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

Abstract

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 Sa ˜o 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 shallowingupward 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%). They grade upward to storm-wave and tide-influenced layers with d 13 C values around 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 d 13 C values (up to + 14%). Regional correlation of Sete Lagoas deposits indicate that they rest atop glaciomarine rocks of the Macau ´bas Group and basal strata show seafloor precipitates with negative d 13 C values. Therefore, it is possible to characterize the Sete Lagoas carbonate as a cap carbonate sequence. The very high d 13 C 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.

C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

“…They confirm δ 13 C values as representative of the isotopic seawater composition at the time of deposition. These isotopic data correlate with the δ 13 C values obtained by Santos et al (2004), Vieira et al (2007b) and Paula-Santos et al (2015), which suggest that the SL and HV sections are positioned above the characteristic C isotopic shift from the upper Sete Lagoas Formation (Fig. 5).…”

“…According to the paleogeographic position of the carbonate sequences with Cloudina in Brazil, Antarctica, Namibia, Argentina and Uruguay, an oceanic connectivity scenario of coeval intracratonic basins in the late Neoproterozoic can be suggested (Grotzinger et al, 2000;Warren et al, 2014). These latter authors obtained δ 13 C values between 0.06 and 0.67‰ (V-PDV) in the fossiliferous carbonates, which place the Cloudina in the middle portion of the Sete Lagoas Formation, according to stratigraphic correlation with C isotopic data from Misi et al (2007), Vieira et al (2007b) and Paula-Santos et al (2015).…”

“…These values are interpreted as representative of the depositional environment and can be correlated with those obtained by Santos et al (2004) and Vieira et al (2007b) in carbonates from the first depositional sequence of the Sete Lagoas Formation, which were above the cap carbonate with negative δ 13 C values around -4.5‰ and below the positive C isotopic shift (Fig. 5).…”

“…It is identified as a cap carbonate of Sturtian age (Babinski et al, 2007;Vieira et al, 2007b). This unit shows strong negative δ 13 C values of -5‰ at the base that increase to 0‰ in the intermediate part of the sequence, and abruptly change to very positive values as high as +16‰ at the top, accompanied by alterations on the sedimentary facies.…”

“…This unit shows strong negative δ 13 C values of -5‰ at the base that increase to 0‰ in the intermediate part of the sequence, and abruptly change to very positive values as high as +16‰ at the top, accompanied by alterations on the sedimentary facies. This enables the subdivision of Sete Lagoas Formation into lower and upper sequences separated by a positive carbon isotopic shift (Vieira et al, 2007b). However, some authors propose that the Sete Lagoas Formation represents a cap carbonate sequence of early Ediacaran age deposited after the Marinoan glaciation, based on global correlations via isotope chemostratigraphy (i.e.…”