Historical:Offerman 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
Link Web page
PDF PDF file
Store ISGS Store

Lithostratigraphy: Hunton Limestone Megagroup >>Kankakee Formation >>Offerman Member
Chronostratigraphy: Paleozoic Erathem >>Silurian System >>Alexandrian Series
Allostratigraphy: Tippecanoe Sequence


H. B. Willman and Elwood Atherton

Name Origin

The Offerman Member of the Kankakee Formation (Willman, 1973, p. 17), which overlies the Drummond Member, is named for Offerman School, three-fourths of a mile southeast of the type section.

Type Section

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).

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.


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