Pongola Supergroup dolostones from the Buffalo River inlier, South Africa: facies, composition and implications for Mesoarchaean carbonate deposition

Abstract

The Mesoarchaean Pongola Supergroup of South Africa hosts the oldest succession of stromatolitic carbonates deposited in an intracontinental setting. In this study, carbonates within two (tectono-) stratigraphic units in the Buffalo River inlier were investigated. Stratiform stromatolite and wave-ripple-laminated dolostone facies are the most common and are interpreted to have formed in a tide-dominated shallow-marine environment. Dolostones typically contain a large amount of siliciclastic detritus, and small domal stromatolites are commonly found scattered in subtidal sandstone facies. The carbonates experienced deformation and greenschist facies metamorphic conditions at peak temperatures of ∼ 450 ± 50 °C. Analyses for major, trace and rare earth elements (REEs) along with carbon, oxygen and strontium isotopes have been undertaken on the best-preserved samples. Their REE + Y distribution patterns reflect their marine origin, with positive La_SN, Gd_SN and Y_SN anomalies, super-chondritic Y/Ho ratios, and depleted light REEs relative to heavy REEs. The δ¹³C_VPDB and δ¹⁸O_VPDB values least affected by diagenetic and metamorphic overprints are 2.2 ‰ and −16.1 ‰, respectively. The least radiogenic ⁸⁷Sr/⁸⁶Sr ratio is 0.704 and reflects isotopic exchange with a siliciclastic component. Despite their alteration, the carbonates of the Buffalo River inlier provide additional constraints on microbial carbonate deposition on the Earth oldest preserved craton. They can be traced laterally for several tens of kilometres and point to environmental conditions suitable for the deposition and preservation of marine carbonate rocks 3.0 Ga ago.