Nannoplankton and 13C and 13O stable isotope stratigraphy record of the mid Cretaceous sequences, Zagros Basin (western Iran)

Abstract

Deposition of black shales and dark argillaceous limestones across the Cenomanian to Turonian interval has been recorded in many parts of the world. In this regard, a stratigraphic section in the Iran's hydrocarbon field in the Kermanshah region in the Zagros Basin has been studied in detail to reconstruct palaeoceanographic conditions under a reliable biostratigraphic framework. Biostratigraphy based on calcareous nannofossils integrated with high resolution carbonate and oxygen stable isotope analyses were carried out on the Cenomanian-early Turonian interval of the upper part of Garau Formation. In this formation, 24 species of calcareous nannofossils were identified in the pelagic marine deposits. Our study allowed the definition of nannoplankton bio zones from the upper part of Eiffellithus turriseiffelii Zone to the lower part of Quadrum gartneri Zone in the mid Cretaceous. This research first recorded the upper part of the Garau Formation at site and it is consistent with the previous bio zones identified in the Tethyan domain in the region. The middle Cretaceous events of calcareous nannofossils proposed here correlate with the isotope global profile for the early Cenomanian-earliest Turonian interval and with those, reported for the southeastern France and south England reference curves, represented by the positive Cenomanian to Turonian δ13C excursion. Thus, the sequences of dark argillaceous limestone rich in organic matter in Kermanshah region can be considered as a continuation of the global trend in the Tethyan domain, where the Cenomanian-Turonian Oceanic Anoxic Event constrained to about 91.5 ± 8.6 millions of years, occurred in an anoxic pelagic sedimentary environment correlates with both, global and local sea-level rises, represented in this study by the characterization of the nannoflora assemblage reflected in a variety of widespread paleoenvironmental changes that coincides with significant shifts in the stable carbon and oxygen isotope signature.