Mixed-layer illite-smectite (I/S) from Middle Ordovician potassium (K)-bentonites was found to be uniformly illitic throughout the southern Appalachian Basin regardless of variable depths of burial of the K-bentonites. The K/Ar ages of illitization are narrowly confined between 272 and 303 Ma (Late Pennsylvanian to Early Permian); this suggests that illitization was a short-lived episode coincident with, and prompted by, the Alleghanian orogeny. The illitization is explainable by the fluid expulsion hypothesis recently proposed by others (e.g., Oliver). Hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny; these fluids are thought to be effective agents of illitization. Some of our K-bentonite samples are stratigraphically and geographically close to Mississippi Valley-type lead-zinc deposits, suggesting a similar mode of origin.

Rapid illitization of shales, in a fashion similar to that observed for the bentonites, should have led to high pore-water pressures that enabled them to act as ideal decollements during Alleghanian thin-skinned deformation.

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