UPPER DEVONIAN ACRITARCHS AND MIOSPORES FROM THE GEIRUD FORMATION IN CENTRAL ALBORZ RANGE, NORTHERN IRAN
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Abstract:
The Geirud Formation consists mainly of clastic sediments which are interbedded with limestone stringers. This rock unit is well-exposed along the Central Alborz Range in northern Iran. The Geirud Formation rests unconformably on the Mila Formation (Upper Cambrian) and is conformably overlain by the Mobarak Formation (Lower Carboniferous). This study was undertaken to determine the geological age of the Geirud Formation, to interpret the depositional sites, and to reconstruct the palaeogeographic relationships between the Central Alborz Range and the southern and northern hemispheres during the Upper Devonian epoch. A total of 59 palynomorph species were described in this study, including 27 acritarch species (21 genera) and 32 miospore species (21 genera). In ascending stratigraphic order, these have been arranged in four miospore and three acritarch assemblages. The assemblage zones I through 111 represent Upper Devonian (Frasnian- Famennian) and zone IV suggests Lower Tournaisian. A major hiatus which includes Ordovician, Silurian, and Lower-Middle Devonian was recognized between the Geirud and Mila formations. This hiatus possibly coincides with the Hercynian orogeny which resulted in the emergence of the Central Alborz Range. In terms of relative frequency, the acritarchs and miospores comprise 48% and 52%, respectively. The presence of acritarchs in the Geirud Formation suggests a shallow marine environment which covered the Central Alborz Range during the Upper Devonian epoch. On the other hand, the presence of miospores in the Geirud Formation shows that spore-bearing plants grew on the adjacent land masses that were transported into the shallow marine basin. Among the encountered tnorphotype species of the Ceirud Formation Archneoper. Perisaccus scabratus a nd Retispora lepirloyhyta are more important than the
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volume 5 issue 2
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publication date 1994-09-01
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