Isotope stratigraphy of the uppermost Vazante Group, Brazil: Insights on paleoenvironments and regional correlation

Rocks of the Vazante Group in the Morro Agudo region (SE-Brazil) integrate a thick clay-carbonate sequence composed of twelve sedimentary facies and four facies associations (FAs), deposited in a mixed marine passive margin environment. This work carried out systematic isotope analyses of δ13C and δ18O on the two upper stratigraphic units of the Vazante Group (Morro do Calcário and Serra da Lapa formations) and on the Serra do Landim Formation. Despite the pervasive secondary dolomitization and hydrothermal overprint, most samples can be classified as least-altered dolostone, with some exceptions. Samples of Serra do Landim Formation are the ones with the highest degree of alteration, together with a small group of dolomites collected in a sphalerite-mineralized zone, at the top of the Morro do Calcário Formation. The isotopic profiles and the facies analysis showed the Morro do Calcário Formation presents homogeneous values of δ13C, and the Serra da Lapa Formation negative values of δ13C at the base, upwards inflected towards positive values. These coupled data suggest a significant change of environment in the depositional setting and conditions between these two formations, passing from calmer and stable (Morro do Calcário) to energetic and disturbed (Serra da Lapa). The strong negative values of δ13C, namely Lapa anomaly, are associated with diamictite facies below thick layer of carbonates, suggesting the influence of the Sturtian glacial event, in the lower Serra da Lapa Formation. The Lapa anomaly is the best chemostratigraphic marker throughout the sequence and guides the regional correlation of the Vazante Group. The basal diamictites of the Serra da Lapa Formation include dolomitic fragments of Morro do Calcário facies, indicating an intra-basinal source. Based on the isotopic profile correlation, it is inferred that the upper units (Morro do Calcário and Serra da Lapa formations) of the north and south portions of the Vazante Group were deposited in the same basin, or in connected basins, possibly separated, in part, by the Rio Escuro basement High.