Palynostratigraphy and palaeogeography of the Cambro-Ordovician strata in southwest of Shahrud City (Kuh-e-Kharbash, near Deh-Molla), Central Alborz Range, northern Iran

A total of 66 surface samples from the Mila, Lashkarak and Ghelli formations of Kuh-e-Kharbash (near Deh-Molla) were palynologically investigated, to determine the geological age of the rock units. This study was also undertaken to assess the palaeogeographic relationships of the study area to Southern and Northern Hemispheres during the Palaeozoic interval represented by these formations. 52 palynomorphs (51 acritarchs and one algal body) were recorded, which permit the recognition of six acritarch-based biostratigraphic zones. Acritarch assemblage zone I is Late Cambrian in age and occurs in the upper part of the Mila Formation. Assemblages zones II through IV are present in the Lashkarak Formation and suggest Lower Ordovician (Tremadoc–Arenig). Assemblage zones V and VI are present in the Ghelli Formation and indicate Upper Ordovician (Caradoc– Ashgill) for this formation. Based on palaeontological data, two hiatuses are present within the studied stratigraphical column. The first hiatus occurs between the Lashkarak and Ghelli formations and encompasses the Middle Ordovician strata. The second hiatus is present between the Ghelli and Geirud formations and includes the whole Silurian and Lower–Middle Devonian deposits. The above-mentioned hiatuses possibly correspond to the Caledonian Orogeny. Diverse acritarch assemblages in the Late Cambrian (Mila Formation), Lower and Upper Ordovician (Lashkarak and Ghelli formations) indicate that a marine environment continued through the entire succession. Comparison of acritarch taxa recorded in Kuh-e-Kharbash (near Deh-Molla) with those reported from other parts of the world, suggests that the Alborz Mountain Ranges have been part of peri-Gondwanan palaeoprovince during the Ordovician. © 2006 Elsevier B.V. All rights reserved.

کلیدواژه ها:

acritarchs ، Cambrian ، Ordovician ، biostratigraphy ، palaeoecology ، palaeobiogeography ، Central Alborz Mountain Range ، Northern Iran