Champlainian and Cincinnatian Series

Ordovician System

Type locality and history of use of name: 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. In 1898 M. R. Campbell used the term Lexington Limestone for 150 feet (45 m) of gray thin-bedded limestone near Lexington, Ky., but he established no type section. As defined by M. R. Campbell, the Lexington 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 (1958a). "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 feet (95 m) thick in the Lexington, Ky., area, and it is subdivided into 11 members consisting of generally gray limestone and variable amounts of skeletal material and thin shale interbeds.

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

Description and distribution: The Lexington Limestone is generally light- to medium-brownish-gray fossiliferous limestone. 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. The Lexington also contains some darker argillaceous zones that are discussed below.

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. The Lexington has a maximum thickness of 256 feet (78 m) in eastern Switzerland County and thins rapidly to the northwest and to as thin as 55 feet (17 m) in eastern Clark County. 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. The thinning of the Lexington is due to a geographically progressive facies change with the Kope (Gray, 1972b Keith, 1985).

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

There are only two named subdivisions of the Lexington Limestone that have been adopted for Indiana use. 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. The other member is the Point Pleasant Member, which is the uppermost unit of the Lexington and is found only in eastern Switzerland County. The reader is referred to the articles on these two units in this compendium for more information. The Lexington also contains as many as three darker argillaceous zones that serve as informal subdivisions above the Curdsville and below the Point Pleasant.

Correlation: 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. 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. (See Gutstadt, 1958a; Gray, 1972b; Waterman, 1975; Droste and Shaver, 1983; and Shaver and others, 1985)

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).