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First record of the Oxfordian positive carbon isotope excursion in the Neuquén Basin, Argentina
CAGNONI, MARIANA; ARMELLA, CLAUDIA; CABALERI, NORA
Simposio; 10º American Symposium on Isotope Geology; 2016
Universidad Autónoma de México
Late Jurassic was a time of global change in the marine sedimentation pattern and in Middle to Late Oxfordian, widespread carbonate sedimentation took place. This change in sedimentation correlates with a positive anomaly in marine carbon isotope records. The positive carbon isotope excursion found in the Alpine Tethys was located in the Mid-Oxfordian, 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.Lower Callovian-Middle Oxfordian in the Neuquén Basin was a time of carbonate deposition represented by the marine carbonates of La Manga Formation. Studies of this Formation have been focused on its sedimentology, palaeontology and biostratigraphy. We present the 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 frequently affected by ocean currents. The scarcity of ammonites in this section, avoided an accurately age assignment but other sections at Sierra de Reyes have been dated by ammonite stratigraphy and correspond to the Perisphinctes- Araucanites Assemblage Zone of 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 on bulk carbonate samples by the usual techniques. To prevent contamination, we carefully avoided alteration crusts, secondary mineralizations and cement while grinding. 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. 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 scatter in the δ18O records may indicate diagenetic alteration of some of the O-isotope signals. We correlate the positive shift in δ13C with the Mid-Oxfordian positive excursion of the carbon isotope global record considering the reorganization of the ocean circulation pattern and the onset of a new east-west current system during the Oxfordian.