AR-40 AR-39 AGE AND CORRELATION OF THE NONMARINE 2 MEDICINE FORMATION(UPPER CRETACEOUS), NORTHWESTERN MONTANA, USA/

The age of the nonmarine Two Medicine Formation of northwestern Montan a is currently based upon correlations with K-Ar-dated Western Interio r ammonite zones. Ar-40/Ar-39 dating of biotite and plagioclase separa ted from four bentonites and one crystal-rich tuff permits for the fir st time direct determination of the age of Two Medicine strata. Biotit e and plagioclase from a bentonite 10 m below the top of the Two Medic ine Formation yield concordant Ar-40/Ar-39 ages of 74 Ma, while biotit e and plagioclase from two bentonites and a crystal-rich tuff from app roximately 100 m above the base of the formation cluster in age around 80 Ma. The total duration of Two Medicine deposition is estimated usi ng these new radio-isotopic ages via extrapolation of an average rock accumulation rate. The new Ar-40/Ar-39 ages facilitate regional correl ation of the dinosaur-dominated paleofauna recovered from the Two Medi cine Formation, and help constrain the timing of the Claggett and Bear paw transgressions. The ages support correlation of the richly fossili ferous upper lithofacies suite of the Two Medicine Formation with expo sures of the Judith River Formation in Dinosaur Provincial Park, Alber ta, Canada. Radioisotopically dated exposures of the Judith River Form ation within Montana that include important Judithian ''age'' mammal l ocalities correlate approximately with middle and lower parts of the m iddle lidiofacies suite of the Two Medicine Formation. The new Ar-40/A r-39 ages further indicate that the transgressions of the Claggett and Bearpaw seas culminated within northwestern Montana at ca. 79.6 and 7 4.0 Ma, respectively.