ASTRONOMICAL TUNING OF THE EARLY TRIASSIC INDUAN STAGE IN THE DISTAL MONTNEY FORMATION, NE BRITISH COLUMBIA, CANADA

The Induan Stage records the recovery of life from Earth’s greatest extinction during a time of continued environmental stress. Current estimates for Induan Stage duration range from 0.7 Myr based on a single radiometric date to 2.2 Myr based on astronomical tuning, pointing to the need for an accurate and precise age model to better understand the timing and rate of recovery. Continuous marine stratigraphic successions that record astronomically-controlled sedimentologic cycles can be used to establish an accurate age model. High-resolution cyclostratigraphic analysis of several XRF data series from a continuous core of the Early Triassic Montney Formation in northeastern British Columbia, Canada revealed clear cycles in sedimentary chemistry. Multi-taper method power spectra, wavelength ratios, filtering, and amplitude modulation analyses all indicate cyclical components in the Griesbachian and Dienerian intervals, which are interpreted to represent the 405 kyr long-eccentricity cycle, 100 kyr short-eccentricity cycle, 33 kyr obliquity cycle, and ~17-20 kyr precession cycle. We calculated a high-resolution astronomical time scale (ATS) for the Induan Stage, based on cyclostratigraphic analysis constrained by conodont biostratigraphy. Our ATS suggests that the duration of the Induan Stage (plus latest Permian) was 1.7 ± 0.1 Myr, with nearly equal durations of ~0.85 ± 0.05 Myr for both the Griesbachian and Dienerian substages. Based on this interpretation, the mean sedimentation rate for the Griesbachian interval is ~5.8 cm/kyr, with the Dienerian interval having a faster rate of ~11.4 cm/kyr. The high sedimentation rate seen globally in the Griesbachian is not observed in this core, possibly as a result of its distal basinal location and subsequent progradation of the Montney during the Early Triassic. Using an age of 252 Ma for the latest Permian transgression as an anchor point for this study, we determine the ages of substage boundaries to be 251.15 ± 0.05 Ma for the base-Dienerian and 250.3 ± 0.1 Ma for the base-Smithian (base-Olenekian Stage). This ATS for the lower Montney Formation provides an independent time calibration for Induan conodont biozone duration, and for the duration and rate of Early Triassic recovery after the end-Permian mass extinction, and will facilitate future global correlation.