Sequence stratigraphy of the Qom Formation in the Sirjan-Abadeh region

Abstract

Sequence stratigraphy of the Qom Formation deposits (in two stratigraphic sections) in the Sirjan-Abadeh region has been studied. The Qom Formation deposits in the Sirjan area (Bujan section; with Rupelian-Chattian age and 156 m thickness), consist mainly of medium to thick-bedded and massive limestones, and marl. They are unconformably underlain by the polygenetic conglomerate with indeterminate age, and unconformably overlain by the quaternary alluvium. The Qom Formation outcrops in northern Abadeh (with Rupelian- Chattian age and 85 m thickness), consist mainly of massive and bedded limestones, shale and conglomerate. They are conformably underlain by the Lower Red Formation and unconformably overlain by the quaternary alluvium. Eleven different microfacies, belonging to open marine (middle ramp), lagoon (inner ramp), beach and braided streams were recognized. Based on the field investigations, deepening and shallowing patterns in microfacies, staking patterns and the distribution of foraminifers, four third-order sequences in the Bujan section and five third-order sequences in the Abadeh 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).