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 Glencoe Member of the Spechts Ferry Formation (Templeton and Willman, 1963, p. 110).
Named for Glencoe, St. Louis County, Missouri, which is 3 miles west of the type section.
The type section of the Glencoe Member is the same as that for the Castlewood Member (near Castlewood, St. Louis County, Missouri (NE SE SE 21, 44N-4E)), where the Glencoe is 5.3 feet thick.
The Glencoe Member overlies the Castlewood Member of the Spechts Ferry Formation.
Extent and thickness
The Glencoe Member is commonly 5-8 feet thick in the area near the Mississippi River, but it thins to the east.
The most persistent and thickest Ordovician bentonite, commonly 1-3 inches thick and locally as much as 8 inches thick, occurs in the lower part of the Glencoe Member, interbedded with green, gray, or brown shale. Locally the bentonite is altered to a distinctive, hard, pink bed, which is almost entirely potash feldspar. The Glencoe is largely green shale, but it contains beds of calcarenite, greenish gray argillaceous limestone, and dark purplish gray coarse-grained limestone.
Well log characteristics
Some beds of the Glencoe Member are a coquina of Pionodema subaequafa. In the Upper Mississippi Valley, the trilobite Isotelus gigas (fig. O-5) is common in an argillaceous limestone bed near the base.
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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