Facies Analysis and Depositional Environment of the Oligocene-Miocene Asmari Formation , Bandar Abbas Hinterland , Iran

The Asmari Formation is a thick carbonate sequence of the Oligocene-Miocene in the Zagros Basin, southwest of Iran. This formation is located in Bandar Abbas and Coastal Fars regions on the following two sections: Anguro anticline (west-northwest of Bandar Abbas) and Gavbast anticline (southwest of Lar County). The Asmari Formation has diameters of 68 and 26 m in the Anguro and Gavbast sections, respectively. This formation is composed of limestone, dolomitic limestone and an altered form of marl. Based on the results of petrographic analyses, 7 facies were identified in the Anguro and Gavbast sections in the study region. The facies were deposited on the following 3 belts: tidal flat (MF 1 3), lagoon (MF 4 5) and open marine (MF 6, 7). According to evidence such as the gradual change of microfacies, the lack of main reef barriers, and the lack of slumping and sliding features, the Asmari Formation was formed in a marine environment of carbonate homoclinal ramp type. This environment is composed of the following two subenvironments: the inner ramp and the middle ramp. The comparison of the facies identified in the Anguro and Gavbast sections indicates that Gavbast section is mainly composed of lagoon facies. Moreover, the Anguro section demonstrates more facies diversity than Gavbast section.


Introduction
The Asmari Formation (the most important reservoir of Iran) is a thick carbonate sequence of the Oligocene-Miocene in the Zagros Basin, southwest of Iran.The Asmari Formation at the type section consists of 314 m of limestone, dolomitic limestone and argillaceous limestone [1].The Asmari Formation, at its type section, is deposited during the late Oligocene (Rupelian)-early Miocene (Burdigalian) (Figure 1).The base of the Asmari Formation varies in age.For instance, toward the coastal Fars area, it is mainly Rupelian while in the Dezful Embayment, it ranges from Rupelian to Chattian [1].
In spite of the extension of the deposits of the Asmari Formation in the Zagros Basin, these deposits have not been studied properly.In this research, numerous sections were studied for the careful examination of the Asmari Formation in the Bandar Abbas region, on the east of the coastal Fars (southwest of Lar County).Due to the lack of deposits associated with this formation in the majority of sections, the significance of this formation, and the limited spread of these deposits in the study area, the Anguro and Gavbast sections were selected (in spite of the low thickness of the Asmari Formation).The main objectives of this research were focused on 1) a description of the facies and their distribution on the Oligocene-Miocene carbonate platform, and 2) describing and interpreting the depositional environments represented by the Asmari Formation.
orogeny [4].The total thickness of the sedimentary column deposited above the Neoproterozoic Hormuzsalt before the Neogene Zagros folding can reach over 8 -10 km [5] [6].The Zagros Basin has evolved through a number of different tectonic settings since the end of Precambrian.The basin was part of the stable Gondwana supercontinent in the Paleozoic, a passive margin in the Mesozoic, and it became a convergent orogen in the Cenozoic [7] [8].During the Paleozoic, Iran, Turkey and the Arabian plate (which now has the Zagros Belt situated along its northeastern border) together with Afghanistan and India, made up the long, very wide and stable passive margin of Gondwana, which bordered the Paleo-Tethys Ocean to the north [9].By the late Triassic, the Neo-Tethys Ocean had opened up between Arabia (which included the present Zagros region as its northeastern margin) and Iran, with two different sedimentary basins on both sides of the ocean [9].The closure of the Neo-Tethys Basin, mostly during the late Cretaceous, was due to the convergence and northeast subduction of the Arabian Plate beneath the Iranian sub-plate [9]- [12].The closure led to the emplacement of pieces of the Neo-Tethyan oceanic lithosphere (i.e., ophiolites) onto the northeastern margin of the Afro-Arabian plate (e.g., [13]- [15]).Continent-continent collision starting in the Cenozoic has led to the formation of the Zagros Fold-Thrust Belt, continued shortening of the mountain range, and creation of the Zagros foreland basin.The late Cretaceous to Miocene rocks represent deposits of the foreland basin prior to the Zagros Orogeny, and subsequent incorporation into the colliding rock sequences.This sequence unconformably overlies Jurassic to Upper Cretaceous rocks.Compressional folding began during or soon after the deposition of the Oligocene-Miocene Asmari Formation [16].On the basis of lateral facies variations, the Zagros Fold-Thrust Belt is divided into different tectonostratigraphic domains that from NW to SE are: the Lurestan Province or Western Zagros, the Izeh Zone and Dezful Embayment or Central Zagros, and finally Fars Province or Eastern Zagros [1] (Figure 2(b)).
Also, from southwest to northeast of the Zagros Basin the following zones are distinguished: Zagros folded belt, fold and thrust belt, High Zagros and crushed zone.The Zagros Basin is also one of the most prolific oil reservoirs in the Middle East.The study area is located in the northeastern part of the Fars Interior Zone.

