IGNIS
Grassy Knob Chert

Age:

Devonian

Type designation:

Type locality: The Grassy Knob Chert was named by 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 ft (30 m) of chert and sandy and siliceous limestone were exposed (Droste and Shaver, 1986).

History of usage:

Summary of early history: 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 1970s (Droste and Shaver, 1986). Droste and Shaver (1986) noted that these problems arose for the following 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.

Extended: Becker (1974, fig. 12 and p. 27-28) extended the use of the term Grassy Knob into southwesternmost Indiana.

Revised and reassigned: 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 (Droste and Shaver, 1986). Further, the Grassy Knob was assigned to the New Harmony Group (Becker and Droste, 1978; Droste and Shaver, 1986). In this recognition an arbitrary vertical cutoff was defined, in effect, and all Backbone-like rocks were reassigned from the Bailey to the Backbone (Droste and Shaver, 1986). The formational cutoff was defined as occurring along the basin most extent of the lower tongue of Backbone-like lithology (Droste and Shaver, 1986). This arrangement has been further detailed for the Illinois, Indiana, and Kentucky areas of the Illinois Basin by Droste and Shaver (1987), so that stratigraphically higher tongues of Backbone-like lithology extend beyond the cutoff and are therefore incorporated in the Grassy Knob.

Description:

The Grassy Knob Chert consists predominantly of yellowish-gray and light-olive-brown very cherty dolomitic limestone and dolomitic chert (Droste and Shaver, 1986). This chert appears to be a nearly complete replacement of bioclastic limestone (Droste and Shaver, 1986). One or more intervals of light-colored granular bioclastic limestone are present. In Indiana the Grassy Knob appears to reach a thickness of more than 400 ft (122 m) (Droste and Shaver, 1987, fig. 7).

Distribution: The Grassy Knob is present in only a small part of southwestern Posey County, Indiana (Droste and Shaver, 1987, fig. 7).

Boundaries:

The Grassy Knob conformably overlies the drab very fine grained Bailey Limestone (Silurian) and conformably underlies the Clear Creek Chert, which is lithologically similar to the Grassy Knob except for the lesser amount of chert in the Clear Creek (Droste and Shaver, 1986). The contact is placed at the level where chert becomes a minor component upward through a 10-foot (3-m) gradational interval (Droste and Shaver, 1986).

Correlations:

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 (Droste and Shaver, 1986). As defined in Indiana, the Grassy Knob has complementary and therefore correlative relationships with the lower part of the Backbone Limestone (Droste and Shaver, 1986). 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 (Droste and Shaver, 1986).

Regional Indiana usage:

Illinois Basin (COSUNA 11)
Supergroup: none
Group: New Harmony Group
Formation: Grassy Knob Chert

Misc/Abandoned Names:

None

Geologic Map Unit Designation:

Dgk

Note: Hansen (1991, p. 52) in Suggestions to authors of the reports of the United States Geological Survey noted that letter symbols for map units are considered to be unique to each geologic map and that adjacent maps do not necessarily need to use the same symbols for the same map unit. Therefore, map unit abbreviations in the Indiana Geologic Names Information System should be regarded simply as recommendations.

COSUNA areas and regional terminology

Names for geologic units vary across Indiana. The Midwestern Basin and Arches Region COSUNA chart (Shaver, 1984) was developed to strategically document such variations in terminology. The geologic map (below left) is derived from this chart and provides an index to the five defined COSUNA regions in Indiana. The regions are generally based on regional bedrock outcrop patterns and major structural features in Indiana. (Click the maps below to view more detailed maps of COSUNA regions and major structural features in Indiana.)

Map showing the COSUNA areas (heavy black line) that approximate regional bedrock outcrop patterns and major structural features in Indiana, and the COSUNA numbers (large bold font) for these areas. The COSUNA boundaries are limited to state and county boundaries that facilitate coding.

COSUNA areas and numbers that approximate regional bedrock outcrop patterns and major structural features in Indiana.

Map showing major tectonic features that affect bedrock geology in Indiana.

Major tectonic features that affect bedrock geology in Indiana.

References:

Becker, L. E., 1974, Silurian and Devonian rocks in Indiana southwest of the Cincinnati Arch: Indiana Geological Survey Bulletin 50, 83 p.

Becker, L. E., and Droste, J. B., 1978, Late Silurian and Early Devonian sedimentologic history of southwestern Indiana: Indiana Geological Survey Occasional Paper 24, 14 p.

Collinson, Charles, James, G. W., Swann, D. H., Becker, L. E., Carlson, M. P., Dorheim, F. H., and Koenig, J. W., 1967, Devonian of north-central region, United States, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 933-971.

Droste, J. B., and Shaver, R. H., 1986, Grassy Knob Chert, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana–a revision: Indiana Geological Survey Bulletin 59, p. 53.

Droste, J. B., and Shaver, R. H., 1987, Upper Silurian and Lower Devonian stratigraphy of the central Illinois Basin: Indiana Geological Survey Special Report 39, 29 p.

Hansen, W. R., 1991, Suggestions to authors of the reports of the United States Geological Survey (7th ed.): Washington, D.C., U.S. Geological Survey, 289 p.

Savage, T. E., 1925, Oriskany rocks in Illinois: American Journal of Science, v. 10, p. 139-144.

Shaver, R. H., coordinator, 1984, Midwestern basin and arches region–correlation of stratigraphic units in North America (COSUNA): American Association of Petroleum Geologists Correlation Chart Series.

Weller, J. M., 1939, Devonian System: Kansas Geological Society Annual Field Conference Guidebook 13, p. 127-130.



For additional information contact:

Nancy Hasenmueller (hasenmue@indiana.edu)
Date last revised: September 19, 2017

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