Historical:Okaw Group

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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: Pope Megagroup >>Okaw Group
Chronostratigraphy: Paleozoic Erathem >>Mississippian Subsystem >>Chesterian Series >>Gasperian Stage
Allostratigraphy: Kaskaskia Sequence


Elwood Atherton, Charles Collinson, and Jerry A. Lineback

Name Origin

The Okaw Group (S . Weller, 1913, p. 120; S. Weller and J. M. Weller, 1939, p. 12; Swann, 1963, p. 76) is named for the Okaw River, now the Kaskaskia River.

Type Section

The type section of the Okaw Group consists of exposures along the river in Randolph County (T4, 5, and 6S, R7 and 8W).


The name is used only in western Illinois for formations from the Beech Creek through the Glen Dean (fig. M-25) in an area where the clastic units of the Hardinsburg are poorly developed and it is difficult to assign many small limestone outcrops to the proper formation.


SWANN, D. H., 1963, Classification of Genevievian and Chesterian (Late Mississippian) rocks of Illinois: Illinois State Geological Survey Report of Investigations 216, 91 p.
WELLER, STUART, 1913, Stratigraphy of the Chester Group in southwestern Illinois: Illinois Academy of Science Transactions, v. 6, p. 118-129.
WELLER, STUART, and J. M. WELLER, 1939, Preliminary geological maps of the pre-Pennsylvanian formations in part of southwestern Illinois: Illinois State Geological Survey Report of Investigations 59, 15 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation