Biofacies Analysis of the Upper Oligocene Deposits ( Qom Formation ) in Urumieh Dokhtar Zone

In order to determine the accurate paleoenvironmental conditions of Qom Formation, one stratigraphic section was studied in the Urumieh-Dokhtar Zone (West of Ashtian). 60 thin sections of the Qom Formation, 71 m thick, were prepared and the distribution of benthic foraminifera was analyzed. On the basis of the recognized foraminifera, the age of Qom Formation in the studied section is assigned to the Late Oligocene (Chattian). In this study, sex different biofacies and two lithofacies types have been recognized, that can be grouped into two depositional environments, back ramp (alluvium channel, lagoon and shoal) and the ramp (inner, middle and outer ramp). The alluvium channel is characterized by Gms (gravel with massive layering and abundant matrix). The shoal is represented by fine to medium sandstone: calcite cemented submature litharenite. The lagoon is dominated by the presence of bioclastic bryozoan corallinacea wackestonepackstone and bioclastic foraminifera corallinacean wackstone to packstone. The inner ramp is characterized by bioclastic benthic foraminifera (perforate and imperforate) wackestone-packstone and the middle ramp represented by bioclastic perforate foraminifera wackstone-packstone and bioclastic Operculina packstone. Outer ramp is characterized by bioclast benthic and planktonic foraminifera mudstone.


Introduction
The Qom Formation is one of the oil reservoirs in Iran and spreads through Central Iran (back arc basin), Urumieh-Dokhtar (Intra arc ban) and Sanandaj-Sirjan (forearc basin) Zones.Loftus (1855) introduced the Qom Formation as Nummulitic series with Oligocene age.Furrer & Suder (1955) divided the Qom Formation into a-f Members and introduced the mountains surrounding of Qom city as type locality.Later, Abaie et al. (1964) divided the c Member of Qom Formation into four Sub-Members (c1-c4).Moreover, Bozorgnia (1966) added an Unnamed Member to the base of Qom Formation in the Kashan area.
Mostly all of the previous works on the Qom Formation are limited to the middle parts of Iran, as the formal lithostratigraphic subdivisions of Qom Formation are limited to the Central Iran.However, surprisingly little information is available concerning the different biostratigraphical and paleoenvironmentical properties of the Qom Formation in the Urumieh-Dokhtar.It is essential and important to study different properties of the oil-bearing Qom Formation because of economic importance in the Iranian Plate at the same time (Mohammadi et al., 2011(Mohammadi et al., , 2013)).This paper examines in detail the biofacies of Qom Formation in the Ashtian area, Urumieh Dokhtar Zone, and provides palaeoenvironmental interpretations of the sedimentary succession.

