CONSTRAINTS ON THE TIMING OF THE STURTIAN GLACIATION FROM SOUTHERN AUSTRALIA; IE FOR THE TRUE STURTIAN

Just as the Rodinian concept encouraged a flurry of research into continental reconstructions, the Snowball Earth concept has rekindled long-term research into determining just when these apparently worldwide Neoproterozoic glacial events really took place. Unfortunately there has recently been a tendency to push for just how small one's U-Pb error ellipses can be rather than overcome the inherent problem that has always existed in Neoproterozoic sediments – what can we actually date? So, we have decided to yet again look at the rocks, searching for the crucial tuffaceous horizon 1-5 cm thick that may have been overlooked, or not appreciated for its geochronologic value. The ability to analyse a large number of samples/zircon grains with minimal destruction to those grains, has enabled us to scan numerous samples that at least have a volcanogenic component, even if not truly tuffaceous. Amongst the wide number of exploratory samples collected from both Australia and North America, a gray vitreous tuffaceous bed in the upper unit of the Sturtian Merinjina Formation southwest of Copley, South Australia (Unit 3 of Link and Gostin, 1981, American Journal of Science, v. 281, p. 353-374), has yielded sufficient zoned igneous zircons for us to report the U-Pb SHRIMP data. The tuffaceous layer overlies a regionally persistent massive diamictite unit and is in turn overlain by about 50 m of dropstone-bearing sandstone, conglomerate, and diamictite below the post-glacial Tapley Hill Formation. SHRIMP data on the youngest zircon population indicates a date of just less than 660 Ma; all concordant analyses. The rock is clearly volcaniclastic and a wide range of Proterozoic and older zircons also are present. Their ages can be easily and rapidly determined in a minimally destructive manner by the ion microprobe. However the ca. 658 Ma date for zoned igneous zircon places a new maximum constraint on the timing from within the Sturtian succession in South Australia. The date requires that glacially influenced sedimentation continued after 658 Ma, and thus prolongs the time interval of worldwide "Sturtian" glaciation from ca. 710 to at least ca. 658 Ma. This date also indicates that the hypothesized synchronous “Sturtian” Snowball Earth events are very problematic.