H. B. Willman and T. C. Buschbach
The Castlewood Member of the Spechts Ferry Formation (Templeton and Willman, 1963, p. 107), the basal member, is largely limestone in its type section near Castlewood, St. Louis County, Missouri (NE SE SE 21, 44N-4E), where it is 6.9 feet thick.
In Minnesota the Castlewood Member is equivalent to the lower part of the Carimona Limestone (Weiss, 1955).
Extent and Thickness
The Castlewood Member is a massive bed 3-7 feet thick in the type region, but it is only a few inches to 1.5 feet thick in northwestern Illinois.
The limestone of the Castlewood Member commonly overlies a thin bentonite, generally less than an inch thick, which rests on dark brown shale up to 3 inches thick. The limestone is fine grained, slightly argillaceous, and more like limestone beds in the Glencoe Member above than the pure lithographic limestone of the Plattin below, from which it is separated by a widespread unconformity.
Fossils are not abundant in the Castlewood Member, but Pionodema subaequata and other Trentonian fossils are present. It has been called the Lingula elderi bed in Minnesota.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
WEISS, M. P., 1955, Some Ordovician brachiopods from Minnesota and their stratigraphic relations: Journal of Paleontology, v. 29, p. 759-774.
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