Age:

Ordovician

Type designation:

Type locality: The name “Curdsville Bed” was given originally by Miller (1905) to 30 ft (9 m) of fossiliferous and partly cherty rocks exposed at Curdsville in Mercer County, Kentucky (Keith, 1986). There it was considered to be the lowermost unit of what is now called the Lexington Limestone (Keith, 1986).

Reference section: An Indiana reference section was designated as the pertinent rocks cored in Indiana Geological Survey drill hole 133 (Indiana Geological Survey Petroleum Database Management System No.126872) on the Robbins farm in sec. 1, T. 2 N., R. 1 W., Switzerland County (Keith, 1986). In this core, the Curdsville is 15 ft (4.6 m) thick (Keith, 1986).

History of usage:

Revised rank: After a period of use at different ranks in Kentucky, the Curdsville was assigned member status by Black, Cressman, and MacQuown (1965) (Keith, 1986).

Extended: Keith (1986) adopted the name “Curdsville Limestone Member” (Lexington Limestone) with slight modification and assigned the name to corresponding rocks in Indiana.

Description:

The Curdsville Member of the Lexington Limestone is generally a tannish-gray fossiliferous limestone and is the lower of two named subdivisions of the Lexington Limestone in Indiana (Keith, 1986).

The Curdsville is absent in Floyd and Harrison Counties; it thickens northeastward to a maximum of 20 ft (6 m) in northeastern Dearborn County. The Curdsville thins and becomes absent through facies change with overlying rocks of the Lexington (Keith, 1986).

Miscellaneous Information: Although the lowermost few feet of the Trenton Limestone in southern Indiana resembles the Curdsville Member in color and composition, that part of the Trenton does not everywhere stand in sharp contrast to the overlying Trenton rocks (Keith, 1986). For this reason, the Curdsville is given member status only in the Lexington in Indiana (Keith, 1986).

Distribution: The Curdsville is present everywhere the Lexington is present except in Floyd and Harrison Counties (Keith, 1986).

Boundaries:

The Curdsville Member is a distinctive lithologic unit as recognized in both cores and samples (Keith, 1986). It represents a sharp lithologic break from the micritic limestones of the underlying Plattin Formation of the Black River Group, but it apparently has a conformable contact with the Plattin (Keith, 1986). It is overlain conformably everywhere by an argillaceous limestone within the Lexington (Keith, 1986).

Correlations:

The Curdsville Member correlates with rocks known by the same name and mapped by Cressman (1973) in north-central Kentucky and, as is implicit from the discussion above, with other rocks of the Lexington that are not named as members (Keith, 1986).

Regional Indiana usage:

Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: none
Formation: Lexington Limestone
Member: Curdsville Member

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Olc

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:

Black, D. F. B., Cressman, E. R., and MacQuown, W. C., Jr., 1965, The Lexington Limestone (Middle Ordovician) of central Kentucky: U.S. Geological Survey Bulletin 1224-C, p. 1–29.

Cressman, E. R., 1973, Lithostratigraphy and depositional environments of the Lexington Limestone (Ordovician) of central Kentucky: U.S. Geological Survey Professional Paper 768, 61 p.

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.

Keith, B. D., 1986, Curdsville 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. 33.

Miller, A. M., 1905, The lead and zinc bearing rocks of central Kentucky, with notes on the mineral veins: Kentucky Geological Survey Bulletin 2, 35 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.