Age:

Ordovician

Type designation:

Type locality: In 1898 M. R. Campbell used the term “Lexington Limestone” for 150 ft (45 m) of gray thin-bedded limestone near Lexington, Kentucky, but he established no type section (Keith, 1986).

History of usage:

Overview (Keith, 1986): The name “Lexington Limestone” was first used by J. L. Campbell (1879) for an Ordovician limestone near Lexington in western Virginia. This unit was later renamed.

As defined by M. R. Campbell in 1898, the Lexington Limestone was overlain by the Flanagan Chert and above that by the Winchester Limestone. These rocks have undergone several reclassifications. The term “Winchester” was dropped by Foerste (1906), the upper Winchester of Campbell was assigned to what has generally been called the Eden Group (Cincinnatian), and the lower, greater part of the Winchester was assigned to the Cynthiana Formation. The name “Cynthiana” was also extended to Ohio and Indiana use by Foerste, and later the term “Lexington-Cynthiana” was used in southeastern Indiana by Gutstadt (1958). "Cynthiana" was dropped from Kentucky use by Black, Cressman, and MacQuown (1965), most of the then-existing Cynthiana section was assigned to the Lexington Limestone, and the interbedded limestone and shale at the top of the Cynthiana were assigned to the overlying Clays Ferry Formation. The present Lexington Limestone, therefore, is about 310 ft (95 m) thick in the Lexington, Kentucky, area, and it is subdivided into 11 members consisting of generally gray limestone and variable amounts of skeletal material and thin shale interbeds.

Extended: The term “Lexington Limestone” was adopted for Indiana use by Gray, Brown, and Lineback (1966) for 250 ft (76 m) of rocks; only 50 ft (15 m) of these rocks are exposed near Patriot in Switzerland County (Keith, 1986). These exposed rocks (limestone and subordinate interbedded shale) are now classified as the Point Pleasant Member of the Lexington Limestone in Indiana (Keith, 1986). Use of the name “Lexington” was extended to the subsurface of Indiana by Gray (1972) in his report on the Maquoketa Group, which overlies the Lexington in Indiana (Keith, 1986).

Description:

The Lexington Limestone is generally light- to medium-brownish-gray fossiliferous limestone (Keith, 1986). Interbeds of shale lithologically similar to the overlying Kope Formation of the Maquoketa Group are found in the upper part of the Lexington in Dearborn County (Keith, 1986). The Lexington also contains some darker argillaceous zones that are discussed below (Keith, 1986).

There are only two named subdivisions of the Lexington Limestone that have been adopted for Indiana use (Keith, 1986). The lowest is the Curdsville Member, which is the basal unit of the Lexington and is found throughout the Indiana area of Lexington recognition except for Floyd and Harrison Counties (Keith, 1986). The other member is the Point Pleasant Member, which is the uppermost unit of the Lexington and is found only in eastern Switzerland County (Keith, 1986). The Lexington also contains as many as three darker argillaceous zones that serve as informal subdivisions above the Curdsville and below the Point Pleasant (Keith, 1986).

Distribution: The Lexington Limestone occurs only in southeastern Indiana, and only the upper few tens of feet are exposed in discontinuous stretches along the banks of the Ohio River in eastern Switzerland County (Keith, 1986). The Lexington has a maximum thickness of 256 ft (78 m) in eastern Switzerland County and thins rapidly to the northwest and to as thin as 55 ft (17 m) in eastern Clark County (Keith, 1986). Northwest of Clark County, in Washington County, the Lexington is absent, and the stratigraphic position of the Lexington is continued northward first by rocks of the Kope Formation and farther on by rocks of the Trenton Limestone (Keith, 1986). The thinning of the Lexington is due to a geographically progressive facies change with the Kope (Gray, 1972; Keith, 1985; Keith, 1986).

Boundaries:

The Lexington Limestone conformably overlies the Plattin Formation of the Black River Group (Keith, 1986). The contact with the overlying Kope Formation is more complex, as described above, but appears to be conformable (Keith, 1986).

