Resumen de Facies variability of a lower Aptian carbonate platform succession, Tirgan Formation, eastern Kopet Dagh Basin, northeast Iran

Parisa Tabatabaee, Yaghoob Lasem, Davood Jahani

• The Lower Cretaceous (Hauterivian-lower Aptian) Tirgan Formation is a carbonate platform succession in the Kopet Dagh Basin and is up to nearly 800 m thick. This study focuses on facies variability and depositional settings of the Tirgan in the eastern part of eastern Kopet Dagh where the formation constitutes the basal part of the lower Aptian deposits. The succession developed during the final stage of evolution of Cretaceous carbonate platform in the southern Eurasian margin and could demonstrate a synchronicity with comparable successions in other areas. In the study area, the Tirgan (48 m thick) overlies Berriasian–Barremian continental strata and underlies the lower Aptian deep marine deposits. Facies analysis of Tirgan determined several facies related to open marine, ooid grainstone shoal, protected lagoon, and lower intertidal facies belts of a carbonate system. Abundant ooids, calcareous green algae, mud size carbonate, lack of evaporite evidence, and the absence of basinal deposits record deposition in warm water shallow marine settings of a carbonate ramp under a relatively humid climate. The Tirgan encompasses a single third-order depositional sequence composed of transgressive and highstand systems tracts. The transgressive package consists of intertidal, lagoon, and open marine facies intercalated with storm and shale layers showing an overall retrogradational trend. However, the highstand package consists mainly of ooid grainstone facies and displays prograding clinoforms. The lowermost lower Aptian Tirgan sequence in the study area corresponds to the basal Aptian global third-order sea level cycle. Contact of Tirgan sequence with the overlying middle to late Aptian deep marine strata is abrupt, suggesting a drowning event, which was likely the result of early Aptian extraordinary global warming.