Materials and Methods
As shown in Figure 1A, the study area is located about 3 km west of Ashtian City.The geographic coordinates of the study section are 49º 59' E, 34º 31' N.
The total thickness of Qom Formation is 70 m in the Ashtian section.60 samples were collected bed by bed.Thin sections were provided for harder litologies whilst softer litologies were disaggregated and the foraminifera picked and analyzed.The samples disaggregated by soaking in water for several days and then washed through 200, 120, 63 and 36 mm sieve series with tap water.Sediment infilling of foraminiferal tests was removed by repeated sonic agitation of the residues for about 15 minutes.The best-preserved specimens of planktonic foraminiferal were picked and mounted on micro slides for permanent record and taking SEM microphotographs.
All rock samples and thin sections have been housed in the Department of Geology, Lorestan University.Definition of microfacies is based on depositional texture, grain composition and fossil content.The classification of carbonate rocks followed the nomenclature of Dunham (1962) and Flügel (2010).
Biogenic components of the Qom Formation consist of different biota including: large benthic and small planktonic foraminifera, red algae, bryozoans, bivalves, echinoderms and gastropoda.The fauna association, particularly the foraminifera, is used for the paleoenvironmental and biostratigraphical interpretation, since they are excellent bioindicators for age dating and the paleoenvironmental interpretation.
Larger benthic foraminifera developed complicated internal structures which can be identified when they are randomly thin sectioned.Beavington-Penny & Racey (2004) showed that because of rapid diversification of foraminifera, these organisms can provide complete and detailed evidence for biostratigraphic analysis of the shelf limestone.There is no formal biozonation for the Qom Formation.However, conspicuous similarity is observed between the foraminiferal assemblages of Qom Formation and Asmari Formation (Zagros region, SW Iran).
In addition to their biostratigraphic utility, foraminifera are also extremely useful in determining the environment of deposition.They are photosymbiontic organisms (Reiss & Hottinger, 1984) and they require light, which requires that they live in a photic zone.Changes in foraminiferal assemblages can indicate fluctuations in light level, and this gives valuable information for the interpretation of palaeoenvironments and paleobathymetry (Hallock & Glenn, 1986).Paleobathymetry itself controls other environmentally important factors such as temperature, oxygenation, substrate type, etc.The size and shape of larger foraminifera gives important clues to environment of deposition, especially  Karvan (2015), Amirshahkarami et al. (2007), Nouradini et al. (2015), Sadeghi et al. (2009) and Sooltanian & Seyrafian (2011) has shown that by shape and size to foraminifera distributions, differing genera of larger foraminifera have differing environmental niches that can be extrapolated into fossil record of the Oligo-Miocene deposits in Iran.
The thickness of Qom Formation in west of Ashtian is 71 m and consists of conglomerate, sandstone, limestone and marl.Based on the lithologic characteristics of these deposits and their stratigraphic relations, five lithostatigraphic units were recognized.The Qom Formation in the studied section is transitionally overlies the Oligocene Lower Red Formation.The contact with the Middle Miocene Upper Red Formation is conformable (Figure 2).
In terms of local foraminiferal zonations applied to the Zagros foreland basin, the interval from base to top of section belongs to Assemblage Zone 56 of Wynd (1965) and to Assemblages Zone   3).
The most diagnostic foraminifera and nonforaminifera in the studied section are shown at figures 4 and 5.

Paleoecology
The foraminifera and algal taxa within Qom Formation are extremely useful in determining the environment of the studied depositional levels.This is particularly true with respect to depth, which itself controls other environmentally important factors such as hydrodynamic energy, nutrient supply, light penetration and temperature.
As stated by Beavingtone-Penney & Racey (2004), the shape of benthic foraminifera reflects their incompatibility in high or low energy environments and their symbiotic relationship with algae.High-energy conditions and increasing water motion causes the test of larger foraminifera to be thick and decreases its growth rate and eventually reduces their test size.
Morphology of Operculina is a clue to Paleobathymetry.As shown in figure 6, Reiss & Hottinger (1984) observed that the test of Operculina showed increasing compression or flattening, as water depth increases in the Gulf of Aqaba, Red Sea, The strongly compressed (flattened) forms illustrated here probably indicate water depth of 50-100 mbsl.The flattening of tests with increasing water depth is a strategy employed by a number of modern and fossil larger foraminifera, presumably due to a large and vast surface area that allows the photosynthetic symbionts to exist as light penetration in sea water decreases with depth.They also need to protect themselves from very high degrees of illumination causing damage by ultraviolet light.Nummulitids with transparent and hyaline walls protect themselves in deeper water from UV-light by producing large and flat walls (Rasser et al., 2005).
Larger benthic foraminifera are oligotrophic to mesotrophic biota have adapted to nutrientdeficient conditions and are extreme K-strategists, characterized by slow growth, late maturity (Renema, 2006), but they cannot respond competitively  when nutrient resources become abundant or eutrophic (Hallock, 1985).Nutrients material enters shallow-water communities principally by upwelling of deeper waters or by run-off from land (Hallock, 1999).
Larger benthic foraminifera acquire their nutrient requirement by algal symbionts.Symbioses are affected by the influences of light which this factor is controlled by depth, suspended material and nutrient supply.Symbiont-bearing large foraminifera are restricted to warm water of tropical realms.Brandano & Corda (2002) and Brandano et al. (2010) showed Larger benthic foraminifera would rather live in warm waters, with temperatures ranging between 18-20ºC.
The unit 1 of Qom Formation at Ashtian section contains of imperforate foraminifera (such as miliolids), red algae and bryozoans that are lightindependent.Although miliolids may be found in a variety of very shallow, hyposaline to hypersaline environments, or even in abundance on fore-reef slopes (Seyrafian et al., 2011) but as Geel (2000) showed, the occurrence of abundant imperforate foraminifera indicates the shallowest water depth of environment.Vaziri Moghaddam et al. (2010) reported imperforate foraminifera of the upper part of shallowest photic zone and in a seagrass-dominated environment, as suggested by the presence of epiphytic porcellaneous (Peneroplis) foraminifera at the Asmari Formation in Zagros Basin.
The presence of porcelaneous foraminifera and lack of hyaline wall structure foraminifera and other normal open marine fauna and low frequency of calcareous agglutinated foraminiferal and echinoid fragments indicate occurrence of the higher level of hyper-saline water.Miliolids become predominant skeletal occurring microfacies in hyper-saline environments (Mossadegh et al., 2009).
There are only bryozoans and red algae in the upper part of unit 1 that maybe is related to eutrophy conditions by run-off from land.
Most important fauna at unit 2 are Elphidium and Pyrgo.Hallock & Glenn (1986) reported that an association of Elphidium and miliolid reflects water depth environment below 35 m that deposition took place in the inner shelf environment.
Abundant of large benthic hyaline foraminifera with red algae in the unit 3 (Neorotalia, Miogypsinoides and Operculina), represents deposition in an oligophotic to mesotrophic zone of a carbonate ramp under tropical to subtropical conditions (Pomar, 2001).
Morphology of Operculina in unit 3 (Figure 6) reflects the water depth environment between 50-100 mbsl.Beavington-Penney & Racey ( 2004) reported that flattened test shapes of Operculina, were deposited in the lower photic zone in the distal middle shelf.Seyrafian et al. (2011) stated that Operculina complanata are found in deepest part of photoc zone of the Oligocene-Miocene Asmari Formation in Zagros Basin.
The abundance of planktonic foraminifera and the absence of calcareous algae in unit 4 suggest that this unit was deposited below the photic zone.It is deepest part of Qom Formation in section study.Geel (2000) and Mossadegh et al. (2009) reported occurrence of planktonic foraminifera, small and large benthic foraminifera with hyaline wall structure reflect normal sea water conditions (34-40 psu).

