Benthic foraminiferal assemblages across the K/Pg transition in northern Patagonia, Argentina: palaeoenvironmental constraints

Abstract Upper Maastrichtian to Danian benthic foraminiferal assemblages of the Jagüel Formation in the eastern sector of the Neuquén Basin, Argentina, were investigated. These assemblages record the first Atlantic transgression that flooded northern Patagonia from the eastern coastal shelf towards the western volcanic arc, establishing shallow marine conditions during the Maastrichtian. Five benthic foraminiferal associations based on species composition were defined, and changes in the benthic fauna were evaluated to infer palaeoenvironmental changes across the K/Pg (Cretaceous/Palaeogene) transition. The upper Maastrichtian associations ( Tehuelchella caminosi , Gavelinella ? neuquense – Planulina spp.) include few cosmopolitan species and abundant endemic taxa, low diversity, and dominance of epifaunal morphogroups. These features suggest stressed palaeoenvironmental conditions with limited organic matter at the sea floor. A mixed interval ( Nonionella robusta – Anomalinoides spp. association) follows, marked by the co‐occurrence of Maastrichtian and Danian species, interpreted as a within‐habitat time‐averaged deposit. In Danian associations, cosmopolitan species of the neritic Midway‐type fauna dominate ( Cibicides spp. – Buliminella pseudoelegantissima , Valvalabamina sp. – Anomalinoides acutus ), showing higher diversity values and mixed shallow infaunal and epifaunal morphogroups. Mesotrophic conditions are interpreted, with organic matter available at and below the sediment–water interface. Endemism is the most distinctive feature of the foraminiferal assemblages at the K/Pg transition in northern Patagonia. Occurrence of endemic species in both Maastrichtian and Danian assemblages suggests that the Neuquén Basin occupied a relatively sheltered palaeogeographic position since the end of the Cretaceous. The presence of the stratigraphic hiatus encompassing Planktonic Foraminiferal Zones P0–Pα cannot be confirmed for the study section.