Comparison of Rb-Sr and K-Ar dates of middle Eocene bentonite and glauconite, southeastern Atlantic Coastal Plain

Abstract

Rb-Sr and K-Ar dates for euhedral biotite from high-Mg, Chetotype smectitic bentonite in glaucony-bearing beds of the middle Eocene (Lutetian) Castle Hayne Limestone indicate deposition of volcanic ash in the North Carolina Coastal Plain at 46 Ma. Five evolved (high-potassium) glauconitic micas from 30-35 cm below the bentonite yield a Rb-Sr isochron date of 45.3 ± 0.3 Ma and an initial 87 Sr/ 86 Sr ratio of 0.70764 ± 0.00015; a single split yields a conventional K-Ar date of 46.7 ± 1.8 Ma. Six evolved (high-potassium) glauconitic micas from 25-30 cm above the bentonite yield a Rb-Sr date of 43.1 ± 1.2 Ma and an initial 87 Sr/ 86 Sr ratio of 0.7079 ± 0.00011; a single split yields a conventional K-Ar date of 44.5 ± 1.7 Ma. Agreement of Rb-Sr and K-Ar biotite dates in the bentonite with Rb-Sr and K-Ar glauconitic mica dates in the underlying and overlying carbonate suggests that carefully selected, evolved glauconitic mica that has neither been deeply buried nor experienced cation exchange with ground water can provide reliable isotopic ages and constraints for the standard geologic time scale.