Palynostratigraphy of lower Paleozoic sediments in southern Fazelabad town (Kholin-Darreh area), southeastern Gorgan city

The lower Paleozoic succession is well-developed in the southern Fazelabad town (Kholin-Darreh area) which is located approximately 46 km southeastern Gorgan city. A total of 62 surface samples were treated and investigated in the palynological laboratory of Exploration Directorate of National Iranian Oil Company. Most of the samples contain acritarchs, chitinozoans, scolecodonts and cryptospores. In this study, the investigation has only focused on identification of acritarch group at the level of species and genera. In general, 55 acritarchs species (29 genera) were identified.The studied rock units of area consist of Lalun, Abastu, and Abarsaj formations. This study points out that Lalun Formation lacks any palynomorphs, yet it is attributed to the early Cambrian, based upon its stratigraphical position. The Abastu Formation contains acritarch taxa among which are: Saharidia downiei, Acanthodiacrodium ubui, Micrhystridium shinetonense, Dactylofusa squama, Athabascaella penika, Vulcanisphaera africana, Cymatiogalea cylindrata and, Cymatiogalea cylindrata. Considering the stratigraphical values of these acritarch species, the Abastu Formation is suggested to be the lowermost Early Ordovician (Tremadocian). The Abarsaj Formation is characterized by presence of abundant acritarchs, chitinozoans, scolecodonts and cryptospores which important acritarch taxa are: Baltisphaeridium perclarum, Baltisphaeridium oligopsakium, Orthosphaeridium insculptum, Veryhachium subglobosum, Villosacapsula setosapellicula, Actinotodissus crassus, Tunisphaeridium eisenackii, Dactylofusa spinata and Multiplicisphaeridium bifurcatum. Based on stratigraphic values of these acritarch species a Late Ordovician is suggested for this formation.The stratigraphical distribution of the encountered acritarch species within the studied rock units reveals two major hiata within the lower Paleozoic sedimentary sequences.The first hiatus appears between Lalun and Abastu formations and encompasses the Upper and Middle Cambrian strata (Mila Formation). The second hiatus occurs between Abastu and Abarsaj formations. This hiatus includes upper part of Early Ordovician (Arenigian) and the whole Middle Ordovician (Llanvirian – Llandelian). This is indicated with some reworked acritarch taxa from Arenigian and Middle Ordovician as well as early Late Ordovician in the Abarsaj Formation. The two mentioned hiata probably correspond to Caledonian orogeny. Based on presence of chitinozoan, scolecodonts, acritarchs and criptospores a shallow marine environment is suggested for these two formations.On the other hand, acritarchs species of Abastu Formation have been reported from Zagros Basin, Central Iranian Basin, other parts of Alborz Mountain Ranges and other parts of the world (Libya, Algeria, Saudi Arabia, Morocco, Tunisia and England), indicating good relationship between the Kholin-Darreh area and Gondwanan supercontinent during the Ordovician period.The acritarch taxa within the Abarsaj Formation show broad similarity with those of Gondwanan and Laurasia paleocontinents, representing cosmopolitan nature. However, based on established chitinozoan biozones in the Abarsaj Formation a Gondwanan realm is suggested for the studied area during the Late Ordovician.