Stratigraphy, microfacies and sedimentary environment of the Qom Formation at Madabad celestite deposit, south of Zanjan

The Qom Formation was deposited at the north-eastern coast of the Tethyan Seaway, in the Oligocene - Miocene, during the final sea transgression, in Central Iran (Reuter et al., 2009). Although the Mohammadi et al. (2013) believe that above 35˚N (including the study area in this research) deposition of the Qom Formation started during the Miocene. It is essential and important to study different properties of the oil-bearing Qom Formation because of economic importance and communicative role between Eastern Tethys (the proto-Indian Ocean) and the Western Tethys region (the proto-Mediterranean Sea) in the Iranian Plate at the same time (Mohammadi et al., 2013). Furrer & Soder (1955) subdivided the Oligocene-Miocene marine strata of the Qom Formation in the type locality of the formation near the town of Qom, into six members (a-f members: a-member basal limestone, b-member sandy marls, c-member alternating marls and limestones, d-member evaporites, e-member green marls and f-member top limestone). In the Zanjan area, only f-member of the Qom Formation has been deposited (Aghanabati, 2004). In general, the f-member consists of light colored, porous, in part chalky and in part cemented limestone. Although many studies have carried out for nearly four decades on the f-member of the Qom Formation outcrops in Central Iran back-arc basin (which they are listed in Mohammadi et al., 2013), stratigraphical, microfacies analysis and sedimentary environments studies of the f-member of the Qom Formation deposits of the Zanjan area has been the subject of only a few studies. So, here for the first time, we document and discuss the results of detailed fieldwork and microfacies analysis from the early Miocene carbonate platform succession in the south of Zanjan (f-member of the Qom Formation).

This study involves one stratigraphic section that was measured bed by bed and investigated sedimentologically.During the fieldwork study, detailed stratigraphic sections were measured, sampled and described with respect to carbonate facies and biota. The petrographic description is based on approximately 73 thin sections. Thin sections were stained using the method of Dickson (1965) to distinguish ferron and non-ferron calcite from dolomite. The petrographic classification for carbonates is based on Dunham limestone classification (Dunham, 1962). Flügel (2010) facies belts and sedimentary models were also used. The composition of associated fauna (presence of red-algae, coral, benthic foraminifer and echinoderm) and non-skeletal grains (e.g. intraclasts and peloids) was considered. Sedimentologic texture and structure (e.g. crossbedding, dolomitization, presence of silt-size quartz grains, boring and burrowing) have been considered qualitatively.

The Qom Formation in the Madabad celestite deposit (south of Zanjan), lithologically composed of 190 m of medium to thick-bedded and massive limestone and marly limestone. In this area, the Qom Formation is conformably overlies the clastic rocks of the Lower Red Formation and is in turn conformably overlain by the Upper Red Formation. In detail, the Qom Formation in the study area consist of 7 lithostratigraphic units as follow from base to top of the formation: 1) thin to medium-bedded limestone with interbedded of thin-bedded argillaceous limestone, 2) thick-bedded coral-bearing limestone, 3) thick-bedded limestone with interlayers of marly limestone, 4) thin-bedded marly limestone, 5)  thick-bedded echinoderm-bearing limestone with interlayers of marly limestone, 6) thin-bedded marly limestone and finally  and 7) thick-bedded to massive limestone with interlayers of marly limestone.The main components of the Qom Formation contain benthic foraminifera with hyaline test, coral, red algae with less frequency of planktonic foraminifera. Due to the abundance of red-algae, larger benthic foraminifera and micrite, the Qom Formation platform facies is referred to as “red algae foraminifera dominated packstone”. Field and microscopic studies led to identification of five microfacies in the limestone units of the Qom Formation in the Madabad area. These microfacies, ordered from shallower to deeper environments, include: A) red algae coral packstone, B) red algae bioclast packstone to wackestone, C) perforate benthic foraminifera packstone to wackestone, D) red algae echinoderm wackestone and E) planktonic foraminifera red algae bioclast wackestone.In general, microanalysis and paleoenviornmental interpretation of the Qom Formation show that this formation was deposited in a variable depositional system. The Qom Formation facies are dividable to four facies as follow: alluvial-deltaic facies carbonate platform-evaporatic facies, slope facies and basin facies (deep sea facies) (Rahimzadeh, 1994). Microfacies analysis including abundant hyaline-test benthic foraminifera as well as the lack of restricted lagoon microfacies show that in the Madabad section, the Qom Formation was deposited in open marine environment. According to recognized microfacies and absences of gravity deposits (turbidites), real and continuous reef, barrier and storm structures, carbonate platform of the Qom Formation developed on an open shelf without effective barriers separating it from the sea. In detail, the distribution of foraminifera and other components, in addition to the vertical microfacies relationships indicate that facies model of the Qom Formation in this section was distal-inner to middle shelf. The distal inner shelf including only the (A) microfacies and the other recognized microfacies (B-E) deposited through the proximal to distal parts of the middle shelf. Proximal middle shelf is characterized by larger benthic foraminifera with hyaline wall in addition to red algae and distal middle shelf is dominated by planktonic foraminifera and red algae.

The Qom Formation in the Madabad celestite deposit (south of Zanjan), lithologically composed of 190 m limestone and marly limestone. Field and microscopic studies led to identification of five microfacies. Distribution of foraminifera and other components, in addition to the vertical microfacies relationships indicate that facies model of the Qom Formation in this section was distal-inner to middle shelf platform.