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

Abstract

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