U-PB ZIRCON GEOCHRONOLOGY OF THE PALEOPROTEROZOIC TRANSVAAL SUPERGROUP, SOUTH AFRICA

The Paleoproterozoic is characterized by major changes in Earth’s surficial environments, which are currently preserved in multiple sedimentary sequences around the world (e.g. occurrence of widespread glaciations, the Great Oxidation Event, milestones in the evolution of life, etc.). Precise absolute time constraints are of paramount importance for establishing timing and duration of these geological processes, and subsequently understanding the mechanisms that caused them. Analytical uncertainties of Paleoproterozoic samples usually exceed 10 Myr; consequently the early surficial evolution of the Earth can be obfuscated by imprecision, making it difficult to interpret their extension in time and space.

Much of current understanding of changes in depositional environments or settings during the Paleoproterozoic come from the Transvaal Supergroup in South Africa, which is a mega sedimentary sequence hosting major igneous events. Here, we present new geochronological data from samples in the Supergroup. Zircon from the youngest clusters of in-situ LA-ICP-MS U-Pb analyses from detrital clastic sediments were further analyzed using higher-precision techniques (CA-ID-TIMS). We provide new maximum depositional ages (MDA) for the Naragas , Heynskop , and Makganyene formations, and a crystallization age for the Hekpoort Formation.

These new ages provide a consistent temporal framework for the Transvaal Supergroup. Our results from the Koegas Subgroup provide a time constraint on the anoxic conditions previously evidenced by the sulfur mass-independent fractionation signal in the Heynskop Formation. Our age for the Makganyene Formation confirms that it is among the oldest known Paleoproterozoic glacial deposits, rather than the youngest as previously interpreted. Our MDAs from CA-ID-TIMS are ~200 Ma older than previous in-situ LA-ICP-MS data demonstrating the importance of accuracy in U-Pb geochronology of detrital zircon in Paleoproterozoic sediments. Another implication of our data is that the proposed correlation between the ~2.4 Ga Ongeluk Formation and the ~2.2 Ga Hekpoort Formation is incorrect, and therefore does not represent a single Large Igneous Province as previously proposed.