First report of gravity flow deposits and depositional environment of the Ilam Formation in the Lurestan subzone, W Iran

The late Cretaceous Ilam Formation in the western part of the Zagros basin (W Iran) was inspected in two outcrops (namely Mehdiabad and Kuh-e Varzarin sections) which are located in the Ilam province (in the Zagros fold belt). The purpose of this research was to identify the lithofacies, microfacies and depositional environment of the Ilam Formation. This formation consists of limestone and argillaceous limestone which comprise three microfacies (planktonic foraminifera wackestone/packstone, planktonic foraminifera-benthic bivalve wackestone/packstone/rudstone, benthic-planktonic foraminifera wackestone/packstone/rudstone that were deposited in two facies zones (c.f. slope and toe of slope). Evidence of the gravity flow including basal truncation (erosional) surface of beds, normal grading, weak amalgamation, abundant deep-water facies, coupled laminae, relatively thick gravite beds, mixed planktonic (deep water) and benthic (shallow water) biota, large scale slump folds and deformed structures, attest the influence gravity flows which transported the shallow water sediments into the deep setting. Poor quality of the outcrops and lack of well-preserved sedimentary structures makes it impossible to discriminate between various types of sediment gravity flow deposits (gravites). Gravites are observed only in the toe of slope facies zone. This facies zone in turn is classified into three parts including upper, middle and deep parts according to the texture and the argillaceous limestone beds thickness. Due to ack of platform top deposits, it was not possible to distinguish the carbonate platform type (ramp versus shelf type). However, the overwhelming resedimented carbonate facies points toward a carbonate shelf rather than a ramp.