Integrated biostratigraphy (orbitolinids and calcareous nannofossils) of the Lower Cretaceous Taft Formation in Isfahan, Central Iran

The so-called “Upper Orbitolina Limestone” of Isfahan is a relatively thick lithological unit of the Taft Formation, cropping out in Central Iran, that is known for its orbitolinid inventory. An integrated biostratigraphy based on agglutinated benthic foraminifera (mainly orbitolinids) and calcareous nannofossils was carried out for the unit. This upper part of the Taft Formation is dated here as early to middle Aptian on the basis of orbitolinids such as Iraqia simplex Henson, Dictyoconus? pachymarginalis Schroeder, Paleodictyoconus cf. cuvillieri (Foury), Mesorbitolina lotzei (Schroeder), and Mesorbitolina parva (Douglass). Calcareous nannofossil index taxa appearing in the upper part of the “Upper Orbitolina Limestone” and also in the underlying shaly unit, including Percivalia fenestrata (Worsley), Percivalia sp., Hayesites irregularis Thierstein, Watznaueria barnesiae Perch-Nielsen, ?Phosterolithus prossii Aguado, ?Rhagodiscus achlyostaurion Hill, R. angustus Stander, Micrantholithus sp., and Eprolithus floralis Stander, support the assigned age by orbitolinids. Based on the discovery of lower Aptian deposits from the “Upper Orbitolina Limestone” (previously assigned to the upper Aptian), the Lower Cretaceous stratigraphy of the Isfahan area is revised.