Sedimentary Environment and Relative Sea Level Changes of the Asmari Formation Deposited in Lurestan Basin , Southwest of Zagros

Asmari Formation is deposited in the Zagros Foreland basin during OligoMiocene time. In the presented research, Sheykh Makan (north of Kabir-Kuh Anticline) section located at Lurestan Basin is studied to achieve an understanding of facies changes, paleoenvironment and also sequence stratigraphic framework (relative sea level changes) of the Asmari Formation. Results of this study revealed that 15 microfacies grouped to four facies belts named as tidal flat, lagoon, barrier and open marine are the main constituents. A homoclinal ramp is determined as the depositional setting of the Asmari Formation. In Sheykh Makan Section, studied interval contains three third-order sequences aging oligmiocene. It overlies gradually shaly Pabdeh Formation and is overlain by evaporitic Gachsaran Formatin.


Introduction
Reservoir geologists and engineers consider that the Asmari Formation is as the first carbonate reservoir rock in the world and one the most important petroleum bearing successions of Iran.It is the most prolific formation in the southwest of Iran and contains two geologic members: Ahwaz Sandstone Member expanded in the south of Dezful Embayment and evaporitic Kalhor Member deposited in the northwestern sections of Dezful Embayment and southwest of Lurestan Basin [1] (Figure 1).Asmari sediments are regarded as the last transgression phase in the Zagros Region [2] documented the first study on the Asmari Formation.[3] proposed 5 biostratigraphic segments for all the Zagros formations.Biostratigraphic characteristics of Asmari are revised and three assemblage zones and two subzones are introduced by [4].[5] revised the biostratigraphy of the Asmari Foemation using strontium isotope.[6] also proposed new biozones for Asmari.The Asmari Formation in Oligo-Miocene, with its extensive expansion in the Zagros Basin, is one of the important lithostratigraphic units in southern Iran (Dezful embayment, Lurestan and Fars basins) to northern Iraq.The existence of a highly developed joint system has led to the specific hydraulic properties of this formation and the formation of large hydrocarbon fields, which can play a significant role in identifying hydrocarbon reservoirs that have not yet been explored.Depositional facies, secondary changes (i.e.diagenetic alterations) and sequence stratigraphic units control the external geometry (body) and internal architecture of both carbonate and silisiclastic reservoirs [6] [7] and [8].About the first factor, reservoir quality is directly influenced by deposited facies traits (acting in fine scale) and/or sedimentary environments (acting in large scale).Diagenetic processes may also be influenced efficiently from primary early essence of the deposited units, so diagenetic history trend is potentially affected by the depositional characteristics [9].The main output of sequence stratigraphy can be addressed as achieving a genetic framework used for correlation [10] [11] and [12].Distinguishing the primary deposited microfacies of Asmari and their relevant facies beslts and depositional setting can be named as the first contribution of presented research.Then, presenting the genetic sequence stratigraphic framework applicable in Lurestan Basing is the second aim.Out nest attempt is interpreting the evolution (structural and depositiona) of Lurestan Basin during Asmari depositon (oligomiocene) to pave the way for more reliable exploration and drillings.

