Ichnostratigraphy of the Ediacaran-Cambrian boundary: new insights on lower Cambrian biozonations from the Soltanieh Formation of northern Iran

Strata in the Central Alborz Mountains, northern Iran, are interpreted to show continuous sedimentation from Ediacaran through Cambrian times. The Soltanieh Formation consists of five members: Lower Dolomite, Lower Shale, Middle Dolomite, Upper Shale and Upper Dolomite members. The clastic units (Lower and Upper Shale members) represent sedimentation in distal marine settings, ranging from the shelf to offshore, and contain abundant trace fossils of biostratigraphic utility. Four ichnozones have been recognized. Ichnozone 1, containing Helminthoidichnites tenuis, Helminthopsis tenuis, and Cochlichnus anguineus, is early Fortunian based on small shelly fossils, and is interpreted as a distal expression of the Treptichnus pedum zone. Ichnozone 2, comprising the first occurrence of T. pedum, is middle Fortunian, and is best regarded as the upper half of the Treptichnus pedum Zone. Ichnozone 3 is late Fortunian–Cambrian Age 2, characterized by a sudden change in abundance and complexity of trace fossils. Main elements in this ichnozone include Cruziana problematica, Curvolithus isp., Phycodes isp., Treptichnus pedum, Treptichnus pollardi, and Treptichnus isp. Ichnozone 4 is of Cambrian Age 2-Age 3 and marked by the first appearances of Psammichnites gigas, Rusophycus avalonensis, and Didymaulichnus miettensis. Integration of trace fossils with small shelly fossils suggests that the Ediacaran–Cambrian boundary should be placed at the base of the Soltanieh Formation or within the Lower Dolomite Member. The delayed appearance of T. pedum and the low ichnodiversity in the Lower Shale and lower interval of the Upper Shale reflect limited colonization of settings below storm wave base during the early Fortunian.