Champlainian and Cincinnatian Series
Ordovician System
Type locality and description: The Trenton Limestone was apparently named by Vanuxem (1838, p. 257) for rocks that were exposed at Trenton Falls in Oneida County, N.Y., and that were about 100 feet (30 m) thick. There is some confusion about what rocks were included in the Trenton by Vanuxem in his 1838 and 1842 reports, but they probably consisted of interbedded dark limestone and shale and a light-gray massive crinoidal limestone. In the type area the Trenton is now given group status and is subdivided into several formations (Ross and others, 1982).
History of use of name in Indiana: The first published use of the name Trenton in Indiana was probably by Phinney (1891), who included rocks presently assigned to the Black River and Ancell Groups except for the St. Peter Sandstone. Phinney was correct, however, in equating only the upper part of the Trenton Limestone of Indiana with the Galena Dolomite of Wisconsin and Illinois, although the exact nature of his correlation is vague. Use of the name Trenton continued in this broad sense in Indiana until Gutstadt (1958a) separated the Black River Limestone from the Trenton Limestone.
Description and distribution in Indiana: The Trenton consists of limestone that becomes increasingly dolomitic in northern Indiana, and in places it is completely dolomitized. The Trenton is tan to light tannish gray to medium tannish gray. The color variation in the limestone is due to the variation in the content of skeletal grains versus micrite the darker color correlates with the higher micrite content. In the dolomite the size of the crystals appears to be the controlling factor the more coarsely crystalline phases are lighter colored.
The Trenton Limestone is everywhere in the subsurface of Indiana except for far southeastern Indiana as noted below. Its only exposure, in the chaotic structure quarried near Kentland in Newton County, was referred by Gutschick (1983) to the Galena Dolomite. The Trenton has a maximum thickness of 265 feet (81 m) in Steuben County in northeastern Indiana, and it thins to zero thickness in far southeastern Indiana through what is believed (although not well understood) to be a geographically progressive facies change with the Kope Formation, which is replaced farther southeastward by the Lexington Limestone through a similar facies change (Gray, 1972b; Droste and Shaver, 1983; and Keith, 1985). This narrow area of dual facies change extends northeastward from Spencer and Perry Counties to eastern Fayette County (Keith, 1985).
The contact of the Trenton Limestone with the underlying Plattin Formation of the Black River Group (Droste, Abdulkareem, and Patton, 1982) appears conformable where it is seen in cores, although a distinct lithologic change occurs between the units. The Trenton generally underlies the shales of the Maquoketa Group (Gutstadt, 1958a, 1958b; Gray, 1972b), but the relationship with the overlying Maquoketa is not wholly clear. The contact in all places where it has been observed appears to represent a discontinuity (hardground) due to a period of probably prolonged submarine exposure. A major unconformity and subaerial erosion as suggested by Rooney (1966) do not apply. Regionally, the contact between these two units becomes younger and rises stratigraphically to the west because the locus of Maquoketa shale deposition was extended progressively westward from the Appalachian Basin during Late Ordovician time. This westward extension therefore accounted for the eventual burying of the Trenton and equivalent carbonate rocks.
Correlation: The Trenton Limestone of Indiana has been variably correlated approximately to exactly with rocks of the same name in Michigan and Ohio; with a section made up of the Lexington Limestone, the Point Pleasant Formation, and the lower part of the Kope Formation in southwestern Ohio; with the Kimmswick and Lexington Limestones and the lower part of the Maquoketa Shale of central and westem Kentucky; with the Galena Group of Illinois and with the Lexington Limestone and the lower part of the Kope Formation of southeastern Indiana. Probably no part of the Trenton of Indiana, however, is as young as the Dubuque Formation in the upper part of the Galena Group of Illinois. (See Twenhofel and others, 1954; Gutstadt, 1958a; Willman and Buschbach, 1975; Willman and Kolata, 1978; Droste and Shaver, 1983; Gutschick, 1983; and Shaver and others, 1985.)
On the basis of both conodont studies (Waterman, 1975; Sweet, 1979) and physical relations with the lower Kope shale of the type area of the Edenian Stage, the Trenton of Indiana is believed to range in age from Rocklandian (Champlainian Epoch) into Edenian (Cincinnatian Epoch).
