The Cambrian System in the Alborz Mountains of northern Iran (Fig. 1A) consists of alternating carbonate and siliciclastic successions deposited in an extensive platform covering the length of the Cimmerian and the Arabian-Iranian plates in northeast Gondwana (e.g., Berberian and King, 1981, Lasemi, 2001). In Alborz, the lower–middle Cambrian transition (Fig. 1B) includes the siliciclastic Top Quartzite of the Lalun Formation (Assereto, 2014) and the chiefly carbonate deposits of Member 1 of the Mila Formation (Stöcklin et al., 1964). We have not applied the stratigraphic nomenclature by Geyer et al. (2014) because their approach has several shortcomings (see Lasemi and Amin-Rasouli, 2017, p. 344–345 and discussions therein). The lower–middle Cambrian transition in northern Iran is plagued by the absence of biostratigraphic criteria; here, we integrate sequence stratigraphy with d13C composition to evaluate the base of the Cambrian third series, the Miaolingian Series, in northern Iran. Our objective is to provide a valuable non-biostratigraphic criteria in the Alborz to permit chronostratigraphic correlation of this interval with other regions. This study provides evidence for the terminal early Cambrian extinction event and the resulting paleoenvironmental changes during the middle Cambrian in northern Iran.