Geological Setting
Sedimentary and structural complexities of Iranian Plateau resulted in dividing it to several regions such as Zagros, Central Iran, Sanandaj-Sirjan, Alborz, eastern flish basin, Kopeh dagh, southern Khazar, Makran and Zabol Embayment [13].Consdition of paleo-environment setting, litho-bio facies varities in coexisting formations of different regions, structural changes and their effects and Figure 1.Asmari Formation distribution during Cenozoic in southwest of Iran [1].especially evidence of the paleotethys and neotethys sutures also play an influential role in segmenting this huge plateau [13] and [14].Southern Iran is devoted to the area southwest of neotethys suture embracing Khuzestan, Lurestan and Fars.This area is placed in the northeastern of Arabian Palte.The gigantic plate comprises Arabian countries such as Saudi Arabia, Jordan, Syria, and Iraq [15] (Figure 2).The Zagros area is divided into three tectonic zones from northeast to southwest: the High Zagros (Zone of tectonic activity), the Zagros simply folded belt and the Zagros foredeep zone [16].The Zagros simply folded belt is subdivided according to its tectonic and sedimentary evolution into three domains: Lurestan, Izeh and Fars areas [17] (Figure 3).Section Sheykh Makan is located in the northern limb of Kabir Kub Anticline and south of Sheykh Makan village belonging to Illam Province (Figure 4(a)).
The access to the area is available in the road of Poldokhtar to Dareh Shahr cities.
Figure 2. The Zagros area located in the northeastern part of the Arabian Plate.The Arabian plate comprises the Arabian Peninsula together with Jordan, Syria and Iraq.A Zagros crush zones bound the plate to the northeast, and is bounded by Arabian Sea and Gulf of Aden to the southeast, and by the Red sea to southwest [15].Figure 3. Tectonic zones of the Zagros area [17] and locations of the surface section.The Zagros area is divided into three tectonic zones from northeast to southwest: the High Zagros (Zone of tectonic activity), the Zagros simply folded belt and the Zagros foredeep zone [16].The Zagros simply folded belt is subdivided according to its tectonic and sedimentary evolution into three domains: Lurestan, Izeh and Fars areas [17].The Fars area is separated into four sectors: coastal, subcoastal, interior Fars and Bandar Abbas Hinterland

Materials and Method
To depict the depositional setting and sedimentary sequences of Asmari in two sections, 150 thin sections are prepared and studied.These slides are prepared from the mentioned sections.A detailed field observation is carried out to achieve a deeper understanding of the facies distribution.[18] updated textural classification presented by [19] and their result is exerted to name the distinguished microfacies and also their depositional setting is compared to Flugel introduced standard ramp microfacies (RMF) [20] System tracts of sequences are defined by tracing two main surfaces (maximum flooding surface (MFS) and sequence boundary (SB)) and Arabian Plate sequences [21] and also universal sequences [22] are compared to results of this study.

Biostratigraphy
Strontium isotope dating method is handled to detect the age of Asmari (Figure 5).
According to this approach, Asmari Formation is divided to three assemblage and one obscure zones including lower, middle and upper.

Microfacies and Facies Belts
Considering the results of microfacies analyses carried out by inspecting thin section, 15 microfacies grouped in 4 facies belts (tidal flat, lagoon, barrier and open marine) (Figure 6 and Figure 7) are documented for Asmari in two sections.Asmari in Sheykh Makan Section covers tidal flat to outer ramp.This facies distribution implicates the deeper condition in Sheykh Makan.These microfacies from land to basin are described in the following:

Sandy Limestone (Mf1)
This microfacies contains quartz clastic debris dispersed in a carbonate background (Figure 6

Dolomudstone Fenestral (Mf2)
This microfacies presents fenestral porosity filled with calcite and occasionally dolomite and evaporatic cements (Figure 6(b)).Bird's eye or fenestral texture originated from explansion and contraction, gas bubble or escape during flooding and even worms and insects burrowing activities [25].This ring structure indicates the tidal flat facies belt.Tidal flat is a part of environment orderly or randomly influenced by tidal currents.This facies coexistence with lagoon ones proposes it is formed under effect of tidal currents that is observed and confirmed in the recent environments.

Mudstone (Mf3)
This microfacies is mud-supported and in some samples lamination, microbial and algal textures, evaporatic crystals and quartz grains are observed that all propose it is formed close to shoreline where clastic debris are entered from terrestrial realm (Figure 6(c)).Carbonate mud abundance and deficit of fossil content confirms the lagoon adjacency to shoreline.Negligible abundance and variety of benthic organism is caused by the unfavourable life condition and also the salinity factor.Generally, these sediments are a reliable depth (i.e.relative sea level changes) index, so they can be exerted as an efficient parametere in sequence stratigraphy studies [26] and [27] in Tang-e-Gargadan, [28] in Khaviz Anticline, [29] in Dill Anticline and [30] reported similar microfacies in the Fars Platform and other parts of the Zagros.

