H. B. Willman and T. C. Buschbach
The Cape Limestone (Templeton and Willman, in Gutstadt, 1958b, p. 524), the oldest Cincinnatian formation, is named for Cape Girardeau, Missouri.
The type section for the Cape Limestone is an exposure on Main Street just north of Broadway in Cape Girardeau, where the formation is 8.5 feet thick, the maximum thickness observed (Templeton and Willman, 1963, p. 134).
The Cape Limestone was for many years called the Fernvale Limestone.
The Cape Limestone is correlated with the Fernvale Limestone in Arkansas and Oklahoma but not with the type Fernvale in Tennessee, which, being Richmondian in age, is much younger.
Extent and Thickness
The Cape Limestone has a patchy distribution in the outcrop area in Missouri from St. Louis to Cape Girardeau. It occurs in the southwestern part of Illinois, where Cincinnatian strata deeply truncate the Trentonian Series, but it is exposed only at Valmeyer, Monroe County (SW 3, 3S-11 W), where it is 1.5 feet thick. A few small outcrops formerly occurred near Thebes in Alexander County but are now covered.
The Cape Limestone occurs mainly in shallow channels eroded in the top of the Dunleith Formation. In the northern part of the area, it is overlain by the Elgin Shale Member of the Scales Shale, as shown by the presence of the Lower Depauperate Zone above it at Valmeyer. In the southern part of the outcrop area, it is overlain by the Thebes Sandstone Sandstone Member of the Scales.
The Cape Limestone is a light gray to reddish gray, coarse-grained, fossiliferous calcarenite. It occurs in medium to thick beds with weak shaly partings.
The Cape Limestone has a large fauna characterized by brachiopods, particularly Lepidocyclus capax, and crinoids.
GUTSTADT, A. M., 1958b, Upper Ordovician stratigraphy in Eastern Interior region: American Association of Petroleum Geologists Bulletin, v. 42, p. 513-547.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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