The carbonate-dominated UpperCretaceous Ilam Formation is representative of an important reservoir in the Zagros Basin. In this study, planktonic foraminiferal biostratigraphy of the Ilam Formationwas investigated in theKuh-e-Surgah section inwestern Iran. In the studied section, the 170mthick Ilam Formation ismainly composed of limestoneswith shale intercalations. The formation conformably overlies and underlies the Surgah and Gurpi formations, respectively. Twelve genera and 24 species of planktonic foraminifera were identified. The following planktonic foraminiferal biozones were identified: the Dicarinella asymetrica Total Range Zone (latest Coniacian– late Santonian), the Globotruncanita elevata Partial Range Zone (Santonian to early Campanian), the Globotruncana ventricosa Interval Zone (middle to late Campanian.), the Radotruncana calcarata Total Range Zone (late Campanian.), the Globotruncanella havanensis Partial Range Zone (late Campanian) and the Globotruncana aegyptiaca Interval Zone (late to latest Campanian). In general, the biozones show that the Ilam Formation in the studied section is latest Coniacian–late Campanian in age. The Santonian–Campanian boundary in theKuh-e-Surgah section correlateswith the Globotruncanita elevata Partial Range Zone. The recognized biozones in the Ilam Formation are comparable with those recorded in other parts of the Tethyan Realm. The crinoid species Marsupites testudinarius is absent in the studied section. Generally, the last occurrence of this cosmopolitan species indicates the base of theCampanian stage. The Santonian/Campanian boundary was placed in the lower part of the Globotruncanita elevata PartialRange Zone at the base of the Ilam Formation in the studied section.

Keywords