Chemostratigraphy of carbonates from the Minas Supergroup, quadrilátero ferrífero (iron quadrangle), Brazil: A stratigraphic record of early proterozoic atmospheric, biogeochemical and climatic change

Abstract

The ca. 2.42 Ga-old Gandarela Formation overlies the finely laminated Cauê Banded Iron Formation (BIF) of the Itabira Group, Minas Supergroup, Brazil and consists of red carbonate facies BIF grading upsection into buff dolomites and limestones, which are locally stromatolitic. Carbonates of the Gandarela Formation have δ13C values ranging narrowly from -1.6 to +0.4 permil versus PDB (n = 93). The Cercadinho Formation, at the base of the overlying Piracicaba Group, is a thick arenaceous succession with rare thin carbonates that have δ13C values ranging from +3.3 to +5.4 permil (n = 12). Stromatolitic dolomites of the overlying Fecho do Funil Formation have Pb-Pb carbonate age 2.11 ± 0.11 Ga (Babinski and others, 1995) that provide the minimum age for this unit and δ13C values ranging from +5.6 to +7.4 permil (n = 69). At present, we find no evidence in Brazil for the any of the three early Paleoproterozoic glacial events recognized in 2.45 to 2.22 Ga sedimentary successions in North America. Given the radiometric constraints, however it is possible that geological and geochemical traces of these ice ages are missing along the unconformity between the Gandarela and Cercadinho formations. If correct, carbonates of the Gandarela Formation may provide a record of the carbon isotope composition of the seawater prior to any of the Paleoproterozoic ice ages. Combined with other available δ13C data results from the Gandarela Formation suggest that seawater preceding the ice ages was not significantly enriched in 13C. On the other hand, the carbon isotopic signature of the Cercadinho carbonates and chemical composition of the Cercadinho arenites are consistent with deposition after the end of the glacial epoch and likely during the early stage of ca. 2.22 to 2.1 Ga carbon isotope excursion. The δ13C values of marine carbonates of the Fecho do Funil Formation are consistent with carbon isotope record of contemporaneous carbonate successions worldwide providing additional evidence for a global biogeochemical anomaly at ca. 2.22 to 2.1 Ga. These compositional and isotopic changes allow integration of the Minas Supergroup into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age.