Chitinozoan biostratigraphy and palaeogeography of Silurian strata (Niur Fm.) at eastern Alborz Range (Kopet-Dagh region) northeastern Iran

The Silurian strata in eastern Alborz Range is called Niur Formation. This formation is well-developed between the Esfarayien and Bojnourd road, approximately ۳۵ km southeastern Bojnourd city(۳۵ km southeastern Caspian Sea). The Niur Formation is ۷۵۰ thick and in ascending stratigraphic order, it has been divided into Member I and Member II. The Member I is ۵۰۰ m thick and consists mainly of black shales with subordinate thin-bedded limestones. The Member II is ۲۵۰ m thick and comprises of white sandstones, changing into red shales towards upward. The lower and upper contacts of Niur Formation is conformable with underlying (Ghelli Fm.) and overlying (Padeha Fm.) formations. A total of ۷۶ surface samples were treated for chitinozoans from this formation, in order to establish a chitinozoan biozonation and to determine the palaeogeographic relationships of eastern Alborz Range (Kopet-Dagh Region) to Zagros Basin of southern Iran as well as other parts of the world.

All samples of Member I, contain well-preserved and abundant palynomorphs (acritarchs, chitinozoans, cryptospores and scolecodonts), but those of Member II are barren. Thirty-two chitinozoan species (۶ genera) were recognized in this study, and arranged in four biozones. Biozone of Ancyrochitina merga appears in the upper part of Ghelli Formation, suggesting the Ashgillian age. The biozones of Ancyrochitina laevaensis, Ancyrochitina ramosaspina / Angochitina aff longicollis and Plectochitina saharica occur through Member I of Niur Formation, that they suggest Early Silurian for this part of formation. Member II lacks of chitinozoans, but some samples contain cryptospores and a few acritarchs, indicating Wenlockian age. ۱۵ chitinozoan species out of ۳۲ are new species, including Ancyrochitina angelinae, A.bojnourdensis, A.pelmisensis, A.silurica, A.heterospinosa, A.alborzensis, A.niurensis, A.longifilosa, A.fatemae, A.nestorae, A.concava, Angochitina kalatensis, Angochitina minor, Angochitina caspiensis and Angochitina multipodspina. Except for the new species, the remainder is similar to those of the same age in Zagros Basin southern Iran, Saudi Arabia, Algeria, Libya, Morocco and Spain, suggesting the eastern Alborz Range has been part of North Gondwana Domain.