Devonian System

Type area and use of name: A sequence of thick-bedded buff granular magnesian limestones exposed around Little Traverse Bay in the northwestern part of the Lower Peninsula of Michigan was named the Little Traverse Group by N. H. Winchell (1871, p. 26-33). The name was later (Lane, 1895, p. 24) shortened to the Traverse Group, and this terminology is now used in the type area and in the Michigan Basin subsurface (Cohee, 1944, 1947), where the term Traverse Limestone is also used. (See Shaver and others, 1985.)

In accord with the correlation of Pinsak and Shaver (1964, pl. 1), the Traverse was formally recognized as a formation in northern Indiana by Schneider and Keller (1970), where it consists of limestones that are coextensive with the Traverse Group of southern Michigan, and it was assigned to the Muscatatuck Group by Shaver (1974a).

Description: The Traverse consists of a variety of limestones, some dolomite, and thin shale beds. These are recognized as making up three stratigraphic units (unnamed to member in Indiana). In ascending order they are: (1) 9 to 15 feet (2.8 to 4.6 m) of interbedded light-gray dense limestone (pellmicrite), light-yellow-brown micritic very fossiliferous (corals, stromatoporoids, and brachiopods) limestone, and crinoidal calcarenite (biosparite); (2) in northwestern Indiana, as much as 50 feet (15 m) of light-colored, highly fossiliferous limestone (including crinoidal, stromatoporoidal, and coralline limestones, partly in bioherms) and lithographic and sublithographic limestones and, in northeastern Indiana, more than 30 feet (9 m) of dark very fossiliferous calcareous shale and argillaceous limestone; and (3) 15 to 50 feet (4.6 to 15 m) of light-colored dense to medium-grained, somewhat cherty dolomite. Sandy (quartz), brecciated, and oolitic rocks are present in some places and are characteristic of given stratigraphic intervals. (See Shaver and others, 1971, p. 50-53; Lazor, 1971; Orr, 1971, p. 8-9 and appendix sections and Doheny, Droste, and Shaver, 1975, p. 35 and appendix sections.)

The Traverse of Indiana overlies the Detroit River Formation unconformably wherever Detroit River rocks are present. This relationship is an overlapping one, so that in updip areas along the north flank of the Kankakee and Cincinnati Arches the Traverse rests on differing parts of the Detroit River and in some places extends past the eroded (pre-Traverse) edge of Detroit River rocks. In these places, therefore, the Traverse rests with major unconformity on Silurian rocks of the Wabash Formation. In northwestern Indiana the Traverse continues onto the crestal area of the Kankakee Arch and coextends with the North Vernon Limestone of central and southern Indiana. (See Doheny, Droste, and Shaver, 1975, p. 33-34.) The Traverse is overlain by the Antrim Shale, both conformably and with modest unconformity according to Orr (1971, fig. 4 and p. 21) and Shaver and others (1985).

The Traverse is limited by post-Devonian erosion and by definition to the northern crestal area of the Kankakee and Cincinnati Arches and to the Michigan Basin, where it ranges in thickness from zero to about 20 feet (6 m) (erosional) along its southern extent to more than 120 feet (37 m) in far northeastern Indiana (Shaver and others, 1971, fig. 9). Not all Indiana geologists, however, have followed this setting forth of a defined southern limit. For example, Orr (1969) used the term Traverse for appropriately correlated rocks in Carroll and Cass Counties, as did the Indiana University Paleontology Seminar (1980, fig. 5 and p. 129).

Correlation: As noted above, the Traverse Formation physically extends into the North Vernon Limestone across the crestal area of the Kankakee Arch in Jasper County and adjacent counties. For the crestal area all across northern Indiana, however, it is a moot point as to which of the many pre-1970 references in the literature to Devonian rocks should have been to the Traverse and which should have been to the North Vernon rather than to the often erroneously identified Jeffersonville Limestone (- Detroit River). North or south of the crestal area, most of the Middle Devonian carbonate rocks of historical record in that area are readily identifiable with the Traverse, whether directly as that formation or in a correlative sense. (See a history of these misidentifications of Traverse-North Vernon rocks in Doheny, Droste, and Shaver, 1975, p. 4-6; see also the North Vernon article and Orr, 1971, p. 9, for now-obsolete names applied to these rocks.)

The Traverse megafauna includes representatives of the Hamilton Group (upper Middle Devonian) of New York (A. C. Brookley as reported by Shaver and others, 1961, p. 40). Two conodont zones are recognized in the Traverse, the Icriodus latericrescens latericrescens Zone in lower Traverse rocks and the Polygnathus varcus Zone in upper Traverse rocks (Orr, 1971). To judge from Orr's work, the division between these two zones is apparently within the middle lithologic unit described above. The Traverse is therefore assigned a late Erian age (North American standard), that is, a Givetian

age (global standard). A small lower part as recognized in northeastern Indiana and that may be equivalent to the upper part of the Dundee Limestone of adjacent Ohio, however, may be latest Eifelian in age. (See Doheny, Droste, and Shaver, 1975, p. 41, unit 9, and pertinent discussion; Shaver and others, 1985, Indiana; column 5 and Klapper and Johnson, 1980, for the Eifelian age of upper Dundee conodonts.)

Given these age determinations and its physical relations, the Traverse of Indiana correlates closely, if not exactly, with these formations: the North Vernon Limestone, central and southern Indiana; the Sellersburg Limestone, western Kentucky; the Lingle Limestone, Illinois; the Traverse Group and probably the underlying Rogers City Limestone if not also some even lower rocks, Michigan; the upper part of the Dundee Limestone, the Silica Formation, and the Tenmile Creek Dolomite, northwestern Ohio; and the Hamilton Group except for the upper (Tully Formation) part, New York. The three lithologic divisions of the Indiana Traverse noted above correspond to these formations in ascending order in northwestern Ohio: the upper part of the Dundee Limestone of Janssens (1970), the Silica Formation, and the Tenmile Creek Dolomite. (See Lazor, 1971, and Shaver and others, 1971, fig. 10.)