Age:

Ordovician

Type designation:

Type locality: The Fort Atkinson Limestone was named by Calvin (1906, p. 60, 98) for Fort Atkinson in eastern Iowa and was designated as the middle formation of the Maquoketa Group (Gray, 1986).

History of usage:

Extended: The Fort Atkinson Limestone has been widely recognized in Illinois, where it is 15 to 40 ft (5 to 12 m) thick (Templeton and Willman, 1963; Kolata and Graese, 1983; Gray, 1986).

Extended: Gutstadt (1958, p. 76) recognized the unit in northwestern Indiana but did not name it. Gray (1972, fig. 5 and p. 20-21) extended the name "Fort Atkinson" into Indiana along with names for other formations in the Maquoketa Group (Gray, 1986).

Description:

The Fort Atkinson Limestone includes light-colored, coarsely crystalline limestone and dolostone, mainly in its upper part, and gray argillaceous limestone and calcareous shale, mainly in its lower part (Gray, 1986). Its wire-line log signature is distinctive–a sharp series of peaks and shoulders in the upper one-third of the formation, tapering in the lower two-thirds to an indefinite lower boundary (Gray, 1986). In the subsurface of much of the northern, central, and western parts of Indiana, the Fort Atkinson is close to 50 ft (15 m) thick (Gray, 1986).

Distribution: The Fort Atkinson Limestone is restricted in Indiana to subsurface usage and is sufficiently distinct to be recognized throughout much of the northern, central, and western parts of the state (Gray, 1986).

Boundaries:

Over most of its extent, the Fort Atkinson Limestone is overlain, probably conformably, by the Brainard Shale (Ordovician) (Gray, 1986). In northeastern Indiana, however, the overlying rocks are appropriately assigned to an extended Whitewater Formation (Ordovician), and in a few places in central northern Indiana the Ordovician-Silurian contact descends to the upper part of the Fort Atkinson, so that in those places it is overlain disconformably by the Sexton Creek Limestone (basal Silurian) (Gray, 1986). The Fort Atkinson overlies the Scales Shale (Ordovician), probably conformably, throughout its extent (Gray, 1986).

Correlations:

According to Willman and Buschbach (1975, p. 86), the Fort Atkinson Limestone of Illinois is equivalent to the Waynesville Formation of former Indiana usage, in part the Waynesville Shale Member of Hay (1981). Subsurface work by Gray (1972) substantiate this correlation (Gray, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: Maquoketa Group
Formation: Fort Atkinson Limestone
Illinois Basin Margin (COSUNA 12)
Supergroup: none
Group: Maquoketa Group
Formation: Fort Atkinson Limestone
Kankakee Arch (COSUNA 14)
Supergroup: none
Group: Maquoketa Group
Formation: Fort Atkinson Limestone
Michigan Basin (COSUNA 15)
Supergroup: none
Group: Maquoketa Group
Formation: Fort Atkinson Limestone

Misc/Abandoned Names:

Waynesville Formation, Waynesville Shale Member

Geologic Map Unit Designation:

Ofa

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:

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

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

Gray, H. H., 1986, Fort Atkinson Limestone, 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. 47–48.

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.

Hay, H. B., 1981, Lithofacies and formations of the Cincinnatian Series (Upper Ordovician), southeastern Indiana and southwestern Ohio: Oxford, Ohio, Miami University, Ph.D. thesis, 238 p.

Kolata, D. R., and Graese, A. M., 1983, Lithostratigraphy and depositional environments of the Maquoketa Group (Ordovician) in northern Illinois: Illinois State Geological Survey Circular 528, 49 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.

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

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.