CORRELATION OF NEOPROTEROZOIC SUCCESSIONS BY ANY MEANS? - COMMENTS ON INTERPRETATIONS MADE BY PRAEKELT ET AL. (2008)

Abstract

Praekelt et al. (2008) add important data concerning the sedimentology of the Cango Caves Group, a critical sedimentary succession of Neoproterozoic age in southern South Africa. They discuss an unconformity in its lower part (between the Kombuis Member of the Matjies River Formation and the Groenefontein Formation) as being a sub-aerial karst surface and argue that this contact is related to glacial processes. Therefore, it is claimed that the unconformity can be used as a correlation horizon on regional scale. Besides the importance of correlation of sedimentary succession, this specific exercise by Praekelt et al. (2008) is part of the efforts to introduce glacial events as correlation horizons especially during the Neoproterozoic in southwest Gondwana. The authors propose therefore a correlation with, in their opinion, glacial deposits of the Nama Group, which delivers the age constraint, as stratigraphically related tuff horizons were dated at c. 548 Ma (Grotzinger et al., 1995). The authors also inform us about radiometric data, which shall confirm the Groenefontein unconformity as being dated, and that in South America unconformities of similar age and sedimentology (glacial and karst deposits) were found and dated. The correlation of these inferred glacial beds together with their ‘age constraints’ led to the proposition that a glacial event affected the entire region of southwest Gondwana during the uppermost Ediacaran. In summary, the interpretation made by Praekelt et al. (2008) and its consequences are based on the opinion that the correlated horizons (which are in some cases interpreted to be karst horizons) (i) are related to glacial environments and of regional significance (southwest Gondwana-wide) and (ii) have well constrained ages. I will demonstrate here that the sedimentology of nearly all referenced correlation horizons is not understood in terms of a glacial origin and that, with the exception of one deposit, reliable age …