Ediacaran/Early Cambrian Serra da Saudade Formation, Bambuí Group: The sedimentary record of a foreland basin in Southeastern Brazil

Abstract

The Serra da Saudade Formation, Ediacaran/Cambrian Bambuí Group, cropping out along the ridge of the same name in Central-Western Minas Gerais, Brazil, contains important deposits of glauconitic siltstones associated with phosphatic rhythmite as well as other siliciclastic, volcaniclastic and carbonate lithofacies. Lithofacies indicate deposition by proximal and distal storm waves on a continental marine shelf. The basal gray siltstone and part of the green siltstone represent a transgressive systems tract. The glauconitic siltstone indicates sea level rise, with the maximum flooding surface located approximately at the interval of highest K2O content. Biogenic carbonate and an upward increase in sandstone beds are interpreted as a highstand systems tract. Gradual lateral facies changes reflect the transition from foredeep (to the west) to forebulge (to the east) deposition in a foreland basin as the final stage of a shallowing-upward second-order sequence. Black organic-rich limestone has δ13C values between -8.78 and 9.79‰, negative δ18O values from -11.50 to -9.35‰, and 87Sr/86Sr around 0.7075. This isotopic signature, similar to that of other Bambuí carbonates, indicates a probably-restricted depositional environment with organic sedimentation, methanogenesis and high evaporation rates from the Ediacaran through the early Cambrian period.