Jurassic–Cretaceous calcareous nannofossils events in the subsurface of Vaca Muerta formation, Northwestern Neuquén Basin, Argentina

Abstract

This work presents the results of a calcareous nannofossil analysis conducted on the Vaca Muerta Formation from the Chihuido de la Sierra Negra well, in the northwestern sector of the Neuquén Basin. A total of 15 cutting samples were processed and analyzed, covering the entire thickness of the formation at this location (~365 meters). Overall, moderately preserved, abundant, and diverse assemblages were recovered. These assemblages are dominated by species of the genus Watznaueria, mainly Watznaueria fossacincta, followed by Watznaueria britannica. A morphometric analysis was carried out on the latter species, distinguishing five morphotypes that will be compared with geochemical data to assess whether the variation in their relative abundance bears any paleoenvironmental significance. In addition, several biostratigraphically significant events were identified, allowing the dating of the studied section. In chronostratigraphic order, these events include: the last occurrence (LO) of Polycostella beckmannii, the first occurrence (FO) of Micrantholithus sp., the LO of Polycostella senaria, the FO of Eiffellithus primus, and the FO of Tegumentum stradneri. Based on these events, a late Tithonian to early Valanginian age is inferred. The studied well is calibrated with regional seismic markers, highlighting the value of calcareous nannofossils for subsurface and surface correlation.