Lithostratigraphy: Ottawa Limestone Megagroup >>Platteville Group >>Plattin Subgroup >>Grand Detour Formation >>Forreston Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Blackriveran Stage
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and T. C. Buschbach
The Forreston Member of the Grand Detour Formation (Templeton and Willman, 1963, p. 87) is named for Forreston, Ogle County, which is 7 miles northeast of the type section.
The type section of the Forreston Member is located in a quarry in Carroll County on the north side of a ravine 1 mile northwest of Brookville (NE SE NW 21, 24N-7E), where it is 17.9 feet thick.
Extent and Thickness
The Forreston Member is commonly 5-25 feet thick in the northern outcrop area and in the northern part of the southern outcrop area, but it thickens southward to 40 feet at Cape Girardeau.
The Forreston Member consists largely of dolomite-mottled lithographic limestone or fine-grained dolomite. It is commonly fossiliferous. In many exposures it can be differentiated into three cyclical units, each consisting of a thin-bedded, shaly unit at the base and a thicker bedded, purer unit at the top. The shaly units generally contain the thickest red-brown shale partings in the Grand Detour. The member contains some chert, and thin beds of calcarenite are common. In subsurface in west-central Illinois, a sandy zone about 5 feet thick is present at or near the base.
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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