Biostratigraphy and Paleogeography of Padeha(upper part) Khoshyeilagh and Mobarak(lower part) formations in the Mighan area, norhwestern Shahrud city, Eastern Alborz Range

A total of 104 surface samples were selected from the measured stratigraphic column (upper part of Padeha, Khoshyeilagh and Lower part of Mobarak formations) in the Mighan village. The samples were analyzed in the palynological Laboratory for palynomorph entities in order to determine the age relationships of the upper part of Padeha Formation and the whole Khoshyeilagh Formation as well as the lower part of Mobarak Formation. In addition to palynomorphs, brachiopod and foraminiferal taxa were investigated in this study. A total of 103 fauna and micro floral species was identified, belonging to 72 palynomorph species (46 genera). Out of 72 palynomorph taxa 32 species belong to miospore group, 38 species related to acritarch group, one chitinozoan and one scolecodont. Moreover, 5 micro faunal genera (10 species) and 21 macro faunal spcies (15 genera) were encountered. Based on stratigraphic distributions of palynomorphs, foraminifers and brachiopods taxa eight assemblage zones were established. The zone I occurs in the upper part of the Padeha Formation and suggests the early Frasnian. The zones of II to VII are present within the Khoshyeilagh Formation and reveals the Middle Frasnian to late Famennian. The zone VIII occurs in the basal part of Mobarak Formation and suggests Early Carboniferous (Tournaisian) for this part of Mobarak Formation. The presence of miospore species of Archaeoperisaccus opiparus in the upper part of Padeha Formation and miospore spcies of Vallatisporites pusillites, Retispora lepidophyta, Retusotriletes phillipsii and Hymenozonotriletes perplexa in the Khoshyeilagh Formation suggest a tropical latitude during the Late Devonian in the study area. On the other hand, based upon the continental elements vs. the marine elements(K=C.E./M.K.) is 1.587, suggesting a shallow marine environment, which is consistent with the sedimentological data. On the other hand, the presence acritarch taxa such as Deltotosoma intosum, Gorgonisphaeridium dicissum, Papulogobata annulata and Maranhites perplexus in the Devonian sediments were compared with those of Zagros, Central Iran, Algeria, Libya, Morocco and Western Australia suggesting broad similarity. This similarity reveals that all part of Iran has been part of Gondwana landmass during the Late Devonian.Keywords: biostratigraphy, Devonian sediments, Khoshyeilagh Formation, Upper part of Padeha Formation, Mighan area, Northwestern shahrud.