Rapid Member

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Lithostratigraphy: Hunton Limestone Megagroup >>Cedar Valley Limestone >>Rapid Member
Chronostratigraphy: Paleozoic Erathem >>Devonian System >>Middle Devonian Series
Allostratigraphy: Kaskaskia 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)

Charles Collinson and Elwood Atherton


Original description

The Rapid Member of the Cedar Valley Limestone (Keyes, 1912, p. 149; 1913, p. 205-206).


Named for a locality in Johnson County, Iowa, not specified by Keyes.

Other names


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Stratigraphic relationships

The Rapid Member of the Cedar Valley Limestone overlies the Solon Member.

Extent and thickness

The Rapid is the most widespread and thickest member. It is as much as 60 feet thick in the Rock Island area, but it thins to the east and south. It forms the upper 53 feet of the quarry southeast of Milan.


The Rapid Member consists of gray to buff, fine-grained, argillaceous, locally dolomitic limestone. Most of it is thick bedded, but the upper and lower parts are thin bedded and have shaly partings.




Well log characteristics


Some beds, especially near the top and bottom, are very fossiliferous. Brachiopods and corals are most common, particularly Atrypa, Stropheodonta, Chonetes, Spirifer, Schizophoria, Pentamerella, Hexagonaria, Favosites, and Heliophyllum (Edmund and Anderson, 1967) (fig. D-6). Conodonts indicate a late Middle Devonian age (Collinson et al., 1967b).

Age and correlation

Environments of deposition

Economic importance



COLLINSON, CHARLES, M. P. CARLSON, F. H. DORHEIM, and J. W. KOENIG, 1967b, Central Iowa Basin, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 963-971; Illinois State Geological Survey Reprint 1968-G.
EDMUND, R. W., and R. C. ANDERSON, 1967, Mississippi River Arch: 31st Annual Tri-State Field Conference Guidebook, Augustana College, Rock Island, Illinois, 64 p.
KEYES, C. R., 1912, Some provincial and local phases of the general geologic section of Iowa: Iowa Academy of Science Proceedings, v. 19, p. 147-151.
KEYES, C. R., 1913, Late Devonic sequence of the Iowa region (synopsis): Iowa Academy of Science Proceedings, v. 20, p. 205-206.

ISGS Codes

Stratigraphic Code Geo Unit Designation