Farmington Shale Member

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Lithostratigraphy: McLeansboro Group >>Modesto Formation >>Farmington Shale Member
Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem >>Desmoinesian Series
Allostratigraphy: Absaroka Sequence

Primary source

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.

Contributing author(s)

M. E. Hopkins and J. A. Simon


Original description

The Farmington Shale Member of the Modesto Formation (Savage, 1927, p. 309).


Named for Farmington Township, Fulton County.

Other names


Type section

Type location

Farmington Township is the type locality (8N-4E) (Wanless, 1956, p. 11).

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The Farmington is the lowest named unit of the Modesto Formation in most of the state. It generally occurs immediately above the Danville (No. 7) Coal Member, but in several places a dark gray, very impure limestone, usually less than a foot thick, lies directly on the coal. In other places black fissile shale, seldom more than 2 feet thick, overlies the coal or occurs between the limestone and the Farrnington Shale.

Extent and thickness

The Farmington ranges from a few feet to as much as 50 feet thick in eastern, western, and southern Illinois and to as much as 100 feet or more in southeastern Illinois.


It is commonly gray shale that becomes coarser grained upward, and the lower part generally contains marine fossils. It has been extensively mined for clay in the Danville region, Vermilion County.




Well log characteristics


Age and correlation

Environments of deposition

Economic importance



SAVAGE, T. E., 1927, Significant breaks and overlaps in the Pennsylvanian rocks of Illinois: American Journal of Science, v. 14, p. 307-316.
WANLESS, H. R., 1956, Classification of the Pennsylvanian rocks of Illinois as of 1956: Illinois State Geological Survey Circular 217, 14 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation