Micropaleontology of the Agrio Formation(Lower Cretaceous) in different areas of the Neuquén Basin

Abstract

The nanoflora and microfauna from the AgrioFormation are analyzed in three different areas of the Neuquén Basin toestablish its age and paleonvironmental variations. In the northern area of thebasin, the oldest Agrio Fomation sedimentites are recognized and assigned tothe Lower Valanginian through ammonites. A monospecific Epistominaloncochensis Ballent microfauna is recovered from these levels, whichindicates disoxic environments. Up to the top of the section, siltstonescontain a diverse foraminifera microfauna of normal shelves, which are replacedby an impoverished fauna related to shallowing processes. The age defined bycalcareous nannofossils is Late Valanginian-Late Hauterivian. In the centraland southern position of the Neuquén Basin, the nannofossil assemblagesindicate the Upper Valanginian-Upper Hauterivian for the Agrio Formation. Thebenthic microfauna suggests frequent eustatic variations ranging from shelvesto shallow marine environments. In the austral sector of the basin, a poorlydiversified foraminiferal assemblage is recorded and a highostracods/foraminifera ratio indicates a restricted marine environment, andnormal salinity. The presence of a level with abundant attached foraminiferaindicates high energy episodes. The high diversified Platycopida ostracodssuggest warm-temperate to subtropical waters.