⁴⁰Ar/³⁹Ar age and correlation of the nonmarine Two Medicine Formation (Upper Cretaceous), northwestern Montana, USA

The age of the nonmarine Two Medicine Formation of northwestern Montana is currently based upon correlations with K-Ar-dated Western Interior ammonite zones. ⁴⁰Ar/³⁹Ar dating of biotite and plagioclase separated from four bentonites and one crystal-rich tuff permits for the first time direct determination of the age of Two Medicine strata. Biotite and plagioclase from a bentonite 10 m below the top of the Two Medicine Formation yield concordant ⁴⁰Ar/ ³⁹Ar ages of 74 Ma, while biotite and plagioclase from two bentonites and a crystal-rich tuff from approximately 100 m above the base of the formation cluster in age around 80 Ma. The total duration of Two Medicine deposition is estimated using these new radio-isotopic ages via extrapolation of an average rock accumulation rate. The new ⁴⁰Ar/³⁹Ar ages facilitate regional correlation of the dinosaur-dominated paleofauna recovered from the Two Medicine Formation, and help constrain the timing of the Claggett and Bearpaw transgressions. -from Authors