Carbonate platform evolution of the Tirgan formation during Early Cretaceous (Urgonian) in the eastern Kopet-Dagh Basin,
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ORIGINAL ARTICLE
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 Tayebeh Golafshani1 · Mohammad Khanehbad1 · Reza Moussavi‑Harami1 · Asadollah Mahboubi1 · Amir Feizy2 Accepted: 28 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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. Keywords Urgonian · Tirgan formation · Depositional environment · Sequence stratigraphy
Introduction During the Barremian–Aptian, because of favorable geographical location and warm climate, the typical Early Cretaceous shallow carbonate facies that is known as Urgonian facies, was widely distributed on both sides of the Tethys (Fig. 1). Tirgan formation as one of the Urgonian platform deposited in north Tethys, the Kopeh-Dagh basin during the Early Cretaceous. The Kopeh-Dagh basin subsided during Jurassic to the latest Cretaceous * Mohammad Khanehbad [email protected] 1
Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
National Iranian Oil Company, Exploration Directorate, Tehran, Iran
2
time, when it was connected, Turkmenistan and northern Afghanistan to the Caspian Basin (Afshar-Harb 1979; Buryakovsky et al. 2001; Ghorbani 2019). This basin was shaped in Middle Triassic
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