(d)). Lack of open marine fauna in MF4
declares that it is deposited in lagoon.Foraminifera with porcelaneous test (miliolid) shows existence of light in the depositional zone of this facies (i.e. it is formed in photic zone) [31] [32] [33] and [34].Detected cementation proposes the notable level of energy in the shoreline part of lagoon.MF-4 is regared as equivalent of RMF 20 designated to laggon.[35] in the Chaman Bolbol Area of Fars and also distinguished similar microfacies from the Asmari Formation.

Bioclastic Echinoderm Wackestone-Packstone (Mf5)
Echinoderm like crinoid and echinoid are the dominant elements of this microfacies.Besides, other allochems such as oyster bivalve, gastropod, benthic foraminifera having porcelaneous test such as miliolid, discorbis, ostracoda, small rotalia and fine string of peyssonneliaceae corals are recorded as minor constituents (Figure 7(a) and Figure 7(b)).Fine clastic quartz grains are randomly dispersed in some samples.Because of the coexistence with lagoon microfacies, it seems MF 5 is deposited in the shoreline front (shallow part) of lagoon.This microfacies is considered as an equivalent of RMF 29 [37] [38] and [39] named almost the same microfacies to MF 7 for Asmari.

Peloids Faverina Grainstone-Packstone (Mf8)
Faverina observed in cross, longitudinal and oblique sections with cemented background is the main allochem of this microfacies.Peloid and corallinaceae red algae are also detected in less frequency (Figure 8

Coral Boundstone (Mf9)
Coral colonies forming in especial environmental condition are the main com-ponent.Corals skeletons has a meander microfabric and pore space is occupied by sparite and micrite [43].In the field observation, this microfacies clearly detectable with naked eyes by its light gray colour and stone debris.In carbonate ramps, coral structures are formed patch shaped in different locations such as mid and inner ramp [43] Corallinaceae red algae, fine hyaline foraminifera, ostracoda debris, equinoderm, discorbis, ooid, bryozoa and sponge needles are less abundant elements (Figure 8

Benthic Foraminifera Peyssoneliacean Boundstone (Mf10)
The microfacies with limited lateral extension is in the upper part of the Sekonj section (Figure 9).The main components of the microfacies are only corals (Figure 9).In the field observations, this microfacies is characterized by gray to cream-colored, medium-bedded limestone.Also, small amounts of other bioclasts can be seen in the microfacies.The micritic cements have filled most of the  suggests a high energy barrier environment.

Corallinacean Echinoids Miogypsina Packstone (Mf11)
Crinoid and large foraminifera such as miogypsina and coralinase red algae are the most abundant components (Figure 8(d)).It is deposited in the open system environment (no restriction) having normal salinity and moderate water flow condirion.Stratigraphic position and marine skeletal fauna confirm the proposed environment for MF 13 and [46].In some samples echinoderm is more abundant than miogypsina, so the microfacies name can be modified to corallinacean miogypsina echinoderm packstone.It is an equivalent of RMF 27.

Bioclastic Nummulitidae Lepidocyclynidae Packstone-Floatstone (Mf13)
The microfacies are mainly composed of large foraminifera with hyaline test.Prolate lepidocyclina is found to be main constituent.Other elements are perculina, heterostegina, red algae, briozoa and echinoid.In some samples the texture is changing to packstone (Figure 9(b)).Lepidocyclina are prolate because in the hyalines coexisting with algae, more light transpasses the shell for reaching to be provided for algae photosynthesis.Then, carbonate severe secretion for generating enough CO 2 is used to compensate pervasive photosynthesis and results in the formation these huge shells [50].Corrallinacea alga and large benthic foraminifera both are indicators for mid ramp facies belt High diversity of prolate and completely saved nummulits and lepicyclina in a micrited texture indocates an open marine condition with low to medium energy and deeper the FWWB.
Therefore, large benthic foraminifera abundance, echinoid, and also red algae all confirm that MF 13 is deposited in oligophotic mid ramp [51] [52] [53].Similar microfacies of Asmari are reported in other parts of the Zagros Region [54].

