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. This formation in the Sirjan area (Bujan section; with Rupelian–Chattian age and 156 m thickness), consists mainly of medium- to thick-bedded and massive limestones and marls. 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, shales, and conglomerates. 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, stacking patterns, and the distribution of foraminifers, four third-order depositional sequences in the Bujan section and five third-order depositional sequences in the Abadeh section have been identified. These depositional sequences are bounded by both type 1 and 2 sequence boundaries. Sequence boundaries are mainly characterized by the abrupt change in lithology and biotic components. Keywords: Qom Formation, Rupelian–Chattian, Sequence stratigraphy, Ramp. Introduction The Qom Formation, Rupelian–Burdigalian in age (Mohammadi et al. 2013, 2015, 2019), was deposited at the northeastern coast of the Tethyan Seaway (Reuter et al. 2009). It consists of thick successions of marine marls, limestones, gypsum, and siliciclastics that were deposited on extensive mixed carbonate-siliciclastic ramps (Reuter et al. 2009), in the Sanandaj–Sirjan fore-arc basin, Urumieh-Dokhtar Magmatic Arc (Intra-arc basin), and Central Iran back-arc basin (Mohammadi et al. 2013, 2015, 2019, Mohammadi and Ameri 2015). The Qom Formation is the main reservoir and source rock for hydrocarbons in middle Iran (Abaie et al. 1964; Rezaei et al. 2000; Mohammadi et al. 2019). This formation is isochronous with the Asmari Formation in the Zagros Basin in southern and southwestern Iran, which is a well-known fractured reservoir that contains 12% of the world's oil (Xu et al. 2007). Although geological investigations on the Qom Formation started more than 160 years ago (Loftus 1855), surprisingly little information is available concerning the sequence stratigraphy of the Qom Formation. Besides, most of the previous works are limited to one stratigraphic section. The aim of this study is, therefore to bridge this gap by studying two stratigraphic sections in the Sirjan–Abadeh region. The stratigraphic sections are located in the Sanandaj–Sirjan fore-arc basin. Material & Methods Bujan and northern Abadeh sections of the Qom Formation in the Sirjan–Abadeh region were studied bed by bed to analyze their sequence stratigraphic framework. A total of 99 and 55 samples were collected (based on field evidence and lithofacies changes) from the Bujan and northern Abadeh sections, respectively. Softer samples were disaggregated and the foraminifera and ostracoda picked and analyzed, while thin sections were prepared from harder samples. All samples were studied in detail and particular attention was paid to main allochems such as foraminifera, corals, corallinacean algae, bryozoans, and ostracoda. The biostratigraphy of the Bujan and northern Abadeh stratigraphic sections are discussed in detail in Mohammadi et al. (2014) and Mohammadi and Ameri (2015), respectively. Likewise, the facies of the Bujan and northern Abadeh stratigraphic sections are analyzed in detail in Mohammadi et al. (2014) and Mohammadi and Ameri (2018), respectively. The facies were investigated by Mohammadi (2020b) to interpret the palaeoenviornment and depositional model. The sequence stratigraphy analysis is based on the model presented by Hunt and Tucker (1992, 1995), Emery and Myers (1996), Hardenbol et al. (1998), Catuneanu (2000, 2006), Simmons et al.(2007), and Catuneanu et al. (2009, 2010, 2011). Discussion of Results & Conclusions Eleven different microfacies/lithofacies, belonging to open marine (middle ramp), lagoon (inner ramp), beach, and braided streams were recognized in the Qom Formation of the two stratigraphic sections from the Sirjan–Abadeh region. The recognized microfacies/lithofacies are as follows: Perforate foraminiferal wack-/packstone, coralline perforate foraminiferal packstone, bioclastic bryozoan corallinacea coral pack-/grainstone, coral boundstone, sandy bioclast wack-/pack-/grainstone, perforate and imperforate foraminiferal bioclast wack-/pack-/grainstone, bioclastic imperforate foraminiferal wack-/packstone, Marl with porcelaneous foraminifera, red shale, muddy sandstone, matrix-supported conglomerate. The latter was deposited within continental braided streams. Four third-order depositional sequences in the Bujan section and five third-order depositional sequences in the Abadeh section have been identified. These depositional sequences are bounded by both type 1 and 2 sequence boundaries. In the Bujan section, only the transgressive systems tracts and the highstand systems tracts were recognized, while in the Abadeh section, the lowstand systems tracts were also identified (in two third-order depositional sequences). Maximum flooding surfaces were coincident with deepening and shallowing in each depositional sequence. Comparison of the relative sea-level curves in the study sections with global sea-level curve of Haq et al. (1988) shows a reasonable correlation around Rupelian–Chattian boundary. However, remaining relative sea-level curve changes probably were related to local factors such as the tectonic and sediment supply in the study area