Age:

Ordovician

Type designation:

Type locality: The Shakopee Dolomite was named (Winchell, 1874, p. 138-139) from outcrops at Shakopee, Scott County, Minnesota, where Shakopee rocks lie on sandstone that was later correlated with the New Richmond Sandstone (Droste and Patton, 1986).

History of usage:

From this original understanding, use of the term “Shakopee” was extended to Illinois, where this unit overlies the New Richmond Sandstone or the Oneota Dolomite and is overlain by the St. Peter Sandstone generally or by the Everton Dolomite in southern Illinois (Willman and Templeton, 1951, and earlier authors) (Droste and Patton, 1986). The Illinois concept was then applied to Indiana but with some modification of its defined stratigraphic relations (Droste and Patton, 1985; Droste and Patton, 1986).

Description:

The Shakopee Dolomite of Indiana is a pure to impure and generally very fine grained to fine-grained dolostone containing some chert and interbeds of shale, siltstone, and sandstone (Droste and Patton, 1986). In southern Indiana, where younger beds of the Shakopee are preserved, fine- to medium-grained dolostone increases in abundance upward (Droste and Patton, 1986). The color ranges from light shades of gray to light to medium shades of brown. Chert in the Shakopee is vitreous, opaque, and tripolitic; is uniformly colored, color banded, or colored with oolitic texturing; and ranges in abundance from sporadic nodules and thin irregular beds in dolostone through zones several feet thick (Droste and Patton, 1986).

Sandstone beds generally as much as several tens of feet thick are interbedded in dolostone through a total thickness of about 400 ft (122 m) in southeastern and south-central Indiana (Droste and Patton, 1986). Most of the interbedded dolostone is not the typical very fine grained to fine-grained rock but instead is medium grained and very light colored (Droste and Patton, 1986).

The Shakopee increases in thickness from its eroded limit in northern Indiana to perhaps 2,000 ft (610 m) in southwestern Indiana (Droste and Patton, 1986). A significant amount of the thickening of the Shakopee results from the addition of younger beds southward, but a regional thickness increase southward indicates a major center of deposition in that direction (Droste and Patton, 1986).

Distribution: The Shakopee is present everywhere in the Indiana subsurface except for an area in northern Indiana where pre-St. Peter erosion completely stripped the formation (Droste and Patton, 1986). Its only Indiana exposure is among the chaotically faulted rocks that are quarried at Kentland in Newton County (Gutschick, 1983; Droste and Patton, 1986).

Boundaries:

The Shakopee overlies the Oneota with gradational contact and is overlain unconformably by rocks of the Ancell Group generally except in southwestern Indiana where the Everton lies superjacent to the unconformity (Droste and Patton, 1986).

Correlations:

The Shakopee Dolomite of Indiana is equivalent to the upper part of the undifferentiated Knox Dolomite of Ohio; the upper part of the undifferentiated Prairie du Chien Group of Michigan; the Roubidoux Formation, the Jefferson City Dolomite, and the Cotter Dolomite of Kentucky; and the New Richmond Sandstone and the Shakopee Dolomite of Illinois (Droste and Shaver, 1983; Shaver, 1984; Droste and Patton, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: Knox Supergroup
Group: Prairie du Chien Group
Formation: Shakopee Dolomite
Illinois Basin Margin (COSUNA 12)
Supergroup: Knox Supergroup
Group: Prairie du Chien Group
Formation: Shakopee Dolomite
Cincinnati Arch (COSUNA 13)
Supergroup: Knox Supergroup
Group: Prairie du Chien Group
Formation: Shakopee Dolomite
Kankakee Arch (COSUNA 14)
Supergroup: Knox Supergroup
Group: Prairie du Chien Group
Formation: Shakopee Dolomite
Michigan Basin (COSUNA 15)
Supergroup: Knox Supergroup
Group: Prairie du Chien Group
Formation: Shakopee Dolomite

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Osh

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:

Droste, J. B., and Patton, J. B., 1985, Lithostratigraphy of the Sauk Sequence in Indiana: Indiana Geological Survey Occasional Paper 47, 24 p.

Droste, J. B., and Patton, J. B., 1986, Shakopee Dolomite, 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. 141–142.

Droste, J. B., and Shaver, R. H., 1983, Atlas of early and middle Paleozoic paleogeography of the southern Great Lakes area: Indiana Geological Survey Special Report 32, 32 p.

Gutschick, R. C., 1983, Geology of the Kentland Dome structurally complex anomaly–northwestern Indiana (Field Trip 5), in Shaver, R. H., and Sunderman, J. A., eds., Field trips in midwestern geology: Bloomington, Indiana, Geological Society of America, Indiana Geological Survey, and Indiana University Department of Geology, v. 1., p. 105–138.

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.

Willman, H. B., and Templeton, J. S., 1951, Cambrian and Lower Ordovician exposures in northern Illinois: Illinois State Academy of Science Transactions, v. 44, p. 109–125.

Winchell, N. H., 1874, Second annual report for the year 1873: Minnesota Geological and Natural History Survey, p. 138–147.