Biostratigraphy and Microfacies of Qom Formation in Urumieh – Dokhtar Zone, Iran

Qom Formation is well developed in Urumieh–Dokhtar Zone, Iran. Three different measured sections were studied in this area in order to interpret the biostratigraphy and microfacies of QomFormation. In the Ashtian and Hezar Abad sections, the Qom Formation disconformably overlies and underlies Lower Red and Upper Red formations, respectively.In the Rakin section, the lower boundary of Qom Formation with tuffs of Urumieh–Dokhtar magmatic arc is sharp and its upper boundary is covered by the recent alluvium. Based on the recognized foraminifera, the age of Qom Formation in the Rakin stratigraphic section is determined as Chattian–Aquitanian and in the Ashtian stratigraphic section as well as the Hezar Abad stratigraphic sections is determined as Chattian. In this study 21microfacies types have been recognized which can be grouped into three depositional environments: Lagoon, shallow and deep open marine. The Qom Formation in three sections represents sedimentation inan open shelf.The great abundance of bryozoans and red algae as well as low diversity of foraminifera indicates that carbonate sediments of the Qom Formation were deposited in a mestrophy to eutrophy conditions. Keyword: Chattian, Open shelf, Urumieh–Dokhtar, Qom Formation ISSN (Print) : 2319-8613 ISSN (Online) : 0975-4024 Zohreh Holakouee et al. / International Journal of Engineering and Technology (IJET) DOI: 10.21817/ijet/2018/v10i4/181004216 Vol 10 No 4 Aug-Sep 2018 1052


INTRODUCTION
Based on the sedimentary sequence, magmatism, metamorphism, structural setting and intensity of deformation, the Iranian Plateau was subdivided into eight continental fragments (Heydari et al., 2003), including Zagros, Sanandaj-Syrjan, Urumieh-Dokhtar, Central Iran, Alborz, Kopeh-Dagh, Lut, and Makran. Urumieh-Dokhtar Magmatic Belt (UDMA) is mainly a magmatic belt with northwest-southeast direction.This belt with some 1500 km length and 100 km width is extended from Sahand to the Bazman and then enters in Pakistan. Mgmatism in the UDMA started since Ypresian in Eocene with a highest activity in the Middle Eocene (Emami, 1981). Sea level drop in Oligocene time caused the removal of underlying Eocene bedsover sunstsntial areas. The pronouncedsea level fall exposed almost the entire the UDMA.The mid-Oligocene global sea level rise constricted marine citculation from the Mediterranean to the Indian Ocean (Rahaghi, 1980, Khaksar andMaghfouri Moghaddam, 2007;Reuter et al., 2008Reuter et al., , 2009). Mohammadi and Ameri (2015)reported that the Qom Formation is the last transgression of the sea in the UDMA.
The Qom Formation is composed of Limestone, argillaceous limestones andnsandstone.It is present throughout theCentral Iran Zone, Sannadaj-Sirjan Zone and UDMA (Mohammadi et al., 2011). The objectives of this study are to establish abiostratigraphic framework and demonstration of the paleobiofacies of the Qom Formation in the UDMA based on the distribution of the larger benthic foraminifera.

STUDY AREA
This research involves three stratigraphic sections from the Qom Formation in UDMA in the central part of Iran. The study area in the Rakin section is located about 70 Km north of Arak (Central Province). The section was measured in detail at 49° 33' 09" N and 34° 46' 01" E. The study area in the Ashitan section is located about 1Km east of Ashtian (Central Province). This section was measured in detail at 50° 02' 09" N and 34° 31' 07" E.The study area in the Hezar Abad section is located about 23 Km southeast of Ashtian. The section was measured in detail at 50° 14' 19" N and 34° 27' 11" E.
The Qom Formation in the Ashtian and Chenar sectionsunderliestheLower Upper Red Fotmation while in Rakin section, its upper boundary is not exposed. The Qom Formation in the Ashtian and Hezar Abad sections conformably overlies the Lower Red Formation.In the Rakin section, the lower contact of the Qom Formation with the underlyingtuff of the Karaj Formation is conformable.The total thickness of the Qom Formation is about 148m, 150 and 148 m in Rakin, Ashtian and Hezar Abad sections, respectively.

