ICHNOLOGY OF THE EDIACARAN–CAMBRIAN TRANSITION: THE SOLTANIEH FORMATION OF NORTHERN IRAN AND ITS SIGNIFICANCE FOR UNDERSTANDING THE CAMBRIAN EXPLOSION

Abstract

The Ediacaran–Cambrian transition is one of the most critical events in Earth history, a time of marked biological and sedimentary changes. The nature of the Cambrian explosion has been a major topic of discussion and numerous explanations have been posited for these dramatic changes. Strata in the Central Alborz Mountains, northern Iran are believed 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. This formation contains abundant trace fossils and displays a significant vertical facies recurrence, allowing comparison of identical environments through time. Four ichnozones have been recognized. Ichnozone I, containing Helminthopsis tenuis and Cochlichnus anguineus, is lower Fortunian based on small shelly fossils, and is interpreted as a distal expression of the Treptichnus pedum zone. Ichnozone II, comprising the first occurrence of T. pedum, is middle Fortunian, and is best regarded as the upper half of the Treptichnus pedum zone. Ichnozone III is late Fortunian–Cambrian Age 2, characterized by a sudden change in abundance and complexity of trace fossils. Cruziana problematica, Curvolithus isp., Helminthopsis tenuis, Palaeophycus tubularis, Phycodes isp., and Treptichnus pedum are common in this zone. Ichnozone IV is of Cambrian Age 2–Age 3, 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 Soltanieh Formation or within the Lower Dolomite Member. An integrated study of sedimentary facies, small shelly fossil zones, sequence stratigraphy, and distribution of associated trace fossils indicates that the delayed appearance of T. pedum and the low ichnodiversity in the Lower Shale reflect a complex interplay of evolutionary and ecological controls. The distribution and preservation of diagnostic early Cambrian trace fossils in the Lower Shale Member of the Soltanieh Formation is very likely to have been controlled by facies, whereas trace-fossil distribution in the Upper Shale Member is primarily controlled by evolutionary innovations. Our systematic review shows that significant diachronism was involved in the generation of the Great Unconformity suggesting that the sea-level fall that resulted in the formation of a sequnce boundary during the Ediacaran¬¬–Cambrian boundary dominantly reflects overprint of local tectonics on pure eustasy. Our results indicate that the sequnce boundary should not be used for inter-basin correlations.