Micropaleontology and sedimentary environment of the Qom Formation in the northwest of Zanjan

Qom Formation carbonate succession unconformably deposited over the red clastic layers of Lower Red Formation after the Eocene tectonic movements in Iran (Berberian & King, 1981; Rahimzadeh, 1994). The type area of the Qom Formation is located in the South East of Qom city. Qom Formation has been divided into six members in the type area (Furrer & Soder, 1955). Based on biostratigraphic studies, Eocene to Middle Miocene age has been proposed for the Qom Formation (Bozorgnia, 1966; Dozy, 1944; Furrer & Soder, 1955; Jaafari, 1963; Zhu et al., 2007). Later studies by (Daneshian & Aftabi, 2010; Daneshian et al., 2008; Ruter et al., 2009) show Oligocene – Miocene age for this Formation. Sedimentological studies in different sections show carbonate shelves and ramps in different locations. So, to biostratigraphy and sedimentary environment reconstruction of Qom Formation, one stratigraphic section has been sampled in the North West of Zanjan. Geological setting  The studied section is located in the North West of Zanjan near And-Abad village. To access the area, we can follow the main road of Zanjan to the Mahneshan. Geological coordination of the studied section is: E 47O 59՜ 10.77     N 36O 48՜ 08.54 . In the studied section, we can see the F member of the Qom formation that created the heights of the area. The And-Abad village is located on the Upper Red Formation in this area. Qom Formation in this area overlies the Lower Red Formation (with the angular unconformity boundary) and underlies the Upper Red formation (with the sharp stratigraphic boundary).

Fifty-three thin sections have been prepared from 168 meters of carbonate layer related to the Qom Formation. We used Adams & Bourgeois (1967) and Leoblich & Tappan (1988) for our biostratigraphic studies. The microfacies studies and reconstruction of the sedimentary environment have been done based on Flugel (2010), and sequence stratigraphy studies carried out according to Van Wagoner et al. (1988). 23 species of foraminifera and 6 microfacies groups have been identified using a transmitted light microscope.

Biostratigraphic studies lead to the identification of 23 foraminifera species: Borelis melo curdica, Textularia sp., Amphistegina sp., Schelumbergerina sp., Quinquloculina sp., Sphaaerogypsina globulus, Pyrgo sp., Dendritina rangi, Ammonia beccarii, peneroplis thomasi, Meandropsina anahensis, Meandropsina irnica, Triloculina trigonula, Orbulina sp., Bolivina sp., Operculina complanata , Globigerinoides sp., Globigerina praebulloides, Triloculina tricarinata, Elphidium sp., Rotalia viennotti, Asterigerina rotula, Archaias kirkukensis. The total range zone of the Borelis melo curdica index species shows Burdigalian age in many studies such as (Adams & Bourgeois, 1967; Ehrenberg et al., 2007; Laursen et al., 2009). In this study, the first occurrence datum (FOD) of this species can be seen at the base of the section from the lowermost part of the Qom Formation layers and the last occurrence datum of this species is located at the top of the section. So we can propose the Burdigalian age for all studies layers of the Qom Formation in this section. Microfacies analysis leads to the identification of 6 different microfacies as follow: 1- Grainestone-packstone with small benthic foraminifera, 2- Floatstone-rudestone with benthic foraminifera and red algae, 3- Wackstone-packstone with peloid, small benthic foraminifera, and sand particles, 4- Boundstone with coral, 5- Wackstone with planktonic foraminifer, 6- Packstone-grainstone with benthic foraminifera. These facies are related to the platform interior, platform margin, and slope and toe of slope parts of the carbonate shelf. Sequence stratigraphic studies is led to identifying one depositional sequence (3rd order) with one sequence boundary type I and one sequence boundary type II. Relative sea-level changes curve and the stratigraphic log show one transgressive-regressive trend during all sections.

To biostratigraphy, microfacies, sedimentary environment, and sequence stratigraphy analysis, one stratigraphic section related to the Qom Formation has been sampled in the North West of the Zanjan. All 53 thin sections have been investigated based on microfacies and fossil contents. 23 foraminifera species have been identified. Based on the total range zone of the Borelis melo curdica index species, Bourdigalian age has been proposed for the studied layers. 6 different microfacies have been identified that show interior platform to lower slope areas of a carbonate platform as a sedimentary environment of the studied samples. On depositional sequence has been determined based on microfacies analysis that shows one transgressive-regressive trend during all sections.