MATERIAL AND METHODS
For this research, 168 samples from the Qom Formation in the selected stratigraphic sections were studied. The rocks were classified in the field using the depositional fabric of Dunham (1962)fossils and facies characteristics were described in thin sections.All rock samples and thin sections have been housed in the Department of Geology, Lorestan University. Facies were determined for each palaeoenvironment according to carbonate grain types, textures and interpretation of functionalmorphology of larger foraminifers.In biostratigraphic studies, 17 foaminifera species belonging to 18 genera were identified and 6 biozones recognizedin these three sections.They are distinguished based on thezonal scheme proposed by Van Buchem et al. (2010).

BIOSTRATIGRAPHY
Three assemblages of foraminifera were recognized in the studied areas and were discussed in ascending stratigraphic order as follows:(e.g.,      Based on biostratigraphy data, the sediments of the Qom Formation is Oligocene in age in theRakin section, whereas of the Qom Formation was deposited in Early Miocene in the Ashtian and Hezar Abad section.

SEDIMENTARY MODEL
Based on the sediments' fabric features and the dominant biotic components, 21 microfacies types were identified in the study sections. Microfacies analyses have allowed reconstruction of the palaeoenvironments of the Qom Formation in the study sections. Larger benthic foraminifera and red algae are common biotic comonents of Qom Formation deposits. Distribution and morphology of larger benthic foraminifera depend on intrabasinal conditions including depth, light, temperature, salinity, nutrient and water energy (Geel, 2000).The red algae, which use blue and green wavelengths of light in their photosynthetic process, are tremendously useful as paleoenvironmental indicatos, particulary for determining paleobathymetry (Banner and simmons, 1994). The presence of large flat Lepidocyclina indicates that sedimentation took place in relatively deep water. Flatter test and thinner walls with increasing water depth reflect the decreased light levels at greater depths (Romero and Rosse, 2002). This biofacies has a fine grained matrix. Other bioclasts include Amphistegina and echinoid debris. The foraminifera assemblage of this facies shows close affinities to that described by     Bioclastic perforate foraminifera wackestone-packstone is characterized by a lithoclastic wackestonepackstone texture with perforate foraminifera such as Operculina andSpiroclypeusand Amphistegina. The presence of large flat Operculina indicates that sedimentation took place in relatively deep water, Wter depth 50-100m (Reiss and Hottinger, 1984).
Shoal condition is characterized bycoral boundstone and has been identified only in the Rakin section (microfacies f).The discontinuous coral boundstone layers indicate a patch reef depositional environment. Coral reef communities are adapted to oligotrophic environments (Flügel, 2010).
The Innerramp is recognised by lagoon environment.There is no evidence of tidal flat and shoreline or beach environments in the Qom Formation sediments in the Hezar Abad and Ashtian sections beacause of the lack of abrasion of the subaerial exposure (such as a vesicular fabric, microcodium, birdseye and fenestral).
The most common microfacies of the inner shelfsection are bioclastic perforate and impoferate wackestone-packstone(faciesg, in Rakin section respectively),sandy miliolidNeorotaliacorallinacea wackstonepackstone; lithoclast miliolidcorallinacea packstone-grainstone and red marl (microfacies f, g and h in Hezar Abad section respectively) and bioclastic miliolids corallinacean packstone-grainstone and Green marl (microfacies e and f in Ashtian section respectively).A semi-restricted lagoon in the Rakin section is recognized by coexistence ofrestricted marine fauna such as imperforate foraminifera and openmarine fauna such as perforate foraminifera (microfacies g).   Restricted conditions are suggested in the Hezar Abad and Ashtian section by the lack of a normalmarine biota and abundant skeletal components of restricted biota (benthic foraminifera such as miliolids).The biotic assemblage of the restrictedlagoon environment suggests deposition in a relatively nutrientrich (mesotrophy) with a slightly hypersaline habitat (Hallock and Gleen 1986;Geel, 2000) colonized by seagrassas suggested by the presence of epiphytic foraminifera suchas: Archaias, Peneroplis and Borelis ).

CONCLUSION
Micropaleontological and biostratigraphical studies indicate that the age of the Qom Formation in the Rakin section ranges from Chattian to Aquitnian. The age of the Qom Formation in the Ashtian and Hezar Abad sections is Chattian.Based on the paleoecology and lithology, three distinct depositional setting can be recognized: inner shelf, middle shelf and outer shelf.
Biogenic components of the Qom Formation are dominated by benthic foraminifera and corallinacea. Based on biogenic components and textures, 21 biofacies have been recognized and grouped into 3 depositional environments that correspond to the inner and middle shelf environments, and are interpreted as a carbonate platform developed in an open shelf settings.
The biotic assemblages of the Qom Formation suggest that carbonate sedimentation took place in mesotrophy to eutrophic conditions.