Three glauconite samples were collected from the Rocky Point Member (Cretaceous, upper Navarro) of the Peedee Formation, below the faunal Mesozoic-Cenozoic boundary in the vicinity of Castle Hayne, North Carolina. Mammillar to lobate, un-abraded, authigenic glauconite pellets were hand-picked for analyses of Rb, Sr, and Sr isotopic composition.
Analyses resulted in four model ages ranging from 65.1 to 71.8 m.y., with an average of 68.5 m.y. λ Rb = 1.39 × 10−11 yr−1). Analysis of an Ostrea sp. shell from the same glauconitic zone yielded a (Sr87/Sr86)0 value of 0.70735.
Results, which are in excellent agreement with recent time-scale estimates for the Mesozoic-Cenozoic boundary, suggest that careful separation of glauconite pellets may be a method of avoiding the generally low Rb-Sr ages associated with glauconites.