Biostratigraphy of Acritarchs and Chitinozoans in Ordovician Strata from the Fazel Abad Area, Southeastern Caspian Sea, Alborz Mountains, Northern Iran: Stratigraphic Implications

Sixty-four surface samples from the Lower Palaeozoic rock units (mainly Lalun,Abastu and Abarsaj formations) near Kholin-Darreh village in the Fazel Abad area,southeastern the Caspian Sea, Northern Iran, were analyzed to determine their agerelationships. The samples of Lalun Formation were barren, but those of Abastu andAbarsaj formations contained well-preserved and abundant palynomorph entities, whichare dominated by acritarchs (23 species belonging to 15 genera) and chitinozoans (29species distributing among 15 genera) with remains of scolecodonts, graptolite and afew cryptospores, although not determined in detail. Based on the restricted stratigraphicrange of acritarch species, an Early Ordovician (Tremadocian) age was assigned to theAbastu Formation; while based upon index chitinozoan and acritarch taxa, a LateOrdovician (Katian to Hirnantian) age was assigned to the Abarsaj Formation. Likewise,based on the presence of diagnostic chitinozoan taxa, the Abarsaj Formation can beassigned to the Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongataand Spinachitina oulebsiri chitinozoan Biozones. These chitinozoan assemblages reflecta clear palaeobiogeographic affinity with the so-called "North Gondwana Domain".Two major hiata are present within the studied Lower Palaeozoic succession in the FazelAbad area. The first hiatus appeared between the Lalun Formation (Early Cambrian) andthe Abastu Formation (Tremadocian) and includes the Middle-Upper Cambrian MilaFormation. The second hiatus occurs between the Abastu Formation (Tremadocian) andthe Abarsaj Formation (Katian-Hirnantian) and spans the interval of the Floian-Sandbian, which corresponds to uplift related to the initial stage of rifting of the Palaeo-Tethys Ocean.