Devonian System

Type area and reference section and use of name in Indiana: The altered volcanic ash that was noted by Fettke (1931, p. 8) in the Tioga Gas Field of Pennsylvania was named the Tioga Bentonite by Ebright, Fettke, and Ingham (1949, p. 10). Presumably the same bentonite was identified in the Illinois Basin by Meents and Swann (1965, p. 10) and North (1969, p. 10), who also described it and gave it the rank of bed in the Grand Tower Limestone. Its identification in Indiana is owed to Droste and Vitaliano (1973, p. 11), who further described it petrographically as noted in 10 cored sections in central and northern Indiana; to Beeker (1974, p. 39-43), who assigned the Tioga bed status in the Jeffersonville Limestone and noted its characteristic signature on many geophysical logs for southwestern Indiana; to Doheny, Droste, and Shaver (1975, p. 30-32), who recognized the Tioga, with the rank of bed, in all three members of the Detroit River Formation; and to still other persons. An Indiana reference section was designated as the cored bentonite rocks at 4,056 feet (1,237 m) in the Superior No. 1 Comm. Braselton well, Gibson County, Ind. (sec. 24, T. 2 S., R. 12 W.) (Becker, 1974, p. 39-43).

Much doubt, however, exists as to the proper correlation of the Tioga from its eastern type area to the Michigan Basin and Illinois Basin areas as is discussed under "Correlation."

Description: The Tioga Bentonite Bed, known only as a shaly parting or a bed as much as a few inches (0.15 m) thick, has the gross physical appearance of other shale partings in the Middle Devonian rocks of Indiana. The drab layer ranges through shades of gray, brown, black, and olive and is distinguished from normal terrigenous shale beds by its distinctive mineralogy. This devitrified ash contains an unusual clay-mineral structure consisting of a superlattice of smectite-illite randomly interstratified with illite, a structure that may be designated a 10-percent smectite layer IMII superlattice. Quartz grains in the bed are sharply wedge shaped to subangular and are partly admixed with well-rounded silt-to sand-size quartz grains of normal terrigenous origin. High-temperature albite and sanadine feldspars are present. Apatite occurs as tiny crystals in clusters, the crystals showing excellent euhedra with typical hexagonal cross sections and hemimorphic forms with basal parting. Zircon in slightly forms with basal parting. Zircon in slightly rounded to sharply angular euhedra is present. Even in thin layers containing much admixed normal terrigenous components, the particular mineralogy of the Tioga ash can be recognized.

As noted above, the Tioga is hardly more than a shale parting where observed in 90 or more wells (some cored) (Droste and Shaver, 1975a, fig. 10) and in a few quarry exposures in both southern and northern Indiana. The thickest accumulations are less than 6 inches, although a split in the Tioga, involving a few feet of rock, has been observed in a Steuben County well (Droste and Vitaliano, 1973, p. 11-12). Whether the lower part of the split represents a stratigraphic leak is unknown. The Tioga has not been found in some cored wells where it would be expected, a circumstance possibly due to bottom stirring in some places after the ash fell and before burial could occur.

Correlation and correlation problems: The K-bentonite layer referred to here has been widely recognized by the one name, Tioga, over much of the northeastern United States. (For example, see Dennison and Textoris, 1970, and references there.) The Tioga of northern Indiana was correlated by Droste and Shaver (1975b, p. 1220) with the Kawkawlin Bentonite of southern Michigan that was described by Gardner (1974) and named by Baltrusaitis (1974).

The Tioga occurs at or near the base of the conodont Zone of Icriodus angustus and, similarly, at or near the base of the brachiopod Zone of Paraspirifer acuminatus. In Indiana the Tioga is present in all three members of the Detroit River Formation and in the Vernon Fork Member of the Jeffersonville Limestone as well as in undifferentiated Jeffersonville rocks.

There have been two notable disagreements with these statements of correlation and, therefore, also of Tioga identification, one by Baltrusaitis (1975) and the other by Conkin and Conkin (1979a, 1979b, 1984). Perhaps Baltrusaitis's most serious objection to the correlation of the Tioga of northern Indiana with his (1974) Kawkawlin Bentonite of the Michigan Basin is the presumed absence (by erosion) of the necessary rocks in Indiana, specifically, upper Detroit River rocks and partly equivalent Dundee rocks. Nevertheless, the evidence presented by Janssens (1970), Doheny, Droste, and Shaver (1975), Droste and Shaver (1975b), Moore and Rexroad (1974), and Sparling (1983), collectively considered, virtually assures that the necessary rocks are present in both northeastern Indiana and northwestern Ohio. A principal obstacle to agreement has been the failure on the part of many stratigraphers to recognize a major facies relationship between the Detroit River and Dundee rocks.

In different kinds of argument, Conkin and Conkin (1979a, 1979b, 1984) have asserted that more than 30 different Middle Devonian bentonites are known by the name Tioga, and they identified the southern Indiana bentonite described here (for example, that of Droste and Vitaliano, 1973, and of Droste and Shaver, 1975b) under a new name, the Onondaga Indian Nation Bentonite.

Probably, the Tioga is poorly named as such in Indiana and in the Illinois and Michigan Basins in general. As suggested by Dale Sparling (written communication, February 1985: see also Sparllng, 1984), one solution would be to create a new name and designate a local type section. This action would at least end the probable ambiguity that attaches to the name Tioga. Another recourse would be to use the name Kawkawlin in Indiana, but it is tainted by the disagreement noted above. A third alternative, the Conkins' (1984) designation of the Onondaga Indian Nation Bentonite in the Jeffersonville Limestone, is also fraught with a problem, one of uncertainty of correlation with the eastern type material.