Upper Jurassic–Lower Cretaceous calpionellid zones in the Neuquén Basin (Southern Andes, Argentina): Correlation with ammonite zones and biostratigraphic synthesis

Abstract

This work presents the first regional biostratigraphic study on the calpionellid zones and assemblages of the Tithonian–lower Valanginian interval in the Neuquén Basin, Southern Andes, Western Argentina. The lower Tithonian–lower Valanginian in the Neuquén Basin is mainly represented by the Vaca Muerta Formation, which is a thick sucession (100–1250 m thick) of rhythmic marlstone and limestone alternations corresponding to the distal hemipelagic facies of a carbonate ramp. This formation is one of the most important unconventional hydrocarbon reservoirs in the world and has become a relevant target in Argentina during the last decade. The Vaca Muerta Formation is characterized by an abundant fossil content and a remarkable stratigraphic continuity along several hundred meters, encompassing the Jurassic/Cretaceous boundary. The detailed study of seven outcrop and well sections (three of them studied for the first time herein), allowed the elaboration of a reliable stratigraphic scheme based on the correlation of ammonites, microfossils, magnetostratigraphy and cyclostratigraphy. The Vaca Muerta Formation contains moderate to poorly preserved calpionellids. Despite that, twenty-six calpionellid species and five calpionellid biozones known in the Tethyan regions have been identified: Chitinoidella, Crassicollaria, Calpionella, Calpionellopsis and Calpionellites. Additionally, nine subzones were recognized: Slovenica, Boneti, Remanei, Massutiniana, Alpina, Elliptica, Simplex, Oblonga, and Darderi. These results allow chronostratigraphic correlations between the Tethys and the Southeastern Pacific domains.