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.
H. B. Willman and T. C. Buschbach
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.
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.
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
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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