M. E. Hopkins and J. A. Simon
The Piasa Limestone Member of the Modesto Formation (Culver, 1925, p. 20) is named for Piasa Creek in Jersey County.
The type section consists of exposures along Piasa Creek (E 1/2 25, 8N-10W) (Wanless, 1956, p. 11).
The Piasa was formerly called the Cutler Limestone in southern Illinois.
Correlations with other units in Illinois are not definite, but the Piasa appears to be equivalent to the lower part of the West Franklin Limestone Member of southeastern Illinois and southwestern Indiana and to the Madisonville Limestone Member of western Kentucky.
Extent and Thickness
The Piasa occurs in southwestern and southern Illinois. It is generally less than 4 feet thick.
In much of southwestern Illinois the Piasa lies only a few feet above the Danville (No. 7) Coal and is separated from it by either variegated claystone, gray shale, or, in places, black fissile shale.
The Piasa is commonly light gray, fine-grained, argillaceous limestone, and is commonly associated with red and variegated claystone.
The Piasa contains an open-marine fauna dominated by crinoids (mainly in the lower part) and brachiopods. In many places it contains numerous fusulinids, especially at the base.
CULVER, H. E., 1925, Coal resources of District III (western Illinois): Illinois State Geological Survey Mining Investigations Bulletin 29, 128 p.
WANLESS, H. R., 1956, Classification of the Pennsylvanian rocks of Illinois as of 1956: Illinois State Geological Survey Circular 217, 14 p.
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