Franconia Formation

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Franconia Formation,

(JBD + JBP)

Munising Group,

Cambrian System

Type section and use of name: The name Franconia Formation was first proposed for about 100 feet (30 m) of sandstone and green shale exposed near Franconia, Chicago County, Minn, (Berkey, 1897, p. 373), and it has since had wide use in the upper Midwest.

Description: In Indiana the Franconia Formation is recognized in the subsurface in the northwestern part of the state (Becker, Hreha, and Dawson, 1978) only where the underlying Ironton Sandstone or the combined Ironton and Galesville Sandstones are recognized. There the Franconia consists of glauconitic fine- to medium-grained dolomite sandstone and enough inter bedded shaly zones to show on geophysical logs a higher natural gamma radiation than the overlying Potosi Dolomite and the underlying Ironton Sandstone. Siliciclastic components dominate the 130 feet (40 m) of maximum thickness of the Franconia of northwestern Indiana. The Franconia becomes more dolomitic and thinner eastward and southward and laterally grades entirely into the upper part of the Davis Formation or into the lower part of the Potosi Dolomite along an arbitrary nomenclatural cutoff in central northern and western Indiana.

Correlation: The Franconia Formation is recognized by the one name in Indiana, Illinois, and Michigan, but in Michigan only the upper Franconia correlates with the Franconia of Indiana. It is also stratigraphically equivalent to the Derby-Doe Run Dolomite and the Davis Formation of Missouri, to parts of the Davis Formation and the Potosi Dolomite in Indiana areas where the name Franconia is not used, and to the middle part of the Elvins Formation of western Kentucky.