Five well-ordered authigenic glauconite samples from the Belfast Member of the Brassfield Formation in south-central Ohio yield a Rb-Sr isochron age of 337 ± 27 m.y. and an initial 87 Sr/86 Sr ratio of 0.7118 ± 0.0043 (2σ uncertainties). This glauconite age is 20% to 25% younger than the approximate stratigraphic age of the Belfast Member determined by fossil correlation, and it probably represents a geologic event in the postdepositional history of the sedimentary strata. This event is interpreted as an uplift of the pre-Mississippian units composing the Cincinnati arch, the glauconites having isotopically equilibrated their strontium with that of associated fluid phases and carbonates within the Belfast Member at the time of uplift.

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