Lithostratigraphy: Ottawa Limestone Megagroup >>Platteville Group >>Plattin Subgroup >>Grand Detour Formation >>Hely Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Blackriveran Stage
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and T. C. Buschbach
The Hely Member of the Grand Detour Formation (Templeton and Willman, 1963, p. 86) is named for Hely's upper quarry on the south side of Cape Girardeau, Missouri.
Hely's upper quarry, with the adjoining Federal and Marquette quarries is the type section (NW NE and SE NW 18, 30N-14E) of the Hely Member.
Extent and Thickness
The Hely Member is 3-5 feet thick in the northern Illinois outcrop area and 5-8 feet in the northern part of the southern outcrop area, but south of Ste. Genevieve it thickens rapidly, and in the type section it is 73 feet thick.
The Hely Member consists of argillaceous, lithographic limestone in thin to medium beds with thin shale partings. Much of the member is calcarenitic or contains thin calcarenite layers. It is locally cherty and generally not fucoidal.
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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