Type locality: The Backbone Limestone was named for the ridge called the Devil's Backbone Ridge along the Mississippi River north of Grand Tower, Jackson County, Illinois (Savage, 1920, p. 173; Droste and Shaver, 1986). The type section is in a quarry in the SE¼SW¼SW¼ sec. 24, T. 10 S., R. 4 W. (Droste and Shaver, 1986).
History of usage:
Extended: The Backbone Limestone was recognized in the subsurface of southwestern Indiana by Collinson and others (1967) (Droste and Shaver, 1986).
The Backbone Limestone is characteristically made up of light-colored medium- to coarse-grained, rather pure bioclastic limestone (Droste and Shaver, 1986). The Backbone also has two prominent intervals of drab cherty dolomitic limestone and dolomitic chert that are overlain and underlain by coarse-grained Backbone limestones (Droste and Shaver, 1986). Some glauconite is present.
The Backbone Limestone conformably overlies the fine-grained carbonate rocks of the Bailey Limestone (Silurian) in the deeper part of the Illinois Basin and probably conformably and unconformably overlies reef and nonreef rocks of the Wabash Formation (Silurian) toward the basin margin (Droste and Shaver, 1986). Similarly, the Backbone has conformable and unconformable relations with overlying rocks, the Clear Creek Chert (Lower Devonian) and the Jeffersonville Limestone (Middle Devonian) respectively (Droste and Shaver, 1986). The boundary with the Clear Creek is placed where light-colored fine- to medium-grained, somewhat cherty rocks (Clear Creek) become dominant over the coarser and purer carbonate rocks below (Backbone) (Droste and Shaver, 1986).
The Backbone of Indiana extends to rocks of the same name in the Illinois and Kentucky parts of the Illinois Basin, but because of varying definitions in the Illinois Basin simple statements on correlation cannot be made (Droste and Shaver, 1986). In the Indiana definition, lower Backbone rocks correlate (as facies) with the Grassy Knob of extreme southwestern Indiana and adjacent Illinois, and upper Backbone rocks probably have a partial complementary age relationship with lower Clear Creek rocks of Indiana and Illinois (Droste and Shaver, 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, and Atherton, Elwood, 1975, Devonian System, in Willman, H. B., Atherton, Elwood, Buschbach, T. C., Collinson, Charles, Frye, J. C., Hopkins, M. E., Lineback, J. A., and Simon, J. A., Handbook of Illinois stratigraphy: Illinois State Geological Survey Bulletin 95, p. 104-123.
Collinson, Charles, James, G. W., Swann, D. H., Becker, L. E., Carlson, M. P., Dorheim, F. H., and Koenig, J. W., 1967, Devonian of north-central region, United States, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 933-971.
Droste, J. B., and Shaver, R. H., 1986, Backbone Limestone, 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. 7-8.
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.
Savage, T. E., 1920, Devonian formations of Illinois: American Journal of Science, v. 49, p. 169-182.
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.
For additional information, contact:
Nancy Hasenmueller (firstname.lastname@example.org)Date last revised: June 19, 2017