THE NEOPROTEROZOIC OXYGENATION EVENT AND BAMBUI GROUP, CENTRAL BRAZIL

The Bambui Group is a ca. 2000-m-thick Cryogenian-Ediacaran mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin of central Brazil. It records a critical interval of Earth history that began with the Marinoan Snowball glaciation followed by a prolonged period of interglacial conditions with increasing atmospheric and oceanic oxygen concentrations during the Neoproterozoic Oxygenation Event (NOE). Detailed analysis of Bambui Group stratigraphy distinguishes six unconformity-bounded depositional sequences reflecting the onset, expansion, and apex of the NOE.

Sequence 1 is composed of Marinoan glacial deposits (ca. 635 Ma) and black shales recording precipitation of pyrite from an euxinic water column. Sequence 2 contains phosphorite that accumulated in coastal environments beneath photosynthetically produced oxygen oases. The widespread occurrence of stromatolites in subtidal carbonates of Sequence 3 implies expansion of oxygenated conditions. Sequence 4 is dominated by basinal pyritiferous siltstone and black shale, suggesting that distal environments were still anoxic. Bacterial sulfate reduction produced framboidal pyrite with δ³⁴S values (+10‰ to +24 ‰) typical of sulfate limitation in Ediacaran seawater and pore water. Sequence 5 is formed of stromatolitic carbonate that reflects persistent oxygenation to at least the depth of the photic zone. Sequence 6 is characterized by shoreface and correlative basinal glauconitic siltstone recording the eventual ventilation of deeper water environments after the Gaskiers Glaciation (ca. 580 Ma).

In each of the six sequences, vertical and lateral stratigraphic trends of redox sensitive lithofacies are interpreted to record the progressive expansion and deepening of the oxygen chemocline in the Sao Francisco Basin through time. Comparison with other Neoproterozic stratigraphic sections in Namibia, Australia and Oman emphasizes local depositional and oceanographic controls on sequence architecture, but also highlights the imprint of the NOE on basin fill. Thus, this revised stratigraphic analysis of the Bambui Group provides a previously unrecognized high-fidelity record of Neoproterozoic environmental change that is an important counterpoint to other type sections.