--> The Vaca Muerta-Quintuco Mixed Depositional System: New Insights From Carbon Stable Isotopes (δ13Ccarb and δ13Corg) and Geochemical Data at the Jurassic-Cretaceous Boundary (Neuquén Basin, West Argentina)

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The Vaca Muerta-Quintuco Mixed Depositional System: New Insights From Carbon Stable Isotopes (δ13Ccarb and δ13Corg) and Geochemical Data at the Jurassic-Cretaceous Boundary (Neuquén Basin, West Argentina)

Abstract

The Late Jurassic-Early Cretaceous Vaca Muerta (VMF) and Quintuco (QF) Formations consist of a mixed system composed of three low frequency sequences identifiable on seismic. Sequences can be traced from basinal organic rich shale and limestone of the VMF, updip to shallow marine mixed carbonates and siliciclastics of the QF. Carbon isotope data (δ13Ccarb and δ13Corg) measured on continuous and sidewall cores from the VMF and the QF, reveal excursions that may aid in higher-resolution stratigraphic correlation across the Neuquén basin. Distinct isotopic trends are recognised along the studied interval, with more positive values found in maximum flooding zones and more negative ones found at sequence boundaries. The maximum flooding interval of the VMF displays a negative δ13Ccarb shift (-5.0‰) influenced by the presence of organic matter, light δ13Corg values (-30.1‰), and high SiO2 and Mo content. Negative isotopic values are followed by a positive δ13Ccarb excursion (2.0‰), related either to continued sequestration of organic matter further offshore in the basin or to a global Tithonian oceanic anoxic event. The overlying HST, characterised by prograding clinoforms identified on seismic, shows low isotopic values (0.5‰) and low SiO2 and Al2O3 concentrations. In the subsequent TST a second positive δ13Ccarb shift (2.4‰) is recorded, coupled with increased concentrations of SiO2 and Al2O3. Main reservoir facies in the QF occur just above this interval where regressive, shallow marine, high-energy grainstones and sandstones display negative isotopic values (−0.2‰) possibly affected by meteoric diagenesis. During the last TST the highest SiO2 and lowest CaO values are observed. No clear δ13Ccarb trend is recorded, due to insufficient carbonate content. The following HST sees the deposition of thick aggrading mixed sequences, characterised by constant δ13Ccarb values (1.0‰) possibly corresponding to the Berriasian and Early Valanginian stages. Overall trends in δ13Ccarb and δ 13Corg curves correlate from well to well, although it is not yet fully understood if they reflect true secular variations in seawater chemistry or result from a combination of global changes and local diagenetic effects. The published isotopic curve is not sufficiently detailed over the Tithonian-Berriasian for direct comparison. This work represents the first continuous isotopic record in the Neuquén Basin at the Jurassic-Cretaceous boundary.