Historical:Mammoth Cave Limestone Megagroup
Elwood Atherton, Charles Collinson, and Jerry A. Lineback
The Mammoth Cave Limestone Megagroup is named for Mammoth Cave, Kentucky. The name was introduced, first as a formation name and later as a series name, for the relatively pure limestone in the interval from the base of the St. Louis Limestone to the base of the Big Clifty Sandstone.
Extent and Thickness
The Mammoth Cave Limestone Megagroup occurs throughout the area of Mississippian rocks in Illinois, but it thins from nearly 2000 feet in extreme southern Illinois to little more than 100 feet in east-central Illinois (fig. M-8). It crops out extensively along the Mississippi and Illinois Valleys in western Illinois and along the Mississippi and Ohio Valleys in southern Illinois.
The Mammoth Cave Limestone Megagroup (Miller, 1917, p. 3; Swann and Willman, 1961, p. 481) consists of the dominantly limestone groups and formations that overlie the siliceous elastic rocks of the Knobs Megagroup and occur below the lowest well developed sandstone formations of late Valmeyeran or early Chesterian age at the base of the Pope Megagroup. The Mammoth Cave is entirely Mississippian in age, but both upper and lower contacts are time-transgressive and are marked by a series of step-like, vertical cut-offs (figs. 14, M-3).
MILLER, A. M., 1917, Table of geological formations of Kentucky: University Bookstore, 7 p.; reprinted in Miller, 1919, p. 9-15.
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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