Biofacies Analysis
As shown in Figures 7 and 8 (A-G), on the basis of the distribution of the skeletal components and facies relationships in the Oligocene succession in the studied area, 6 biofacies and 1 lithofacies have been identified.Among the most important facies, one might point out the following samples.

Biofacies A: Bioclast Benthic and Planktonic Foraminifera Mudstone
This biofacies occurs in unit 4 and is dominated by rhythmically alternating thin olive green to grey marl and represented by association of hyaline benthic and pelagic foraminifera, gastropoda and bivalvia with dominant mud-supported texture.
Interpretation: Absence of calcareous algae represents an aphotic zone (Cosovic et al., 2004).The high amounts of micrite and lack of sedimentary structures reflect a relatively low turbulence environment suggest that this microfacies was deposited in calm, low energy hydrodynamic and deep normal salinity water (Scholle et al., 1983).The precence of planktonic foraminifera accompanied by perforate foraminifera indicated an outer ramp depositional setting below storm wave base in the lower limit of the photic zone (Corda & Brandano, 2003;Romero et al., 2002;Vaziri-Moghaddam et al., 2006).Amin Rasouli (2005) reported a similar microfacies from the Qom Formation in south Central Iran and also Mohamdi et al. (2010) reported similar microfacies from Qom Formation in west of Saveh.

