Historical:Grand Tower Limestone
Charles Collinson and Elwood Atherton
The Grand Tower Limestone (Keyes, 1894, p. 30, 42), which forms the lower part of the Middle Devonian Series in Illinois south of the Sangamon Arch, is named for Grand Tower, Jackson County (fig. D-3A).
The type section of the Grand Tower Limestone, not designated by Keyes, was probably intended to be, and currently is accepted as, the exposure in the Devil's Bake Oven, an isolated hill half a mile north of Grand Tower (SW SE NE 23, 10S-4W), where the formation is 157 feet thick.
The Grand Tower correlates with all but the upper part of the Wapsipinicon Limestone in northern Illinois and Iowa, with the Geneva Dolomite and Jeffersonville Limestone in Indiana, and with the Onondaga in New York (Collinson et al., 1967a).
Extent and Thickness
The Grand Tower thins out northward against the Sangamon Arch, westward against the Ozark Uplift, and southward at the southern margin of Illinois (fig. D-14). It apparently was not deposited on the Sparta Shelf, an eastward projection of the Ozarks, but it does occur in the Wittenberg Trough, along the south side of the Sparta Shelf, and it is as much as 160 feet thick there (Meents and Swann, 1965). From the Sparta Shelf it thickens eastward to reach a maximum of 250 feet.
The base of the Grand Tower is a major unconformity- the base of the Kaskaskia Sequence- and in the northern part of its area the Grand Tower sharply overlaps the entire Lower Devonian Series and rests on the Silurian System.
The Grand Tower is mostly coarse-grained, light gray, medium- to thick-bedded, cross-bedded, pure, fossiliferous limestone, but it also contains lithographic limestone, which becomes more abundant upward (North, 1969). At the base a calcareous sandstone or sandy limestone with sandstone beds is differentiated as the Dutch Creek Sandstone Member. In the central and eastern parts of the state the Grand Tower is largely dolomite, the lower part of which is brown dolomite differentiated as the Geneva Dolomite Member, and the upper part is a laminated, gray to tan, fine-grained dolomite that has not been named. In the northern part of its area, the Grand Tower is represented primarily by a light gray, lithographic limestone differentiated as the Cooper Limestone Member. A thin bentonite widely present near the top of the formation is the Tioga Bentonite Bed.
The Grand Tower Limestone is abundantly fossiliferous, and a large macrofauna (S. Weller, 1897; Savage, 1920; Grimmer, 1968) and conodont fauna (Orr, 1964; Collinson et al., 1962) have been described.
COLLINSON, CHARLES, L. E. BECKER, G. W. JAMES, J. W. KOENIG, and D. H. SWANN, 1967a, Illinois Basin, in International symposium on the Devonian System: Alberta Society of Petroleum Geologists, v. 1, p. 940-962; Illinois State Geological Survey Reprint 1968-G.
COLLINSON, CHARLES, A. J. SCOTT, and C. B. REXROAD, 1962, Six charts showing biostratigraphic zones and correlations based on conodonts from the Devonian and Mississippian rocks of the Upper Mississippi Valley: Illinois State Geological Survey Circular 328, 32 p.
GRIMMER, J. C., 1968, Stratigraphy of the Middle Devonian shales of southern Illinois: Illinois Academy of Science Transactions, v. 61, p. 407-415.
KEYES, C. R., 1894, Paleontology of Missouri (Part I): Missouri Geological Survey, v. 4, 271 p.
MEENTS, W. F., and D. H. SWANN, 1965, Grand Tower Limestone (Devonian) of southern Illinois: Illinois State Geological Survey Circular 389, 34 p.
NORTH, W. G., 1969, Middle Devonian strata of southern Illinois: Illinois State Geological Survey Circular 441, 45 p.
ORR, R. W., 1964, Conodonts from the Devonian Lingle and Alto Formations of southern Illinois: Illinois State Geological Survey Circular 361, 28 p.
SAVAGE, T. E., 1920, Devonian formations of Illinois: American Journal of Science, v. 49, p. 169-182.
WELLER, STUART, 1897, Correlation of the Devonian faunas in southern Illinois: Journal of Geology, v. 5, p. 625-635.
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