Correlations:

The Lexington Limestone of Indiana has been correlated with varying degrees of probability and certainty with the Trenton Limestone (Formation, Group) of Indiana, Michigan, and Ohio, the Kimmswick and Lexington Limestones of Kentucky, the Galena Group of Illinois, and the lower parts of the Kope Formation and the Maquoketa Group (Shale) of Indiana and Kentucky (Keith, 1986). The Lexington also correlates with rocks in southwestern Ohio extending up from the Lexington Limestone, through the Point Pleasant Formation, and into the lower part of the Kope Formation (Keith, 1986). (See Gutstadt, 1958; Gray, 1972; Waterman, 1975; Droste and Shaver, 1983; and Shaver, 1984.)

On the basis of conodont studies (Waterman, 1975; Sweet, 1979) the Lexington of Indiana is believed to range in age from Rocklandian (Champlainian Epoch) into Edenian (Cincinnatian Epoch) (Keith, 1986).

Economic Importance:

Petroleum production: The following petroleum fields have produced oil from the Trenton Limestone/Lexington Limestone (Ordovician) in Indiana: Ashley, Baldwin, Barlettsville, Broad Ripple, Broad Ripple South, Decatur, Edgerton, Edgerton North, Fort Wayne, Kingsland, Lakeside, Monroeville, Monroeville South, Monroeville West, New Haven, Peru, Peru East, Richvalley, Royal Center, Trenton, Unionville, Urbana, Urbana North, Walton, Woodburn, and Woodburn North (Cazee, 2004).

Regional Indiana usage:

Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: none
Formation: Lexington Limestone
Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: none
Formation: Lexington Limestone

Misc/Abandoned Names:

Cynthiana Formation, Lexington-Cynthiana

Geologic Map Unit Designation:

Ol

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.

See also:

Trenton Limestone

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.

Campbell, J. L., 1879, Silurian formation in central Virginia: American Journal of Science, ser. 3, v. 18, p. 16–29.

Campbell, M. R., 1898, Description of the Richmond quadrangle (Kentucky): U.S. Geological Survey Geologic Atlas, Folio 46, 4 p.

Cazee, J. T., 2004, Petroleum exploration, development, and production in Indiana during 2002: Indiana Geological Survey, Mineral Economic Series 49, 50 p.

Droste, J. B., and Shaver, R. H., 1983, Atlas of early and middle Paleozoic paleogeography of the southern Great Lakes area: Indiana Geological Survey Special Report 32, 32 p.

Foerste, A. F., 1906, The Silurian, Devonian, and Irvine formations of east-central Kentucky, with an account of their clays and limestones: Kentucky Geological Survey Bulletin 7, 369 p.

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

Gray, H. H., Brown, G. D., Jr., and Lineback, J. A., 1966, Physical techniques of correlation applied to Upper Ordovician rocks of southeastern Indiana [abs.]: American Association of Petroleum Geologists Bulletin, v. 40, p. 615–616.

Gutstadt, A. M., 1958, Cambrian and Ordovician stratigraphy and oil and gas possibilities in Indiana: Indiana Geological Survey Bulletin 14, 103 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., 1985, Map of Indiana showing thickness, extent, and oil and gas fields of Trenton and Lexington Limestones: Indiana Geological Survey Miscellaneous Map 45, scale 1:500,000.

Keith, B. D., 1986, Lexington 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. 77–78.

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.

Sweet, W. C., 1979, Conodonts and conodont biostratigraphy of post-Tyrone Ordovician rocks of the Cincinnatian region, in Pojeta, John, Jr., ed., Contributions to the Ordovician paleontology of Kentucky and nearby states: U.S. Geological Survey Professional Paper 1066-G, 26 p.

Waterman, A. S., 1975, Conodont biostratigraphy, paleontology, and paleoecology of the Trenton and Lexington Limestones in southeastern Indiana: Bloomington, Indiana University, master's thesis, 60 p.