Cretaceous–Paleogene transition interval in the north flank of the Alborz Mountains (N Iran); Planktic foraminiferal biostratigraphy and stable isotopes evidence

Abstract

A Cretaceous/Paleogene (K/Pg) boundary interval in the north flank of Alborz Mountains (Aboksar section), northern Iran, has been studied to investigate the biological and geochemical variations of this event in the eastern Tethys. Abathomphalus mayaroensis at the late Maastrichtian, Pα (Parvularugoglobigerina eugubina), P1a (Parasubbotina pseudobulloides) and P1b (Subbotina triloculinoides) at the early Danian are the recognized planktic foraminiferal biozones and subzones in the studied area. The Plummerita hantkeninoides subzone at the uppermost Maastrichtian and P0 Biozone at the base of Danian were not defined in the studied section. This proposes a short-term hiatus and sedimentary discontinuity at the base of Paleocene and probably uppermost Maastrichtian, which most likely caused by winnowing of sediments after a cooling phase and development of bottom current in the Albors Basin. The K–Pg transition in the Aboksar section is characterized by a bloom in the Guembelitria cretacea, evolution of first Danian species (Pv. eugubina, Pv. longiapertura), δ13C negative shift (about 0.5‰), and a gentle Ir anomaly (0.1 ppb). Planktic foraminifera were diverse and abundant during the late Maastrichtian of studied section in a warm (relatively lower δ18O contents) and well stratified environment. Bloom of the opportunist species, G. cretacea, and decrease of planktic foraminiferal abundance and diversity at the base of Paleocene interval propose an abnormal stressful environment in this time probably after the asteroid impact and its following temperature decline. The appearance of more complete species with more specialized shells and larger sizes, such as Globoconusa daubjergensis, Parasubbotina pseudobulloides, Eoglobigerina eobulloides and Subbotina triloculinoides in the P1a and P1b subzones and return of carbon isotope diagram into the pre-boundary values testify an environmental recovery and optimization of ecosystem conditions.