Globally Synchronous Marinoan Deglaciation Indicated by U-Pb Geochronology of the Cottons Breccia, Tasmania, Australia
U-Pb zircon data from the uppermost Cottons Breccia, representing the Marinoan glacial-postglacial transition on King Island, Tasmania, provide the first direct age constraint on the Cryogenian-Ediacaran boundary in Australia. Zircons in four samples from the topmost meter of the Cottons Breccia, dated by sensitive high-resolution ion microprobe, exhibit two modes ca. 660 Ma and ca. 635 Ma. The younger component predominates in the uppermost sample, a possibly volcanolithic dolomitic sandstone, apparently lacking glacially transported debris, in the transition to cap carbonate. Chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb dating of euhedral zircons from that sample yields a weighted-mean age of 636.41 ± 0.45 Ma. Equivalence to published TIMS ash bed dates from Cryogenian-Ediacaran transitional strata in Namibia (635.51 ± 0.82 Ma, within glacial deposit) and China (635.23 ± 0.84 Ma, 2 m above glacial deposit) supports correlation of those strata to the Australian type sections and globally synchronous deglaciation at the end of the Cryogenian Period.
Calver, C. R.; Crowley, J. L.; Wingate, M. T. D.; Evans, D. A. D.; Raub, T. D.; and Schmitz, M. D.. (2013). "Globally Synchronous Marinoan Deglaciation Indicated by U-Pb Geochronology of the Cottons Breccia, Tasmania, Australia". Geology, 41(10), 1127-1130. https://doi.org/10.1130/G34568.1