaver, R. H., 1970, Dillsboro Formation, in Shaver, R. H., Burger, A. M., Gates, G. R., Gray, H. H., Hutchison, H. C., Keller, S. J., Patton, J. B., Rexroad, C. B., Smith, N. M., Wayne, W. J., and Wier, C. E., Compendium of rock-unit stratigraphy in Indiana: Indiana Geological Survey Bulletin 43, p. 45–48.

Frey, R. C., 1995, Middle and Upper Ordovician nautiloid cephalopods of the Cincinnati Arch region of Kentucky, Indiana, and Ohio: U.S. Geological Survey Professional Paper 1066, p. 1–126.

Gray, H. H., 1972, Lithostratigraphy of the Maquoketa Group (Ordovician) in Indiana: Indiana Geological Survey Special Report 7, 31 p.

Gray, H. H., 1986, Dillsboro Formation, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana–a revision: Indiana Geological Survey Bulletin 59, p. 37–38.

Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7^th ed.): Washington, D.C., U.S. Geological Survey, 289 p.

Hay, H. B., 1981, Lithofacies and formations of the Cincinnatian Series (Upper Ordovician), southeastern Indiana and southwestern Ohio: Oxford, Ohio, Miami University, Ph.D. thesis, 238 p.

Hay, H. B., Pope, J. K., and Frey, R. C., 1981, Lithostratigraphy, cyclic sedimentation, and paleoecology of the Cincinnatian Series in southwestern Ohio and southeastern Indiana (Field Trip 1), in Roberts, T. G., ed., Geological Society of America Cincinnati '81 Field Trip Guidebooks: Falls Church, Va., American Geological Institute, v. 1, Stratigraphy, Sedimentation, p. 73–86.

Patton, J. B., Perry, T. G., and Wayne, W. J., 1953, Ordovician stratigraphy, and physiography of part of southeastern Indiana: Indiana Geological Survey Field Conference Guidebook 6, 29 p.

Shaver, R. H., coordinator, 1984, Midwestern basin and arches region–correlation of stratigraphic units in North America (COSUNA): American Association of Petroleum Geologists Correlation Chart Series.