Deep Marine Trace Fossil Assemblages and their Palaeo- Environmental Signicance from the Paleocene Amiran Formation in SW Lorestan

Amiran Formation (Paleocene) in Lorestan، folded Zagros zone، contains a diverse and exceptionally well preserved ichnofauna. A quantitative study of trace fossil in the Paleocene deep-marine clastic systems، this Formation، shows that they are powerful discriminators of submarine fan and related environments. The host lithologies are conglomerates، sandstones، siltstones and mudstones. These sediments are interpreted to be deposits in upper to lower-fan palaeoenvironment. Channel deposits consist of thick-bedd turbidite and interchannel deposits are interpreted to be deposited by unconfined debris flows and high concentration turbidity currents. The ichnofauna with high diversity is most frequently، and best، preserved within the Tcde interval of turbidities، which are interpreted as interchannel deposits، produced by low concentration turbidity flows. On the basis of trace fossil diversity، ichnotaxonomic composition، ethology، and morphologic complexity، trace fossils assemblage were grouped into pre-depositional or post-depositional forms. The predepositional assemblage is rich in graphoglyptids and grazing trails، and feeding structures. The ichnodiversity، ethology، and morphologic complexity of the predepositional association are indicative of the Nereites ichnofacies. The postdepositional association essentially consists of dwelling and feeding traces. The post-depositional association includes elements of the Skolithos ichnofacies. Overall characteristics of trace fossils suggest that from the proximal to the distal environment of Amiran Formation، proportions of domichnia trace-fossil assemblages decreases while agrichnia، fodinichnia and pascichnia increases.