Historical:Grassy Knob Chert
Charles Collinson and Elwood Atherton
The Grassy Knob Chert (Savage, 1925b, p. 139-144), which overlies the Bailey Limestone, is named for Grassy Knob, a prominent high area on the Mississippi River bluffs 4 miles east of Grand Tower, Jackson County.
The type section of the Grassy Knob Chert is in the west face of the knob (SW SW 27, 10S-3W).
Extent and Thickness
The formation is exposed in and near the Mississippi River bluffs in Jackson, Union, and Alexander Counties and occurs in subsurface through the deep part of the Illinois Basin (Collinson et al., 1967a). It is about 200 feet thick in the outcrop area but thickens to about 300 feet in the basin to the east.
The Grassy Knob is gradational to the Bailey Limestone below. It is more sharply differentiated from the Backbone Limestone above, although no unconformity is evident. In the southern part of the Devonian outcrop area, where the Backbone is absent, the Grassy Knob is not readily separated from the Clear Creek Chert.
The Grassy Knob differs from the Bailey Formation below in being lighter colored and in containing more thick beds of solid chert, especially in the middle part of the formation (Weller, 1940), which is well exposed 2 miles southeast of Reynoldsville, Union County (NW cor. 32, 13S-2W).
Fossils are rare in the Grassy Knob Chert.
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.
SAVAGE, T. E., 1925b, Oriskany rocks in Illinois: American Journal of Science, v. 10, p. 139-144.
WELLER, J. M., 1940, Geology and oil possibilities of extreme southern Illinois: Illinois State Geological Survey Report of Investigations 71, 71 p.
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