Type section: The Floyds Knob Limestone Member was originally named as a formation for exposures along U.S. Highway 150 through the Knobstone Escarpment at Floyds Knobs in the center of sec. 21, T. 2 S., R. 6 E., 0.75 miles (1.2 km) east of the Floyds Knobs post office in Floyd County, Indiana (Stockdale, 1929, p. 170; 1931, p. 195-196; Gates and Rexroad, 1970; Rexroad, 1986).
History of usage:
Revised rank: The unit was reduced to the rank of a member of the Muldraugh Formation by Smith (1965) (Gates and Rexroad, 1970; Rexroad, 1986).
The Floyds Knob is primarily a limestone as far north in Indiana as Jackson County but is "expressed by a peculiar light buff to ocherous zone of slightly calcareous, shaly and arenaceous rock with many variations" north of there (Stockdale, 1931, p. 197). In its type area and southern extent the Floyds Knob approximates 5 ft (1.5 m) in thickness (Rexroad, 1986). From Monroe County northward bioherms and lenticular limestone beds are locally prominent in the Edwardsville; however, these generally appear to be somewhat above the Floyds Knob stratigraphically (Rexroad, 1986).
The Floyds Knob is conformable within the Borden Group, although there is some reworked material in its basal part and a bed of glauconite in many places marks the base of the overlying part of the Edwardsville (Rexroad, 1986).
Conodonts recovered from the type section of the Floyds Knob (Gates and Rexroad, 1970; Whitehead, 1978) show that the Floyds Knob is correlative with part of the Keokuk Limestone of the upper Mississippi Valley (Rexroad, 1986).
Regional Indiana usage:
Illinois Basin Margin (COSUNA 12)
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.
Gates, G. R., and Rexroad, C. B., 1970, Floyds Knob Member, 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. 57–58.
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.
Rexroad, C. B., 1986, Floyds Knob Limestone Member, 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. 46–47.
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.
Stockdale, P. B., 1929, Facies of the Borden rocks of southern Indiana [abs.]: Ohio Journal of Science, v. 29, p. 170.
Stockdale, P. B., 1931, The Borden (Knobstone) rocks of southern Indiana: Indiana Department of Conservation Publications 98, 330 p.
Whitehead, N. D., III, 1978, Lithostratigraphy, depositional environments, and conodont biostratigraphy of the Muldraugh Formation (Mississippian) in southern Indiana and north-central Kentucky: Southeastern Geology, v. 19, p. 83–109.
For additional information, contact:
Nancy Hasenmueller (email@example.com)Date last revised: May 27, 2016