Study Area and Methodology
Anguro section is situated on the Anguro anticline with a length of 45 km and a width of 12 km.It is located on the west-northwest of Bandar Abbas City.It has a latitude of 27˚16' and longitude of 55˚50'.Gavbast section is situated on the Gavbast anticline with a length of 30 km and width os 7 km.It is located on the southwest of Lar County (Fars Province).It has a latitude of 27˚14' and longitude of 53˚52'.In both of the sections under study, the lower boundary of the Asmari Formation is placed on the Jahrum Formation through a paraconformity.The upper boundary is conformable with the Gachsaran Formation (Figures 3(a)-(c)).Following field inspections, 70 samples were collected from the sections of interest to study the Asmari Formation.
Thin sections of the samples were also obtained for the purpose of microscopic studies.The resulting thin sections were stained using Dickson's method [19] and alizarine red to spot the calcite and dolomite contents.In this study, the Dunham classification was used for the classification of carbonate rocks [20].Facies were also classified based on Fugel's standard facies [21] and sedimentary environments were described based on Buxton and Pedley classification [22].

Previous Works
Interest in the study of the paleontology, stratigraphy, and sedimentary environment of the Asmari Formation has been largely motivated by the exploration for oil and gas, because it contains more than 90% of Iran's oil.The Asmari Formation was adopted after the Asmari anticline located in the northern Dezful Embayment and was referred to a sequence of Cretaceouse-Eocene in age [23].The Asmari Formation was measured and defined as an Oligocene nummulitic limestone by Richardson [24] and described by Thomas [25] as an Oligocene-Miocene carbonate interval.James and Wynd [2] summarized previous viewpoints and finally formally defined the Asmari Formation.Recently, the studies of biostratigraphy, depositional environment and sequence stratigraphy have been undertaken by Seyrafian et al. [26], Seyrafian [27], Seyrafian and Mojikhalifeh [28], Vaziri-Moghaddam et al. [29], Amirshahkarami et al. [30] and Hakimzadeh and Seyrafian [31].Ehrenberg et al. [32] and Laursen et al. [33] examined the Asmari Formation based on Sr isotope stratigraphyand revised age ranges mostly for the lower and middle parts of the Asmari Formation.Moreover, salinity changes duringthe late Oligocene to early Miocene for deposition of the Asmari Formation have been described by Mossadegh et al. [34].

