Stillman Member

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Lithostratigraphy: Ottawa Limestone Megagroup >>Platteville Group >>Plattin Subgroup >>Grand Detour Formation >>Stillman 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 Stillman Member of the Grand Detour Formation (Templeton and Willman, 1963, p. 85).


Named for Stillman's Run, a stream 1.5 miles east of the type section.

Other names


Type section

Type location

The type section of the Stillman Member is located in a quarry just north of Byron, Ogle County (NE NE SE 30, 25N-11E), where it is 5.3 feet thick.

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

Extent and thickness

The Stillman Member is 5-8 feet thick in the northern outcrop area, but in the southern outcrop area it thickens from 5 feet in Calhoun County to 64 feet at Cape Girardeau, Missouri.


The Stillman Member consists of dolomite-mottled, lithographic limestone, or, in places, vuggy, fine- to medium-grained dolomite. It is generally pure, cherty, fucoidal, and thick bedded or massive. In the southern area it is divisible into an upper fucoidal unit, a middle argillaceous to shaly, partly cherty unit, and a basal calcarenitic fucoidal unit.




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