The Cenomanian/Turonian boundary interval deep-sea deposits in the Zagros Basin (SW Iran): Bioevents, carbon isotope record and palaeoceanographic model

Abstract

The Cenomanian/Turonian (C/T) boundary interval, spanning the upper part of the Sarvak Formation in the Zagros Basin (Izeh Zone), has been studied to determine the timing and the possible cause of biotic turnovers, coinciding with the intense environmental perturbations during the Oceanic Anoxic Event 2 (OAE2). The upper Cenomanian strata in the study section, composed of light-colour marlstone/organic-rich marlstone and shale bedding couplets, record fluctuations in microfossil assemblages and organic carbon contents. We suggest that the organic-poor layers, characterized by high abundance of radiolarians, were deposited during cold periods under surface water eutrophication (upwelling) and ventilated seafloor. In contrast, water column stratification during warm periods enhanced organic matter preservation and allowed deposition of organic-rich strata. Continued stratification and also increased runoff and primary productivity during warm intervals of OAE2 led to expansion of oxygen minimum zone (OMZ), which may have caused extinctions of deep followed by intermediate water planktic foraminifera. Such conditions may have led to significant decreases in the abundance and diversity of planktic foraminifera and dominance of the biserial genus Planoheterohelix (“Heterohelix” shift event). The δ13C curve in the study interval reveals the diagnostic features of global C/T boundary carbon isotope record, including three positive peaks (A, B and C) within the Whiteinella archaeocretacea Biozone. These carbon isotope features and the most important bioevents in the study section are stratigraphically listed below: 1- Highest occurrences (HOs) of Th. greenhornensis and Th. deeckei, 2- HO of R. cushmani and δ13C peak A, 3- HO of “G”. bentonensis, 4- Onset of “Heterohelix” shift event, 5- δ13C peak B, 6- δ13C peak C, 7- Filament event, 8- Lowest occurrence (LO) of H. helvetica, and 9- Holywell event.