Historical:Ottawa Limestone Megagroup
H. B. Willman and T. C. Buschbach
The Ottawa Limestone Megagroup (Wilson, 1938, 1946, p. 21; Swann and Willman, 1961, p. 478) is named for Ottawa, Ontario, Canada.
Extent and Thickness
The Ottawa Megagroup underlies most of the state and thickens from about 300 feet in northern Illinois to 1300 feet in southern Illinois (fig. O-14).
The Ottawa Limestone Megagroup consists of the dominantly carbonate rocks overlying the St. Peter Sandstone and underlying the clastic rocks of the Cincinnatian Series. It consists largely of the Galena and Platteville Groups, but in southwestern Illinois it includes at the top the Cape Limestone, which underlies the Maquoketa Shale Group, and at the base it includes the dominantly dolomite Dutchtown and Joachim Formations in the upper part of the Ancell Group. At the base in northern Illinois it locally includes the Daysville Dolomite Member of the Glenwood Formation. In local areas where the St. Peter Sandstone is absent, the Ottawa Megagroup rests on the Knox Dolomite Megagroup but generally is readily distinguished from it.
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.
WILSON, A. E., 1938, Legend on Ottawa sheet (east half): Canada Geological Survey.
WILSON, A. E., 1946, Geology of the Ottawa-St. Lawrence lowland, Ontario and Quebec: Canada Geological Survey Memoir 241, 65 p.
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