Type Locality: The name “Stephensport,” though having some earlier mention (see Swann, 1963, p. 83), was formally proposed in a group sense by Gray, Jenkins, and Weidman (1960, p. 37), who redefined it to consist in descending order of the Glen Dean Limestone, the Hardinsburg Formation, the Golconda (now Haney) Limestone, the Big Clifty Formation, and the Beech Creek Limestone (Gray, 1986). The group was named for Stephensport, Breckinridge County, Kentucky (Gray, 1962, table 2 and fig. 4) (Gray, 1986).
The Stephensport Group consists of about equal parts of limestone, shale, and cliff-forming sandstone (Gray, 1962, table 2 and fig. 4; Gray, 1986). The total thickness of the Stephensport Group is 150 to 300 ft (45.7 to 91.4 m) (Droste and Keller, 1995, fig. 4). It is recognized on the outcrop from eastern Greene County southward to the Ohio River (Droste and Keller, 1995, fig. 4). In the subsurface it extends from Clay County southwestward (Gray, 1986; Droste and Keller, 1995).
The Stephensport conformably overlies the West Baden Group (Chesterian) and is overlain conformably by the Buffalo Wallow Group (Chesterian) or disconformably by the Mansfield Formation (Morrowan) (Gray, 1986).
The Stephensport Group is exactly correlative with the Okaw Group of southwestern Illinois (Swann, 1963, p. 53) but is distinct in usage by including prominent clastic formations; the Okaw is dominantly limestone (Swann, 1963, p. 45-46; Gray, 1986). On the basis of their conodont faunas, formations of the group represent the Gnathodus bilineatus-Cavusgnathus altus and Gnathodus bilineatus-Kladognathus mehli Assemblage Zones of the North American standard (Collinson, Rexroad, and Thompson, 1971; Gray, 1986). The group spans the Visean-Namurian boundary of European usage and correlates with rocks within North American foraminiferal Zones 16s and 17 of Mamet and Skipp (1971) (Gray, 1986).
Regional Indiana usage:
Illinois Basin (COSUNA 11)
Geologic Map Unit Designation:
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.
Collinson, Charles, Rexroad, C. B., and Thompson, T. L., 1971, Conodont zonation of the North American Mississippian: Geological Society of America Memoirs 127, p. 353–394.
Gray, H. H., 1986, Stephensport Group, 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. 150–151.
Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7th ed.): Washington, D.C., U.S. Geological Survey, 289 p.
Mamet, B. L., and Skipp, B. A., 1971, Lower Carboniferous calcareous Foraminifera–preliminary zonation and stratigraphic implications for the Mississippian of North America: Sixieme Congres International de Stratigraphie et de Geologie du Carbonifere Sheffield, 1967, Compte rendu, v. 3, p. 1,129–1,146.
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.
Swann, D. H., 1963, Classification of Genevievian and Chesterian (Late Mississippian) rocks of Illinois: Illinois State Geological Survey Report of Investigations 216, 91 p.
For additional information, contact:
Nancy Hasenmueller (firstname.lastname@example.org)Date last revised: May 23, 2017