Historical:Orchard Creek Shale Member
H. B. Willman and T. C. Buschbach
The Orchard Creek Shale Member of the Scales Shale (Savage, 1909, p. 515) is named for a small exposure along Orchard Creek, 2 miles south of Thebes, Alexander County. This section is now covered.
The Orchard Creek is exposed in the Mississippi River bluffs a mile north of Thebes (SE 5, 15S-3W), where it is 27 feet thick, overlies the Thebes Sandstone Member, and is overlain by the Girardeau Limestone.
Extent and Thickness
The Orchard Creek Shale Member is exposed locally in and near the Mississippi bluffs in Alexander County from Gale southward almost to Fayville. It is 10-30 feet thick in the outcrop area.
The Orchard Creek Member is largely blue-gray shale, but in the thickest exposures the upper part is interbedded with fine-grained limestone. The Orchard Creek is probably a lateral equivalent of part of the Elgin Shale Member, but its precise position is uncertain, and it could be equivalent even to the Brainard Shale. It is differentiated only where it overlies the Thebes Sandstone Member.
The Orchard Creek contains a large fauna (Savage, 1917). It was long assigned to the Silurian System, but the presence of Climacograptus putillus indicates Cincinnatian age (Pryor and Ross, 1962).
PRYOR, W. A., and C. A. Ross, 1962, Geology of the Illinois parts of the Cairo, La Center, and Thebes Quadrangles: Illinois State Geological Survey Circular 332, 39 p.
SAVAGE, T. E., 1909, Ordovician and Silurian formations in Alexander County, Illinois: American Journal of Science, v. 28, p. 509-519.
SAVAGE, T. E., 1917, Thebes sandstone and Orchard Creek shale and their faunas in Illinois: Illinois Academy of Science Transactions, v. 10, p. 261-275.
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