Historical:Tioga Bentonite Bed

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Handbook of Illinois Stratigraphy
Series Bulletin 95
Author H. B. Willman, Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, Jack A. Simon
Date 1975
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Lithostratigraphy: Hunton Limestone Megagroup >>Grand Tower Limestone >>Tioga Bentonite Bed
Chronostratigraphy: Paleozoic Erathem >>Devonian System >>Middle Devonian Series
Allostratigraphy: Kaskaskia Sequence


Charles Collinson and Elwood Atherton

Name Origin

The Tioga Bentonite Bed of the Grand Tower Limestone (Ebright et al., 1949, p. 10) is named for Tioga County, Pennsylvania, where it was first discovered in drill cuttings from the Tioga gas field.

Extent and Thickness

The Tioga Bentonite Bed is widely present in Illinois 10-30 feet below the top of the formation, but no outcrops have been found (Meents and Swann, 1965), except recently in a quarry at Tuscola in Douglas County. Although possibly as much as 6-8 inches thick in southeastern Illinois, it most commonly is only 1-2 inches thick. Its occurrence is patchy, but it has been observed in more than 100 wells and can be recognized on sonic logs even in areas where it is only a quarter of an inch thick.


The Tioga Bentonite Bed is greenish to brownish gray shale that contains biotite flakes and an abundance of mixed layer clay minerals that distinguish it from the normal shales.


EBRIGHT, J. R. , C. R. FETTKE, and A. I. INGHAM, 1949, East Fork-Wharton gas field, Potter County, Pennsylvania: Pennsylvania Geological Survey Bulletin M30, Series 4, 43 p.
MEENTS, W. F., and D. H. SWANN, 1965, Grand Tower Limestone (Devonian) of southern Illinois: Illinois State Geological Survey Circular 389, 34 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation