Ichnological model of siliciclastic sediments of Shirgesht Formation in Kalmard block, Centeral Iran

The mixed siliciclastic succession of Shirgesht Formation (Ordovician) have a diverse trace fossils in Kuh-e Asheghan and Kuh-e Rahdar sections of Kalmard block in the Central Iran. Ichnofacies development is primarily controlled by environmental factors, e.g. sediment type, food accessibility, oxygenation and energy level. The Shirgesht Formation was deposited in some subenvironments including shoreface, offshore and barrier island-lagoon. F ive ichnofacies have been recognized in these subenvironments: three ichnofacies include Cruziana, Skolithos and Psilonichnus are controlled by bottom factors and mixed Skolithos–Cruziana ichnofacies controlled by local conditions, such as salinity. Erosinal transgressive surfaces (i.e. ravinement surfaces) are the most elegant manner of generating widespread, substrate-controlled ichnofacies assemblages such as, Glossifungites ichnofacies. Upper shoreface/foreshore sediments accumulated on a storm-dominated, prograding barrier island coast and are characterized by low-diversity Skolithos and Psilonichnus elements. Middle Shoreface and proximal lower shoreface sediments of Shirgesht Formation are characterized by a diverse mixture of by suspension to deposit-feeding trace-makers (mixed Skolithos–Cruziana), reflecting the presence of adequate food resources in both the substrate and in the water column. Washover fan/lagoonal sediments are characterized by a mixed Skolithos–Cruziana ichnofacies consistent with deposition in a setting subject to periodic salinity and oxygenation stresses. Finally, the offshore-lower shoreface trace-fossils of shirgesht formation is diverse, consistent with fully marine equilibrium communities in unstressed environments, which predominance of deposit-feeding structures,with associated grazing structures (cruziana ichnofacies), supports a lower shoreface and offshore setting.