- © 2006 Geological Society of South Africa
The Paleoproterozoic Voëlwater Subgroup in Griqualand West, South Africa, is an exceptionally well preserved succession of banded iron formation, braunite lutite and shallow marine limestone/dolostone. A detailed chemostratigraphic study was carried out on this succession, resulting in a comprehensive set of whole rock geochemical data, including stable isotope (δ13Ccarb, δ13Corg, δ18Ocarb) and 87Sr/86Sr isotope data. The results are in excellent agreement with previous investigations and reveal trends that correspond closely to mineralogical variations and reflect systematic changes in the depositional environment.
Total inorganic carbonate contents (TIC) and δ13Ccarb values increase systematically from manganiferous iron formation at the base of the succession to dolostones that mark the top of the Voëlwater Subgroup. These first order trends transgress across all lithologies and mirror the interplay of synsedimentary carbonate formation (utilizing dissolved marine bicarbonate), and Fe-Mn-rich carbonate formation during suboxic early diagenesis. First order trends are modulated by second order variations associated with lithological changes in the lower portion of the Voëlwater Subgroup, constituted by three symmetrical cycles of iron formation and braunite lutite deposition. These second order trends, delineated by TIC and δ13Ccarb, as well as δ18Ocarb and major element concentrations (Mn, Fe, SiO2) can be attributed to repeated physicochemical changes in the MnF-BIF depositional environment. Variations in 87Sr/86Sr isotope composition do not relate to lithological changes. Instead, carbonate-rich and/or Sr-rich samples of MnF and BIF with well-preserved early diagenetic carbonate textures define a tight cluster with 87Sr/86Sr signatures of ~0.703 to ~0.706, in good agreement with Paleoproterozoic oceanic crust. Carbonate- and Sr-poor BIF samples with markedly more radiogenic 87Sr/86Sr signatures (~0.711 to ~0.722), in contrast, show evidence of extensive post-depositional recrystallization, apparent effects of dia- or epigenetic fluid flow.
Chemostratigraphy and lithostratigraphy of the Voëlwater Subgroup are in good agreement with models that predict a layered Paleoproterozoic ocean, with a hydrothermally-dominated Fe-Mn-rich deep water mass, overlain by a mass of oxygenated shallow marine water. A facies distribution model for the Voëlwater Subgroup is proposed, taking into consideration not only the presence of these two isotopically distinct water masses, but also abundance and transport of organic carbon and shallow marine carbonate muds, as well as the transformation of primary iron- and manganese oxihydroxide precipitates into carbonate minerals during early suboxic diagenesis.