Ordovician tephra distribution, tephrochronology and geochronology

Abstract

The study of Ordovician tephras yields a wealth of valuable information about regional tectonism, sedimentation, stratigraphic correlation, and process rates. As such, these layers are prized by geologists and are the subject of a rich literature. Ordovician tephra studies were pioneering, particularly in the development of chemical fingerprinting to improve precision in tephrochronology. Modern radioisotope geochronology utilizes zircons and other phenocrysts from these layers to generate eruption ages with uncertainty on the order of a hundred thousand years. When integrated with biostratigraphy, chemostratigraphy and astrochronology, tephra ages provide an unparalleled opportunity to constrain process rates. Fifty such Ordovician tephra ages have been published over the last decade from U–Pb analysis of individual zircon phenocrysts, providing geochronolog-ical coverage across all stages of the Ordovician. Laurentia dominates this coverage (24) followed by the Baltic Basin (12), North Gondwana (11), Cuyania (four) and the Siberian Tungus Basin (one). Future tephra studies should seek to fill the numerous remaining gaps in the Ordovician time scale.