Isotope stratigraphic evidence for a neoproterozoic cap carbonate in Sierras Bayas group, Tandilia system, Argentina

Abstract

C-, O-, Sr-isotope data for undeformed and unmetamorphosed dolostones (40-80m thick) from the basal section of the Sierras Bayas Group, Tandilia system, Argentina, provide new useful information for regional and global correlations. In this sense, the model of its diagenetic evolution is consistent with trends of primary marine origin.Detailed chemostratigraphic correlations between four complete profiles of dolostones of the Villa Mónica Formation allow to identify three sections, from base to top, regarding C-O trends, sedimentary facies in its marine paleoenvironmental, and redox context: (1) subtidal dolomitic domed biostromes with negative δ13C values ~ -2 to -1‰; (2) laminated dolostones related to subtidal platform in which δ13C are near 0‰, and (3) subtidal mounds of domed stromatolites with shales and marls on top related to supratidal flat environment where δ13C turns to positive values ~ +1.5 to 2‰. The δ13C curves for dolostones of the Villa Mónica Formation show in the basal section signals of a stratified ocean associated with important change from icehouse to greenhouse. Similar trends are observed in δ18O VPDB which show an excursion from -3.5 to - 7.8‰, related to temperature increase. The positive Ce anomaly registered in phosphatic levels, just in the base of this succession, confirms an ocean of stratified nature. C-and O- isotope excursions are related to sea-level fall from deeper to shallow facies to a marine stromatolitic platform. Isotope trends plus presence of a facies of laminated mudstones with dropstones, just below the dolostones, suggest this succession is a probable cap carbonate. The karstic surface on top of the unit is associated to important sealevel fall whose relation to glaciation is uncertain. Despite the age of this unit is still under debate, new Sr isotope data in dolostones (0.7068 to 0.7070), and the δ13C curve (-2 to +2‰) suggest a Cryogenian age for this unit, bounded by a glaciation at its base.