Stratigraphy of the Albian–Turonian Sarvak Formation in Mish Anticline, Zagros Basin (southwestern Iran), with emphasis on the OAE2 interval

Abstract

The Cenomanian/Turonian (C/T) boundary, marked by the global oceanic anoxic event 2 (OAE2), coincides with a significant tectonic event and widespread unconformity across much of the Zagros Basin (southwestern Iran), except within intrashelf basins. The northern Mish Anticline section, situated in one such intrashelf basin, preserves a continuous sedimentary record of OAE2 within the Albian–Turonian Sarvak Formation. This study examines the stratigraphy of the Sarvak Formation, with particular emphasis on δ13C and environmental variations during the OAE2, to evaluate the interplay between global events and regional tectonics in this region. Analysis of a 517 m thick section reveals a succession assigned to the late Albian–Turonian based on the Cretaceous planktonic foraminiferal biozones for tropical and subtropical regions. The δ13C profile shows a positive excursion across the C/T boundary, capturing the global carbon isotope record's three diagnostic peaks (A, B, and C). The late Albian and most of the Cenomanian intervals reflect deposition in a relatively uniform, deep, and quiet environment, characterized by abundant planktonic foraminifera and oligosteginids. In contrast, the C/T interval records significant environmental changes, driven by OAE2 and regional tectonic activity in the Arabian Plate. A planktonic foraminiferal turnover, including the extinction of rotaliporids and Laeviella bentonensis, as well as the “Heterohelix” shift, is observed at the onset of the carbon isotope excursion between peaks A and B. These events are associated with the expansion of organic-rich layers, which point to the development of an oxygen minimum zone in the late Cenomanian. The regional tectonic activity culminated in two uplift phases within the basin, leading to the appearance of shallow-water carbonate facies during the early Turonian and the development of an unconformity at the top of the Sarvak Formation in the middle Turonian. © 2025 Elsevier B.V.