Embayment Megagroup

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Lithostratigraphy: Embayment Megagroup
Chronostratigraphy: Mesozoic Erathem

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)

H. B. Willman and John C. Frye


Original description

The Embayment Megagroup (Swann and Willman, 1961, p. 482).


Named for the Mississippi Embayment, the northward extension of the Gulf Coastal Plain into southernmost Illinois.

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Stratigraphic relationships

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.

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.





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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.

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