Bio- and Chemostratigraphic Constraints Establish an Early Ediacaran Age for the Volta Basin in Ghana

Ornamented, organic-walled microfossils are important for understanding eukaryotic evolution in the aftermath of the Snowball Earth and prior to the rise of macroscopic life. They are also useful to biostratigraphically constrain age of and correlate Ediacaran successions. Here we report an assemblage of Doushantuo-Pertatataka acritarchs (DPA) from the Volta Basin in Ghana, a part of the West African Craton. The Neoproterozoic Ota-Pendjari Group contains tillite and cap carbonates at the base, overlain by approximately 1-km-thick green to grey mudstones. Biostratigraphic and chemostratigraphic (δ13C and δ15N) data for the Kodjari and Pendjari formations were integrated to provide age constraints and correlations with other Ediacaran basins. The Kodjari carbonates have negative carbonate carbon isotope values, consistent with those for Marinoan cap carbonates. The fossiliferous samples are from the middle megasequence, the Pendjari Formation. Palynological acid-maceration yielded DPA microfossils characteristic of lower Ediacaran strata and compositionally most similar to assemblages of South-Central Australia and Siberia. The assemblage contains 26 taxa, dominated by acanthomorphs Tanarium conoideum and Variomargosphaeridium litoschum. Species richness declines upsection. The assemblage broadly corresponds to Tanarium-Schizofusa-Variomargosphaeridium zone established in Australia. However, in Ghana, some taxa diagnostic of other zones (Ceratosphaeridium, Appendisphaera) appear throughout the sequence together with this assemblage. Organic C and N isotope data suggest deposition in an oxic setting. These data establish the early Ediacaran age for the Pendjari Formation and confirm the global stratigraphic significance of DPA. However, this also shows that biozonation established in Australia and South China might not be entirely applicable to other Ediacaran sedimentary basins. The Pendjari Formation microfossils further demonstrate a rapid diversification of eukaryotic microbiota following the Marinoan glaciation.