H. B. Willman and T. C. Buschbach
The Edenian Stage of the Cincinnatian Series (Newberry, 1873, table opp. p. 89; Twenhofel et al., 1954, chart), the basal stage, is based on the Eden Group, named for Eden Park in Cincinnati, Ohio.
In Illinois the Edenian Stage includes the Cape Limestone and the lower part of the Scales Shale (essentially the part underlying the Upper Depauperate Zone), which consists of the Thebes Sandstone Member and probably the Orchard Creek Shale Member in extreme southern Illinois and the middle and lower parts of the Elgin Shale Member elsewhere.
The presence of dark brown to nearly black shale is characteristic of the Elgin Shale Member and lower Edenian strata in the type region of the Cincinnatian.
NEWBERRY, J. S., 1873, General geological relations and structure of Ohio: Ohio Geological Survey Report 1, part 1, p. 1-167.
TWENHOFEL, W. H., et al., 1954, Correlation of the Ordovician formations of North America: Geological Society of America Bulletin, v. 65, p. 247-298.
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