Sequence stratigraphy of the Qom Formation in the Natanz-Qom region

Sequence stratigraphy of the Qom Formation deposits (in two stratigraphic sections) in the Natanz-Qom region has been studied. The Qom Formation outcrops in the Natanz section (with Rupelian?-Chattian-Aquitanian age and 330 m thickness), consist mainly of medium to thick-bedded and massive limestones, reefal limestones, shale, and marl. They are underlain by the Eocene volcanic rocks and unconformably overlain by the quaternary alluvium. The Qom Formation deposits in southeastern Qom (Khurabad section; with Rupelian-Burdigalian age and 260-m thickness), consist mainly of medium to thick-bedded and massive limestones, marly limestones, marl, evaporate deposits and siliciclastic deposits. They are underlain by the Eocene volcanic rocks and disconformably overlain by the Upper Red Formation (URF). Fourteen different microfacies, belonging to open marine (middle ramp), lagoon (inner ramp) and peritidal zone were recognized. Based on the field investigations, deepening and shallowing patterns in microfacies, staking patterns and the distribution of foraminifers, three third-order sequences in the Natanz section and five third-order sequences in the Khurabad section have been identified. These depositional sequences are bounded by both type 1 and type 2 sequence boundaries. Sequence boundaries are mainly characterized by the abrupt change in lithology and biotic components. In the Natanz section, only the transgressive systems tracts (TST) and the highstand systems tracts (HST) were recognized; while in the Khurabad section, the lowstand systems tracts (LST) were also recognized (in two third-order sequences). The transgressive systems tracts (TST) of sequences show deepening-upward facies trend, whereas the highstand systems tracts (HST) have a shallowing-upward facies trend.