Microfacies and sedimentary environment of Tirgan Formation in the west of Kopeh-Dagh sedimentary basin, NE Iran

The Kopet-Dagh Basin in northeast Iran, is an inverted basin (Allen et al., 2013) extending from the east of Caspian Sea to NE Iran, Turkmenistan and north Afghanistan (Afshar Harb, 1979; Buryakovsky et al., 2001). This basin separates Central Iran from the Turan plate (Alavi et al., 1997). Following the closure of Palaeo-Tethys in the Middle Triassic (Alavi et al., 1997) and the opening of Neo-Tethys during the Early to Middle Jurassic (Buryakovsky et al., 2001), the Kopet Dagh Basin formed in an extensional regime during the Early to Middle Jurassic (Garzanti and Gaetani, 2002). The objectives of this study are facies analysis and depositional model of the Tirgan Formation in the study areas.

In this study, 3 outcrop sections from the Tirgan Formation (Fig 1), were measured bed-by-bed, and samples were taken systematically. Over 524 fresh carbonate samples were collected, from which 500 thin sections were made. Carbonates were classified using the Dunham (1962) scheme. Siliciclastic rocks were classified using the Folk (1980) scheme.

Petrographic studies in the study area, 17 different microfacies types have been distinguished in four facies belts including tidal-flat, lagoon, shoal, and open marine and 2 petrofacies including subarkose and quartzarenite. According to the field observations, petrographic studies, and comparison of vertical and lateral changes of the facies, the carbonate facies assemblage was deposited in a homoclinal carbonate ramp, while the siliciclastic lithofacies were deposited in tidal environments at high detrital entry rates.