Mesozoic Erathem

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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 Mesozoic Era is the interval of "middle life."

Other names

The Mesozoic was formerly called the "Secondary Group" of rocks, and is also known as the Age of Reptiles.


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Reference section

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Stratigraphic relationships

The Mesozoic Erathem - the rocks deposited during the Mesozoic Era (Phillips, 1840) - is represented in Illinois only by the Cretaceous System, the youngest deposits of the era. As the youngest of the Paleozoic rocks (the Permian System), the Mesozoic, Triassic, and Jurassic Systems and the Comanchean Series of the Cretaceous System are not represented in Illinois, the base of the Cretaceous System is a major unconformity (fig. 1). However, the general distribution of the marine Permian rocks over the continental United States suggests that Permian seas covered much of Illinois and that their absence is attributable to erosion, either in latest Permian time or during the early and middle parts of the Mesozoic Era. There is no evidence to suggest that extensive deposits, either marine or continental, accumulated in Illinois during the pre-Cretaceous periods of the Mesozoic Era. Although such deposits could have been present and entirely eroded, the development of the sub-Cretaceous unconformity required a long interval of erosion, and it seems likely that Illinois was continuously in an erosional environment during early Mesozoic time. The Little Bear Soil, which developed on the Paleozoic rocks and is present at the base of the Cretaceous sediments in southern Illinois (fig. K-2), probably formed during the later part of this interval.

Extent and thickness

The Mesozoic rocks occur only in extreme southern Illinois and locally in western Illinois (figs. K-3, K-4). The southern Illinois Cretaceous strata are part of the deposits at the head of the Mississippi Embayment of the Gulf Coastal Plain. With the overlying Tertiary formations, they compose a distinctive unit differentiated as the Embayment Megagroup.


The Mesozoic rocks are largely unconsolidated sand incorporating some silt and clay.




Well log characteristics


The huge dinosaurs of the Mesozoic may have been common in Illinois, although no remains have been found. The era also saw the first appearance of birds, mammals, and angiosperms.

Age and correlation

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Economic importance



PHILLIPS, J., 1840, Penny cyclopedia: v. 17, p. 153-154.

ISGS Codes

Stratigraphic Code Geo Unit Designation