Synsedimentary fault control on the deposition of the Duitschland Formation (South Africa) : implications for depositional settings, Paleoproterozoic stratigraphic correlations, and the GOE

Abstract

The Paleoproterozoic Great Oxidation Event (GOE) marks the first significant oxidation of atmosphere and surface environments, and is causally associated with the global disappearance of mass-independent sulfur isotope fractionation (MIF-S). However, fundamental sedimentary aspects of sedimentary successions recording this event (e.g. depositional environment, tectonic setting and stratigraphic correlation) are poorly constrained and often debated, restricting full understanding of causes and effects related to the GOE. In South Africa, MIF-S disappears across the ‘mid-Duitschland unconformity’ (MDU) in the Duitschland Formation (Transvaal Supergroup). New sedimentological observations of the lower Duitschland Formation have identified up to 5 times thicker and more diverse chert-pebble conglomerates than previously documented. New facies observed include lenticular conglomerates which incise cross-bedded dolomites, and imbricated conglomerates. The overlying MDU is angular in nature, recording an ∼15° N dip of the lower Duitschland strata; elsewhere it possesses a disconformable geometry. A new depositional model is proposed where shallow-marine carbonate (ramp) deposition interfaced with wave-influenced Gilbert-type fan deltas in an isolated depocentre produced during synsedimentary faulting. There is no evidence that the MDU formed due to direct glaciation as proposed previously, however glacio-eustatic changes may have had an influence. This study supports lithostratigraphic correlations between the Duitschland and Rooihoogte formations which both register the MIF-S disappearance, but are considered separate lithostratigraphic units, which implies oscillations in MIF-S. The correlation proposed in this study implies a unique MIF-S signal and has important consequences for differentiating true spatiotemporal oscillations in MIF-S chemistry from artificial variations caused by unresolved stratigraphic relations.