Historical:Okaw Group

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

Authors

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).

Definition

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.

Fig. M-25 -- Columnar section of the Chesterian Series (after Swan, 1963), showing biozones (after Collinson et al., 1971). In the columnar section, the blank areas are shale.

References

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
4260	Mo