Devonian System

Type area and use of name: The term Clear Creek Limestone was first applied to a few hundred feet of cherty carbonate rocks exposed in Jackson, Union, and Alexander Counties, southwestern Illinois, that were thought to be Helderbergian and Deerparkian (Oriskany) in age (Worthen, 1866, p. 126-129). The name was taken from Clear Creek in Union County. The lower part of this sequence was later assigned to the Bailey Limestone (E. O. Ulrich as published by Buckley and Beuhler, 1904, p. 110). The Bailey at that time was considered to be Devonian in age. The term Clear Creek Chert was used in 1908 (Savage, p. 431-443) for the restricted unit, and in 1920 (Savage, p. 169-178) the Clear Creek Chert of southwestern Illinois was further defined as overlying the Backbone Limestone. Its distribution in the Illinois Basin was delineated by Collinson and others (1967) and, more specifically for the southwestern Indiana subsurface area, by Becker (1974, p. 28-29), Becker and Droste (1978, p. 5), and Droste and Shaver (in preparation).

Description: The Clear Creek Chert consists of light-yellowish-gray to whitish fine- to medium-grained limestone that is cherty and somewhat glauconitic. The Clear Creek in its subsurface Indiana area of about four-county size (Becker, 1974, fig. 11D; Droste and Shaver, in preparation, fig. 8) almost every-where overlies the Backbone Limestone in an apparently conformable and on lapping relationship. The contact is placed at the change from finer grained and somewhat cherry carbonate rocks above to coarser grained and purer carbonate rocks below. Only in a small part of southwestern Posey County in its Indiana distribution does the Clear Creek rest directly on the Grassy Knob Chert. This contact is placed at the level within a 10-foot (3-m) gradational interval where chert becomes a major component downward. (See the articles on the Backbone Limestone and the Grassy Knob Chert for an explanation of these spatial relationships.) The Clear Creek is overlain both conformably (farthest southwest) and unconformable by the Jeffersonville Limestone (Middle Devonian), including the Dutch Creek Sandstone Member. Where it is conformable, the contact is placed amid transitional lithology.

The thickness of the Clear Creek ranges from an erosional zero along its northeastern limits to 300 feet (92 m) at the tristate corner of Illinois, Indiana, and Kentucky (Droste and Shaver, in preparation, fig. 10).

Correlation: The Clear Creek of Indiana extends into the Clear Creek of Illinois and Kentucky, but because of differing manners of defining the units of the New Harmony Group among stratigraphers of the Illinois Basin, precise statements on correlation involve semantics. Further, in the interpretation of an onlapping Backbone-Clear Creek relationship, the Clear Creek is not wholly isochronous and has some age equivalency with the Backbone itself.

Indicator fossils from the Clear Creek of Indiana have not been described, but an abundant fauna in exposures in southwestern Illinois suggested a late Siegenian and Emsian (late Ulsterian) age to Collinson and others (1967) but only an Emsian age to Boucot and Johnson (1968, p. B3).