Carbon cycle, cyclostratigraphy and U/Pb ages of the Tithonian from the Neuquén Basin, Argentina

PELLENARD, P.; MARTINEZ, M.; LONDERO, A.; COLON, D.; AGUIRRE-URRETA, BEATRIZ; VENNARI, V.V.; LESCANO, M.; LÓPEZ-MARTÍNEZ, R.; JOHAN SCHNYDER; SCHALTEGGER, U.

Budapest

Congreso; 11th International Congress on the Jurassic System; 2022

Calibrating the Jurassic/Cretaceous boundary is of prime importance for interbasinal correlations and identification of global vs local geodynamic events. We present here the first high-resolution carbon isotope curve (13Corg) provided in the Vaca Muerta Formation from the Las Alcantarillas section (Neuquén Basin, western Argentina) during the Early/Late Tithonian and Early Berriasian, to compare carbon cycle variations from this well-constrained and continuous series across the Jurassic-Cretaceous boundary with other basins worldwide. The section of Las Alcantarillas (127 m-thick) is newly dated using ammonites, calcareous nannofossils and calpionellids allowing South America biostratigraphic zonation to be established. The numerous volcanic ash layers (K-bentonites and consolidated carbonated ash layers) intercalated in the mixed carbonate-clastic organic-matter-rich marine sediments serve for the temporal calibration of the section: zircon from five volcanic levels have so far been dated using CA-ID-TIMS U-Pb techniques, yielding very precise ages ranging from 146.147 ±0.036 Ma in the P. zitteli ammonite biozone of the Early Tithonian to 143.199 ±0.040 Ma in the S. koeneni ammonite biozone of the Late Tithonian. In parallel, bulk rock was sampled to measure magnetic susceptibility every 6 cm, with spectral analyses providing an astrochronological framework that is anchored to the U-Pb dates obtained from volcanic ash layers.Despite a marked burial diagenetic overprint in the Neuquén Basin reaching the oil window at least, the carbon isotopic signal from organic matter is still preserved, as indicated by the non-scattered signal, the coherent absolute values and consistent trends recognized in equivalent Late Jurassic basins worldwide. A net decrease of 13Corg values from ~-25 ‰ in the P. zitteli biozone (Early Tithonian) to ~-30 ‰ close to the C. alternans/S. koeneni ammonite biozone boundary (Late Tithonian) is recorded, reaching a plateau before increasing to values comprised between -26 ‰ and -28 ‰ in the Late Tithonian (S. koeneni Zone). Although the Jurassic/Cretaceous boundary is still not precisely defined on the Las Alcantarillas section from coupled data of ammonites, nannofossils and calpionellids, the recovery to common values (around -27 ‰) appears either in the very Late Jurassic or in the very Early Cretaceous (i.e. Berriasian, A. Noduliferum ammonite Zone). This large negative carbon isotope excursion (5 ‰ in amplitude) recorded in the Late Jurassic can be compared to the VOICE event (VOlgian Isotopic Carbon Excursion) recognized in the boreal domain, which would suggest a global disruption of the carbon cycle at the J/K boundary whose causes must now be sought.