Hely Member

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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

Primary source

Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Contributing author(s)

H. B. Willman and T. C. Buschbach


Original description

The Hely Member of the Grand Detour Formation (Templeton and Willman, 1963, p. 86).


Named for Hely's upper quarry on the south side of Cape Girardeau, Missouri.

Other names


Type section

Type location

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.

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

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.




Well log characteristics


Age and correlation

Environments of deposition

Economic importance



TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation