Petrography, modal composition, and tectonic provenance of some selected sandstones from the Swaershoek and Alma Formations (Waterberg Group) and Glentig Formation, Limpopo Province, South Africa: evidence from framework grains

The Swaershoek and Alma Formations (Waterberg Group) overlie the unconformity-bounded Glentig Formation. The Glentig Formation has been correlated with the Swaershoek and Alma Formations, but neither the source rock characteristics nor the tectonic provenance of these formations have been studied in the past. Hence, this study uses petrographic and modal composition analyses to decipher the provenance and tectonic setting of the Swaershoek, Alma, and Glentig formations. The Swaershoek, Alma, and Glentig Formations attained a maximum thickness of about 300 m, 190 m, and 400 m, respectively. Petrographic analysis indicates that the detrital components of the sandstones are dominated by monocrystalline quartz, feldspar, and lithic fragments. The sandstones are moderately to well-sorted with mostly sub-rounded to rounded grains and can be classified as subarkosic arenite and sub-lithic arenite. The QFL tectonic provenance diagrams show that the sandstones are largely derived from granites and granite–gneisses of a continental block tectonic provenance, suggesting that the sandstones were derived from stable shields and uplifted areas. These sandstones could have been derived from the whining stages of the emplacement of the Bushveld igneous complex. The QFL tectonic diagram also shows that the sandstones are related to trailing edge setting, where the continental margin is facing a spreading center or rifting, which resulted in the development of the Waterberg Basin on the trailing edge of the continent. In the regional context of the evolution of the Waterberg Basin, the results presented in this study inferred that the basin developed on a rift passive setting (trailing edge) of the stable continental margins.