Historical:Clear Creek Chert
Charles Collinson and Elwood Atherton
The Clear Creek Chert (Worthen, 1866, p. 126-129; Savage, 1920, p. 174-175), the uppermost Lower Devonian formation, is named for Clear Creek in Union County.
Although the Clear Creek Chert is exposed for about 5 miles near Clear Creek (T11-12S, R2W), no specific type section has been designated.
Extent and Thickness
The Clear Creek Chert is exposed along the Mississippi Valley from the south end of the Backbone in Jackson County, south to the vicinity of Tamms, Alexander County. It is well exposed in a quarry on the west bluff of Cache Valley northwest of Tamms (NE NW 36, 14s-2W). The thickness of the Clear Creek is difficult to determine, but it is at least 300 feet thick in the outcrop area. Bald Knob, the highest point in southern Illinois, rises 600 feet above Clear Creek and appears to be all Clear Creek Chert, but the thickness is complicated by structure. The formation thickens eastward from the outcrop area to about 600 feet in the southern part of the basin.
As originally defined by Worthen (1866), the Clear Creek may have included the Backbone Limestone and possibly the Grassy Knob Chert, but Savage (1920) restricted the name to the chert formation above the Backbone. The Clear Creek is dominantly chert; much of it is white, or generally lighter in color than the chert in the lower formations (Weller, 1940). It contains some beds of gray, very fine-grained, siliceous limestone, which in places are separated from the chert beds by stylolitic partings. Such partings are also common in the chert. The proportion of limestone is generally small in most outcrops, but it is larger near the top of the formation and increases in subsurface north and east from the outcrop area. Solution of the limestone beds and fracturing of the chert beds has produced modifications of the original deposit that are variously described as novaculite, ganister, and tripoli (Lamar, 1953). The novaculite and ganister have been used for gravel and for making silica brick. The tripoli, which consists of finely divided particles of silica in vein-like bodies, is used in abrasives.
The Clear Creek is much more fossiliferous than the Grassy Knob, and some chert beds are largely casts of fossils, particularly of the brachiopod Eodevonaria melonicus.
LAMAR, J. E., 1953, Siliceous materials of extreme southern Illinois: Illinois State Geological Survey Report of Investigations 166, 39 p.
SAVAGE, T. E., 1920, Devonian formations of Illinois: American Journal of Science, v. 49, p. 169-182.
WELLER, J. M., 1940, Geology and oil possibilities of extreme southern Illinois: Illinois State Geological Survey Report of Investigations 71, 71 p.
WORTHEN, A. H., 1866, Geology: Geological Survey of Illinois, v. 1, 504 p.
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