Chemostratigraphic constraints on early Ediacaran carbonate ramp dynamics, Río de la Plata craton, Uruguay

摘要

C- and Sr-isotope data for marine carbonates can provide a detailed record of isotopic variations in seawater through time and have proven to be a valuable tool for interpreting biogeochemical events and correlating Neoproterozoic sedimentary successions worldwide. Negative carbon isotope excursion to values as low as -4.5‰ in the Ediacaran Polanco Limestones Formation, Uruguay has been interpreted as recording the aftermath of a post-Gaskiers glacial event occurred elsewhere on southwest Gondwana. The record of both deep- and shallow-water settings in the Polanco Limestone Formation provides an opportunity to examine δ ~(13)C variability across the platform and to evaluate whether the carbon isotope values reflect local or global processes. Herein we provide high-resolution δ ~(13)C-chemostratigraphy and Sr-isotope data from stratigraphic sections comprising proximal and distal settings on the carbonate platform. Carbon isotopic values are negative in deep-water facies but progressively rise towards positive values in shallow-water settings. This previously unrecognized trend suggests that deposition of Polanco carbonates occurred in a stratified marine basin where degradation of organic matter below the redox boundary led to lower δ ~(13)C values at greater depths. The negative δ ~(13)C excursion is restricted to specific horizons in shallow-water facies and is interpreted as being originated due to higher levels of reworking under storm-dominated conditions, which produced a strong local oxidation of organic matter and ultimately, the negative carbon isotope signatures. 87Sr/ 86Sr and δ ~(13)C chemostratigraphy coupled with new radiometric data allow us to revise the previously proposed age for the unit. This new data support an age between 590 and 560Ma for the deposition of the Polanco Limestones Formation.