Historical:Thebes Sandstone Member
Lithostratigraphy: Maquoketa Shale Group >>Scales Shale >>Thebes Sandstone Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Cincinnatian Series >>Maysvillian Stage
Allostratigraphy: Tippecanoe Sequence
Authors
H. B. Willman and T. C. Buschbach
Name Origin
The Thebes Sandstone Member of the Scales Shale (Worthen, 1866, p. 139; Savage, 1909, p. 515) is named for Thebes, Alexander County.
Type Section
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).
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.
Stratigraphic Position
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.
Description
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.
References
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.
ISGS Codes
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