Reconstruction of paleoecological conditions of the Qom Formation in the Kahak, Nowbaran, and Andabad areas (back-arc Qom basin)

The Tethyan seaway was located between the Gondwana and Eurasia supercontinents (Reuter et al., 2009). The Qom basin developed at the northern margin of the Tethyan seaway (Reuter et al., 2009). In addition, the Qom Formation was deposited in this basin during the Oligocene-Miocene ages (Reuter et al., 2009). The researchers indicated that the Qom basin was divided into three sub-basins by the Urumieh-Dokhtar magmatic arc, and the sub-basins are inclusive of the fore-arc sub-basin (Esfahan-Sirjan basin), the back-arc sub-basin (Qom basin), and the Intra-arc sub-basin (Reuter et al., 2009; Mohammadi et al., 2013). The study areas (Andabad, Nowbaran, and Kahak areas) lie in the back-arc sub-basin (Qom basin). In this research, the paleoenvironmental conditions (such as light, nutrient supply, salinity, substrate nature, depth of seawater, and temperature) of the Oligo-Miocene deposits (Qom Formation) were studied.

The Andabad area, with the coordinates of N 36˚ 48' 12.6" and E 47˚ 59' 28", is located 19 km away from northwestern Mah Neshan city. The Nowbaran area (coordinates of 35˚ 05' 22.5" and E 49˚ 41' 00") which is situated at 53 km from northwestern Saveh city, as well as the Kahak area (coordinates of N 34˚ 23' 01.1" and E 50˚ 50' 20.2"), lies 30 km from south Qom city. The Qom Formation in the Andabad (with 220 m thickness), Nowbaran (with 458 m thickness), and Kahak (with 645 m thickness) areas mainly contain shale and limestone rocks. The lower boundary between the Qom and Lower Red formations is unconformable with the Andabad and Nowbaran areas. In the Kahak area, Eocene volcanic rocks are covered by the Qom Formation sediments. The Upper Red Formation unconformably overlies the Qom Formation in the Andabad and Kahak areas. The upper boundary between the Qom Formation and Recent Alluvial deposits is unconformable. A total of 401 rock samples from the study areas were collected. Microfacies analysis and paleoecological studies were performed by resources such as Dunham (1962), Embry and Klovan (1971), Pomar (2001), Mutti & Hallock (2003), Mossadegh et al. (2009), Mateu-Vicens et al. (2009), Brandano et al. (2009), and Pomar et al. (2014).

The deposits of the Qom Formation were formed in the Kahak, Andabad, and Nowbaran areas during the Rupelian-Chattian, Aquitanian, and Burdigalian ages, respectively. The paleoecological studies show the high marine salinity to hypersaline conditions in the Kahak (Rupelian-Chattian), Nowbaran (Aquitanian), Andabad (Burdigalian) areas. The euphotic condition can be considered for the deposition environment of the Qom Formation in the Kahak (Rupelian-Chattian), Nowbaran (Aquitanian), Andabad (Burdigalian). In the study areas, the oligotrophic to mesotrophic conditions can be observed. The paleo depth of seawater in the Kahak and Andabad areas was variable from less than 10 m to more than 30 m. The seawater depth in the Nowbaran area fluctuated from less than 10 m to more than 40 m. Therefore, the sea in the Aquitanian stage was deeper than the Rupelian-Chattian and Burdigalian ages. The abundance of coral, red algae, and large benthic foraminifera in the study areas indicate tropical and subtropical environments (18 to 25 °C). The grain associations such as foramol and foralgal (belong to heterozoan association) were identified in the study areas.