Late Precambrian and Cambrian geological time-scale

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Abstract
Summary: The suggestion by the Subcommission on Precambrian Stratigraphy (of the International Union of Geological Sciences) that the beginning of Late Proterozoic times should be defined in the geochronometric scale at 900 Ma is accepted; this probably correlates with the chronostratic scale at the Middle-Upper Riphean boundary. The Upper Riphean-Vendian boundary on the chronostratic scale seems to correlate with approximately 650 Ma. The chronostratic Precambrian-Cambrian boundary is poorly correlated with the chronometric scale on a global basis at present but may be more accurately related in the next decade or so. In the meantime a range of 530-600 Ma can be considered but no conclusions are offered here. An attempt is made to give the present state of knowledge and techniques, with only a selection of radiometric data, especially those bearing more or less directly on the calibration of chrono-stratigraphic units. The geographic coverage is not intended to be complete.