摘要
The Cryogenian Period was first established in 1988 along with other Precambrian eon,era and period-level subdivisions that were defined numerically by Global Standard Stratigraphic Ages(GSSAs).As absolute age constraints have improved,some of these time intervals no longer bracket adequately the geological event(s),for which they were named.For example,the age discrepancy between the basal Cryogenian GSSA at 850 Ma and the onset of widespread glaciation ca.
