C-O-Sr isotope stratigraphy of the Neoproterozoic carbonate successions of the Sierras Bayas Group, Río de La Plata Craton, Argentina: Palaeoclimatic implications

Abstract

The Neoproterozoic undeformed and unmetamorphosed carbonate successions from the Tandilia System are constituted by dolostones of Villa Mónica Formation and limestones of Loma Negra Formation (Sierras Bayas Group). The present study provide a record of carbon, oxygen and strontium isotopic stratigraphic variations considered useful for regional and global correlations. Detailed chemostratigraphic correlations between four complete profiles of dolostones (36-52 m thick) of Villa Mónica Formation allow to differenciate three sections, regarding C-O trends, sedimentary facies in its marine paleoenvironmental context. Its diagenetic evolution is consistent with trends of primary marine origin. The dolostones δ13C curves (from -2 to +2‰) show signals of a stratified ocean associated with climatic change from icehouse to greenhouse. Similar trends in δ18O VPDB from -3.5 to -7.8‰ are 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 into a marine stromatolitic platform. These, plus the presence of a facies of laminated mudstones with pebble-dropstones, below the dolostones, suggest a probable cap carbonate. The age of Villa Mónica Formation is still under debate, however Sr isotope data (0.7068 -0.7070) and the δ13C curve from dolostones suggest a Cryogenian age as well as their stromatolites. The Loma Negra Formation is composed almost exclusively of micritic limestones originated by suspension fall-out in open marine ramp and lagoonal environments. The 13C values range from +2.2 to +4.5‰ and 18O(V-PDB) values vary from –7.1 to –14.1‰ and all C- and O- isotope shows similar excursions in all sections and localities studied. 18O excursion towards the top of the unit is associated with decrease in pH and incursion of rich 16O fluids, moreover this stratigraphic excursion shows a regional connotation in the Tandilia System. On the contrary, the typical trend expected for an exposed surface with negative 13C was not observed. Preservation of organic matter of the limestones of Loma Negra Formation was referred by δ13Corg, Δδ13C, Mn/Sr, Fe/Sr, Ca/Sr and Rb/Sr which support a primary isotopic signature. Despite, early marine-meteoric and burial diagenesis do not modified significantly its primary signature, later uplift of the succession drive in subaerial exposure and meteoric diagenesis in relation with the implantation of the karstic surface just on top of the sequence. This karst, knew as Barker Surface (regional unconformity) is related to an important sea level fall which could be linked to a glaciation. The age Loma Negra Formation (580-590Ma) was suggested from δ13C and 87Sr/86Sr trends and supported by Cloudina. Furthermore, such data are very consistent with most of the SW Gondwana Late Ediacarian carbonate successions. In summary, two strong evidences of icehouse influence are registered in these carbonate successions, one in the base of the dolostones probably linked with the Bitter Springs Stage (~800 Ma), and the second one on top of the Loma Negra Formation related to Gaskiers glacial event (~ 580 Ma).