Upper Cretaceous oceanic red beds (CORBs) of the eastern Sakarya Zone back-arc basin, NE Türkiye: evidence from planktonic foraminiferal biostratigraphy, facies architecture, and chronostratigraphy

Upper Cretaceous sequences of the eastern Sakarya Zone (ESZ), NE Türkiye, comprise 2–5-km-thick volcano-sedimentary successions deposited in a back-arc basin associated with northward Neotethyan subduction. Intercalated Cretaceous Oceanic Red Beds (CORBs) provide key archives of Late Cretaceous biostratigraphy and basin evolution. We investigated planktonic foraminiferal assemblages in 340 limestone samples from six CORB horizons in the Giresun and Artvin areas. Assemblages dominated by double-keeled marginotruncanids and dicarinellids indicate well-oxygenated, open-marine pelagic conditions. In Giresun, the co-occurrence of Dicarinella asymetrica and “pill-box”-like Globotruncana linneiana constrains four CORB horizons to the Santonian. An early Santonian (85.4 Ma) intrusion crosscutting the two lower horizons further constrains the lower pair to the earliest Santonian, and the upper pair to the middle–late Santonian. In Artvin, the older CORB horizon is similarly Santonian, whereas a younger horizon contains sparse assemblages indicating a late Campanian age. Multiple Santonian and late Campanian CORBs distinguish the northern ESZ from the central Sakarya and İstanbul zones, where CORBs are largely Cenomanian–Turonian and Santonian. These results refine Late Cretaceous pelagic sedimentation in the ESZ and demonstrate that CORBs sensitively record tectono-oceanographic evolution in convergent-margin back-arc basins.