Foraminiferal biostratigraphy of the Qom Formation, southeast Delijan

The Qom Formation is known with layers of shallow water limestone and marl which its special geological features and distinct color distinguish it from the Lower Red and Upper Red formations. This formation were deposited in back-arc and for-arc basins and is one of the lithostratigraphic units of Central Iran Zone with Oligocene to Miocene age that shows the last transgression in central Iran (Rahimzadeh, 1994). This formation is diachronous in areas which have outcrops and the changes of age is significant considering the time of sea transgression and the beginning of sedimentary cycles. The Qom Formation maybe shows part of Rupelian, Chattian, Aquitanian, Burdigalian and even Tortonian (Aghanabati, 2004). According to Berberian (1983), generation of sedimentary basin of the Qom Formation has been indication subduction of Tethys oceanic crust beneath central Iran by opening back-arc and associated with deposition of marine sediments of the Qom Formation and alkaline volcanic processes. Changes of  age and thickness in the Qom deposits of adjacent areas of the studied section such as Aranjan and Doreh sections with Rupelian age (Daneshian & Raziei, 2004), Maragh section the Aquitanian (Raziei, 2004), Jazeh sections the Rupelian-Chattian (Mohammadi et al., 2009), Navab section the Aquitanian-Burdigalian (Daneshian & Aftabi, 2010), Kaftarkuh section the Aquitanian (Estakhr, 2010), Khafr section the Aquitanian (Daneshian & Naderi, 2013) and in northeast Delijan the  Rupelian-Chattian (Karevan et al., 2014) were considered. So, Farnagh stratigraphic section was selected for lithostratigraphic study and age determination in southeast Delijan. Although the Qom Formation deposits were attributed to Oligo-Miocene in this area, we expect the existence of deposits with Oligocene and Miocene (Aquitanian) age. But geological and paleontological studies in this research, confirm the end of Early Miocene. This situation is possibly because of tectonic and performance which caused considerable changes in lithology, facies and age with little distance.

After reconnaissance and choice appropriate section, a systematic sampling was done of the Qom Formation. Preparation of samples contains making thin sections of hard samples and washing soft samples then separation foraminifera in usual methods. Study of thin sections was carried out with optical microscope and stereomicroscope was used for soft samples. For identification of benthic and planktic foraminifera were used many references such as Henson (1950), Blow (1959), Huang (1964), Wynd (1965), Adams & Bourgeois (1967), Postuma (1971), Kennett & Sirinivasan (1983), Papp & Schmid (1984), Bolli & Saunders (1985) and Loeblich & Tappan (1988). Based on presence of different species, range chart of foraminifera was drawn and compared with biozonation of Adams & Bourgeois (1967) and Wynd (1965).

Study of 210 collected samples including 151 hard and 59 soft led us to recognition 45 genera and 64 species, contains 38 genera and 54 species of benthic and 7 genera and 10 species of planktic foraminifera. While 27 genera and 39 species of benthic and 4 genera and 5 species of planktic foraminifera were identified of thin sections and 15 genera and 18 species of benthic and 6 genera and 7 species of planktic foraminifera from soft samples. Important index foraminiferal taxa of Farnagh section such as Ammonia beccari, Ammonia stachi, Austrotrillina asmariensis, Austrotrillina howchini, Borelis melo curdica, Bozorgniella sp., Globigerinella obesa, Globigerinoides subquadratus, Globorotalia archeomenardi, Rotalia viennoti and Valvulina sp.1, confirm the Early Miocene (Burdigalian). The Burdigalian age is corroborated specially by Borelis melo curdica in the major part of studied sequence. So from 558 m thickness, about 15 m (in bottom) are Aquitanian and 543 m belong to Burdigalian. While, the presence and stratigraphic position of species such as Ammonia beccari, Austrotrillina howchini, Globigerinella obesa, Globigerinoides subquadratus and Globorotalia archeomenardi with above species are confirmed an Early Miocene (Burdigalian) age for most part of this succession.
According to presence of index species and similarity to Asmari Formation’s fauna, this section is comparable to Borelis melo group-Meandropsina iranica Assemblage Zone (biozone 1) and Miogypsinoides-Archaias-Valvulinid Assemblage Zone (biozone 2) of Adams & Bourgeois’ biozonation (1967) and Austrotrillina howchini - Peneroplis evolutus assemblage zone (biozone 59) and Borelis melo-curdica assemblage zone (biozone 61) of  Wynd biozonation (1965). So that samples 1 to 9 specifies biozone 2 and samples 10 to 160, biozone 1 of Adams & Bourgeois’ biozonation (1967). These results show that the outcrops of the Qom Formation in Farnagh section are different from the foraminiferal contents and age with adjacent areas and possibly is affected and performance of tectonic and caused Oligocene and main part of Aquitanian sediments not present in this section.