Estratigrafia e geocronologia da Formação Serra da Saudade (Grupo Bambuí): registro de bacia de antepaís no Ediacarano/Cambriano

Abstract

The Serra da Saudade ridge, located between the municipalities of São Gotardo, Matutina, Santa Rosa da Serra and Cedro do Abaeté, Alto Paranaíba region, Minas Gerais State, is the better exposure of the Serra da Saudade Formation, Bambuí Group. In this region outcrops green glauconitic siltstones, with thickness varying from 50 to 80 m, that have potential as potash source for agriculture. There were differentiated two lithofacies, glauconitic siltstone and green siltstone. The glauconitic siltstone is dark-green and shows K2O grades up 10% and 40 to 80% of glauconite. The green siltstone, popularly known as verdete, contains minor amounts of glauconite (<37%) and K2O grades from 6.0 to 7.3%, occurring interbedded to rhythmites and sandstones. Due to the glauconite geochemistry and its crystalline structure, in which potash is easily available to soil as result of the cation exchange, glauconitic siltstone could be used as potash source for agriculture. This work performed the first U-Pb direct dating of the Bambuí Group from zircon grains recovered from a lenticular layer of a volcaniclastic rock with 0.5 m-thick, interbedded within the glauconitic siltstones, identified on drill cores. A total of 107 U-Pb ages were obtained by LA-ICP-MS. The provenance signature comprises a wide range of detrital zircon ages (2.8 to 1.0 Ga). 10 highly concordant, prismatic zircon grains, clustered into a well constrained age of 520.2  5.3 Ma, represent the depositional age of the upper Serra da Saudade Formation, indicating that the upper Bambuí Group is Cambrian in age. A detailed stratigraphic analysis (sedimentary facies) was performed and recognized the following lithofacies: massive and laminated siltstones, silty-clay rhythmites, green and glauconitic siltstones, volcanoclastic rock, sandstones with hummocky cross-stratification, phosphatic rhythmites, reworked carbonates (calcarenites) and a black organic-rich micritic limestone. The W-E lateral facies changes reflect the transition from foredeep to forebulge deposits in a foreland basin, and the transition from a deeper to a shallower marine platform. A transgressive system tract (TST) was identified at the base and at the uppermost part, a regressive system tract (RST), separated by a maximum flooding surface (MSF), represented by the glauconitic siltstone. The black micritic limestone was sampled for isotopic analyses and display δ13C values between -9.26 and 9.79‰, negative δ18O values ranging from -11.50 to -9.35‰, and 87Sr/86Sr ratio around 0.7075. The isotopic signature, very similar to other limestones from Bambuí Group, indicates that the depositional environment was probable restricted from Ediacaran through early Cambrian period.