Planktonic Foraminifera Lepidocyclinidae Nummulitidae Bioclastic Wackestone-Packstone (Mf14)
Prolate large benthic foraminifera such as lepidocyclina and nummulites besides other fauna such as ditropa, echinoderm debris, red algae, bryozoa, planktonic foraminifera (globigerina and globotruncana) and calcisphere and also nonskeletal grains like peloid are the main elements of this microfacies (Figure 9(C)).Abundance of fauna living in normal salinity like hyaline tested large foraminifera and also planktonic ones implicate the deposition in lower parts of the mid ramp and close to FWWB boundary with storm wave base (SWB).The same MFs are also addressed in other parts of the Zagros Region for Asmari.

Planktonic Foraminifera Bioclastic Wackstone-Packstone (Mf15)
Main allochems are planktonic foraminifera, globigerina and globotruncana without geel, calcisphere, red algae and echinoderm debris while less frequent ones are ditropa, peloid bryozoa and benthic foraminifera debris dispersed in a micritic texture (Figure 8(d)).This microfacies is detected in the gradual contact of Asmari with Pabdeh.Planktonic foraminifera abundance and mud texture implicate the MF 15 is formed in low energy condition below the SWB.
Planktonic microfacies are composed of planktons living in the deeper parts of the basin.The abundance and variety of planktonic fauna of this microfacies confirms its formation in outer ramp setting MF 15 is similar to RMF 5 introduced.The same microfacies existing in the lower parts of Asmari are reported in other zones of the Zagros.

Depositional Model
Regarding evidence such as absence of reef building facies, transitional changes

Sequence 1
This sequence is noted only in Sheykh Makan Section, including a 34.5 meters highstand system tract (HST) aged late chattian -early achitanian.The sequence lithology is marny limestone (argillaceous limestone) and marn.It covers the upper parts of the Pabdeh Formation and lower zone of Asmari.The succession is formed as a shallowing up complex, starting with shale and marn and continuing with microfacies belonging to mid and outer ramp facies belts.Finally, a 1 meter sandy limestone formed in tidal flat (MF 2) caps lower mid ramp facies.
As mentioned, MF 2 deciphers a firced regression and plays the rule of SB.Inspecting the microfacies from base to top of this sequence, a shallowing up trend is acquired.

Sequence 2
In Sheykh Makan, it is 112 meters and contains three system tracts: lowstand system tracts (LST), HST and transgressive system tract (TST) aged to achita-

Sequence 3
This sequence is also found in both evaluated section.It is 54 meters and include TST and HST (shallowing up trend) in Sheykh Makan Section.Its age is late achitanin to burdigalian.SB 2 is revealed as the lower contact of this sequence.

Comparing Studied Sections Sequences to Arabian Plate (Hag et al., 2005) and Global Sequences (Ogg et al., 2016)
Relative sea level changes curve provided for Sheykh Makan Section is in great accordance with Arabian Plate sequences.In this section, upper and lower contacts of the sequences can be correlated with global sea level fall, but the sequences are different from globals that possibly originates from internal tectonic processes (Figure 11).
Peyssoneliacea boundstone microfacies detected in third sequence of Sheykh Makan indicate its difference from the other studied section.The first section is surely influenced by regional tectonic that prepared the especial favourable life condition needed for this rare coral.This property has made this area unique.Regarding this comparison, it is clearly understood that factors beside the eustasy controlled Asmari facies distribution.The global factor influence Asmari generally, but small scale changes in sequences are caused by regional and local ones.Tectonic movement in Zagros Region can be addressed as the most influential factor resulting in Sheykh Makan difference.The movements finally led to closure of neotethys and generation of foreland basin.Evidence of tectonic activity that tectonic parameter played the most dominant role in accommodation space changes of Zagros Basin while eustasy has merely influenced the formation of stratigraphic geometries.It seems movements finally closed the basin (i.e.followed by subduction and collision).

