New biostratigraphic data from the Callovian-Oxfordian La Manga Formation, Neuquén Basin, Argentina: Evidence from an ammonite condensed level

Abstract

The La Manga Formation constitutes most of the sedimentary record of the Callovian-Oxfordian in the Neuquén Basin. This stratigraphic unit represents the middle part of the Lotena Mesosequence, which is dominated by carbonates with interbedded shales, marls, and occasional sandstones. Based on detailed stratigraphic work in the stratotype section (La Manga Creek, Mendoza), the depositional environments and the sea-level history of the La Manga Formation have been interpreted. Petrographic and field observations led to identification of nine facies distributed in two informal units. Unit 1 was deposited on a outer ramp setting, while deposition of unit 2 took place in the intertidal-supratidal environments. A condensed level close to the base of unit 1 has yielded Rehmannia sp., Rehmannia cf. paucicostata (Tornq.) and Homoeoplanulites sp., from the Lower Callovian Bodenbenderi-Proximum Zones, and Peltoceratoides sp. and Rursiceras sp. from the Upper Callovian and also in the Lower Oxfordian Peltoceratoides-Parawedekindia Zone. The condensed level is overlain by a Middle Oxfordian succession characterized by Perisphinctes (?Arisphinctes) sp., Perisphinctes (?Kranaosphinctes) sp., Mirosphinctes sp., Perisphinctes (?Antilloceras) cf. prophetae Gygi & Hill., Perisphinctes (?Otosphinctes) sp., Perisphinctes (?Subdiscosphinctes) sp., and Perisphinctes (?Kranaosphinctes) cf. decurrens (Buck.) of the Perisphinctes-Araucanites Zone, which was correlated with the upper part of the Cordatum Standard Zone to the Transversarium Standard Zone, and probably to the lower part of the Bifurcatus Standard Zone. Unit 2 has yielded small Mirosphinctes sp., indicating an Oxfordian age. The sharp contact between the outer ramp facies of unit 1 and the overlying intertidal-supratidal facies of unit 2 can be interpreted as the result of an abrupt fall (forced regression) of the relative sea level during the end of Middle Oxfordian or Upper Oxfordian. These results could be used for comparison with other localities in the Neuquén Basin providing additional data for Lower Callovian-Middle Oxfordian deposits.