Palynostratigraphy and palaeogeography of a Palaeozoic sequence in the Hassanakdar area, Central Alborz Range, northern Iran

Abstract

A total of 129 samples from the Palaeozoic Lashkarak, Geirud and Dorud formations of the Hassanakdar area were examined palynologically in order to determine more precisely the stratigraphical age of these units. The study was also undertaken to assess the palaeogeographical relationships of the Alborz Ranges to Southern and Northern Hemispheres during the Palaeozoic interval represented by these formations. 90 palynomorph taxa were recorded including 58 acritarch, 24 spore and 8 pollen taxa, which permit the recognition of six ascending stratigraphic zones. Zones I-III represent the Lower Ordovician (Tremadoc-Arenig), Zones IV-V indicate the Upper Devonian (Frasnian) and Zone VI suggests a Lower Permian (Sakmarian) age. Two major “hiata” are recognised within the studied interval; the first “hiatus” appears between the Lashkarak Formation and Geirud Formation and extends from the Upper Ordovician through the Silurian and Lower-Middle Devonian, possibly equating with the Caledonian Orogeny. The second “hiatus” occurs between the Geirud Formation (Late Devonian) and Dorud Formation (Lower Permian) and spans the interval of Famennian, the entire Carboniferous and part of the Lower Permian. It possibly corresponds to the Hercynian Orogeny. Diverse acritarch taxa in Lower Ordovician (Lashkarak Formation), Late Devonian (Geirud Formation) and Lower Permian (Dorud Formation) indicate a marine environment for each formation. Comparison of palynomorph taxa recorded in the Hassanakdar area with those reported from other parts of world indicates that the Alborz Ranges have been part of the Gondwanan Supercontinent during the Palaeozoic Era.