U-Pb zircon date from the Neoproterozoic Ghaub Formation Namibia: Constraints on Marinoan glaciation

Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds. U-Pb zircon geochronology of an ash layer constrains the deposition of the glaciomarine sediments to 635.5 ± 1.2 Ma, providing an age for what has been described as a "Marinoan-type" glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the terminal Proterozoic (Ediacaran) system and period. Combined with reliable age constraints from other Neoproterozoic glacial units-the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland -these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during Neoproterozoic time with interglacial periods, characterized by prolonged positive δ13C excursions, lasting at most ∼50-80 m.y. © 2004 Geological Society of America.