Ancient Coastlines in the Continental Shelf of the State of Ceara, Northeast Brazil: Evidence from Sediment Chemistry and Stable Isotopes

摘要

Continental shelf sediments from the state of Ceara, northeastern Brazil, have been studied to investigate meteoric diagenesis of marine sediments of previously defined ancient coastal lines. Major elements for 131 samples of carbonate sediments and C and 0 isotopes for 51 carapace samples of benthonic foraminifers of the Amphistegina radiata and Peneroplis planatus species were analyzed. Geochemical signatures of delta~(13)C, delta~(18)O, SiO_2 (stoichiometrically calculated as quartz percent), Sr and Mn, and XRD document the impact of meteoric diagenesis on relict sediments from current depths of 25-30 m, 45 m, and 80 m. Morphologies of the marine shelf at these depths, interpreted from previous bathymetric profiles, correspond to holocenic marine terraces. Despite a recent sedimentary cover, it was possible to confirm that these terraces possess relict detrital, biogenic, and continental material, that resulted from erosional reworking of ancient coastlines during the Flan-drian transgression.