Biofacies B: Bioclastic
Operculina Packstone (Figure 8A) This facies occurs in upper part of unit 3. The main characteristic feature of this facies is the presence of large and flat epifauna benthic foraminifera (Elphidium sp., Miogypsinoides sp.).Nummulitidae (Operculina, Hetrostegina) with small size test (A form) are abundant biogenetic components in microfacies B. Other components of this facies include echinoids, gastropoda and bivalves.The matrix is fine-grained micrite.Fine medium grained detrital quartz is recognized in this microfacies.The Packstone is poorly medium sorted and grains are angular to subangular.Megascopically, this microfacies is composed of medium bedded green marly limestone.
Interpretation: Large rotaliids (such as Amphistegina) are in fact restricted to the photic zone, since all of them house symbiotic algae (Pomar, 2001) and diatoms (Mateu-Vicens et al., 2009) that they are able to use a wide range of light wavelength.They can be found in a variety of palaeobathymetric setting even below depth 180 mbsl (Beavington-Penny & Racey, 2004).Bassi et al. (2007) divided the photic zone into upper and lower parts; in this classification, Neorotalia live in the upper part of the upper photic zone, and Heterostegina, Operculina are dominant in the lower part of the photic zone.
A form dominated fossil communities are likely to have formed in the shallowest or deepest part of depth range.These two environments can be distinguished on the basis of the matrix and stratigraphic position (Beavington-Penny & Racey, 2004).The relatively low degree of fragmentation of the Nummulitidae indicates that these deposits formed in the distal part of the middle ramp, well below the fair-weather wave base since there are no signs of wave hydraulic turbulence in these microfacies.The abundance of large and flat epifauna benthic foraminifera (Operculina and Amphistegina) and quartz grains in micritic texture may indicate that the sedimentary environment was situated in   (Geel, 2000).A similar microfacies has also been reported of Qom Formation in Navabn section (Sadighi et al., 2009).

Facies C: Bioclastic Porferate
Foraminifera Wackstone-Packstone (Figure 8B) This facies occurs in lower part of unit3.The main characteristic feature of this facies is the abundance of perforate hyaline benthic foraminifera (Amphistegina sp., Elphidium sp., Neorotalia sp.).Other components of this facies include echinoids, gastropoda and bivalves.The matrix is fine-grained micrite.Inorganic particles are medium sorted and rounded quartz.Megascopically, this facies is composed of very soft green marly limestone.
Interpretation: The presence of high diverse stenohaline fauna such as echinoid and larger foraminifera indicate that the sedimentary environment was situated in the oligophotic zone and normal salinity in a shallow open marine environment or near a fair-water wave base on the proximal middle sramp (Pomar, 2001;Vaziri Moghaddam et al., 2010) and in the upper part of the upper photic zone (Bassi et al., 2007).Wilson & Vecsei (2005) show an association of red algae with large foraminifera such as Neorotalia deposits in an oligophotic zone under tropical to subtropical conditions and in a shallow open marine environment or near a fair-water wave base on the middle shelf.Murray (1991) suggested for the Neorotalia an open condition of open marine platform conditions with water depth in the order of 20-50 mbsl.Amin Rasouli (2005) reported a similar microfacies from middle ramp from the Qom Formation in south Central Iran.
Interpretation: Hallock & Glenn (1986) reported that an association of Elphidium and miliolid reflects water depth environment below 35 m that deposition took place in the inner shelf environment.Today, porcelaneous larger foraminifera thrive in tropical carbonate platforms within the upper part of the photic zone (Bassi et al., 2007).Some biogenic components such as miliolids indicate stress conditions within restricted environments.The predominance of mud-rich lithologies with oligotypic fauna (such as miliolids) and the presence of a low-diversity foraminiferal association indicate deposition in a low-energy with poor connection with open marine and/or extreme salinities circulation and probably reduced oxygen contents or euryhaline conditions (Geel, 2000).Such an assemblage described to be associated with an inner ramp environment (Vaziri-Moghaddam et al., 2004).
A similar facies with imperforate foraminifers and perforate foraminifers was reported from the inner ramp of the Oligocene-Miocene sediments of the Zagros basin at Kabir Kuh, Darre Shahr and Mamolan sections (Vaziri-Moghaddam et al., 2006).

