Tonian basins record rifting of Kalahari from Rodinia and no evidence of a pre-Sturtian Kaigas glaciation

Abstract

Neoproterozoic strata record evidence for two Snowball Earth glaciations during which ice extended to the equator. Geochronological data have confirmed the multi-million year duration, synchroneity, and global extent of the Cryogenian Sturtian and Marinoan glaciations between 717 and 635 Ma. It has been proposed that an earlier ca. 750 Ma glaciation, referred to as the Kaigas glaciation, included glaciomarine deposits that formed at low-latitude. Here we evaluate this hypothesis at the eponymous location with detailed stratigraphy and geochronology through the Kaigas, Rosh Pinah, and Numees formations on the western margin of the Kalahari craton in southern Namibia. We find that glacial deposits previously assigned to the Kaigas Formation are instead ca. 717–661 Ma diamictites of the Sturtian Numees Formation. Pre-Numees strata, including the Kaigas Formation, host facies associations diagnostic of fan delta deposition along an active normal fault. Interbedded volcanic rocks in the Rosh Pinah Formation overlying the Kaigas Formation were dated with U-Pb CA-ID-TIMS on zircon at ca. 752 Ma. These Tonian deposits are interpreted to have been deposited in an active rift basin without evidence for glaciation. Rosh Pinah magmatism could be broadly correlative with the Mount Rogers Complex in Virginia, USA, consistent with a scenario of the Kalahari craton actively rifting from Laurentia and associated terranes within 20° of the equator at the time. We conclude that, at least in marine settings, evidence for low-latitude glaciation is limited to the 717–635 Ma Cryogenian Period.