Paleoenvironmental reconstruction of the Ediacaran Araras platform (Western Brazil) from the sedimentary and trace metals record

Abstract

Ocean and atmosphere oxidation in the Ediacaran Period paved the way for the dawn of animals. In an attempt to better document the record of seawater redox state at the onset of the Ediacaran, we performed a paleoenvironmental study of post-Marinoan (635Ma) carbonate strata from the southeastern margin of the Amazon craton, western Brazil. Five sections were sampled along the Araras carbonate platform. Outcrop-based facies analysis, complemented by petrographic description of representative samples, was performed on these sections. Seven facies associations (FA) were recognized. Four FA are encountered in the inner-shelf, from the basal glacio-marine deposits of the Puga Formation to shallow and moderately deep platform facies, which are systematically covered by deeper CaCO3 over-saturated facies. Five FA occur on the outer-shelf, including storm-wave influenced facies, below storm-wave base facies and slope intraformational breccia. These facies associations indicate a transgressive systems tract over the carbonate platform. Trace metals (U, Mo, Zn, Pb, Cd, Cu, Ni, V) and Al concentrations, pyrite abundance and total organic carbon (TOC) contents in these sections are generally low. They nonetheless present, from base to top of the sections, significant stratigraphic variations which can be traced along the platform, with successive enrichments of (i) Pb and Zn, (ii) U, and (iii) both U and Mo in the thin marl levels containing the highest amount of organic carbon and pyrite (0.4% and 1.9%, respectively). We interpret this succession to record the progressive evolution of sediment pore-waters from oxic to more reducing conditions, driven by an increase in sedimentary organic matter accumulation. The lack of evidence for persistent sulphide mineralization and associated enrichments in Mo suggests that sulfidic conditions were only achieved in sediment pore waters. In the aftermath of the Marinoan glaciation the water column must then have been essentially oxic on the Araras platform, with anoxia only sporadically reaching the sediment–water column interface in the deepest parts of the platform.