Depositional environments and ichnology of the deep-marine succession of the Amiran Formation (upper Maastrichtian–Paleocene), Lurestan Province, Zagros Fold–Thrust Belt, Iran

Abstract

The paleoenvironmental significance of deep marine trace fossil assemblages in the upper Maastrichtian–Paleocene Amiran Formation has been done for the first time in Lurestan Province (NW Zagros) from the Zagros Fold–Thrust Belt. A semi-quantitative study of trace fossil abundance (bioturbation indices) in the Amiran Formation shows that trace fossil assemblages can be very powerful discriminators of deep-marine clastic fan and related environments. The Amiran Formation is a sand-rich, thick-bedded, and coarse-grained turbidite succession. Deep marine paleoenvironments from basin slope to basin floor settings are preserved. These strata contain a diverse and abundant pre- and post-depositional ichnofauna. Proximal and axial parts of the sandy systems show low-diversity and low-density trace fossil assemblages dominated by post-depositional trace fossils, but distal environments show a general increase in trace fossil diversity, abundance and number of pre-depositional trace fossils. The post-depositional association essentially consists of dwelling, feeding, and grazing traces. The pre-depositional assemblage is rich in graphoglyptids and grazing trails, and feeding structures also occur.The ichnodiversity, composition, ethology, and morphologic complexity of the pre-depositional association of the heterolithic successions of thin to thick-bedded turbidite sandstone and inter-turbidite mudstone are characteristic of three ichnosubfacies of the Nereites ichnofacies. These ichnosubfacies are identified in Amiran Formation deposits, the typical succession of ichnosubfacies can express a bathymetric trend from shallower to deeper parts and from higher-to-lower hydrodynamic conditions. They are: (1) the Ophiomorpha rudis ichnosubfacies (the proximal and axial parts with thick bedded channel and lobe related environments), (2) the Paleodictyon ichnosubfacies (channel-lobe transition in the distal lobe facies), and (3) the Nereites ichnosubfacies (distal part of off-axis lobe or fan fringe–basin floor transition and overbank settings). There is also mixed O. rudis to Paleodictyon ichnosubfacies (channel–margin, lobe–fringe in the proximal lobe facies). The dominance of Zoophycos and Chondrites in poor-oxygen basin floor facies may represent Zoophycos ichnofacies. The number of graphoglyptids increases from distal (basin floor) to proximal (fan fringe–basin floor transition, fan fringe, lobe complex fringe/lobe complex off-axis) settings. They are absent in the most proximal/axial lobe environments due to increased volume, frequency, erosive power of turbidity current events and sedimentation rate, grain size, decreased preservation potential of shallow tier, and pre-turbidite trace fossils. The ichnosubfacies method used here, as utilized by other authors, has the potential to improve paleoenvironmental analysis of other deep marine depositional settings, and in outcrop investigation of turbidite systems.