Facies E: Fine to Medium Sandstone -Calcite Cemented Submature Litharenite (Figure 8F)
This facies occurs in unit 5 and lower part of unit 1. Megascopically, this facies is composed of whith sandstone.Quartz is the main component in this facies.Frequency of plagioclase is negligible.Also, chert, siltstone, calcareous rock fragments exist.The glauconite minerals rarely present.The textural and maturity of the sandstones of these deposits is sub-mature or mature.
Interpretation: As Tucker (2001) showed, presence of lithic grains revealed that the sedimentary basin was a near-origin environment.The features of components and the stratigraphic position and the presence of marine trace fossil (Kuphus sp.) support that it has been probably barrier, and it is comparable to barrier of between back ramp and inner ramp Biofacies Analysis of the Upper Oligocene Deposits (Qom Formation) in Urumieh Dokhtar Zone, Iran Iraj Maghfori Moghadam; Zohre Holakouee; Mehdi Yazdi & Bizhan Yousefi (Burchette & Wright, 1992).Flügel (2010) showed, in the coastal environment, the clastic sediments may originate from the bedload of estuaries.The coastal sandstone beds of the Qom Formation have been already identified (Amirahahkarki et al., 2015).
Interpretation: Brandano et al. (2010) showed corallinacean algae are present in water depth environments ranging from 0 to 286 m.Hallock (1999) proposed that the occurrence of abundant miliolids and agglutinate foraminifera (Schlumbergerina sp.) and corallinacean algae indicates the shallowest water depth of environment and may point to the depositional environment being slightly hypersaline.Lee (1990) reported that some porcelaneous imperforate foraminifera (Peneroplis) live in recent tropical and subtropical shallow water environments, hosting dinoflagellate, rhodophycean and chlorophycean endosymbionts.Therefore, Brandano et al. (2008) knew that the presence of epiphytic foraminifera in the microfacies is due to sea-grass-dominated environments.The predominance of mud-rich lithologies with oligotypic fauna (such as miliolids) indicates deposition in a low-energy with poor connection with open marine.Recent miliolids are euryhaline forms living in shallow restricted environments with low turbulence thriving on soft substrates.When they are present in great abundance, Geel (2000) described, miliolids may indicate nutrient-enriched conditions.Since nutrients are abundant and water is shallow, and due to the foraminiferal assemblage, F biofacies belongs to the shallow environment, and probably to the semi restricted lagoon.

Facies G: Bryozoans Corallinacea
Bioclastic Wackestone-Packstone (Figure 8F) This facies occurs in upper part of unit 1.The textures are wackestone-packstone.The matrix consists of micritic.Bryozoans and corallinacea red algae are dominant components in this microfacies.Other bioclasts are rare but include bivalvia.No foraminifera were found in this unit.
Interpretation: Hallock (1985) reported that larger benthic foraminifera have highly adapted to stable, oligotrophic and nutrient-deficient conditions, but they cannot respond competitively when nutrient resources become abundant.Unlike foraminifera, bryozoans have adapted to the eutrophic conditions.Probably, the presence of abundant bryozoan on facies G indicates that the condition was highly eutrophic.Moreover, Nebelsick et al. (2005) reported that the presence of well-preserved coralline algae indicates a relatively quiet-water environment with stable substrate and low sedimentation rates.Brandano & Corda (2002) showed the presence of bryozoans indicate an increase in heterotrophy.They proposed that red algae with bioerosion suggest a change from oligotrophic conditions to a high level of nutrients.Depositional textures, fauna and flora and the stratigraphic position show that sedimentation may have taken place in warm, euphotic and shallow water, with low energy conditions.G biofacies belongs to the semi-restricted lagoon.Amirshahkarami & Karavan (2015) reported a similar microfacies from Qom Formation.

Facies H: Gms (Gravel With Massive
Layering and aAbundant Matrix (Figure 8G) This facies occurs in lower part of unit 1 and represents layers whose main parts are made of gravels.Gravel particles roundness varies from semi-rounded to rounded.Most of the space among the gravels consists of fine-grained sands and muddy sediments.The sediments are fairly well sorted.The amount of mud in these sediments is very few.Interpretation: According to Mial (2006) and Rust (1978), gravels of conglomerate are wholly rounded because of their remoteness to the origin, and they include very large to very small gravels which are often very poorly sorted.This reflects the high energy of the environment at the time of depositional conditions.According to Mial (1977), the layers in pile situation are due to their formation in high energy and turbulent activity conditions.So, this facies is formed by debris actions with high viscosity and high power of environment.Fluvial channel current and debris flow are recommended on the basis of facies and lateral and vertical expansions in Gms facies.

