Improved Geochronologic Accuracy and Precision for the ICS Chronostratigraphic Charts: Examples from the late Cambrian–Early Ordovician

Abstract

The utility of ICS International Chronostratigraphic Charts (ICS Charts) relies on the accuracy and precision of reported geochronologic dates and their relationship to the bases of global chronostratigraphic units. However, an examination of some early Paleozoic dates (i.e., Cambrian-Ordovician boundary and lowest Ordovician) on the ICS Charts shows that they differ from originally reported and late recalculated dates. Thus, they record a numerical precision that differs from these revised dates. The dates assigned to the bases of formal early Paleozoic chronostratigraphic units may be interpreted by the lay reader as precise determinations, although they are almost all estimates based on dated horizons that stratigraphically bracket a chronostratigraphic unit's base. The bases of informal Cambrian chronostratigraphic units (e.g., Stage 10, ∼489.5 Ma) are also assigned relatively precise ages although consensus does not exist on the horizon that should define an informal unit's base, and several alternative GSSP horizons with differing ages may exist. The uncertainties of the ICS Chart's dates on stage bases within the traditional Upper Cambrian are so great that the tilde (e.g., ∼500.5) indicating about might be replaced by question marks or an abbreviation for estimated. A preferred alternative is to replace the age estimates on the bases of informal and almost all formal chronostratigraphic divisions in the Phanerozoic with precise dates that bracket the bases of the divisions. The ICS Chart should include (perhaps on-line) brief discussions on and references to the history and procedures of age determination, accuracy, and precision for each dated horizon. An even more accurate, precise, and informative ICS Chart would result from review by a new ICS subcommission on geochronology. The subcommission could evaluate proposed dates for the ICS Chart for accuracy, precision, and suitability for determining/bracketing the ages of chronostratigraphic unit bases and provide updated reviews of the procedures used to determine all ICS Chart dates.