Ichnological discrimination of hemipelagic‐dominated slope versus basin‐floor mudstones: Insights from early Viséan successions of a distally steepened ramp, northern Iran

Integrating sedimentological and ichnological data from the uppermost unit of the Mobarak Formation (early Viséan, Northern Iran), deposited in a distally steepened ramp, provides insights into deep‐water muddy gravity flows in mud‐rich slope settings. The succession is characterised by mudstones formed from hemipelagic suspension settling and diluted, muddy calciturbidite currents, hosting four ichnocoenoses: Chondrites ‐ Trichichnus ( Cho‐Tr ), Phycosiphon ‐ Planolites ( Ph‐Pl ), large Nereites ‐ Zoophycos ( Ne‐Zo ) and Thalassinoides‐ Zoophycos ( Th‐Zo ). Basin‐floor mudstones exhibit low bioturbation intensity, small trace fossils and patchy distributions, with abundant opportunistic chemosymbionts ( Chondrites and Trichichnus ), alongside pyritised burrows. Slope mudstones show higher bioturbation intensity, larger burrows and greater ichnodiversity due to diluted calciturbidite currents, increased oxygen levels and nutrient input. Critically, hemipelagic processes remain ecologically decisive on slopes (40%–60% hemipelagic facies), evidenced by: (1) Cho‐Tr traces in hemipelagic interbeds confirming sustained suspension settling; (2) vertically accreting mudstones homogenising substrates and suppressing Nereites and graphoglyptids; and (3) background redox conditions shaping tiering patterns, with turbidites causing only temporary perturbations. This explains the ubiquity of the Zoophycos ichnofacies and absence of Nereites ichnofacies across all settings. Slope deposits display a higher ratio of shallow‐tier deposit‐feeding traces ( Phycosiphon , Nereites ) to deep‐tier traces ( Zoophycos , Thalassinoides ), reflecting better oxygenation and nutrient availability. The absence of graphoglyptids and other Nereites ichnofacies indicators stems directly from persistent hemipelagic sedimentation and high sediment homogeneity disrupting oligotrophic conditions required for farming strategies. The study demonstrates that integrated ichnological‐sedimentological criteria effectively distinguish between basin‐floor and slope mudstones. Variations in trace fossil assemblages and ichnocoenoses reflect changes in environmental factors such as hydrodynamic regime, oxygen levels, organic content and sedimentation rates, providing a robust framework for interpreting deep‐water muddy gravity flows in mud‐rich slope‐basin settings.