Lithology
According to the results of field examinations of the study area, the thickness of the Asmari Formation increases relatively from the north to the south in the Bandar Abbas region.Therefore, the thickness of this formation in the Anguro and Gavbast anticlines is approximately 68 and 26 m, respectively.A total of 59 samples were obtained from the deposits of the Asmari Formation which were located in the Anguro section with a thickness of 68 m.This formation forms the heights and resistant areas due to its limestone lithology and high resistance to erosive factors.The lower boundary of the Asmari Formation is placed on top of shallow limestones of the upper part of the Jahrum Formation through a paraconformity (as a result of the absence of assemblage zone no.58).The Asmari Formation contained within the Anguro section includes sequences of limestone, dolomitic limestone, and an alternation of marl and gray limestone.At the base of the Asmari Formation located on this section, gray limestones enriched with large benthic foraminifera accumulations (such as Nummulites intermedius-fichteli) were found next to other skeletal components (such as bivalves and echinoid debris) (Figure 4).The upper parts of the Asmari Formation contain sequences of thick dolomitic limestone and an alternation of limestone and marl.Large benthic foraminifera and bivalve fossils are also abundant in this section.There are also numerous fractures in this formation.The upper boundary of the Asmari Formation is conformable with the Gachsaran Formation.The evaporative Gachsaran Formation contains sequences of evaporate rocks located on the limestones of the Asmari Formation.The Gachsaran Formation demonstrates a milder topography as compared to the heights and walls formed by the Asmari Formation.That part of the Asmari Formation which is located on the Gavbast section has a total thickness of 26 m (Figure 4).This formation in this section was fully examined and 11 samples were obtained from it.Paleontological studies suggest that the lower boundary of the Asmari Formation is placed on the shallow limestones of the upper section of the Jahrum Formation.The part of the formation that lies on this section includes sequences of gray limestone.At the base of the section, the Asmari Formation includes gray limestone enriched with large benthic foraminifera accumulations (such as Archaias operculiniformis) and skeletal components (such as bivalves and echinoid debris).The upper parts of the formation embrace sequences of thick limestone as well as plenty of large benthic foraminifera and bivalve fossils.In the study area, the Gachsaran Formation lies on the Asmari Formation with in the conformable.It also demonstrates a milder topography as compared to the heights and walls formed by the Asmari Formation.

Biostratigraphy
Biostratigraphic criteria of the Asmari Formation were established by Wynd [35] (Table 1) and reviewed by Adams and Bourgeois [36] (Table 2) in unpublished reports only.Based on the foraminiferal assemblages, the Asmari Formation is divided into lower, middle, and upper units.From base to top, three foraminiferal assemblages were recognized in the study area: 1) Assemblage l is characterized by the presence of Rotalia viennotti, Ditrupa sp., Planorbulina sp., Spirolina cylindracea, Austrotrillina asmariensis, Paragloborotalia spp.and coral and echinoid debris.This microfauna correspond to the Nummulites intermedius-Nummulites vascus Assemblage zone of Wynd [35] and Eulepidina-  Nephrolepidina-Nummulites Assemblage Zone of Adams and Bourgeois [36].The faunal assemblage of this zone suggests a Rupelian-Chattian age.

Facies Description and Depositional Environment
Seven carbonate sedimentary facies were recognized for the Asmari Formation in the study area.These facies are related to three depositional settings (tidal flat, lagoon and open marine) of inner and middle portions of a carbonate platform.

Tidal Flat Facies Association MF 1: Dolo Mudstone
This microfacies has been observed in the upper part of the Asmari Formation.The MF 1 consists of dolomicrite with fine dolomite crystals 5 to 16 µm (Figure 5(a)).Bioturbation is also common (Figure 5 bioclast in this microfacies.Silt-sized quartz grains are scattered in the dolomicrite matrix (<10%) (Figure 5

(c)).
Based on the presence of dolomicrite, detrital quartz grains, bioturbation, and the lack of bioclasts and comparison to the standard microfacies of Flugel [21], it can be concluded that microfacies MF 1 has been deposited in supratidal to upper intertidal environment.This microfacies is equivalent to RMF 22 of Flugel [21] and Facies Belt 1 of Buxton & Pedley'sclassification [22].

