Microfacies, depositional environment and diagenetic processes of the Qom Formation in the Khani Abad section, southeast of Kashan

During the Late Oligocene -Early Miocene time, the western border of central Iran (Qom, Hamedan, Kashan and Tafresh) to the southern margin of Jazmurian and some regions of Azerbaijan as well as in the Zagros a carbonate platform formed with evidence as a vast marine progradation that was a part of the horst of Pyrenean tectonic phase. The carbonate succession is named Asmari Formation in Zagross and Qom Formation in Azerbaijan and Central Iran (Aghanabati, 2006). Due to the economic importance of the Qom Formation as a hydrocarbon reservoir and the presence of celestite and chalk deposits, this formation is important to study. Although lots of stratigraphic and lithologic studies are done on the Qom Formation, due to variations in depositional environment of the Qom Formation in different locations, it is not possible to consider one type of sedimentary environment for this formation (e.g. Seyrafian et al., 2007; Mohammadi et al., 2009; Reuter et al., 2009; Safari et al., 2014; Daneshian et al., 2017; Nasiri Ghareh Shiran, 2017). In this research, the Qom Formation was studied in Khani Abad stratigraphic section in the southeast Kashan. The aim was to investigate microfacies, sedimentary environment and diagenetic features of Qom Formation in this area.

During the field study, sampling was performed systematically. 98 samples were collected from the Qom Formation in Khani Abad section with a thickness of 70 meters. It contains 36 soft and 62 hard samples. The sampling distance in carbonate parts was several centimeters in some areas, based on facies variations. Thin sections were prepared from hard samples. All thin sections were stained with potassium ferricyanide and alizarin-red S solution (Dickson, 1965). The petrographic classification for carbonates is based on the Dunham (1962) and Embry and Klovan (1971) classification schemes. In this study, Dolomite textural classified based on Sibley and Gregg (1987) and Mazzullo (1992) and for the size of dolomite crystals from the scale provided by Folk (1974). Wilson (1975), Pomar (2001) and Flügel (2010) facies belts and sedimentary models were also used.

The Qom Formation in Khani Abad stratigraphic section, southeast of Kashan has 70 m thickness and is composed of thick, medium and thin-bedded limestone, argillaceous limestone, dolomitic limestone and marl. This formation with erosional boundaries located between the red color non-marine rocks (the Lower and Upper Red formations). Investigations of 98 selected samples in this outcrop led to the recognition of 9 microfacies in 2 lagoonal and shoal facies belts and a marl petrofacies. The lagoonal facies are A1: Benthic Foraminifera, Bioclast Grainstone, A2: Bioclast Wackestone, A3: Benthic Foraminifera Packstone to Wackestone, A4: Bryozoan Bioclast Packstone, A5: Bivalve Packstone. A6: Bryozoa, Bivalve, Bioclast Packstone and A7: Benthic Foraminifera Red Algae Packstone to Wackestone. The shoal facies are B1: Coral Boundstone and B2: Bioclast Red Algae Grainstone. Micritization, bioturbation, cementation, dissolution, neomorphism, replacement and dolomitization are the main diagenetic features in the studied area.

According to field observations (discontinuous reef traces), the abundance of packstone facies which consist of encrusted organism related to lagoon toward the reef such as algae and bryozoa, abundance of coral boundstone facies in a low volume but large quantity and absence of gravitational sediments, the sedimentary modal of the Qom Formation in Khani Abad section (southeast of Kashan) had to be considered as non-rimmed carbonate shelf (open shelf). In the middle and top of the stratigraphical column of the Qom Formation, the amount of benthic foraminifera with porcelaneous walls reduced, while the amount of bioclast increased, which indicates normal salinity during deposition of these parts. Regarding to the degree of light penetration and according to the porcelaneous and hyaline foraminifera, the euphotic to mesophotic zones can be suggested for the study section. Since most of the carbonate sediments were deposited in shallow depth of lagoonal environment, the matrix is dominant, therefore these sediments have undergone less diagenetic processes. Cementation and dolomitization were seen mostly in shoal microfacies parts.