Charles Collinson and Elwood Atherton
The Backbone Limestone (Savage, 1920, p. 169-178), which overlies the Grassy Knob Chert, is named for an isolated ridge called the Backbone along the Mississippi River north of Grand Tower, Jackson County.
The type section of the Backbone Formation is in a quarry at the south end of the ridge (SE SW SW 24, 10S-4W), where only the upper 38 feet of the formation, overlain by the Clear Creek Chert, is exposed.
The fossils indicate a correlation with the Oriskany in New York. The Backbone is continuous with the Little Saline Limestone in Missouri.
Extent and Thickness
The Backbone Limestone appears to be as much as 200 feet thick where it rims the deep part of the Illinois Basin, but it becomes thinner within the deeper part and in places is absent (Collinson et al., 1967a). In the outcrop area in Jackson and Union Counties, the Backbone Limestone is about 100 feet thick, but it thins southward and is absent (or becomes cherty and is not recognized) in the Devonian outcrop area in Alexander County.
The Backbone Limestone consists of light gray, massive, crystalline, pure limestone that commonly contains many large crinoid stems and a few scattered chert nodules.
The Backbone has a large fauna that is dominated by brachiopods, notably Acrospirifer murchisoni and Costispirifer arenosus, but gastropods, bryozoans, conodonts, and trilobites are common.
COLLINSON, CHARLES, L. E. BECKER, G. W. JAMES, J. W. KOENIG, and D. H. SWANN, 1967a, Illinois Basin, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 940-962; Illinois State Geological Survey Reprint 1968-G.
SAVAGE, T. E., 1920, Devonian formations of Illinois: American Journal of Science, v. 49, p. 169-182.
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