BIOSTRATIGRAPHIC AND PALEOENVIRONMENTAL STUDY OF RIO TURBIO FORMATION (MIDDLE-UPPER EOCENE) IN SOUTHWEST OF PATAGONIA (ARGENTINA) BASED ON DINOFLAGELLATE CYSTS

During the middle and late Eocene the Austral Basin, in southern Patagonia (Argentina), was flooded by an Atlantic Ocean transgression, which was responsible for the deposition of the upper member of the Rio Turbio Formation. We have analyzed the dinoflagellate cyst assemblages from this stratigraphic section in order to provide a biostratigraphic framework and reconstruct the paleogeographic and paleoceanographic conditions in this area. Dinoflagellate cyst events recorded were compared and interpreted based taking into account other records recognized for the South Pacific Ocean. The comparison allowed us to suggest an age between 45.5 Ma (mid-Lutetian) and 33.5 Ma (Priabonian) for the upper member of the Rio Turbio Formation. We recognized three zones of dinoflagellate cysts. Zone I presents an alternating dominance between Enneadocysta dictyostila, Deflandrea antarctica and Vozzhennikovia apertura indicating two sea level rises. Zone II is dominated by V apertura suggesting high trophic levels and cool waters in a shallow-marine coastal environment. The lower part of the Zone III shows a high abundance of Turbiosphaera filosa with different morphotypes, which may correspond to physicochemical changes in the water column. The uppermost part of the Zone III is characterized by Protoperidinaceae and typical forms of T. filosa suggesting an oceanic environment influenced by upwelling processes. Zones I and II are defined by the dominance of middle Eocene endemic-Antarctic assemblage, while Zone III shows a significant replacement of these taxa by cosmopolitan species. This turnover seems to be forced by the deepening of the Southern Atlantic Ocean gateways and the changes in the ocean circulation patterns.