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

Abstract

Published time scales provide discrepant age estimates for Jurassic stage boundaries and carry large uncertainties. The U-Pb or 40Ar/39Ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration. A radiometric age database consisting of fifty U-Pb and 40Ar/39Ar ages was compiled to construct a revised Jurassic time scale. Accepted ages have a precision of ±5 Ma (2σ) or better and are confined to no more than two adjacent stages. The majority of these calibration points result from integrated bio- and geochronologic dating in the western North American Cordillera and have not been previously used in time scales. Direct dates are available only for the Triassic-Jurassic boundary and the initial boundary of the Crassicosta chron and the Callovian stage. The chronogram method was used to estimate all Early and early Middle Jurassic zone boundaries (attempted here for the first time), late Middle Jurassic substage boundaries, and Late Jurassic stage boundaries. Significant 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 isotopic database remains too sparse, therefore chronogram estimates are improved using interpolation based on magnetochronology. The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous): 141.8+2.5&#150 1.8 Ma; Tithonian: 150.5+3.4&#150 2.8 Ma; Kimmeridgian: 154.7+3.8&#150 3.3 Ma; Oxfordian: 156.5+3.1&#150 5.1 Ma; Callovian: 160.4+1.1&#150 0.5 Ma; Bathonian: 166.0+3.8&#150 5.6 Ma; Bajocian: 174.0+1.2&#150 7.9 Ma; Aalenian: 178.0+1.0&#150 1.5 Ma; Toarcian: 183.6+1.7&#150 1.1 Ma; Pliensbachian: 191.5+1.9&#150 4.7 Ma; Sinemurian: 196.5+1.7&#150 5.7 Ma; Hettangian (Triassic-Jurassic): 199.6 ± 0.4 Ma.