Biostratigraphy of acritarchs and chitinozoans in Ordovician strata from the Fazel Abad area, southeastern Caspian Sea, Alborz Mountains, northern Iran: Stratigraphic and paleogeographic implications
Published:December 21, 2017
Mohammad Ghavidel-Syooki, 2017. "Biostratigraphy of acritarchs and chitinozoans in Ordovician strata from the Fazel Abad area, southeastern Caspian Sea, Alborz Mountains, northern Iran: Stratigraphic and paleogeographic implications", Tectonic Evolution, Collision, and Seismicity of Southwest Asia: In Honor of Manuel Berberian’s Forty-Five Years of Research Contributions, Rasoul Sorkhabi
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Sixty-four surface samples from the Lower Paleozoic rock units (mainly the Lalun, Abastu, and Abarsaj Formations) near the village of Kholin-Darreh in the Fazel Abad area, southeastern Caspian Sea, northern Iran, were analyzed to determine their age relationships. The samples from the Lalun Formation are barren, but those from the Abastu and Abarsaj Formations contain well-preserved and abundant palynomorph entities, which are dominated by acritarchs (23 species belonging to 15 genera) and chitinozoans (29 species distributed among 15 genera). Scolecodonts and graptolite remains as well as a few cryptospores were also observed, although these were not determined in detail. Based on the restricted stratigraphic range of acritarch species, an Early Ordovician (Tremadocian) age is assigned to the Abastu Formation, while based upon index chitinozoan and acritarch taxa, a Late Ordovician (Katian to Hirnantian) age is 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 elongate, and Spinachitina oulebsiri chitinozoan biozones. These chitinozoan assemblages reflect a clear paleobiogeographic affinity with the so-called “North Gondwana Domain.” The composition of acritarch assemblages also appears to be consistent with newly proposed hypotheses of a Late Ordovician phytoplanktonic biogeographical differentiation between a Laurentian/Baltica realm and a Gondwanan realm, with the present assemblages belonging to the latter. The presence of some chitinozoan and acritarch taxa from the Baltic and Laurentia in Gondwanan chitinozoan biozones of the Fazel Abad area suggests the existence of counterclockwise marine currents, which brought planktonic organisms (acritarchs, chitinozoans, and graptolites) from lower latitudes (Baltica) to higher latitudes (North Gondwanan Domain) settings. The presence of a low-diversity acritarch assemblage in the Abastu Formation and taxonomically diverse chitinozoan, acritarch, and scolecodont assemblages in the Abarsaj Formation suggests a marine depositional environment for these two formations. Two major hiatuses are present within the studied Lower Paleozoic succession in the Fazel Abad area. The first hiatus appears 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 Paleo-Tethys Ocean.