Thebes Sandstone Member
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 Thebes Sandstone Member of the Scales Shale (Worthen, 1866, p. 139; Savage, 1909, p. 515).
Named for Thebes, Alexander County.
The type section of the Thebes Member of the Scales Formation consists of exposures in the Mississippi River bluffs in Thebes (SW SE 8, 15S-3W).
The Thebes Member overlies the Cape Limestone and grades or intertongues eastward and northward into the lower part of the Elgin Shale Member. It is overlain by the Orchard Creek Shale Member, which probably is also laterally equivalent to part of the Elgin Shale Member.
Extent and thickness
The Thebes Sandstone Member is exposed in Illinois only in the vicinity of Gale and Thebes, and it occurs in subsurface only in the extreme southwestern part of the state. About 65 feet of the Thebes Sandstone Member is exposed in the bluff north of Thebes (SE 5, 15S-3W), but it has a maximum thickness of about 160 feet.
The Thebes consists of dark brown, silty, fine-grained sandstone, largely medium to thick bedded and locally cross bedded. In places it is largely brown siltstone, but it locally contains beds of gray to brown shale several feet thick. Several types of fucoid marks are common in the upper 35 feet.
Well log characteristics
Age and correlation
Environments of deposition
SAVAGE, T. E., 1909, Ordovician and Silurian formations in Alexander County, Illinois: American Journal of Science, v. 28, p. 509-519.
WORTHEN, A. H., 1866, Geology: Geological Survey of Illinois, v. 1, 504 p.
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