Age determination and evolution of the paleoenvironmental conditions of Oligocene-Miocene sediments (Qom Formation) in the Qom Back-Arc Basin (northern margin of Neotethys), Central Iran

The age and evolution of paleoenvironmental conditions in the Oligocene-Miocene sedimentary sequences (Qom Formation) from the Andabad, Nowbaran, and Kahak areas were studied from the distribution of benthic foraminifera, microfacies, and microtaphofacies. We identified 56 species from 40 genera and two subspecies of planktic and large benthic foraminifera belonging to three biozones. The Qom Formation spans the Rupelian to Chattian, Aquitanian, and Burdigalian ages, in the Kahak, Nowbaran, and Andabad areas, respectively. Nine microfacies, five microtaphofacies, and one terrigenous facies were identified in the Qom Formation in the studied areas. Moderate (40–50 psu) to high (> 50 psu) salinity and euphotic conditions were dominant during the lower Rupelian-Chattian (Kahak area), lower Aquitanian (Nowbaran area), upper Aquitanian (Nowbaran area), lower Burdigalian (Andabad area), and upper Burdigalian (Andabad area). Variable salinity conditions from normal (34–40 psu) to high (>50 psu) and photic conditions from euphotic to mesophotic-oligophotic characterize the upper Rupelian-Chattian (Kahak area), middle Aquitanian (Nowbaran area), and middle Burdigalian (Andabad area) stages. Mesotrophic-oligotrophic conditions are observed during the Oligocene (Rupelian-Chattian) and the Miocene (Aquitanian and Burdigalian). Furthermore, hydrodynamic energy during the Rupelian-Chattian was higher than during the Aquitanian and Burdigalian, and seawater depth was greater during the Aquitanian age and was deeper than during the Rupelian-Chattian and Burdigalian in the Qom basin (back-arc basin). The connection between the Tethys seaway and Paratethys basins was completed during the early and middle Miocene (Burdigalian and Langhian).