Calcareous nannofossils, stable isotopes, and cyclostratigraphy of the upper Qom Formation (Burdigalian, Central Iran): Implications for paleoclimate and paleoecology

During the Miocene, the Tethyan Seaway documented in the Qom Basin of Central Iran experienced a stepwise closure in a changing climate. Calcareous nannofossils in the upper part of the Qom Formation (Burdigalian) provide useful markers for investigating biostratigraphy, paleoecology, and paleoclimatic evolution and for unravelling the closure process of the Qom Basin. Calcimetry, isotope stratigraphy and cyclostratigraphy have also been undertaken on the studied Dochah section in the Qom Basin to estimate the overall time period of the closure. Relative species abundances and statistical evaluation of nannofossil assemblage changes are used to reconstruct ecological parameters such as nutrient availability, productivity, oxygen levels, and temperature, dependent on changing water masses. Two biozones of Burdigalian age, consisting of NN2/CNM4 (Disocoaster druggii zone, early Burdigalian) and the lower part of NN4/CNM6 (Sphenolithus heteromorphus zone, middle/late Burdigalian), have been documented in the studied, predominantly marly member e of the Qom Formation. An increase in paleoproductivity took place in the early to middle Burdigalian on the basis of nannofossil taxa and carbon isotope values. Regarding nannofossil evidence (missing NN3/CNM5), stable isotopes, and cyclostratigraphy, this study provides evidence of a regional uplift-related gap lasting ca. 900 kyr. Furthermore, cyclostratigraphy shows four 405 kyr cycles below this hiatus (base of studied section at ca. 20.4 Ma) and eight 100 kyr cycles above the hiatus (base at 17.687 kyr). Based on nannofossil assemblages dominated by Discoaster and Sphenolithus spp., warm-water oligotrophic conditions were dominant in the late Burdigalian.