Abstract

The Tongwane Formation (~2.4 Ga) conformably overlies banded iron formations (BIF; Penge Iron Formation) on the Kaapvaal Craton, South Africa. As such, it provides a unique window into depositional processes and environmental conditions in the aftermath of major Archean-Paleoproterozoic BIF deposition, and on the eve of irreversible environmental oxygenation in the Great Oxidation Event (GOE, ~2.35 Ga). This study presents the first sedimentological and bulk-rock geochemical characterization of the Tongwane Formation to provide a sedimentological and stratigraphic framework for further studies of early Paleoproterozoic environments.

The Tongwane Formation is 220 m thick and consists from the base up of shales, siliceous mudstones with local BIF facies, interbedded mudstones and dolomites, and a massive dolomite unit at the top. Strata record the progressive shallowing of depositional environments from deep shelf (BIF) to a wave-swept carbonate ramp. Intervening slope environments record increased detrital sedimentary input in the form of shales and distal turbidites. The carbonate ramp had a distally steepened margin as documented by an important margin collapse breccia. Extension due to seismic forces and/or slope steepening caused progressive deformation of slope deposits, from slumping and fracturing through sedimentary boudinage, to brecciation, and mass wasting.

Termination of BIF deposition could have been related to (a) shutdown of Fe-precipitating processes, (b) shutdown of the hydrothermal Fe source, (c) shallowing of environments to restrict BIF deposition to deeper parts of the basin, (d) masking of Fe deposition by increased detritus, or a combination of these. Although a partial or complete shutdown of the Fe source or of Fe precipitating processes cannot be excluded, the weight of evidence from the Tongwane Formation favors external factors such as relative sea level fall and Fe dilution by increased detrital input as the main drivers for the BIF-carbonate transition. All samples fall on a mixing curve between hydrothermal and detrital end members, and despite metamorphic overprint, a weak hydrothermal signature is observed up to below platform deposits. These results stress the importance of understanding sedimentary factors in studies of Archean-Paleoproterozoic environments.

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