STATE-OF-THE-ART OF THE PHANEROZOIC ISOTOPIC-GEOCHRONOLOGIC SCALE

Abstract

The modern substantiation of the boundaries in the Phanerozoic scale of geologic time is discussed with reference to absolute geochronology. The critical analysis of the isotopic-geochronologic data that form the basis of modern scales shows that only a few of them are suitable for calibration of an adequate scale. At present, reliable dating is available for the following boundaries: the Cambrian lower boundary (535 Ma); the Ordovician-Silurian (440 Ma); the Jurassic-Cretaceous (more than 137 Ma); the Cretaceous-Paleogene (65 Ma); and the Paleogene-Neogene (23 Ma). Moreover, the available data allow determination of many age boundaries in the Ordovician, Late Cretaceous, Paleogene, and Neogene and evaluation of boundary ages between several epochs: the Early-Late Silurian (421 Ma); the Middle-Late Devonian (367 Ma); the Early-Middle Triassic (238 Ma); and the Early-Middle Jurassic (less than 185 Ma). The principles of calibrating the modern scale, in particular the priority of the "geochronologic" approach, are discussed, as also the prospects of future work on the scale and the necessity to elaborate the geologic time scale in this country.