THE BIOSTRATIGRAPHIC STUDY OF ILAM FORMATION IN THE ZAGROS MOUNTAIN, SOUTHWESTERN IRAN

The Zagros Mountain chain is located in southwestern Iran and trends NW-SE. The Zagros mountain chain is associated with the Zagros thrust fault which marks the collision boundary between the Arabian and Eurasian plates. An almost continuous sedimentation with minor and local gaps has happened since the early Paleozoic through early Pleistocene in the Zagros trough. The Ilam Formation is a pelagic calcareous unit with the late Cretaceous age in the Simply Folded zone of Zagros mountain. The thickness of Ilam Formation in the studied area, approximately 5 miles north of Poldokhtar is 510 feet. Ilam Formation consists of thin to medium, well-bedded, gray micro-crystalline limestone. Based on lithostratigraphic features, the Ilam Formation is readily recognizable from the lower and upper units. Late Cretaceous planktonoic foraminifera are the most abundant microfauna in the Ilam Formation. Several well preserved genera of Globotruncanids are found, which indicate a Santonian-early Campanian age for the Ilam Formation. Different species found are Dicarinella concavata, Dicarinella primitiva, Dicarinella asymetrica, Marginotruncana sigali, Marginotruncana renzi, Marginotruncana sinousa, Globotruncana arca, Globotruncana linneiana,and Rosita fornicata. The lithofacies of Ilam Formation ranges from spars to packed biomicrite. The combination of biofacies and lithofacies indicates a deep marine environment during Santonian to early Campanian for Ilam Formation in simply folded zone of Zagros Mountain in Southeastern Iran. Furthermore, the presence of the aforementioned cosmopolitan microfauna, also indicates the connection between the depositional environment of the Ilam Formation and the late Cretaceous seaways.