BAYESIAN AGE MODELING AND INTEGRATIVE CHRONOSTRATIGRAPHY OF THE CHUAR AND TONTO GROUPS OF THE GRAND CANYON

The superlative strata of the late Tonian (Neoproterozoic) Chuar Group and Cambrian Tonto Group of the Grand Canyon host important records of single-celled eukaryotic and metazoan diversification, respectively. Efforts to provide sedimentological, stratigraphic, and geochronologic context for these successions have spanned over a century, and have accelerated over the past decade with new analytical tools and experimental designs. Geochronological constraints, for example, have accumulated from diverse radioisotopic chronometers, such as Re-Os dating of organic-rich shales, the ⁴⁰Ar/³⁹Ar method in diagenetic minerals, U-Pb zircon dates from volcanic tuffs, and maximum depositional ages from detrital zircons. Cyclostratigraphic proxies for time have been documented in the Chuar Group, while trilobite biochronology has played a preeminent role in telling time in the Tonto Group.

In order to merge these disparate data types into an integrative chronostratigraphic framework for each sequence, we have applied a deep time Bayesian age-depth modeling scheme adapted from the open-source package Bchron that combines an uninformed compound Poisson-gamma sediment accumulation prior conditioned by radioisotopic, cyclostratigraphic, and/or biochronologic age likelihood functions. The Chuar and Tonto Group strata present rich opportunities and challenges for Bayesian age-depth modeling. We document a method for constructing appropriate nonparametric likelihoods for detrital zircon maximum depositional age constraints important to both Chuar and Tonto Groups. We also propose a strategy for constructing a likelihood function based upon cyclostratigraphic information, and apply it to Chuar Group strata to reduce the uncertainty envelope on its age model. Finally, we tie global biochronological constraints from trilobite taxa into the Tonto Group stratigraphy to quantitatively assess their consistency with new detrital zircon maximum depositional ages throughout the succession.

In both case studies, the resulting chronostratigraphic frameworks not only represent the most robust available amalgams of rock and time for these pivotal chapters in Earth history, but also provide a road map for future integrated stratigraphic studies.