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This study presents the first detailed data on calcareous nannofossil assemblages from the Sabiñánigo Sandstone Formation in the Jaca Basin (central south Pyrenees). This formation is mainly composed of deltaic and outer-shelf sediments. These siliciclastic deposits contain nannofossil assemblages that are moderately to well-preserved, particularly in fine-grained levels. They contain a calcareous nannofossil assemblage dominated by the species Cyclicargolithus floridanus, Coccolithus pelagicus, Coccolithus formosus, Clausicoccus fenestratus, Zygrhablithus bijugatus, and several species of Sphenolithus and Chiasmolithus. The biostratigraphic results enabled the characterization of the Middle Eocene biohorizons, based on global stratigraphic scales and the improvement of the temporal correlation and lateral evolution of this basin's deposits. The sedimentary sequence of the Sabiñánigo Sandstone was deposited during the Middle Eocene, between the upper part of biozone NP16 and the base of NP17, in the Bartonian. The calcareous nannoplankton assemblage suggests warm and oligotrophic surface waters for the Bartonian interval in the Jaca Basin. [ABSTRACT FROM AUTHOR]