Ordovician of North Iran: New lithostratigraphy, palaeogeography and biogeographical links with South China and the Mediterranean peri-Gondwana margin

The Ordovician litho-and biostratigraphic framework of Alborz, Kopet-Dagh and the East-Central Iranian blocks is outlined and significantly updated, and a broad summary of the current state of knowledge of the Ordovician deposits and faunas across Iran is documented. Four tectono-stratigraphical units (including the Alestan, Damghan, Saluk and Talesh domains) are distinguished in northern Iran. They differ considerably from one another in their lithology, facies, fossil record and completeness of their sedimentary record. A comprehensive revision of the Ordovician stratigraphy in the eastern Alborz and the Kopet-Dagh regions leads to the definition of the (i) Simeh-Kuh Formation (new), (ii) the Qumes Formation (new) and its subdivision into the Gerd-Kuh (new) and Raziabad (new) members, (iii) the Lashkarak Formation and its subdivision into the Cheshmeh-Ali (new) and Hajiabad (new) members, (iv) the Abarsaj Formation (formalised) in the Alestan Domain, (v) the Qyzlar (new) and Pelmis formations in the Saluk Domain, and (vi) the Tatavrud Formation (new) in the Talesh Domain. The Ordovician strata of the eastern Alborz and Kopet-Dagh Mountains comprise a rifting volcanism emplaced within an active horst-and-graben palaeotopography. Despite the strong Ordovician biogeographical affinities with South China and, to a lesser extent, with Mediterranean peri-Gondwana, zircon populations analysed from Cambro-Ordovician sandstones in the area point to the Arabian-Nubian Shield of the western Arabian Peninsula and northeastern Africa as the principal continental sources. Consequently, an open oceanic communication between the Mediterranean and the northern (Alborz) margins of Gondwana and the southern margin of South China favoured the establishment of strong biogeographical affinities between these mid-latitude basins.