Carbonate chemostratigraphy of the Vazante Group, Brazil: A probable Tonian age

Abstract

The thick carbonate-siliciclastic succession of the Vazante Group was deposited on the western border of the São Francisco craton, central Brazil, which was deformed during the Brasiliano Orogeny that ended at 540 Ma. In this research, we studied five drill cores and outcrops to obtain integrated isotope data (δ13Ccarb, δ18Ocarb, and 87Sr/86Sr) as well as U-Pb ages of detrital zircons to understand the chronostratigraphic relationships between the Tonian and Cryogenian periods. The Vazante Group forms a succession of more than 3 km thick of limestones, dolostones, shales and rare sandstones. These rocks were deposited on a marine shelf that shows distinct δ13C variations throughout the stratigraphic succession. The lower limestone of this group (Lagamar Formation) has low positive δ13C values (0.3–1.0‰) and 87Sr/86Sr ratios of 0.70646–0.70692 (with 423–1,719 ppm Sr), and possible vase-shaped microfossils (VSMs) that may be equivalent to those found in Huaibei Group in northern China (lower Tonian) or has stratigraphic correlations to the VSMs-bearing units of the upper Tonian. The Serra do Poço Verde Formation has a plateau of moderate δ13C positive values (0.0–4‰) followed by dolostones of the Morro do Calcário Formation, which is a 50-m-thick interval with negative values (as low as −4‰). The upper unit of the Vazante Group (Serra da Lapa Formation) begins with a sharp negative δ13C excursion locally overlying a few layers of diamictite, which may be the equivalent of the Sturtian glacial age. Restrictions on establishing the depositional age of the Vazante Group continue; however, the combination of biostratigraphic and chemostratigraphic data and the detrital zircons spectra suggest a Tonian age for the Vazante Group.