Organic carbon isotope chemostratigraphy, geochronology and cyclostratigraphy in the Vaca Muerta Formation (Tithonian-Berriasian, Neuquén Basin, Western Argentina)

MARTINEZ, MATHIEU; PELLENARD, P.; LONDERO, A.; AGUIRRE-URRETA, BEATRIZ; COLON, DYLAN; VENNARI, V.V.; JOHAN SCHNYDER; SCHALTEGGER, U.

Brest

Simposio; 18° French Sedimentological Meeting; 2022

IAS

The Jurassic-Cretaceous boundary (Tithonian-Berriasian boundary) is marked by a global cooling, aridification event and a major eustatic regression. The forcing factors of these climatic changes are poorly understood due to the inaccuracy of the correlations between basins, linked to faunal endemism. In the Boreal Realm, the 13Corg shows a characteristic negative excursion in the Tithonian that ends with a rapid positive shift close to the Jurassic-Cretaceous boundary, so-called VOlgian Isotope Carbon Excursion (VOICE; Hammer et al., 2012). As this excursion was not documented elsewhere, it is usually attributed to local processes, such as change in the source of organic matter or oxidation of buried organic matter (Galloway et al., 2020). Here, we provide organic geochemistry data from the Vaca Muerta Formation, in the Neuquén Basin in Argentina to test the global or local nature of the VOICE. The section studied, the Las Alcantarillas section is 127-m thick ranging from the Lower Tithonian to the Berriasian. It is composed of organic-rich silt and claystone in which intercalate sandstone beds, some of which showing planar laminations, HCS, gutter cast and shell accumulations, carbonate nodules, pyritic and beef layers and altered volcanic ash layers (K-bentonite). CA-ID-TIMS U-Pb ages performed on 4 k-bentonites demonstrate an average sedimentation rate of 2.1 cm/kyr, the age of the interval studied ranging from ~146.6 to ~140.6 Ma. The 13Corg shows a negative excursion during the Tithonian, ending in the upper part of the Tithonian (Pseudolissoceras zitelli to the middle part of Substeueroceras koeneni, Andean ammonite zones). It corresponds to a decrease in the TOC content in the Vaca Muerta Formation. The organic matter remains of marine origin, so the change in 13Corg does not correspond to local change in the source of organic carbon. Stratigraphically, this is consistent with the VOICE identified in the Boreal realm. Spectral analyses conducted on 13Corg and TOC content reveal a sedimentary cycle of 8 m, which corresponds to an average duration of 0.38 Myr, considering the average sedimentation rate. This pacing is close to the period of the 405-kyr eccentricity cycle, also identified in Boreal Realm. Thus, our data highlight that the global nature of the VOICE and the pacing of the carbon cycle in the latest Jurassic, which provides an opportunity for establishing a global marker for the Jurassic-Cretaceous boundary.