MF 3: Ooid Packstone to Grainstone
The texture of this microfacies varies from packstone to grainstone, and Ooid is the main allochem of this microfacies.Skeletal debris are Miliolides.Benthic foraminifera like Peneroplis sp., Peneroplis evolutus archaias sp., Austrotrillina howchini and bivalve debris are present.Superficial and two layer Ooids are well sorted.Lagoonal bioclasts formed the core of the Ooids (Figure 5(h) and Figure 5(i)).The presence of well-sorted, superficial, fine grained (>0.5 mm) Ooids, lagoonal fauna as Ooid cores, the presence of grain-supported texture, and the vertical change of this microfacies to a tidal flat microfacies (MF 1 and MF 2) indicate deposition of MF 3 in a tidal channel environment with medium to high energy condition [37].
The wide variety of marine fauna and microfauna (corals, echinoids, bivalve, and bryozoan (Tubucellaria spp.)) suggests deposition in open and oxygenate marine conditions, perhaps above the storm wave base (cf.[42]).The MF 6 microfacies is equivalent to RMF-7 of Flugel [21] and compares to the model of Buxton & Padley [22].It should be equivalent to facies belt 5, deposited in mid ramp environment.
The presence of hyaline test of microfauna like Nummulites and also stenohaline fauna such as echinoids, indicates deposition in a proximal open marine environment.Abundance and size of the Nummulites are good indicators of the depositional environments [43].According to Racey [43], elongate and large Nummulites have occurred offshore, in deeper water conditions with respect to other types of Nummulites, indicating increase in accommodation space.This microfacies is equivalent to the RMF-13 of Flugel [21] and facies belt 5 of Buxton & Pedley [22].The association of red algae and larger benthic foraminifera is known to inhabit the Oligo-photic zone of the middle ramp environment [39] [41] [44] [45].
The results presented in the present report, based on the gradual changes of microfacies, the lack of main barrier reefs, and the slumping and sliding features, clearly suggest that the Asmari Formation has been deposited on a homoclinal ramp environment.Our results are in complete concordance with Pedley [50] who has proposed that during the Oligocene-Miocene, distally steepened and homoclinal ramps were widespread in Mediterranean areas.

Conclusions
The thickness of the Asmari Formation increases from north to south in the Bandar Abbas area.In this area, this formation thickness is 68 and 26 m respectively in the Anguro and Gavbast sections.
The Asmari Formation in the Gavbast section has been deposited in an inner ramp.Fossil contents indicate that the lower contact of the Asmari Formation is with shallow marine carbonates of the Jahrum Formation.The upper contact of the Asmari Formation with the Gachsaran Formation is a disconformity.
The Asmari Formation contained within the Anguro section includes sequences of limestone, dolomitic limestone, and an alternation of marl and gray limestone.
In the Anguro and Gavbast sections, seven types of microfacies have been identified.These microfacies are interpreted as have been deposited in 3 microfacies belts, including tidal flat (MF 1 -3), lagoon (MF 4,5) and proximal open marine (MF 6, 7) environments.Gradual transition between microfacies, the lack of main barrier reef, and sliding and slumping features indicate the Asmari Formation has been deposited in a homoclinal ramp.Two subenvironments have been identified: inner ramp and mid-ramp.The lagoonal microfacies is the more abundant, followed by lagoon facies belt, the tidal flat (inner ramp) and proximal open marine facies belt are developed respectively.

Figure 2 .
Figure 2. Location and Geological map of the study area.(a) General map of Iran showing eight geologic provinces, adapted from Lacombe et al. (2006) [17] and Mobasher and Babaie (2008) [18]; (b) Sub-divisions of the Zagros Mountains and Fars Subbasin, after Motiei (1994) [1], with situation of the study sections in Fars Province.

Figure 3 .
Figure 3. Field photographs showing: (a) A west view of the Anguro Section (lower boundary of the Asmari Formation with Jahrum Formation); (b) Upper boundary of the Asmari Formation with Gachsaran Formation is identified with paleosoil horizon in Anguro Section; (c) A east view of the Gavbast Section (Lower boundary and upper boundary of the Asmari Formation with Jahrum Formation and Gachsaran Formation respectively).

Figure 7 .
Figure 7. Schematic block diagram of depositional environment of the Asmari Formation in the Bandar Abbas area.Lateral distribution of microfacies in the sedimentary model, petrographic characteristics, and main carbonate particles of various facies are shown.