Coevolution of life and environment during the Ediacaran-Cambrian transition: Clues from litho-, bio-, and chemostratigraphy of the Corumbá Group, Brazil

Abstract

This study presents new litho-, bio-, and chemostratigraphic data of the late Ediacaran Corumbá Group (Brazil) obtained from two drill cores that were collected as part of the GRIND-ECT project (Geological Research through Integrated Neoproterozoic Drilling – Ediacaran-Cambrian Transition). This is the first study encompassing, continuously, the Bocaina (ca. 570–555 Ma) and Tamengo (ca. 550–540 Ma) formations, which are key carbonate units from the upper Corumbá Group and optimal archives for investigating the ECT in Western Gondwana. Sequence stratigraphic analysis reveals two third-order transgressive–regressive cycles. A sequence boundary at the Bocaina-Tamengo Limit (BTL) formed during a major, potentially glacio-eustatic, regression, after which the morphology of the carbonate platform changed from a rimmed/open shelf to a distally-steepened ramp. Biostratigraphic analysis constrains the lowest regional occurrence of Cloudina shortly above the BTL. Four species of acritarchs from the genus Leiosphaeridea and Germinosphaera are documented in the Tamengo Formation, revealing an association with the Late Ediacaran Leiosphere Palynoflora. Cloudina and Leiosphaeridea show biostratigraphic trends linked to third-order cycles, being more abundant during shallower intervals. The δ13Ccarb curve in the Bocaina Formation exhibits a relatively stable trend around +2 ‰, followed by a minor negative excursion (n1) down to −1.7 ‰ at the BTL, which is also present in other sections on the Corumbá platform. Above n1, the Tamengo Formation exhibits a positive excursion reaching values of +6 ‰. Confidence in precise correlation between the global carbon isotope curve and the Corumbá Group curve is somewhat hampered, given the uncertain age of the lowermost Bocaina Formation. It is possible that n1 is temporally equivalent to the globally documented negative excursion at 550 Ma, suggesting that these excursions, in part, result from diagenetic alteration or local effects that were exacerbated during a major glacio-eustatic regression. Despite having values consistent with the Late Ediacaran Positive carbon Isotope Plateau, the Tamengo Formation lacks negative excursions that may exist in the global curve, such as A0, which may cast doubt on the global nature of this event. The links between sequence-, bio-, and chemostratigraphy presented herein indicate an intricate coevolution of life and environment during the ECT.