Cretaceous/Tertiary boundary in the area of Bjala, eastern Bulgaria – biostratigraphical results

Abstract

The Cretaceous/Fertiary boundary is identified in the vicinity of the town of Bjala on the Black Sea coast, within a n continuous sedimentary succession. It is located in the Bjala Formation. The latter has been studied in four sections on the coast in separate, tectonically dislocated blocks. The K/T boundary is lithologically marked by a boundary clay bed, from 1 to 6 cm thick. Biostratigraphically, it is defined by the disappearance of Cretaceous nannoplankton and appearance of Biantholithus sparsus and Cyclagelosphaera alta. Seven nannofossil zones fro m CC25C (Micula murus Zone) in the Upper Maastrichtian to NP 5 (Fascicultihus tympaniformis Zone) in the Upper Palaeocene, have been identified in an uninterrupted succession. The Bjala 2b section is one of the few Upper Maastrichtian sections in the world in which macro-invertebrate fossils occur commonly. Three ammonite zones are recognized based on the representatives of Pachydiscidae: Anapachydiscus freisvillensis Zone, P. gollevillensis gollevillensis Zone and Anapachydiscus terminus Zone. Evidence from the Bjala 2b section indicates that ammonites gradually became less diverse during the Late Maastrichtian. The "youngest" ammonites were found 40cm below the K/T boundary. The calcareous nannoplankton assemblages vary considerably across the K/T boundary: a rapid turnover of taxa occurred within the limits of the first Palaeocene zone NP 1.