Historical:Seahorne Limestone Member

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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: Kewanee Group >>Spoon Formation >>Seahorne Limestone Member
Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem >>Desmoinesian Series
Allostratigraphy: Absaroka Sequence


M. E. Hopkins and J. A. Simon

Name Origin

The Seahorne Limestone Member of the Spoon Formation (Wanless, 1931a, p. 191) is named for Seahorne Branch in Fulton County.

Type Section

The type section consists of exposures along the Seahorne Branch stream (S 1/2 SE 5, 3N-3E) (Wanless, 1956, p. 9; 1957, p. 76, 200).

Extent and Thickness

The Seahorne is best known in western Illinois, where it varies from limestone nodules in claystone to a solid ledge of limestone more than 6 feet thick. The Seahorne is not persistent but occurs at many places in Illinois and adjacent states.


It is usually light gray. In some places it is conglomeratic or brecciated and consists of dark gray fragments that contain abundant brachiopods embedded in a light gray matrix that is dominated by a diverse gastropod fauna. Another fauna, Spirorbis and ostracodes, is found at places and is generally considered nonmarine. It is the caprock of the Tebo coal in Missouri (Wanless, 1957).


WANLESS, H. R., 1931a, Pennsylvanian cycles in western Illinois: Illinois State Geological Survey Bulletin 60, p.
WANLESS, H. R., 1956, Classification of the Pennsylvanian rocks of Illinois as of 1956: Illinois State Geological Survey Circular 217, 14 p.
WANLESS, H. R., 1957, Geology and mineral resources of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois State Geological Survey Bulletin 82, 233 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation