Biostratigraphy, microfacies and depositional model of the Qom Formation in northeastern Natanz (southeastern Qom back arc basin)

The marine Qom Formation was deposited at the north-eastern coast of the Tethyan Seaway (Reuter et al., 2009). Its deposition took place in three NW-SE-trending basins: Sanandaj-Sirjan fore-arc basin, Urumieh-Dokhtar magmatic arc (Intra-arc basin) and Central Iran back-arc basin (Mohammadi et al., 2013, 2015). Transgression of the Qom Sea started from the southeast and continued northwestward gradually. Evaporate deposits of the Qom Formation were deposited merely in a rather small area of the Central Iran back-arc basin and deposited in the Early Miocene (Aquitanian-Burdigalian). Because of high facies changes of the Qom Formation and its situation in intermountain basins, defining a depositional model for the Qom Formation for the whole of the Central Iran is impossible. The aim of this research is to study of the biostratigraphy, microfacies and depositional model of the Qom Formation in Natanz area.

One stratigraphic section of the Qom Formation was measured and sampled in Natanz area with the aim to study its biostratigraphy, microfacies and depositional model. A total of 166 samples were collected systematically and based on facies and texture changes. All samples were studied in detail by microscope. Based on different studies the obtained foraminifera were identified and age of the study section was determinate. The classification of carbonate rocks followed the scheme of Dunham (1962) and Embry & Klovan (1971). Finally, depositional model of the study section was reconstructed. 

The study succession with 330 m thickness is one of the most cliff-forming outcrops of the Qom Formation and lithologicaly are composed of two parts: the lower part with 210 m thickness consists mainly of medium to thick bedded and massive limestone, reefal limestone and shale; while, the upper part with 120 m thickness is composed of medium to thick bedded limestone and marl. On the basis of recognized foraminifera and their vertical distribution, the Qom Formation is Rupelian?-Chattian-Aquitanian in age in the study area. Field and laboratory studies caused to identification of 9 microfacies, they are: A) Shale; B) Bioclastic imperforate foraminifera packstone; C) Sandy bioclastic miliolida wackestone; D) Coral boundstone; E) Bioclastic perforate and imperforate foraminifera packstone; F) Marl; G) Bioclastic perforate foraminifera packstone; H) Bioclast wackestone; I) Bioclastic bryozoan packstone. The investigation results indicate that the lower part of the study section deposited mainly in lagoonal settings; while its upper part deposited entirely in the open marine setting. At the end of the deposition of the lower part, increasing of the colloidal grain inputs (which characterized by the abundance presence of the marly facies) and depth (which characterized by the presence of typical fauna of the deeper settings) prevented the growth of the reef-forming corals. According to recognized microfacies and field investigations, deposition of the Qom Formation in northeastern Natanz took place on a carbonate ramp. The proposed carbonate ramp can be divided into inner ramp and middle ramp. Inner ramp is characterized by high abundance of imperforated foraminifera and mainly includes the lower part of the study section. Inner ramp is dominated mainly by A to E microfacies. Middle ramp is characterized by high abundance of larger benthic foraminifera with hyaline wall and mainly includes the upper part of the study section. Middle ramp is dominated mainly by F to I microfacies. Deposits of the outer ramp, which is characterized by absence of larger benthic foraminifera together with the presence of planktonic foraminifera and light-independent organisms, are not recognized in the study section.