Facies analysis and depositional environment of the Upper Devonian Geirud Formation in the Tuyeh area, eastern Alborz Mountains, northern Iran

Abstract

The Upper Devonian Geirud Formation is a mixed carbonate and siliciclastic succession in the Alborz Mountains of northern Iran. It was deposited on the passive Paleo-Tethys margin of northern Gondwana. It is over 250 m thick and is bounded by the post-Lower Ordovician and the basal Carboniferous unconformities. Facies analysis of the Geirud Formation in the Tuyeh and adjoining areas of eastern Alborz recognized various clastic, mixed carbonate-clastic, carbonate and storm facies related to a ramp platform setting. The clastic facies comprise inter-bedded sandstone and shale demonstrating fining upward (fluvial) and coarsening upward (deltaic/shoreface) cycles. Cross- lamination/cross-bedding and hummocky cross-stratifications have been recognized in the deltaic and shoreface environments. Mixed carbonate-clastic shoal facies consist of cross-bedded sandy echinoderm bioclast grainstone and fossiliferous subarkose. Carbonate facies comprise girvanella bioclast grainstone (inter-tidal), gastropod/foraminifer mudstone to packstone (lagoon), echinoderm/brachiopod grainstone (barrier) and bioturbated fossiliferous mudstone to packstone (open marine). The carbonate and mixed clastic-carbonate facies are arranged into meter-scale shallowing upward cycles. Both clastic and carbonate storm deposits related to delta/shoreface, lagoon, barrier, proximal open-marine and distal openmarine environments were recognized. The storm facies fine upwards and are characterized by the presence of basal erosional surface, hummocky cross-stratification, intraformational conglomerates and mixed component of various facies.