UPPER DEVONIAN ACRITARCHS AND MIOSPORES FROM THE GEIRUD FORMATION IN CENTRAL ALBORZ RANGE, NORTHERN IRAN

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