Essentially composed of alternations of marls and marly limestones, the Gurpi Formation extends through the major part of the Zagros Mountains, in west and southwest of Iran. Due to the lack of integrated stratigraphic data for the Upper Cretaceous of the eastern Tethys, and in order to improve both the local stratigraphy and the Late Cretaceous Geologic Time Scale, four stratigraphic sections including Shahneshin, Bowan, Sepiddar and Tang-e Dasht-e Bahman were chosen in the central Zagros, Fars Province, Iran. This work was supplemented by the inclusion of the Gandab section in the Lurestan Province, northwestern Zagros. Presenting the combined features of continuous exposure through the upper Coniacian to Maastrichtian, no signs of major faulting and easy access, the Shahneshin section was chosen as a potential reference for the Gurpi Formation. A high-resolution biostratigraphic analysis of the Shahneshin section has been undertaken, based on calcareous nannofossils, planktic foraminifera, and dinoflagellate cysts, supplemented by bulk carbonate carbon and oxygen stable isotopes which allowed for the establishment of an integrated stratigraphic scheme for the late Coniacian to late Danian of the eastern Tethys. This study brings a thorough discussion on the definition of Late Cretaceous stage boundaries. A paleoecological study of calcareous nannofossils of Shahneshin indicate a major influence of Late Cretaceous climate change and associated environmental changes in Zagros on the biodiversity and abundance of calcareous nannoflora. The long-term cooling trend is expressed in the Gurpi Formation by a calcareous nannofossil turnover and by the increased abundance in Cribrosphaerella ehrenbergii and Arkhangelskiella spp., which are considered here to be the best cool-water indicators for the Zagros Basin. A first episode of cooling occurs in the late early Campanian. Cooling intensified during the late Campanian to early Maastrichtian, in coincidence with the significant increase in the abundance of benthic foraminifera that indicates a relative sea-level fall. The nannofossil assemblage also indicates that a less significant second episode of cooling occurs in the late Maastrichtian. Comparison of these data with that of Stevns-1 (Denmark) reveals that the cooling in Zagros occurred significantly earlier than the early Maastrichtian cooling of the Boreal Realm while the timing of the mid-Maastrichtian warming and late Maastrichtian cooling episodes is similar between these two geographic areas. The coincidence of this earlier cooling in the Zagros Basin with an interval characterized by a significant increase in benthic foraminifera suggests an amplified response to environmental changes associated with local tectonics in Zagros. The late Campanian calcareous nannofossil assemblage turnover in central Zagros is either a response to an early cooling trend in the eastern Tethys or to sea-level fall or both.
Transient size variations and a shift in the morphology of Cribrosphaerella ehrenbergii and Microrhabdulus spp. have been documented across the Late Cretaceous of Shahneshin and led us to define two new species Microrhabdulus zagrosensis n.sp. and Microrhabdulus sinuosus n. sp. A first common increase in size of the two studied lineages occurs across the late Campanian-early Maastrichtian cooling episode and highlights the major influence of this event on both ecophysiology and biodiversity. A second shift in size and shape of Microrhabdulus spp. is recorded across the mid-Maastrichtian interval, coinciding with a time of major changes in macro-invertebrates, and probably associated with climatic instability.
An integrated stratigraphic study has been undertaken for the Bowan section, albeit limited to nannofossil biostratigraphy and stable carbon and oxygen isotope stratigraphy. Correlation of Shahneshin and Bowan allows for the establishment of a sequence stratigraphic interpretation across the Gurpi Formation. Large-scale correlations to reference Tethyan and Boreal sections demonstrate a local expression in the evolution of seawater δ13C in the Zagros area that we link to the influence of tectonic uplift on the carbon pool in the Zagros Basin. In addition to previously defined carbon isotope excursions, our work allows for the definition of three new useful Late Cretaceous carbon isotope events named ‘basiplana’ in the late Campanian, ‘pre-MME’ and ‘junior’ events in the Maastrichtian.
Additional sections of the Gurpi Formation in central Zagros, Sepiddar and Tang-e Dasht-e Bahman sections, have been logged and dated by calcareous nannofossil biostratigraphy. These sections add to a better understanding of the sequence stratigraphic scheme, regional sea-level variations and their links to global eustasy and tectonic uplift. A late Campanian condensed interval with a significant hiatus appears to be of particular importance throughout the studied sections and systematically corresponds to a pronounced increase in abundance of benthic foraminifers. By means of nannofossil, foraminifer and carbon isotope stratigraphy applied to the section of Gandab in the Lurestan area, we correlate the base of this interval to the base of the late Campanian-early Maastrichtian interval, a level that is particularly rich in Lopha oysters but also in crinoids and brachiopods. The presence of this shallow facies in the Lurestan area imply a supra-regional late Campanian sea-level fall and following lowstand throughout the early Maastrichtian of Zagros. Comparison of sediment accumulation rates and of the completeness of all studied sections across this late Campanian to early Maastrichtian sea-level fall suggests that the northeastern part of central Zagros was generally affected earlier and for a longer duration than the southwestern part of the basin, while the Lurestan area in northwestern Zagros, has been much more affected by tectonic uplift than the Fars area in central Zagros.