CHEMOSTRATIGRAPHY OF NEOPROTEROZOIC 'CAP' CARBONATES FROM THE VOLTA BASIN, WEST AFRICA

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 occurs, generally as part of a stratigraphic `triad' consisting 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, d<sup>13</sup>C (ppmil v. PDB) begins at ~-2 to -3 and drops to -5 to -6, correlated with a drop in both d<sup>18</sup>O and a transition from dolomite to limestone. This pattern -- both the drop in d<sup>13</sup>C and the correlation between d<sup>13</sup>C, d<sup>18</sup>O, 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 between lithology, d¹³C , and d¹⁸O suggests that lithological variation may be controlling isotopic variation, implying that the dolomite is either primary or very early diagenetic, precipitating in equilibrium with seawater. This would require low levels of sulfate, consistent with available S-isotope data, the conditions of seawater chemistry hypothesized in the aftermath of a `snowball Earth' and the presence of barite crusts in association with some cap carbonates.

The pattern of carbon isotope variation in the Volta Basin `cap'carbonates suggests that underlying tillites may be Marinoan in age. This interpretation is supported by distinctive lithological characters observed in Volta Basin `cap' carbonates and other Marinoan correlatives. Tillites elsewhere on the craton have been suggested to be latest Precambrian or Early Cambrian in age. Either the basis for these proposals requires reexamination, or West African tillites record more than one glacial event, and the `triad' is diachronous.