Historical:Coggan Member

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Handbook of Illinois Stratigraphy
Series Bulletin 95
Author H. B. Willman, Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, Jack A. Simon
Date 1975
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Lithostratigraphy: Hunton Limestone Megagroup >>Wapsipinicon Limestone >>Coggan Member
Chronostratigraphy: Paleozoic Erathem >>Devonian System >>Middle Devonian Series
Allostratigraphy: Kaskaskia Sequence


Charles Collinson and Elwood Atherton

Name Origin

The Coggan Member of the Wapsipinicon Limestone (Norton, 1894, p. 23-24), at the base of the Wapsipinicon in Illinois, is named for Coggan, Linn County, Iowa.

Type Section

The type section of the Coggan Member is at the Illinois Central Railroad crossing of the Buffalo River near Coggan, Linn County, Iowa.

Extent and Thickness

The Coggan Member is not exposed in Illinois, but it occurs locally in subsurface and is as much as 20 feet thick in the area between Galesburg, Knox County, and Muscatine, Iowa.


The Coggan Member consists of massive, buff, cherty dolomite. It also contains a thin bentonite that has been correlated with the Tioga Bentonite Bed, which occurs near the top of the Grand Tower Formation southeast of the Sangamon Arch.


NORTON, W. H., 1894, Notes on the lower strata of the Devonian Series in eastern Iowa: Iowa Academy of Science Proceedings 1, part 4, p. 22-24.

ISGS Codes

Stratigraphic Code Geo Unit Designation