NEW GEOCHRONOLOGICAL DATA AND AGE MODEL APPROACHES FOR THE LATE JURASSIC THROUGH EARLY CRETACEOUS GEOLOGIC TIME SCALE (Invited Presentation)

The global geologic time scales for the Late Jurassic and Early Cretaceous Periods are prime examples of how rigorous radioisotopic analytical data and metadata standards in the production of the Geologic Time Scale 2012 (GTS2012) led to a depauperate numerical age database for time scale calibration. In the absence of precise radioisotopic anchors, the Late Jurassic and Early Cretaceous time scales were constructed from an amalgam of spline fitting to sparse radioisotopic ages, modified by estimated durations of stages and/or zones from floating astrochronologies, across a magnetostratigraphic composite framework derived from a cycle-scaled spreading rate model for the Pacific M-sequence. It is difficult to estimate the magnitude of systematic errors that may have propagated through this complex time scale calibration. Indeed these difficulties were underscored in GTS2012 by Ogg, Hinnov and Huang in their closing remarks on Earth Cretaceous scaling: “Therefore, the age model for scaling events within the Valanginian, Hauterivian and Barremian stages should be considered as a temporary model that awaits detailed enhancement.”

The much less complacent time scales that resulted in GTS 2012 have renewed efforts to produce more precise and accurate time scales through modern radioisotopic and astrochronological studies. Precise U-Pb geochronological results within robust biostratigraphic contexts have highlighted discrepancies with GTS2012 on the scale of several million years, much larger than the stage boundary uncertainties estimated in GTS2012. This is just one of many examples where it has become apparent that classical regression methods commonly underestimate the confidence intervals about the least-squares derived best fit time scale calibration. In this presentation I will review the progress in radioisotopic geochronology and astrochronology in Late Jurassic and Early Cretaceous strata, highlighting how new data may or may not be reconciled with legacy models. I will also introduce a methodology for Bayesian age modeling particularly suited to time scale calibration, and apply this computational framework to diverse data types to illustrate how this statistical approach may produce more robust age models and time scales for the Late Jurassic and Early Cretaceous, and indeed the entire Phanerozoic.