CRETACEOUS CALCAREOUS NANNOFOSSILS AND BIOSTRATIGRAPHY OF SOUTHEASTERN AUSTRAL BASIN, PATAGONIA, ARGENTINA. This study presents the results of a quantitative analysis of the calcareous nannofossils recovered in five localities in the southeastern sector of the Austral Basin. On the basis of the assemblages recovered and its comparison with previous biozonations for the Austral Atlantic Ocean, it was possible to identify Valanginian—early Hauterivian, early Albian—middle Albian, late Albian—Cenomanian, Coniacian/Santonian—Campanian and late Maastrichtian assemblages. Two discontinuities evidenced by biostratigraphical hiatuses for the Cretaceous sequence were deduced: one for the late Hauterivian—Barremian and the other for at least the Turonian. The Early Cretaceous assemblages are comparable to those of the Falkland Plateau and they would be developed on an external platform having cold surface waters and high availability of nutrients. The Late Cretaceous assemblages are similar to those found in the Neuquina and Colorado basins and they would be developed on an internal platform having cold to temperate surface waters.