Offerman Member

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Lithostratigraphy: Hunton Limestone Megagroup >>Kankakee Formation >>Offerman Member
Chronostratigraphy: Paleozoic Erathem >>Silurian System >>Alexandrian Series
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 Elwood Atherton


Original description

The Offerman Member of the Kankakee Formation (Willman, 1973, p. 17).


Named for Offerman School, three-fourths of a mile southeast of the type section.

Other names


Type section

Type location

The type section of the Offerman Member is located in the same exposure as the Drummond Member, in a railroad cut on the southeast side of the Des Plaines River Valley, 2 miles southwest of Joliet, Will County (NW SE SW 30, 35N-10E).

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The Offerman Member overlies the Drummond Member of the Kankakee Formation.

Extent and thickness

In the type section the Offerman Member is only 2.5 feet thick, a minimum thickness, but it more commonly is about 3 feet thick along the Kankakee River and 6-11 feet thick in quarries at Joliet, Elmhurst, and Hillside.


The Offerman Member differs from the underlying and overlying members in being more argillaceous, less vesicular, and thinner bedded, and in having smoother bedding surfaces and a lighter colored weathered surface.




Well log characteristics


Age and correlation

Environments of deposition

Economic importance



WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation