The so-called Cryogenian Period of the Neoproterozoic rests on evidence for coincident glaciation at all latitudes. It has been supposed to include at least two, maybe three and perhaps more frigid ‘snowball’ events, each with a pattern of lower diamictites and an upper carbonate cap rock. The most widely supposed glacial epochs are the Sturtian at 712 Ma, the Marinoan at 635 Ma and the Gaskiers at 580 Ma, but Precambrian sedimentary sequences are notoriously difficult to tie down in time. Only if dateable igneous events bracket evidence for glaciation is an age truly valid. Yet the global 3-fold division depends largely on correlation of stratigraphic and carbon-isotope sequences with the odd few that are dated in an absolute time-frame. The developing field of rhenium-osmium (Re-Os) radiometric dating offers a more universal check, since it provides a means of dating highly reduced black shales, that are abundant in the Neoproterozoic. The first reported results come as a blow to the ‘Snowball Earth’ community (Kendall, B. et al. 2006. Re-Os geochronology of postglacial black shales in Australia: constraints on the timing of the ‘Sturtian’ glaciation. Geology, v. 34, p. 729-732).
Bruce Kendall and colleagues from the Universities of Alberta, Canada and the Durham, UK have constrained some of the principal occurrences of the Sturtian event in Australia to between 643 and 657 Ma, by dating the shales which envelop the diamictites and cap carbonates. They are younger than even the widest range previously suggested for the Sturtian: either the glaciation was grossly diachronous, or this is yet another glaciation of ‘Sturtian’type. The best that can be concluded is that the ‘Cryogenian’ was cold but glaciation shifted from place to place – a ‘slushball’ model?