Chemostratigraphy of Neoproterozoic Cap Carbonates from the Volta Basin, West Africa

Abstract

The Neoproterozoic Era includes some of the most extreme ice ages in Earth history. The exact number of glaciations is unknown, although there were at least two events of global reach and possibly an equal number of lesser advances. Neoproterozoic glacial deposits in West Africa have proven particularly difficult to correlate with better constrained successions elsewhere. In most West African successions, only a single glaciogenic deposit is present, generally within a stratigraphic 'Triad' of tillite, carbonate, and bedded chert; the age of these deposits and their synchroneity across the craton remain controversial. We report isotopic data from carbonates that cap tillites in the Volta Basin (Sud-Banboli Group). In the three sections measured, ∂¹³C (%o PDB) begins at similar to-2 to -3 and drops to -5 to -8, correlated with a drop in both ∂¹⁸O and Mg/Ca. This pattern-both the drop in ∂¹³C and its correlation with ∂¹⁸O and lithology-has been observed in Neoproterozoic cap carbonates on other cratons, suggesting that it reflects primary depositional variations. In addition, the strong positive correlation among lithology, ∂¹³C, and ∂¹⁸O suggests that lithological variation may have exerted a principal control on isotopic variation, implying that the dolomite formed in isotopic C equilibrium with seawater, as a primary precipitate or very early diagenetic replacement. This would require low levels of sulfate (estimated to be similar to 1 mM or less), consistent with available S-isotopic data, the presence of barite crusts in association with some cap carbonates, and the conditions of seawater chemistry hypothesized in the aftermath of a 'snowball Earth.' The stratigraphic pattern of carbon isotope variation in the Volta Basin cap carbonates suggests that underlying tillites are likely Marinoan (similar to 650-600 Ma) in age. This interpretation is supported by distinctive lithological characters observed in Volta Basin cap carbonates and accepted Marinoan correlatives. Tillites elsewhere on the craton have been assigned latest Proterozoic or Early Cambrian ages. Either the basis for these proposals requires reexamination, or West African tillites record more than one glacial event.