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 age relationships. The samples of Lalun Formation were barren, but those of Abastu and Abarsaj formations contained well-preserved and abundant palynomorph entities, which are dominated by acritarchs (23 species belonging to 15 genera) and chitinozoans (29 species distributing among 15 genera) with remains of scolecodonts, graptolite and a few cryptospores, although not determined in detail. Based on the restricted stratigraphic range of acritarch species, an Early Ordovician (Tremadocian) age was assigned to the Abastu Formation; while based upon index chitinozoan and acritarch taxa, a Late Ordovician (Katian to Hirnantian) age was assigned to the Abarsaj Formation. Likewise, based on the presence of diagnostic chitinozoan taxa, the Abarsaj Formation can be assigned to the Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongata and Spinachitina oulebsiri chitinozoan Biozones. These chitinozoan assemblages reflect a clear palaeobiogeographic affinity with the so-called "North Gondwana Domain". Two major hiata are present within the studied Lower Palaeozoic succession in the Fazel Abad area. The first hiatus appeared between the Lalun Formation (Early Cambrian) and the Abastu Formation (Tremadocian) and includes the Middle-Upper Cambrian Mila Formation. The second hiatus occurs between the Abastu Formation (Tremadocian) and the 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.