Ironton Sandstone

Munising Group,

Cambrian System

Type locality and use of name: The term Ironton Member (Thwaites, 192:3, p. 550) was first applied to exposures of several feet of coarse-grained sandstone, basal most in the Franconia Formation, near Ironton, Sauk County, Wis. Most later use of the name in the upper Midwest has been at formation rank.

Description: The Ironton Sandstone was identified (Becker, Hreha, and Dawson, 1978) in the subsurface of northwestern Indiana as overlying the Galesville Sandstone and underlying the Franconia Formation. There it consists of medium- to coarse-grained sandstone and some interbedded dolomitic sandstone. Eastward and southward these sandstones become more dolomitic and shaly, so that nomenclaturally the Ironton is arbitrary cut off in central northern and western Indiana and replaced by the term Davis Formation. In several counties in northwestern Indiana the Ironton can be distinguished from the underlying Galesville Sandstone, but in other counties where the name Davis Formation is not used the two formations cannot be recognized separately and are mapped together as a single unit.

The Ironton and Galesville together range in thickness from about 100 feet (30 m) at the position of arbitrary cutoff with the Davis Formation to more than 200 feet (61 in) in the northwest corner of Indiana. Where the Ironton is distinguishable from the Galesville, the formation is about 100 feet (30 in) thick.

Correlation: The Ironton Sandstone is recognized by the one name in Indiana, Illinois, and Wisconsin, and it has stratigraphic equivalency with part of the Davis Formation east and south of the cutoff line in northwestern Indiana. The Ironton is also equivalent to the Kerbel Formation in Ohio as recognized by Janssens (1973) and the lower part of the Franconia Formation in the Lower Peninsula of Michigan (Droste and Shaver, 1983; Shaver and others, 1985).

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