Palaeoenvironmental Model
As shown in Figure 9A, the recognized facies have allowed the differentiation of two major depositional environments.These include ramp (inner, middle and outer) and back ramp (alluvium channel, restricted and semi-restricted) environments.Marl with abundant planktonic foraminifera was deposited in an outer ramp setting.As Flügel (2010) maintained, the presence of mudsupported textures and the apparent absence of wave and current structures indicate a low energy environment below storm wave base.
The mid-ramp is the zone between the fairweather wave and the storm wave.Wackestonepackstone with Operculina and Amphistegina were deposited under low energy conditions, below the fair weather wave base (FWB) and above the storm wave base (SWB) in the middle ramp setting.The variation in the shape of the test reflects the differences in water depth.Foraminifera are the dominant microfauna of the middle ramp, probably because, as Romero et al. (2002) suggested, they were the best-adapted fauna to the paleoenvironmental conditions such as low hydrodynamic energy, lower limit of the photic zone, oligothrophy and normal salinity.The distal mid-ramp is differentiated from the shallower depth by decreased flatness and size of the perforate foraminifera and reduces of the number of porcelaneous foraminifera.
Proximal inner ramp conditions above the fair-weather wave base are characterized by mixed open marine fauna (such as perforate foraminifera) and the presence of protected environment fauna (such as miliolids).
Back-ramp sediments originate in restricted lagoonal areas (mudstone, wackstone and packstone) (Flügel, 2010).In the study areas, the back-ramp deposits consist of a semirestrcted lagoon, restricted lagoon and alluvium channel.Semi protected lagoon environments are characterized by biofacies types that include mixed open marine bioclasts (such as perforate foraminifera) and protected environment fauna (such as imperforate foraminifera).The diversity association of skeletal components represents a shallow subtidal environment, with optimal conditions with regard to salinity and water circulation.The most abundant biofacies are medium to coarse-grained larger foraminifera with imperforate wall-bioclast wackestone-packstone.The presence of imperforate foraminifera indicates a low-energy, upper photic, shallow shelf lagoon depositional environment.Generally the upper photic zone is dominated by porcellaneus larger foraminifera, predominantly living in symbiosis with dinophyceans, chlorophyceans or rhodophyceans (Romero et al., 2002).
Lagoonal plants and animals occupy different zones depending on their ability to tolerate salt concentration (salinity), wave action, river flow, tidal changes, and sedimentation levels.
In study section, the occurrence of large number of bryozoans and red algae indicate a eutrophic condition.High nutrient levels may be related to a high nutrient flux from land.
Facies E (Benthic foraminifera (perforate and imperforate) bioclast wackestone-packstone), facies F (Bioclastic foraminifera corallinacean wackstone to packstone) and facies H (gravel with massive layering and abundant matrix) are comparable to barrier, restricted and semi restricted lagoon and alluvium channel respectively.

Conclusion
On the basis of the foraminifera recognized at the studied section, the age of the Qom Formation is Late Oligocene (Chatting).Biogenic components of the Qom Formation are dominated by foraminifera, coralline red algae in association with Bryozoa.Based on biogenic components and textures, 6 biofacies and 2 lithofacies have been recognized and grouped into two depositional environments that correspond

Figure 1 (
Figure 1 (A) Location of studied section; (B) Geological subdivisions of Iran (adapted from Heydari et al., 2003) and the distribution map of Qom Formation in Iran.

Biofacies
Figure 6 Variations in the morphology of Operculina with changing in water depth (after Reiss & Hottinger, 1984); (A) specimen is from ware depth of 20-50m in the modern Gulf of Aqabea; (B) specimen is from water depth of 50-100m; (C) Operculina sp.sample number 32.

BiofaciesFigure 8
Figure 7 Vertical facies distribution of the Qom Formation at west of Ashtian.

Biofacies
Analysis of the Upper Oligocene Deposits (Qom Formation) in Urumieh Dokhtar Zone, Iran Iraj Maghfori Moghadam; Zohre Holakouee; Mehdi Yazdi & Bizhan Yousefito the ramp (inner, middle and outer ramp) and backramp (semi restricted lagoon, restricted lagoon and alluvium) environments.The biotic assemblages of the Qom Formation suggest that carbonate sedimentation took place in subtropical waters with oligotrophic conditions.Moreover, the abundance and association of Bryozoa with coralline red algae in the basal part of Qom Formation are referred to shallow warm water environments of the photic zone where eurytrophic condition was prevalent.

Biofacies Analysis of the Upper Oligocene Deposits (Qom Formation) in Urumieh Dokhtar Zone, Iran
Iraj Maghfori Moghadam; Zohre Holakouee; Mehdi Yazdi & Bizhan Yousefi the oligophotic zone in a shallow open marine environment with low energy or near a fair-water wave base on the proximal middle shelf