Fort Atkinson Limstone

Lithostratigraphy: Maquoketa Shale Group >>Fort Atkinson Limstone
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Cincinnatian Series >>Richmondian Stage
Allostratigraphy: Tippecanoe Sequence

Primary source

Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Contributing author(s)

H. B. Willman and T. C. Buschbach

Name

Original description

Fort Atkinson Limestone (Calvin, 1906, p. 60, 98).

Derivation

Named for exposures at Fort Atkinson, Winneshiek County, Iowa.

Other names

The Fort Atkinson Limestone has been called the Divine Limestone (Lamar and Willman, 1931) and the "middle limestone" (DuBois, 1945).

Stratigraphic relationships

The Fort Atkinson Limestone is the "middle limestone" formation in the Maquoketa Group.

Extent and thickness

The Fort Atkinson Formation is exposed in northeastern Illinois, particularly at Wilmington, Will County (SE SW NW 25, 33N-9E) and near the Dresden Island Dam, Grundy County (SE NE NW 34, 34N-8E). It is widely distributed in subsurface in Illinois and is commonly 15-40 feet thick.

Lithology

In some areas, for instance in much of the Maquoketa outcrop area in northwestern Illinois, the Fort Atkinson becomes very shaly and is inseparable from the shales above and below. In other areas it includes limestone laterally equivalent to the shale formations above and/or below. The Fort Atkinson changes within short distances from white or pink, coarse-grained, crinoidal limestone to brown fine-grained dolomite or gray argillaceous limestone. In some localities the upper part is limestone, the lower part dolomite. The amount of interbedded shale varies greatly.

Fossils

The Fort Atkinson is generally fossiliferous with a large fauna of brachiopods and bryozoans (Savage, 1925a).

Age and correlation

The Fort Atkinson Formation correlates with the Waynesville Limestone in Ohio and Kentucky.

References

CALVIN, SAMUEL, 1906, Geology of Winneshiek County: Iowa Geological Survey, v. 16, p. 37-146. DUBOIS, E. P., 1945, I. Subsurface relations of the Maquoketa and "Trenton" Formations in Illinois: Illinois State Geological Survey Report of Investigations 105, p. 7-33.
LAMAR, J. E., and H. B. WILLMAN, 1931, High-calcium limestone near Morris, Illinois: Illinois State Geological Survey Report of Investigations 23, 26 p.
SAVAGE, T. E., 1925a, Correlation of the Maquoketa and Richmond rocks of Iowa and Illinois: Illinois Academy of Science Transactions, v. 17, p. 233-247.

ISGS Codes

Stratigraphic Code	Geo Unit Designation
7430	On