Paleodictyon in shallow-marine settings – an evaluation based on Eocene examples from Iran

The graphoglyptid trace fossil Paleodictyon, characterized by stratiform hexagonal meshes, typically occurs preserved at the base of deep-marine turbidites. There is, however, a growing number of occurrences of Paleodictyon in shallow-marine deposits as evidenced by new finds in the Eocene of Iran. The Paleodictyon-containing Asara Shale Member of the Karaj Formation accumulated in a shallow backarc basin. Parallel-crested wave-ripple marks and microbially induced sedimentary structures occur closely above and below the Paleodictyon-bearing strata. Shallow-marine Paleodictyon have so far been reported from morphologically structured, extensive, epicontinental seas, rift basins, and young, prograding passive continental margins, but mainly from foreland and backarc basins. In the two latter cases, the Paleodictyon producers appear to represent adaptive survivors. Initially they settled in abyssal to bathyal turbiditic settings that rapidly aggraded and/or became tectonically uplifted with slight changes to depositional conditions. Finally, the Paleodictyon producers lived in rather shallow water and became preserved by tempestites. This scenario argues against the continuous presence of Paleodictyon producers in shallow-marine settings, suggesting instead they appeared there recurrently.