Lithostratigraphy: Ottawa Limestone Megagroup >>Platteville Group >>Plattin Subgroup >>Grand Detour Formation >>Dement Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Blackriveran Stage
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and T. C. Buschbach
The Dement Member of the Grand Detour Formation (Templeton and Willman, 1963, p. 84) is named for Dement Avenue in Dixon, Lee County, 2 miles south of the type section.
The type section of the Dement Member is part of the type section for the Grand Detour which is located in a quarry in the west bluff of the Rock River, on the Walgreen estate 2 miles north of Dixon (NE 20, 22N-9E), where it is 8 feet thick.
Extent and Thickness
The Dement Member is generally 3-8 feet thick in the northern outcrop area and in the Calhoun County exposures, but it thickens southward to 20 feet in the Cape Girardeau area.
In northern Illinois the Dement Member is relatively pure, dolomite-mottled, gray, lithographic, thick-bedded limestone, but locally it is fine-grained dolomite. It contains a few thin calcarenitic layers and a few scattered chert nodules. In the southern area it is brown, fucoidal, and only locally cherty, but near St. Louis it is a conglomeratic, vuggy, very fine-grained calcarenite.
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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