Planktonic foraminiferal and isotope stratigraphy of the upper Cenomanian-Turonian interval in the Zagros Basin, SW Iran

Abstract

A high-resolution bio- and chemostratigraphic study based on planktonic foraminifera and carbon stable isotope of the late Cenomanian–late Turonian time interval of the Sarvak Formation was undertaken in the Zagros Basin (SW Iran). In this research, a hemipelagic succession, named the Tang-e Bawlek section, was inspected. Significant documented bio-events in stratigraphic order include lowest occurrence (LO) of Praeglobotruncana algeriana, the highest occurrences (HO) of Thalmanninella appenninica, Th. deeckei, Rotalipora montsalvensis, Th. greenhornensis and R. cushmani, LO of Helvetoglobotruncana praehelvetica, HO of La. bentonensis, “Heterohelix” shift, LO of Helvetoglobotruncana helvetica and finally HO of H. helvetica. These bio-events have led to the recognition of four biozones (from Rotalipora cushmani Zone to Dicarinella primitiva-Marginotruncana sigali Zone) from the Tethyan realm. Integrating of the well-documented bio-events with a high-resolution δ13C curve, particularly around the Cenomanian/Turonian boundary (CTB), enabled comprehensive documentation of OAE2 and its three distinct associated peaks (a, b and c) as well as facilitated correlation with key reference curves. These results reveal that in contrast to the other parts of the Zagros Basin, where the CTB is represented by a marked disconformity or sea level fall, this portion of the Lurestan Subzone exhibits evidence of deep marine conditions. However, the detailed lithological and microfacies observations point to the development of a condensed horizon with a low sedimentation rate, rich in glauconite and phosphate located just above the peak c of the δ13C curve, which can be associated with a short hiatus close to the CTB. Palaeoecological reconstructions of the studied interval revealed three major changes in the depositional conditions, establishing a notable eutrophic condition during the OAE2 interval accompanied by a well-developed oxygen minimum zone (OMZ).