Age:

Mississippian

Type designation:

Type section: At its type section in the NW¼SE¼ sec. 5, T. 13 S., R. 8 E., Hardin County, Illinois (Atherton, 1948, fig. 8), the Downeys Bluff Limestone consists of limestone with minor amounts of shale and is 41 ft (12.5 m) thick (Carr and Gray, 1986).

Reference section: Indiana–The principal reference section for the Paoli Limestone, an abandoned quarry just north of the railroad tracks on the west side of Paoli, Indiana (SW¼SE¼SE¼ sec. 35, T. 2 N., R. 1 W.), was designated by Carr and Gray (1986, p. 39) as a reference section for the member, which at this locality consists of about 13 ft (4 m) of oolitic, skeletal, and micritic limestone.

History of usage:

First use of name: According to Atherton (1948, p. 129), the name “Downeys Bluff” was proposed in an unpublished manuscript by F. E. Tipple for a member of the Paint Creek Formation (Carr and Gray, 1986).

Revised rank: The unit was given formational status by Atherton and others (1975, p. 153).

Extended: Carr and Gray (1986, p. 39) adopted the name “Downeys Bluff” for use in Indiana for the uppermost member of the Paoli Limestone in the outcrop areas.

Revised extension: Droste and Carpenter (1990, p. 40) recognized the Downeys Bluff as the upper continuous zone of limestone at the top of the Paoli in the subsurface and outcrop areas in Indiana.

Description:

The Downeys Bluff is light-gray to medium-gray to light-brownish-gray oolitic and biofragmental grainstones and packstones (Droste and Carpenter, 1990). Echinoids are the dominant skeletal type, followed in decreasing abundance by bryozoans and brachiopods (Kissing, 1967, p. 83, 86). Along the outcrop in Indiana the Downeys Bluff Member is a few feet thick to absent, probably depositionally, north of Monroe County, but it thickens irregularly southward, generally ranging from 6 to 12 ft (1.8 to 3.7 m) and to as much as 19 ft (6 m) in southern Crawford County near the Ohio River (Carr and Gray, 1986). Droste and Carpenter (1990) reported a maximum subsurface thickness of 16 ft (4.9 m) in Warrick County.

Distribution: The Downeys Bluff is thin or completely absent beneath the West Baden clastic belt, particularly in Martin and Daviess Counties (Droste and Carpenter, 1990).

Boundaries:

The Downeys Bluff generally lies conformably below the rocks of the West Baden Group or unconformably below Pennsylvanian rocks (Droste and Carpenter, 1990).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Blue River Group
Formation: Paoli Limestone
Member: Downeys Bluff Member
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Blue River Group
Formation: Paoli Limestone
Member: Downeys Bluff Member

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Mpdb

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:

Atherton, Elwood, 1948, Some Chester outcrop and subsurface sections in southeastern Illinois: Illinois State Academy of Science Transactions p. 122–139.

Atherton, Elwood, Collinson, Charles, and Lineback, J. A., 1975, Mississippian 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. 123–163.

Carr, D. D., and Gray, H. H., 1986, Downeys Bluff Member, 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. 38–39.

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.

Kissling, D. L., 1967, Environmental history of lower Chesterian rocks in southwestern Indiana: Bloomington, Indiana University, Ph.D. thesis, 367 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.