New occurrences of the trace fossil Paleodictyon in shallow marine environments:: Examples from the Triassic-Jurassic of Iran

The graphoglyptid trace fossil Paleodicyon, characterized by a stratiform hexagonal net, is a diagnostic member of the deep-water Nereites ichnofacies and usually found in deep-sea flysch successions. Scattered records of the trace fossil from shallower, lower-shelf-toupper-slope environments, particularly since the mid-Mesozoic, have been substantially augmented by new data from Upper Triassic and Jurassic sedimentary basins of Iran. Paleodictyon has been found on the soles of mid-to-lower-shelf event beds produced by storm-induced currents and on the soles of prodelta turbidite deposits in the Upper Triassic Nayband Formation of east-central Iran, the lower Middle Jurassic Shemshak Formation of north-central Iran, the Middle Jurassic Kashafrud Formation of the Koppeh Dagh, and the Middle Jurassic Dalichai Formation of the Binalud Mountains. It appears that the bathymetric range of Paleodictyon throughout the Phanerozoic was considerably wider than generally assumed. The observed dominance of the trace fossil in deep-water flysch successions we interpret as being at least partially due to preservational effects. The conservation of the networks requires limited erosion, immediate casting and sealing of the exhumed negative epireliefs by sand, and limited subsequent bioturbation at deeper tiers. This restricts the trace fossil to event beds, either deep-sea turbidites or shallow-water storm beds in environments characterized by high-sedimentation rates. For bathymetric interpretations of paleoenvironments, whole ichnoassemblages should be used rather than single ichnotaxa to avoid erroneous conclusions.