Charles Collinson and Elwood Atherton
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.
Extent & 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).
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).
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.
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.
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