Peri-Gondwanan acritarchs and chitinozoans, from Ordovician succession in the Alborz Mountain Ranges, northern Iran: Regional stratigraphic significance and paleogeographic implications

A total of 1500 surface samples were collected from the Ordovician outcrops of the Alborz Mountains and treated for acritarch and chitinozoan taxa to determine the exact age of Ordovician rock units and paleogeographic position of these mountain ranges. The Alborz Mountains stretches from the border of Azerbaijan along the western and entire southern coast of the Caspian Sea and finally runs northeast and merges into the Aladagh Mountains in the northern parts of Khorasan province. The Alborz Mountains are divided into the Western, Central, and Eastern Alborz Mountains that the Ordovician rock units (e.g. Lashkarak, Abastu, Ghelli, and Abarsaj formations) extend throughout this mountain ranges. The Lashkarak Formation is well-developed in the whole Alborz Mountains, whereas Ghelli, Abastu, Abarsaj formations, and Gorgan Schists have exposure in the southern Caspian Sea and the northern parts of Khorasan province. The study localities consist of the Alam-Kuh, Hassanakdar, Simeh-Kuh, Gerd-Kuh, Deh-Molla, Kuh-e Saluk, Fazel Abad, and Deraznau areas. Except for the Gorgan Schists in the Deraznau area which has low metamorphic grade, the rest of Ordovician rock units are siliciclastic sediments. The study materials either metamorphic or non-metamorphic samples contain abundant and well-preserved palynomorph entities (e.g. acritarchs and chitinozoans), which permit the exact age of rock units and relationships of the Alborz Mountains to the paleocontinents (e.g., Gondwanan vs. Laurentia palaeo-provinces).
Based on palynological analyses, the Lashkarak and Abastu formations contain several diagnostic acritarchs, consisting of Arbusculidium filamentosum, Barakella felix, Coryphidium bohemicum, Dactylofusa velifera, Striatotheca principalis, Cymatiogalea messaoudensis var. messaoudensis, Aureotesta clathrata var. simplex, and Vavrdovella areniga var. areniga. On the other hand, the presence of Eremochitina brevis, Velatachitina veligera, Desmochitina ornensis, Belonechitina henryi, Siphonochitina formosa, Cyathochitina jenkinsi, and Armoricochitina armoricana chitinozoan Biozones confirm the inclusion of the Alborz Mountains in the peri-Gondwanan paleoprovince during the Lower-Middle Ordovician. The Ghelli Formation is well-developed in Kuh-Saluk at the northern parts of Khorasan province, whereas the Abarsaj Formation and the Gorgan Schists are present in the southeastern Caspian Sea. The Ghelli and Abarsaj formations are characterized by the presence of Late Ordovician diagnostic acritarch taxa, consisting of Acanthodiacrodium crassus, Villosacapsula setosapellicula, Multiplicisphaeridium irregulare, Multiplicisphaeridium bifurcatum, Orthosphaeridium insculptum, Orthosphaeridium bispinosum, Orthosphaeridium quadrinatum, Ordovicidium elegantulum, Dactylofusa cabottii, Baltisphaeridium oligopsakium, Baltisphaeridium perclarum, Dorsennidium hamii, Diexapllophasis denticulata, Neoveryhachium sp. cf. carminae, Veryhachium lairdii, Navifusa ancepsipuncta, Leiofusa litotes, Muzivum gradzinskii, and Veryhachium inflatum. The aforementioned acritarch taxa are associated with the Belonechitina robusta, Tanuchitina fistulosa, Acanthochina barbata, Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongata, and Spinachitina oulebsiri chitinozoan Biozones, which have been previously established in the ‘northern’ Gondwanan Domain. Herein, it should be mentioned that the Tanuchitina elongata and Spinachitina oulebsiri Biozones are present in the glacial member of the Ghelli Formation in Kuh-e Saluk. Moreover, the treated samples of Gorgan Schists contain the same acritarch and chitinozoan taxa of the Ghelli and Abarsaj formations which reveal the Late Ordovician for the Gorgan Schists. Therefore, based on palynological data, not only the Zagros Mountains but also the Alborz Mountains were part of the peri-Gondwana/Gondwanan paleocontinent.

Mohammad Ghavidel-Syooki

Keywords: Ordovician, Alborz Mountains, chitinozoans, acritarchs, Peri-Gondwana.

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