The age and correlation of the Postmasburg Group, southern Africa: Constraints from detrital zircon grains

Abstract

Age determinations were conducted on detrital zircon grains from two stratigraphic levels in the Postmasburg Group, Transvaal Supergroup – the top of the Makganyene Formation and the base of the Hotazel Formation – to constrain the age of the group and for comparison with purported correlatives in the Segwagwa Group of the Kanye basin and Pretoria Group of the Transvaal basin. Detrital zircon grains are interpreted as being derived mainly from the underlying Ghaap Group and its basement of Ventersdorp Supergroup and Kraaipan granite/greenstone rocks, indicating a proximal source to the east and north on the Vryburg arch. The maximum age of the Postmasburg Group was constrained by the youngest detrital-zircon age to 2436±7Ma and the broad age range of the group, as a consequence, to somewhere between 2.43 and 2.38Ga. Comparison with various stratigraphic units in the Kanye and Transvaal basins indicated close similarities in zircon populations with the Duitschland Formation in the Transvaal basin. The Pretoria and Segwagwa Groups contain younger zircon populations at 2350–2320 and 2240–2200Ma that are related to syn-depositional volcanic activity and are not present in the Postmasburg Group samples, indicating the younger age of these groups. Based on compatibility of zircon populations, new stratigraphic correlations between the upper groups of the Transvaal Supergroup in the Griqualand West, Kanye and Transvaal basins are proposed. These include the probable restriction of the lower Timeball Hill – Hekpoort portion of the Pretoria Group to the Transvaal basin. An older age for the Postmasburg Group – ≈2.4Ga as opposed to the generally accepted 2.22Ga – will affect various models proposed for the evolution of the Earth’s atmosphere/hydrosphere that have been based on studies of rocks of the group, supporting an older ≈2.4Ga age for global oxidation transformations.