Age:

Ordovician

Type designation:

Type locality: The Brainard Shale was named by Calvin (1906, p. 60, 97) for exposures of bluish-gray shale near Brainard, Fayette County, Iowa (Gray, 1986).

History of usage:

Extended: Use of the name was extended into Illinois by Ulrich (1924, p. 71).

Extended: Gutstadt (1958, p. 76) recognized the Maquoketa in northwestern Indiana and noted that it consisted of "upper and lower shale units separated by interbedded shale and limestone or dolomite units," but he did not apply names to the divisions (Gray, 1986).

Gray (1972, fig. 5 and p. 20-21) adopted the name “Brainard Shale” for the uppermost formation in the Maquoketa Group in western Indiana.

Description:

The Brainard Shale is restricted to subsurface usage in Indiana and depends for its recognition on identification of the underlying Fort Atkinson Limestone (Gray, 1986).

The Brainard consists principally of gray to greenish-gray shale that contains a few thin interbeds of limestone (Gray, 1986). Over most of the area of its recognition, the Brainard Shale is 75 to 100 ft (23 to 30 m) thick, but in a narrow area from central Indiana northward to the southern shore of Lake Michigan it is thin to absent because of erosion before deposition of overlying Silurian rocks (Gray, 1986).

Distribution: Gray (1972) recognized the Brainard primarily in northwestern Indiana, but later work showed that the Fort Atkinson and, therefore, also the Brainard is identifiable over a wider area that includes much of northern, central, and southwestern Indiana (Gray, 1986). Flugeman and Pope (1983) recognized thin outliers or tongues of Brainard Shale on the outcrop in southeastern Indiana.

Boundaries:

The Brainard Shale is overlain in central and western Indiana by the Sexton Creek Limestone (Gray, 1986; Hohman, 1998, pl. 3) and in northeastern Indiana by the Manitoulin Dolomite Member of the Cataract Formation (Gray, 1986).

The Brainard overlies the Fort Atkinson Limestone through nearly all of its known extent, but in the outliers mentioned above the Brainard rests on a hardground developed on the underlying Whitewater Formation (Flugeman and Pope, 1983). Regional significance of this feature is unclear (Gray, 1986).

Correlations:

Willman and Buschbach (1975, p. 86) stated that fossils in the upper part of the Brainard in Illinois suggest equivalence to the Elkhorn Formation of former Indiana usage (uppermost Richmondian); however, Flugeman and Pope (1983) concluded that "the Maquoketa Group is in part younger than the entire Cincinnatian Series in southeastern Indiana."

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Maquoketa Group
Formation: Brainard Shale
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Maquoketa Group
Formation: Brainard Shale
Kankakee Arch (COSUNA 14)
Supergroup: none
Group: Maquoketa Group
Formation: Brainard Shale
Michigan Basin (COSUNA 15)
Supergroup: none
Group: Maquoketa Group
Formation: Brainard Shale

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Ob

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.

See also:

Whitewater Formation

References:

Calvin, Samuel, 1906, Geology of Winneshiek County: Iowa Geological Survey, v. 16, p. 37–146.

Flugeman, R. H., Jr., and Pope, J. K., 1983, Brainard Shale outliers (Upper Ordovician Maquoketa Group) in southeastern Indiana [abs.]: Geological Society of America Abstracts with Programs, v. 15, p. 574.

Gray, H. H., 1972, Lithostratigraphy of the Maquoketa Group (Ordovician) in Indiana: Indiana Geological Survey Special Report 7, 31 p.

Gray, H. H., 1986, Brainard Shale, 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. 19.

Gutstadt, A. M., 1958, Cambrian and Ordovician stratigraphy and oil and gas possibilities in Indiana: Indiana Geological Survey Bulletin 14, 103 p.

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.

Hohman, J. C., 1998, Depositional history of the Upper Ordovician Trenton Limestone, Lexington Limestone, Maquoketa Shale and equivalent lithologic units in the Illinois Basin: an application of carbonate and mixed carbonate-siliciclastic sequence stratigraphy: Bloomington, Indiana University, Ph.D. dissertation, 186 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.

Ulrich, E. O., 1924, Notes on new names in table of formations and on physical evidence of breaks between Paleozoic systems in Wisconsin: Wisconsin Academy of Sciences, Arts and Letters, v. 21, p. 71–107.

Willman, H. B., and Buschbach, T. C., 1975, Ordovician System, in Willman, H. B., Atherton, Elwood, Buschbach, T. C., Collinson, Charles, Frye, J. C., Hopkins, M. E., Lineback, J. A., and Simon, J. A., Handbook of Illinois stratigraphy: Illinois State Geological Survey Bulletin 95, p. 47–87.