Age:

Cambrian

Type designation:

Type area: The name "Franconia Formation" was first proposed for about 100 ft (30 m) of sandstone and green shale exposed near Franconia, Chisago County, Minnesota (Berkey, 1897, p. 373; Droste and Patton, 1986).

History of usage:

Extended: The Franconia Formation was recognized in the subsurface in the northwestern part of the Indiana by Becker, Hreha, and Dawson (1978) where the underlying Ironton Sandstone or the combined Ironton and Galesville Sandstones are recognized (Droste and Patton, 1986).

Description:

The Franconia Formation consists of glauconitic fine- to medium-grained dolostone sandstone and enough interbedded shaly zones to show on geophysical logs a higher natural gamma radiation than the overlying Potosi Dolomite and the underlying Ironton Sandstone (Droste and Patton, 1986). Siliciclastic components dominate the 130 ft (40 m) of maximum thickness of the Franconia of northwestern Indiana. The Franconia becomes more dolomitic and thinner eastward and southward (Droste and Patton, 1986).

Distribution: The Franconia 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 (Droste and Patton, 1986).

Correlations:

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 (Droste and Patton, 1986). 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 (Droste and Patton, 1986).

Regional Indiana usage:

Illinois Basin Margin (COSUNA 12)
Supergroup: Potsdam Supergroup
Group: Munising Group
Formation: Franconia Formation
Kankakee Arch (COSUNA 14)
Supergroup: Potsdam Supergroup
Group: Munising Group
Formation: Franconia Formation
Michigan Basin (COSUNA 15)
Supergroup: Potsdam Supergroup
Group: Munising Group
Formation: Franconia Formation

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Cf

Note: Hansen (1991, p. 52) in Suggestions to authors of the reports of the United States Geological Survey noted that letter symbols for map units are considered to be unique to each geologic map and that adjacent maps do not necessarily need to use the same symbols for the same map unit. Therefore, map unit abbreviations in the Indiana Geologic Names Information System should be regarded simply as recommendations.

COSUNA areas and regional terminology

Names for geologic units vary across Indiana. The Midwestern Basin and Arches Region COSUNA chart (Shaver, 1984) was developed to strategically document such variations in terminology. The geologic map (below left) is derived from this chart and provides an index to the five defined COSUNA regions in Indiana. The regions are generally based on regional bedrock outcrop patterns and major structural features in Indiana. (Click the maps below to view more detailed maps of COSUNA regions and major structural features in Indiana.)

COSUNA areas and numbers that approximate regional bedrock outcrop patterns and major structural features in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

References:

Becker, L. E., Hreha, A. J., and Dawson, T. A., 1978, Pre-Knox (Cambrian) stratigraphy in Indiana: Indiana Geological Survey Bulletin 57, 72 p.

Berkey, C. P., 1897, Geology of the St. Croix Dalles: American Geologist, v. 20, p. 345–383.

Droste, J. B., and Patton, J. B., 1986, Franconia Formation, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana–a revision: Indiana Geological Survey Bulletin 59, p. 48.

Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7^th ed.): Washington, D.C., U.S. Geological Survey, 289 p.

Shaver, R. H., coordinator, 1984, Midwestern basin and arches region–correlation of stratigraphic units in North America (COSUNA): American Association of Petroleum Geologists Correlation Chart Series.