Historical:Embayment Megagroup

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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
Chronostratigraphy: Mesozoic Erathem


H. B. Willman and John C. Frye

Name Origin

The Embayment Megagroup (Swann and Willman, 1961, p. 482) is named for the Mississippi Embayment, the northward extension of the Gulf Coastal Plain into southernmost Illinois.

Extent and Thickness

The Embayment Megagroup includes seven formations (figs. K-2, T-2) that occur largely south of the Cache Valley in Pulaski and Massac Counties but also extend westward across Alexander County from the Cache River Valley to the Mississippi River Valley. The megagroup thickens rapidly southward into the Embayment area and has a maximum thickness of nearly 1000 feet in subsurface near Cairo, Alexander County.


It consists in Illinois of the dominantly clastic formations of Cretaceous and Tertiary age that differ markedly in character from the underlying Paleozoic and the overlying Quaternary formations and are separated from both by major unconformities.


SWANN, D. H., and H. B. WILLMAN, 1961, Megagroups in Illinois: American Association of Petroleum Geologists Bulletin, v. 45, p. 471-483; Illinois State Geological Survey Reprint 1961-N.

ISGS Codes

Stratigraphic Code Geo Unit Designation