Carbonate platform evolution of the Tirgan formation during Early Cretaceous (Urgonian) in the eastern Kopet-Dagh Basin, northeast Iran: depositional environment and sequence stratigraphic significance

Tirgan formation of the Kopet-Dagh Basin (northeast Iran) represents one of the Urgonian carbonate platforms that were deposited during the Early Cretaceous time in the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, six stratigraphic sections of the shallow-water platform sediments (Tirgan formation) were measured based on microfacies and fauna abundance. Detail study of petrography, fossil content, and sedimentary structures led to the identification of fifteen microfacies belonging to four facies belts including open marine, shoal, protected lagoon, and tidal flat. The sediments of the Tirgan formation exhibit calcareous green algae, abundant ooids, oysters, bryozoans, and crinoids in inner and middle platform ramp facies and planktonic bivalves and sponge spicules in outer-platform facies. Furthermore, the absence of basinal deposits and lack of evaporate evidence point to deposition under warm-temperate and humid climate conditions. Sequence stratigraphy analysis of Tirgan formation led to distinguish a single depositional sequence in all of the sections which are composed of transgressive and highstand systems tracts with sequence boundaries of type II (SB2). The lowermost lower Aptian Tirgan sequence in the study area relatively shows a similar trend in comparison with the global curve. This basin was deepened over time so that shaly and marly sediments of Sarcheshmeh formation were placed over Tirgan conformably and may suggest a drowning event that was likely related to unusual global warming. Last, this study contributes to the better understanding of the high distribution of facies assemblages in the Urgonian carbonate platforms.