First record of the Oxfordian positive carbon isotope excursion in the Neuquén Basin, Argentina

CABALERI, N.G; ARMELLA, C; CAGNONI, M

Puerto Vallarta

Simposio; 10th South American Symposium on Isotope Geology; 2016

Universidad Nacional Autónoma de México

Late Jurassic was a time of global change in the marine sedimentation pattern during the Middle to Late Oxfordian, when widespread carbonate sedimentation took place. This change in sedimentation correlates with a positive anomaly in marine δ13Crecords. This δ13C positive excursion, found in the Alpine Tethys, occurred in the Mid-Oxfordian, at the Plicatilis and Transversarium ammonite zones, and was proposed to be used as reference for carbon isotope stratigraphy. During the Oxfordian, the connection between Tethys and the Pacific through the Atlantic, Hispanic Corridor, became deep and wide enough to influence the general oceanic circulation with significant water mass exchange between the basins. The Lower Callovian-Middle Oxfordian was a time of carbonate deposition represented by the marine carbonates of La Manga Formation in the Neuquén Basin. Studies of this Formation have focused on its sedimentology, palaeontology and biostratigraphy. We now add carbon and oxygen isotope data of a studied section in Aguada de Ñaco, Sierra de Reyes, Mendoza. This section represents carbonate sedimentation in a shallow open lagoon recurrently affected by ocean currents. The scarcity of ammonites in this section prevented us from an accurate age determination. However, ages in other sections at Sierra de Reyes have been determined with ammonite stratigraphy, and correspond to the Perisphinctes- Araucanites Assemblage Zone from the Upper Jurassic of west-central Argentina, which is equivalent to Latest Cordatum to Transversarium (?Bifurcatum) Zone (Latest Early and Middle-Oxfordian). Carbon and oxygen isotope analyses were done in bulk carbonate samples by the usual techniques. In order to prevent for contamination, alteration crusts, secondary mineralization and cement we carefully avoided. Isotopic ratios were determined in a Delta S Finnigan Mat triple collector mass spectrometer, at INGEIS. The isotopic composition is reported as deviation per mil (δ?) relative to the V-PDB standard. The precision of measurement is ±0.1?for carbon and oxygen. The base of the carbon isotope record from the Aguada de Ñaco section shows an increasing trend of δ13C values from 0.4 ? to 1.2? and continues with a progressive increase to a maximum of 3.0?. A drop towards a value of 1.4? marks the end of the positive carbon isotope shift giving amplitude of 1.6?. δ13C and δ18O do not show covariance (r=0.16) indicating that the C-isotope record of La Manga Formation represents primary oceanic values. However, the scattered δ18O values may indicate diagenetic alteration of some of the O-isotope signals. We link the positive shift in δ13C with the Mid-Oxfordian positive excursion of the carbon isotope global record, as the reorganization of the ocean circulation pattern and the onset of a new east-west current system occurred during the Oxfordian.