Foraminifera and Rb-Sr glauconite ages of a Paleocene Beaufort Formation outcrop in North Carolina

Abstract

A 3.3-m section of the Paleocene Beaufort Formation, which crops out along Mosley Creek at the Lenoir-Craven county line, North Carolina, consists of alternating, unconsolidated, sandy, foraminiferal-glauconitic sediments and thinner, slightly glauconitic, foraminiferal biomicrosparites. A single Beaufort Formation sample collected from unconsolidated sediments about 1.5 m below the Paleocene-Eocene boundary was analyzed for foraminifera and radiometrically dated by glauconites. The occurrence of Globorotalia aequa Cushman and Renz, G. pseudomenardii Bolli, G. pseudobulloides (Plummer), and Globigerina triloculinoides Plummer indicates that the sample is Thanetian in age and is part of the P4 planktonic foraminiferal zone. The zone, identified by the occurrence of Globorotalia pseudomenardii , has a suggested absolute age range of 56 to 58 m.y. Three hand-picked, mammillated to lobate, glauconite concentrations were separated from the same sample and analyzed for Rb, Sr, and Sr-isotopic composition. Model ages of 64.5, 55.7, and 57.8 m.y. (λRb 87 = 1.39 × 10 −11 yr −1 ) using an initial Sr 87 /Sr 86 ratio of 0.7078 were determined. Although the older age (64.5) is anomalous, the average model age of 56.8 m.y. for the two younger determinations is in excellent agreement with recent time-scale estimates for the late Paleocene. The results suggest that hand-picked glauconites with a well-documented diagenetic history can yield Rb-Sr radiometric ages accurate to within 1% to 2%.