Historical:Porters Creek Formation

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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: Embayment Megagroup >>Porters Creek Formation
Chronostratigraphy: Cenozoic Erathem >>Tertiary System >>Paleocene Series

Authors

H. B. Willman and John C. Frye

Name Origin

The Porters Creek Formation (Safford, 1864, p. 361, 368) (fig. T-2) is named for exposures on Porters Creek in Hardeman County, Tennessee.

Extent and Thickness

It is present in Alexander and Pulaski Counties in southern Illinois (Lamar, 1928b; Pryor and Ross, 1962) and is well exposed in pits south of Olmsted, Pulaski County (NE SE 27, 15S-1E), in the Ohio River bluffs near U.S. Dam 53, and along the Cache River near Unity. It is 50-150 feet thick, but north of the Cache Valley it is completely truncated by the Mounds Gravel.

Stratigraphic Position

The Porters Creek is overlain unconformably by the Wilcox Formation, which locally cuts deeply into the Porters Creek.

Description

The Porters Creek is largely massive, compact, essentially unbedded or weakly bedded, dark gray to nearly black clay that has a blocky fracture. It is gray-tan to white where weathered. Because it does not readily become plastic when wet, it can stand in steeper faces than most clays. It is more than 80 percent montmorillonite. Because the clay has the absorptive and filtering properties of fuller's earth, it has been mined commercially south of Olmsted, Pulaski County.

Fossils

Microfossils (Radiolaria, Hystrichospheridae, and Foraminifera) are abundant, but fish scales, shark teeth, and casts and molds of pelecypods and gastropods are also common.

Depositional Environment

The Porters Creek is a marine deposit representing an advance of the sea over essentially the entire area of the embayment.

References

LAMAR, J. E., 1928b, Preliminary report on the fuller's earth deposits of Pulaski County: Illinois State Geological Survey Report of Investigations 15, 31 p.
PRYOR, W. A., and C. A. ROSS, 1962, Geology of the Illinois parts of the Cairo, La Center, and Thebes Quadrangles: Illinois State Geological Survey Circular 332, 39 p.
SAFFORD, J. M., 1864, On the Cretaceous and superior formations of west Tennessee: American Journal of Science, v. 37, p. 360-372.

ISGS Codes

Stratigraphic Code Geo Unit Designation
1280
Tpc