Facies and sequence stratigraphy of Tirgan Formation in Abgarm and Sirzar sections, northeastern Kopet-Dagh Basin

Tirgan Formation with Lower Cretaceous age (Barremian-Aptian) in the Kopet-Dagh basin consists of carbonate and siliciclastic rocks (calcareous shale and sandstone) which conformably lay between siliciclastic rocks of Shurijeh Formation below، and marly Sarcheshmeh Formation at the top. In this study، Tirgan Formation in two stratigraphic sections of Abgarm (77 m in thickness) and Sirzar (46 m in thickness) in northeast Mashhad were measured and sampled. Facies variations and lithological characteristics in the carbonate sequences led to the recognition of 19 facies (17 carbonate and 2 silisiclastic lithofacies). Facies analysis indicates that they were deposited in four facies belts including open marine، shoal، lagoon and tidal flat in a shallow carbonate ramp. Three third-order sedimentary sequences in these sections were identified based on field observations، facies analysis and sequence stratigraphy. The transgressive parasequences (TST) including a predominance deep water facies and high stand tract (HST) contains shallow facies parasequens. Sequences are bounded by type 2 sequence boundaries. Interpreted sea level curve is comparable with the worldwide sea level curve. However، some differences are probably due to regional events.