GEOCHRONOLOGY REVEALS MULTI-MILLION YEAR DURATION OF THE END-CRYOGENIAN (MARINOAN) GLACIATION, NAMIBIA

Cryogenian (850-635 Ma) time is characterized by at least two global glaciations in which ice sheets existed at sea level in the tropics and oceans worldwide were covered by thick sea ice. The duration of the older glacial episode (Sturtian) was c. 55 My (from c. 717-662 Ma) based on U-Pb and Re-Os geochronology from the same continental margin in northwest Canada (Macdonald et al. 2010, Science; Rooney et al. 2014, PNAS). U-Pb zircon age dates from disparate continental margins in Namibia (Hoffmann et al. 2004, Geology), South China (Condon et al. 2005, Science) and Tasmania (Calver et al. 2013, Geology) have established both the global nature and synchronous termination of the younger glaciation (Marinoan) at c. 635.5 Ma, however, the timing of initiation, and thus duration, of that glaciation has yet to be constrained.

Here we report new CA-ID-TIMS U-Pb (zircon) data obtained from ash beds within Marinoan-equivalent deposits of northern and central Namibia. One new age comes from one of several (0.5-15.0 cm thick) felsic ash beds within an interval of discontinuous stratified ice-rafted debris layers between massive diamictites of the Ghaub Formation deposited at the Otavi shelf-slope break along the southern margin of the Congo craton; the age is c. 639.3 Ma. We have also reanalysed zircons from the previously dated felsic ash bed in the upper part of the Ghaub Formation from basinal equivalent rocks (Hoffmann et al. 2004, Geology) in order to calculate their age using the results of new U-Pb calibration experiments and pre-treatment methods to minimise the impact of Pb-loss, and that confirms the reported age of c. 635.5 Ma. A third age comes from volcaniclastic strata at the top of the Blässkranz Formation, which represents the younger glacial unit deposited along the continental margin of the Kalahari craton in south-central Namibia; this age is c. 636 Ma. Together with published dates from China and Tasmania, these new age dates corroborate the synchronous deglaciation at c. 635 Ma and provide the first constraints on the duration of the end-Cryogenian glaciation of at least c. 4 million years.