A U-Pb and Ar-40/Ar-39 time scale for the Jurassic

Published time scales provide discrepant age estimates for Jurassic stage b oundaries and carry large uncertainties. The U-Pb or Ar-40/Ar-39 dating of volcaniclastic rocks with precisely known stratigraphic age is the preferre d method to improve the calibration. A radiometric age database consisting of fifty U-Pb and Ar-40/Ar-39 ages was compiled to construct a revised Jura ssic time scale. Accepted ages have a precision of +/- 5 Ma (2 sigma) or be tter and are confined to no more than two adjacent stages. The majority of these calibration points result from integrated bio- and geochronologic dat ing in the western North American Cordillera and have not been previously u sed in time scales. Direct dates are available only for the Triassic-Jurass ic boundary and the initial boundary of the Crassicosta chron and the Callo vian stage. The chronogram method was used to estimate all Early and early Middle Jurassic zone boundaries (attempted here for the first time), late M iddle Jurassic substage boundaries, and Late Jurassic stage boundaries. Sig nificant improvement is achieved for the Pliensbachian and Toarcian, where six consecutive zone boundaries are determined. The derived zonal durations are disparate, varying between 0.4 and 1.6 Ma. The latest Jurassic isotopi c database remains too sparse, therefore chronogram estimates are improved using interpolation based on magnetochronology. The initial boundaries of J urassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous): 1 41.8(-1.8)(+2.5) Ma Tithonian: 150.5(-2.8)(+3.4) Ma Kimmeridgian: 154.7(-3. 3)(+3.8) Ma Oxfordian: 156.5(-5.1)(+3.1) Ma Callovian: 160.4(-0.5)(+1.1) Ma Bathonian: 166.0(-5.6)(+3.8) Ma Bajocian: 174.0(-7.9)(+1.2) Ma Aalenian: 1 78.0(-1.5)(+1.0) Ma Toarcian: 183.6(-1.1)(+1.7) Ma Pliensbachian: 191.5(-4. 7)(+1.9) Ma Sinemurian: 196.5(-5.7)(+1.7) Ma Hettangian (Triassic-Jurassic) : 199.6 +/- 0.4 Ma.