The Hatton Tuff and the Chickasaw Creek Tuff occur near the bottom and top, respectively, of the Stanley Shale in the Ouachita Mountains. Seven whole-rock samples of the Hatton Tuff and four whole-rock samples of the Chickasaw Creek Tuff were analyzed for their Rb and Sr isotopic content. Analysis of the Hatton Tuff samples yielded an isochron age of 310 ± 15 m.y. (Rb87 = 1.39 × 10−11yr−1). Analysis of the Chickasaw Creek Tuff samples did not yield an isochron. The age of the Hatton Tuff, when compared to the currently accepted geologic time scales, indicates that the lower Stanley Shale is Pennsylvanian in age.

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