Miocene foraminifera from the Neuquén Basin, Argentina: stratigraphic and paleoenvironmental implications

Abstract

At the Bajada del Jagüel and Opaso sections (Añelo low, Neuquén basin), the Maastrichtian to Danian mudstones of the Jagüel Formation, are overlain by a succession of ostreid coquinas, sandstones, and mudstones. This succession has been usually referred to the Roca Formation, a Paleocene marine unit with its type section located about 120 km to the southeast. A contrasting interpretation proposed by the late seventies suggests that the beds assigned to the Roca Formation in the Añelo low were actually the base of the overlying Barranca de los Loros Formation, of terrestrial paleoenvironment and Miocene age, and that their very abundant Paleocene invertebrate remains were reworked. In the present revision, the specimens of the foraminiferal family Elphidiidae recovered from a muddy horizon intercalated between the coquinas at the Opaso and Bajada del Jagüel sections, previously assigned to Protelphidium sp. cf. P. hofkeri Haynes, are identified as Porosononion granosum (d’Orbigny). Porosononion granosum is one of the most common foraminiferal species of the shallow Paranense transgression, which covered large part of Argentina during the middle-late Miocene. It has been traditionally referred to as Protelphidium tuberculatum (d’Orbigny) in Argentina. The occurrence of this species indicates that the coquinas, sandstones and mudstones overlying the Jagüel Formation at the Opaso and Bajada del Jagüel sections should not be assigned to the Roca Formation, but are related to the Miocene Paranense transgression, thus supporting the later stratigraphic interpretation. In the muddy horizon, Porosononion granosum constitutes a monospecific assemblage, and its paleoenvironment might be either marginal marine or lacustrine. The elphidiid specimens from the Roca Formation at the Sierras Blancas area, ca. 12 km southwest from the Opaso section, remain identified as Protelphidium sp. cf. P. hofkeri. Both species, P. sp. cf. P. hofkeri and P. granosum, are described and illustrated, and paleoenvironmental and paleogeographic features are considered.