H. B. Willman and T. C. Buschbach
The Pecatonica Formation (Hershey, 1894, p. 175; 1897), the basal formation of the Platteville Group, is named for exposures along the Pecatonica River north of the Illinois-Wisconsin state line.
A section in the quarries and a roadcut on the East Branch of the Pecatonica River just north of Woodford, Lafayette County, Wisconsin (W 1/2 NW NE 14, 2N-5E), where it is 22.2 feet thick, has been designated the type section (Templeton and Willman, 1963, p. 73).
The Pecatonica is a widely distributed unit in the Mississippi Valley, and it is equivalent to the Ridley Formation in Tennessee and to the lower beds in the Lowville of New York.
Extent and Thickness
In Illinois the Pecatonica Formation is exposed only in the northern area, but it is a distinctive unit widely recognized in subsurface (Buschbach, 1964). It is locally absent in northern Illinois over anticlines, in extreme western Illinois, and in the southern outcrop area north of Perry County, Missouri. It is commonly about 20 feet thick in northern Illinois, but it thickens to 150 feet in extreme southern Illinois.
The Pecatonica Formation overlies the Glenwood Formation or the St. Peter Sandstone in northern Illinois and the Joachim Dolomite in southern Illinois. It is overlain by the Mifflin Formation and is separated from both overlying and underlying units by sharp contacts that are at least regional diastems, perhaps unconformities.
North of a line from southeastern Missouri, through north-central Illinois, and into northwestern Indiana, the Pecatonica Formation is mainly brown, finely vuggy dolomite in medium to thick beds and has large chert nodules at a few horizons. South of there it is largely brownish gray lithographic limestone mottled with dolomite. The Pecatonica is subdivided into six members: the Hennepin (at the base) varies but is generally dolomitic siltstone, the Chana is sandy, the Dane is argillaceous, the New Glarus is pure, the Medusa is weakly argillaceous and strongly fucoidal, and the Oglesby at the top is coarse calcarenite.
The Pecatonica has a dominantly brachiopod-molluscan fauna but generally is less fossiliferous than the overlying Platteville formations.
BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.
HERSHEY, O. H., 1894, Elk Horn Creek area of St. Peter Sandstone in northwestern Illinois: American Geologist, v. 14, p. 169-179.
HERSHEY, O. H., 1897, The term Pecatonica Limestone: American Geologist, v. 20, p. 66-67.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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