Age:

Ordovician

Type designation:

Type area: The Pecatonica Formation (Hershey, 1894, p. 175; 1897) was named for exposures along the Pecatonica River in southern Wisconsin (Droste and Patton, 1986).

History of usage:

In Illinois, the Pecatonica Formation was classified as the lowest formation of the Platteville Group (Droste and Patton, 1986). There it is overlain by the Plattin Subgroup of the Platteville Group (Templeton and Willman, 1963; Droste and Patton, 1986).

Description:

The Pecatonica Formation as traced in Indiana from Illinois (Droste, Abdulkareem, and Patton, 1982) is the lower formation of the Black River Group and except for faulted blocks in the quarried Kentland structure (Gutschick, 1983) is known only in the subsurface in Indiana (Droste and Patton, 1986). The Pecatonica overlies the Ancell Group with minor erosional discontinuity generally and the Knox Supergroup with major unconformity where the Ancell rocks are missing because of nondeposition (Droste and Patton, 1986).

In northwestern Indiana medium- to dark-grayish-brown dolostone dominates the formation (Droste and Patton, 1986). Elsewhere in Indiana grayish-brown to dark-brownish-gray lithographic limestone and fine-grained burrow-mottled limestones and dolostones compose the Pecatonica Formation (Droste and Patton, 1986). Throughout most of Indiana a zone a few feet thick of dark argillaceous limestone or silty calcareous shale is near the base of the Pecatonica, and in northwestern Indiana this zone also contains thin sandy carbonates beds of dolostone showing fine- to medium-grained sand grains surrounded by dolomite (Droste and Patton, 1986).

The Pecatonica thickens from 30 ft (9 m) in northwestern Indiana to 130 ft (40 m) in southwestern Indiana (Droste and Patton, 1986).

Correlations:

The Pecatonica Formation in Indiana is correlated with the Pecatonica Formation of Illinois and Kentucky and becomes parts of the Black River Group in Michigan and of the Black River Limestone in Ohio (Droste and Shaver, 1983; Shaver, 1984; Droste and Patton, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Black River Group
Formation: Pecatonica Formation
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Black River Group
Formation: Pecatonica Formation
Cincinnati Arch (COSUNA 13)
Supergroup: none
Group: Black River Group
Formation: Pecatonica Formation
Kankakee Arch (COSUNA 14)
Supergroup: none
Group: Black River Group
Formation: Pecatonica Formation
Michigan Basin (COSUNA 15)
Supergroup: none
Group: Black River Group
Formation: Pecatonica Formation

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Opc

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., Abdulkareem, T. F., and Patton, J. B., 1982, Stratigraphy of the Ancell and Black River Groups (Ordovician) in Indiana: Indiana Geological Survey Occasional Paper 36, 15 p.

Droste, J. B., and Patton, J. B., 1986, Pecatonica 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. 110.

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.

Hershey, O. H., 1894, Elk Horn Creek area of St. Peter Sandstone in northwestern Illinois: American Geologist, v. 14, p. 169–179.

Hershey, O. H., 1897, The term Pecatonica Limestone: American Geologist, v. 20, p. 66–67.

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.

Templeton, J. S., and Willman, H. B., 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.