Conclusions
Results of this study revealed 15 microfacies, can be determined for Asmari in Sheykh Makan section.Thick ooid grainstone facies indicates the ramp depositional setting and other evidence such as transitional change of facies and lack of turbidite facies also confirm the existence of a homoclinal ramp in the studied time range for both the section.In the Sheykh Makan Section, inner, mid and outer ramp facies are deposited.Frequency of deeper facies in the first section proposes that deeper basin existed in southern parts of Zagros.
Asmari is covered by evaporatic Gachsaran Formation in this section.In Sheykh Makan, Asmari overlies shale bearing Pabdeh gradually.Sequence stratigraphic analyses resulted in characterizing three third order sequences in Sheykh Makan: an incomplete sequence in Pabdeh and two complete shallowing up ones in Asmari.
Asmari is aged oligomiocene and contains lower (chattian), middle (achitanian) and upper (burdigalian) units.The third sequence of Asmari in Sheykh Makan is totally different from the first one.Peyssoneliacea boundstone substituted coral red algae facies in this sequence.Relative sea level changes curve provided for Sheykh Makan Section is in great accordance with Arabian Plate sequences.In this section, upper and lower contacts of the sequences can be correlated with global sea level fall.But the sequences are different from globals that possibly originates from internal tectonic processes.Tectonic activities closed the neotethys are the most probable factor causing this unique property.

Figure 4 .
Figure 4. (a) A view of Sheykh Makan Section in the south of picture; (b) Lower contact of Asmari formation with Pabdeh formation in the Sheykh Makan Section; (c) Gachsaran formation overlying Asmari formation in Sheykh Makan.
(a)).Quartz grain is in fine sand size.This microfacies is related to intertidal setting and appears suddenly on top of the microfacies deposited in lower part of mid ramp.It is observed in the lower parts of Asmari presenting its lower gradual contact with Pabdeh in Sheykh Makan Area.Similar

