Devonian System

Type locality, identification problems, and use of name in Indiana: The Grassy Knob Chert was named (Savage, 1925, p. 139) for Grassy Knob, a prominent high area on the Mississippi River bluffs in Jackson County, southwestern Illinois, where more than 100 feet (30 m) of chert and sandy and siliceous limestone were exposed. Consistent use of the term in the Illinois Basin (Weller, 1939, p. 128; Collinson and others, 1967, p. 940-941; and Becker, 1974, p. 27-28) was beset with problems until the late 1970's. These problems arose for these reasons: (1) statements differed as to the conformable or unconformable relations with the underlying Bailey Limestone (Silurian, but long considered to be Devonian in age); (2) the Grassy Knob could not be distinguished from the stratigraphically higher Clear Creek Chert unless coarse carbonate rocks of the Backbone Limestone intervened; (3) Backbone-like rocks had been included in the Bailey; and (4) stratigraphers had not recognized that the Backbone was a facies of both the Grassy Knob and Clear Creek.

After the term Grassy Knob was introduced in an area of four-county size in southwesternmost Indiana (Becker, 1974, fig. 11C and p. 27-28), the classification of Grassy Knob and associated rocks was revised (Becker and Droste, 1978, p. 3-5) to accord with a basin-to-shelf facies relationship, one of intertonguing Grassy Knob- and Clear Creek-like lithology and Backbone-like lithology. Further, the Grassy Knob was assigned to the New Harmony Group (Becker and Droste, 1978). In this recognition an arbitrary vertical cutoff was defined, in effect, and at Backbone-like rocks were reassigned from the Bailey to the Backbone. The formational cutoff was defined as occurring along the basin most extent of the lower tongue of Backbone-like lithology. This arrangement has been further detailed for the Illinois, Indiana, and Kentucky areas of the Illinois Basin by Droste and Shaver (in preparation), so that stratigraphically higher tongues of Backbone-like lithology extend beyond the cutoff and are therefore incorporated in the Grassy Knob also, the Grassy Knob occupies in Indiana only a small part of southwestern Posey County (Droste and Shaver, in preparation, fig. 8).

Description: The Grassy Knob Chert consists predominantly of yellowish-gray and light-olive-brown very cherry dolomitic limestone and dolomitic chert. This chert appears to be a nearly complete replacement of bioclastic limestone. One or more intervals of light-colored granular bioclastic limestone are present.

The Grassy Knob conformably overlies the drab very fine grained Bailey Limestone (Silurian) and conformably underlies the Clear Creek Chert, which is lithologic ally similar to the Grassy Knob except for the lesser amount of chert in the Clear Creek. The contact is placed at the level where chert becomes a minor component upward through a 10-foot (3-m) gradational interval.

In Indiana the Grassy Knob appears to reach a thickness of more than 400 feet (122 m) (Droste and Shaver, in preparation, figs. 6 and 9).

Correlation: The Grassy Knob of Indiana extends under the same name into the basin parts of Illinois and Kentucky, but differing schemes of classification preclude simple statements on correlation. As defined in Indiana, the Grassy Knob has complementary and therefore correlative relationships with the lower part of the Backbone Limestone. Fossils have not been described from the Grassy Knob of Indiana, but on the basis of interval stratigraphy and what is known of fossils from the New Harmony Group and the Bailey Limestone wherever they occur in the Illinois Basin, the Grassy Knob has been assigned a Gedinnian and Siegenian (early Ulsterian) age. (See Shaver and others, 1985.)