CYCLOSTRATIGRAPHY OF THE LOWER TRIASSIC (INDUAN-OLENEKIAN) MONTNEY FORMATION IN NE BRITISH COLUMBIA, WESTERN CANADA

The Early Triassic records the recovery of life from Earth’s greatest extinction during a time of continued environmental stress. Understanding the rate and timing of this recovery depends on having an accurate and precise age model for the Early Triassic. Continuous marine stratigraphic successions that record astronomically-controlled sedimentologic cycles can be used to establish an accurate age model. A near-continuous core of the Early Triassic Montney Formation in northeastern British Columbia was measured with an ITRAX x-ray fluorescence core scanner at cm-scale resolution, providing an excellent opportunity for cyclostratigraphic analysis for this interval. The cored sedimentary succession reveals clear depositional cycles, with calcispheric algal dolosiltstone beds appearing at regular intervals throughout almost the entire Montney Formation. These appear to reflect well-known astronomical forcings, including 405-kyr long-eccentricity, ~100-kyr short-eccentricity, 33-kyr obliquity, and 20-kyr precession cycles. Using these cycles, a high-resolution astronomical time scale was established for the entire Early Triassic Epoch, constrained by biostratigraphy. The resulting ATS suggests that durations of the Induan and Olenekian stages are 1.1 ± 0.1 Myr and 4.0 ± 0.1 Myr, respectively. When anchored to the base of Montney Formation, with a high-precision date for the beginning of the latest Permian transgression, the time scale suggests ages of 250.9 ± 0.1 Ma and 246.9 ± 0.1 Ma for the Induan/Olenekian (Dienerian/Smithian) boundary and Olenekian/Anisian boundary, respectively. This ATS for the Lower Triassic Montney Formation allows us to time key events in the Early Triassic recovery, such as the reappearance of carbonate in the form of Claraia biostromes in the late Dienerian, recurrence of anoxia in the Smithian, or the resumption of coastal upwelling in the earliest Spathian. It provides an independent time calibration for the duration of Early Triassic conodont biozones, and possibilities for global correlation.