An earliest Ediacaran oxygenation episode in the Wilpena Group, Adelaide Superbasin, South Australia

Abstract

The first appearance of animals during the Ediacaran is arguably related to an increase in oceanic oxygenation during this time. However, there is considerable ambiguity in the global record of Ediacaran oxygenation, making it difficult to assess the potential links between oxygen and metazoan evolution. Here, we examine the earliest Ediacaran Nuccaleena Formation cap dolomite and basal Brachina Formation of the Adelaide Superbasin, South Australia, to determine the redox landscape in which these units were deposited. Red shales are present at the base of the Brachina Formation (lower Moolooloo Siltstone Member) over much of the Adelaide Superbasin but these transition laterally into green shales in the north, correlating with a facies transition into a deeper water setting.

Fibrous dolomite cements within sheet cavities of the Nuccaleena Formation cap dolomite display evidence of a primary marine origin. Cathodoluminescence microscopy and laser ablation ICP-MS trace element analysis of these marine dolomite cements indicates a transition from an oxic environment in the south (with low Fe, Mn, and a Ce anomaly) to an anoxic (ferruginous) setting in the north (with high Fe, Mn and no Ce anomaly). This cap carbonate trace element geochemistry is spatially consistent with the overlying red to green shale transition in the basal Brachina Formation. Together, these data suggest the existence of a deep-water chemocline in this basin, separating an oxic upper water column from a ferruginous deeper water mass. This oxic interval directly post-dates the end-Cryogenian Marinoan Glaciation and is synchronous with an earliest Ediacaran oxygenation event previously described from South China.

This evidence from the Adelaide Superbasin provides direct evidence for an earliest Ediacaran oxic water mass penetrating to a substantial paleodepth. The synchronous development of oxic intervals in both Australia and South China supports the notion of a globally developed oceanic oxygenation event and is consistent with the hypothesis that Ediacaran continental margin settings were periodically bathed in oxic water, conducive to the evolution of metazoans.