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The distribution of conventional subichnofacies in deep‐water fan systems (i.e. Ophiomorpha rudis–Paleodictyon–Nereites‐subichnofacies) depends on the presence of fully turbulent non‐cohesive flows such as low‐density turbidites and high‐density turbidites. However, changes in flow state between laminar and turbulent conditions can affect the distribution style of conventional subichnofacies. This study deals with the effects of environmental or ecological stressors as a result of hybrid sediment gravity flows (hybrid event beds) on distribution and composition of trace fossils in the deep‐marine depositional system of the Ordovician Ghelli Formation in the north‐eastern Alborz Range. The spatial distribution and sedimentological character of hybrid sediment gravity flows, in particular their abundance in the down‐dip parts of lobe‐related facies associations, are interpreted as products of longitudinal and lateral transformation from turbidity currents to debris flows promoted by incorporation of significant claystone and mudstone clasts into the flow by seafloor erosion and flow bulking. Seven trace‐fossil assemblages were recognized in the channel and lobe‐related facies associations. The observed spatial distribution of trace‐fossil assemblages in the interpreted frontal and lateral lobe fringes, somewhat contrasts with conventional subichnofacies distribution models. The development of cohesive flows and rapid emplacement of turbidite, banded and debrite beds could be accompanied by fluctuations in physico‐chemical conditions and, hence, the time available for colonization and the lifestyle of producers. This study suggests that, compared to the archetypal models of the subichnofacies in down‐dip parts of lobe‐related facies associations, ichnodiversity and density of bioturbation are drastically reduced, and trace fossils are limited to simple, shallow‐tier feeding strategies. Commonly, the presence of cohesive flows may have forced the producers in the Paleodictyon and Nereites‐subichnofacies to migrate into lateral lobe fringe with more participation of low‐density turbidites. This study shows that, in addition to sedimentological and architectural criteria, ichnological criteria can also be used to distinguish between frontal and lateral lobe‐fringe deposits in unconfined systems. [ABSTRACT FROM AUTHOR]

This study integrates ichnological and sedimentological data to interpret depositional environments in the carbonate sediments of the Mobarak Fm. (Lower Carboniferous) in the Alborz Mountains of northern Iran. Seven ichnofabrics including Thalassinoides (Th), Zoophycos‒Phycosiphon (Zo-Ph), Skolithos‒Thalassinoides (Sk-Th), Zoophycos‒Diplocraterion (Zo-Di), Thalassinoides‒Rhizocorallium (Th-Rh), Zoophycos‒Arenicolites (Zo-Ar) and Chondrites‒Phycosiphon (Ch-Ph) have been identified in the studied succession. Th ichnofabric is characterised by the dominance of the deposit-feeding trace fossils, revealing low sedimentation rate and activity of the organisms in a loose substrate. Vertical succession of the Zo-Ph, Sk-Th and Zo-Di ichnofabrics indicates change in the firmness of the substrate. Th-Rh and Zo-Ar ichnofabrics reveal two different energy phases including a lower energy condition with the activity of deposit-feeders and a higher energy phase identified by the activity of suspension-feeders. Ch-Ph ichnofabric is characterised by low diversity and low abundance of trace fossils that were identified in black, limy shale, displaying a long lasting outer ramp setting with low energy, low sedimentation rate, and dysoxic-anoxic conditions of the substrate. Ichnological features of the Mobarak Fm. are controlled by subsidence-inducing changes in the accommodation space of the depositional system dictating variations in paleoenvironmental conditions such as oxygen levels and hydrodynamic energy. [ABSTRACT FROM AUTHOR]