اثر فابریک ها و اهمیت محیطی اثر فسیل ها در سازند جیرود، جنوب غرب شاهرود و شمال شهمیرزاد، البرز مرکزی

Geirud Formation (Frasnian-Famenian) crops out in the north of Iran. This formation is composed of siliciclastic sediments in the lower part and carbonate sediments in the upper part. In the Studied area Geirud Formation disconformably overlies the marine shale of the Milla Formation (Ordovician) and is conformably overlain by the black limestone of the Mobarak Formation (Lower Carboniferous) at the Shahmirzad locality and by Shemshak Formation with a faulted surface at the Dehmolla locality. Based on studies of the ichnofossils of the Geirud Formation, were identified nine ichnofabrics that display a general deepening up-ward trend from low to upper part of the formation. Skolithos-Arenicolites, Thalassinoides- Rhizocorallium and Helminthopsis-Chondrites ichnofabrics in bidirectional and current wave ripples, Wight to red sandstone indicate a high stress and instable substrate with high sedimentation rate environment like estuary. Thalassinoides-Rhiocorallium-Palaeophycus, Arenicolites- Thalassinoides and Arenicolites ichnofabrics in fossiliferous sandstone with trough cross bedding and HCS display a medium to high energy of the shoreface setting with suitable water circulation and oxygen level and enough nutrients in substrate. Arenicolites-Diplocraterion, Thalassinoides-Rhizocorallium-Palaeophycus, Arenicolites-Thalassinoides, Arenicolites-Protovirgularia and Protovirgularia ichnofabrics with high diversity of the trace fossils in thin-medium bedded, dark limestone with abundant skeletal elements indicate establishing a normal marine condition with high nutrients and enough oxygen level in a shallow shelf setting.