Historical:Potsdam Sandstone Megagroup
T. C. Buschbach
The Potsdam Sandstone Megagroup (Emmons, 1838, p. 214-217, 230) is named for Potsdam, St. Lawrence County, New York.
The name "Potsdam" was widely used in early reports for the basal Cambrian sandstone, but later its use was restricted to the type region. It was reinstated for the megagroup by Swann and Willman (1961) (figs. 14 and Є-3).
Extent and Thickness
The Potsdam Sandstone Megagroup underlies all of Illinois but is not exposed. It has a maximum thickness of more than 2600 feet in northeastern Illinois.
The Potsdam Sandstone Megagroup consists of the dominantly sandstone formations that compose the lower part of the Cambrian strata in the Illinois Basin. Traced southward from southern Wisconsin, where the entire Cambrian section is included in the Potsdam, the clastics become finer grained and grade to dolomite that is included in the Knox Megagroup. Throughout most of Illinois, however, the megagroups are separated by a zone of mixed dolomite, siltstone, and sandstone that is not included in either group. In the northern tier of counties in Illinois, the top of the Potsdam is commonly the top of the Franconia Formation. In the rest of the northern and in the central part of the state, the top is placed at the top of the lowest member (Elmhurst) of the Eau Claire Formation. In southern Illinois, the top of the Potsdam is the top of the Mt. Simon Sandstone.
EMMONS, EBENEZER, 1838, Report of the Second Geological District of the State of New York: New York Geological Survey, 2nd Annual Report, p. 186-252.
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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