Knobs Megagroup

Lithostratigraphy: Knobs Megagroup
Chronostratigraphy: Paleozoic Erathem
Allostratigraphy: Kaskaskia Sequence

Primary source

Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Contributing author(s)

Charles Collinson and Elwood Atherton

Name

Derivation

The Knobs Megagroup is named for the Knobs, an area of highly dissected Devonian and Mississippian shales that partially surrounds the Bluegrass Region and lies in front of the Highland Rim Escarpment of Kentucky and Indiana.

Stratigraphic relationships

The Knobs Megagroup (Swann and Willman, 1961 , p. 480-481) consists of the Devonian and Mississippian clastic rocks that overlie the Hunton Limestone Megagroup and underlie the Mammoth Cave Limestone Megagroup throughout the Illinois Basin (fig. 14).

Fig. 14 -- Pre-Pennsylvanian megagroups and sequences of Illinois (Swann and Willman, 1961).

Extent and thickness

The Knobs has a maximum thickness of about 1000 feet, but it is much thinner in western Illinois and it thins out in places (fig. D-16).

Fig. D-16 -- Thickness of the Knobs Megagroup.

Lithology

In much of the Illinois Basin, the Knobs Megagroup includes shale, siltstone, and sandstone from the base of the Blocher Shale of the New Albany Group to the top of the Borden Siltstone; but on the flanks of the basin, where shallow-water carbonates were deposited, the stratigraphic interval is restricted at both top and bottom. The Knobs is mainly black shale in most of the basin, but in central Illinois it is dominated by the great volume of gray siltstone and silty shale in the Borden delta.

References

SWANN, D. H., and H. B. WILLMAN, 1961, Megagroups in Illinois: American Association of Petroleum Geologists Bulletin, v. 45, p. 471-483; Illinois State Geological Survey Reprint 1961-N.

ISGS Codes

Stratigraphic Code	Geo Unit Designation
5260	Dk