Identification of a Sturtian cap carbonate in the Neoproterozoic Sete Lagoas carbonate platform, Bambui Group, Brazil

A sedimentological and C-O isotopic study has been carried out in nine sections of the Sete Lagoas Formation at its classical outcropping area, in the southern tip of the Sao Francisco craton (central Brazil), with the objective of refining its stratigraphic position within the Neoproterozoic. At the study area, the Neoproterozoic Sete Lagoas Formation comprises two shallowing-upward megacycles, corresponding to more than 200 m in thickness. Each cycle is limited by a flooding surface amalgamated with a third-order sequence boundary. The first megacycle presents deep-platform deposits with abundance of crystal fans (aragonite pseudomorphs). These deposits are characterized by negative C-isotope values (-4.5%(0)). They grade upward to storm-wave and tide-influenced layers with delta C-13 values around 0%(0). In the second megacycle, a new transgression drowned the platform, depositing a thick, mixed sub-storm wave-base succession. This megacycle comprises deposits of lime mudstone-pelite rhythmite, which grade to crystalline limestone rich in organic matter, both with unusually positive delta C-13 values (up to + 14%(0)). Regional correlation of Sete Lagoas deposits indicate that they rest atop glaciomarine rocks of the Macaubas Group and basal strata show seafloor precipitates with negative delta C-13 values. Therefore, it is possible to characterize the Sete Lagoas carbonate as a cap carbonate sequence. The very high 613C in the second megacycle together with geochronologic data suggest that this unit correlates better with post-Sturtian sequences. Some differences in the depositional record are observed between Sete Lagoas and the other post-Sturtian units previously described in North America, Australia, and Namibia. Those differences may in part be due to deposition in shallower settings of the Sete Lagoas carbonates, thus preserving a thick record of storm- and wave-influenced sedimentation not found elsewhere. Alternatively, they may also be attributed to diachronic deposition of the so-called post-Sturtian cap carbonate sequences.