5 . 7 .
Diversity Framinifera Grainstone-Packstone (Mf6)Benthic foraminifera with porcelaneous test are the most abundant constituent of this microfacies and have considerabe variety.They include miliolid, borelis, pygro, schlumbergerina, triloculina, peneroplis, archaias, discorbis, amphistegina, ammonia, faverina, rotalia, dendritina, mindropsina, austrotrillina.Minor constituents in the studied thin section slides are echinoderm, gastropod, bivalve and red algae debris, ostracoda, bryozoa, and peloid (Figure7(c)).Skeletal fauna related to semi-restricted marine environment including foraminifera with porcelaneous test and grainstone texture both are signatures confirming MF 8 can be deposited in the barrier facies belt that has high levels of energy (a zone shallower than fair weather wave base (FWWB)).Calcareous mud and cement existence acting as orthochems shows the energy fluctuation.This microfacies is found in both the sections and correlates with RMF 27 Similar microfacies are reported in Tang-e-Gargadan[35] and in Tang-e-Bibi Narjes and Tang-e-Aboulfars of Fars for Asmari.Bioclastic Ooid Grainstone (Mf7)Ooid is the dominant component of this microfacies.Ooids are uni or multilayered and have good sorting.Compaction influenced the grains and they have point (meniscus cement) or tangent contact in an anhydrite sparitic cemented background.Minor allochems consist miliolid, dendritina, bivalves, bryozoa, faverina, ostracoda, echinoderm, and red algae.Miliolid, dendritina and echinoderm are playing as the most of ooids' nucli (Figure7(d)).In some slides, ooids are micritized, also lost the primary texture and deformed to peloid.Grainstone texture, ooids, marine cement, transform relicts and good sorting altogether propose high levels of energy existed in the depositional setting of MF 7. Ooids can be formed in a wide range of recent carbonate environment.The most favourable condition is hypersaline shallow (less than 2 meters) enegentic environments with warm weather and high saturation of calcium carbonate[36].
(a)).Peloids are possibly faverina fossils crushed by the wave currents.It is mostly observed with ooid grainstone microfacies.This microfacies is known comparable to RMF 27 and shows mid and inner ramp boundaries.Faverina is a fecal pellet produced by a marine crab and reported from Mesozoic shallow limestone, Jurassic sandy limestone and Cretaceous stricted lagoon This facies is also addressed in lower cretaceous Germany sediments formed in the lagoon environment influenced repeatedly by storm microfacies are also reported in other parts of Zagros Tang-e-Band, Tang-e-Aboulfars and Tang-e-Nayab [40] [41] [42].
(b)).Discussed microfacies are found between algal (lower interval) and ooid (upper interval) facies indicating MF 9 is formed around the mid and inner ramp boundaries.It can be correlated to RMF 12 Similar microfacies are also reported in Fars by some authors and also other parts of the world its equvalents are documented[44] [45].
(a)).Red algae and large foraminifera indicate the deposition of this microfacies in oligophitic realm [47] [48].Sediments enriched with nummulite and lepidocyclina are lenticular and round.They usually form in shallower setting than sediments having large and prolate ones [49].Stratigraphic position and marine skeletal fauna (echinoid and red algae) confirms the interpretation presented for depositional environment of MF 12.It can be equal of RMF 7 located at mid ramp.Almost similar microfacies are revealed in other parts of Zagros.
of microfacies from shoreline to deeper zones (outer ramp), lack of shallow belts fossils mixing with deeper ones (mainly observed in rimmed shelf and, depositional setting of the Asmari Formation is distinguished as a carbonate ramp (Figure10).Meanwhile, absence of breccia, mass flow and turbidite in deep parts of the basin, introduced ramp as a homoclinal one.Asmari has a gradual boundary with Pabdeh in lower contact and this section lithological units are deposited in the deeper settings with marn to marn limestone lithology and planktonic fauna of outer ramp.Upper parts of the Asmari Formation is formed in shallower mid to inner ramp setting.Finally, regression occurred and evaporitic Gachsaran succession emerged on top of the Asmari.Analogous environments can be found in Persian Gulf.

Figure 10 .
Figure 10.Conceptual depositional model of the Asmari Formation in Shekh Makan section located at Lurestan Basin.

The
TST presents a gradual deepening up trend from laggon and barrier facies belts toward coral patch reefs (MF11).This unit resembles type 1b TST introduced by Tucker et al., 1993.Upper parts of this sequence have aggradaing carbonate succession deposited in barrier and lagoon setting caped with tidal flat facies (indicator of SB1), implicating a shallowing up trend (i.e.HST).Evaporatic Gachsaran Formation is observed in the top of this boundary in field patrol.Third sequence in Asman Dul Section is about 56 meters and includes LST and HST.In fact, it merely covers the upper Asmari Formation, so aged burdigalian.Two sequence boundaries exist at the top and base of this sequence.The lower SB is picked where the conglomerate layer emerges (1.5 meters), implicationg the pervasive regression.The upper SB separates the evaporatic Gachsaran Froramtion from Asmari.This complex shows a clear shallowing up trend.
nian.Lower boundary sits on the sandy limestone layer deposited in the tidal flat (forced regression).Bellow this layer, shallowing up trend is clearly observable, whereas deepening up trend is formed above it.Lagoon carbonate succession covers MF 2 and deciphers the relative sea level rise resulted in carbonates deposition.
About upper parts of this sequence, thick to medium layered limestone succession deposited in deep parts of the mid ramp is distiguished.MF 16 is the deepest microfacies of sequence 2, so picked as the MFS.Transgression of this sequence is deciphered observing evidence such as stratigraphic position of deposits in upper parts, revealed MFS, and existence of shallowing up succession (HST) on upper interval.Sequence 2 is an analogous to type 1a.93.5 meters of shallowing up carbonate interval (HST) composed of mid ramp sediments (patch reef, then barrier and finally lagoon) covers discussed TST.