Sequence Microbiostratigraphy of Jahrum and Asmari Formation in Shiraz Area , Zagros , Fars , Iran

The study of sequence microbiostratigraphy of Jahrum and Asmari formation in Shiraz, Fars, Zagros, SW of Iran and recognition of relationship between depositional sequence parameters and microbiostratigraphy 3 stratigraphic sections (Beyza, Sarvestan A, B and Estahban sections) have been selected and studied. Jahrum and Asmari formations in study area with unconformity overlying together have been exposed. Based on Field and laboratory studies such as microfossils, microbiostratigraphy, microfaceis, sedimentary environment models and sequence stratigraphy evidences recognized 3 depositional sequences in study area that presented during Paleocene-Oligomiocene. Jahrum formation consists of one depositional sequence and Asmari formation consists of two depositional sequences. Microfossils and microbiostratigraphy studies will lead to the identification of 6 biozones in the studied area. In Beyza and Estahban sections biozones 1, 2 and 3 are related to Jahrum formation and suggest Paleocene to Middle Eocene age for this formation and biozone 4 is related to Asmari formation and suggests Oligocene age for this formation. In Sarvestan section biozones 1, 2 and 3 are related to Jahrum formation and suggest Paleocene to Middle Eocene age for this formation and biozones 4 and 5 are related to Asmari formation and these biozones indicate Oligocene-Miocene (Aquitanian) age for Asmari formation. Sequence microbiostratigraphy studies indicate that biozones of Jahrum formation are formed in DS1 and biozones of Asmari formation are formed in DS2 and DS3. Also, some relationship between depositional sequence parameters and distribution of microfossils and biozones as in the case of in SB1 and LST facies didn’t found any microfossil and we can’t suggest biozonation. In study area upper part of HST of DS1 (Jahrum formation) and some biozones related to Upper Eocene have been eliminated, for example in Estahban, Sarvestan B and Beyza sections all biozones related to Upper Paleocene to Middle Eocene and eliminated biozones of Upper Eocene. In study area SB2 existed in lower boundary of DS1 (between Sachun and Jahrum formation) and between DS2 and DS3 of Asmari formation. Some parts of biozones in study area could correlate with SB2. For example for upper limit of biozones no 4 in Estahban section overlies to SB2 of DS2 in Asmari formation. Pa-


Introduction
Paleocene-Oligomiocene sediments in study area cover two formations: Jahrum and Asmari.Jahrum formation consists of limestone, dolomitic limestone and dolomite.Lower contact of Jahrum with Sachun formation is transitional and upper contact with Asmari formation is erosional.The lower boundary of the Asmari Formation is exposed and underlain by the Jahrum Formation and the upper boundary is exposed and overlain by the Razak Formation and erosional.This formation is well-known as a major hydrocarbon reservoir in Zagros Basin, southwestern Iran and dominantly formed of limestone and dolomitic limestone [1].In this research, relationship between sequence stratigraphy, relative sea level change, system tract microfossils content and microbiostratigraphy has been studied.

Materials and Methods
This paper is the first report on the relative sequence stratigraphy evidences and microbiostratigraphy of the Jahrum and Asmari Formations at the Shiraz area, Fars, SW of Iran.For study of sequence microbiostratigraphy of Jahrum and Asmari formation 3 stratigraphic sections selected and studied.More than 650 thin sections were analyzed for description of lithofacies and biofacies characterizes.The most samples contain well preserved and abundant larger benthonic foraminifera such as Nummulitidae, Alveolinidae, Soritidae, Peneropelidae and Miliolidae families.Theses faunas are good tools for biofacies analysis, recognition of the relationship between sequence stratigraphy evidences and microbiostratigraphy. Biostratigraphic data of the Jahrum formation was established by Wynd in 1967 [2].Table 1 shows biozonation of Jahrum formation.Adams and Bourgeois in 1967 [3] designed four assemblage zones for the Asmari Formation, Table 2 shows this biozonations.In 2009 Laursen et al. [4].Published a new biozonation of the Upper Oligocene-Lower Miocene sediments by distribution of larger benthonic foraminifera for Asmari formation that Table 3 shows this biozonations.

Geologic and Stratigraphic Setting
The study area is located SW of the Zagros Mountains, which is a Paleocene-Oligomiocene sediments in foldthrust belt Zagros that located in Fars area.The Zagros is the result of a multi-phased collision between the Arabian plate, the Neo-Tethys Ocean, and the central Iran micro plate [5].As such, both 3 studied sections include two formations; Jahrum and Asmari, Figure 1 shows the geological setting and Geographical location of the studied sections.Figure 1 shows the geological setting and Geographical location of the studied sections.

Lithostratigraphy of the Studied Sections
The geographic coordinates of the base of the selected sections, together with their lithostratigraphical character-    ristics are summarized in Table 4. Figure 2 shows lithostratigraphical relationship between formation in Fars area, SW of Iran and Figure 3 shows some field photos of study area.

Sequence Microbiostratigraphy of Jahrum and Asmari Formation in Study Area
Sequence stratigraphy is the modern knowledge of dividing sediments of a basin to sequences which are between equivalent discontinuities and continuities.Depositional sequence (DS) is the main unit of sequence stratigraphy, the upper and lower limit of which have appeared during one of the main cycles of the rise and fall of the relative sea level.Chronologically, the sequence is equivalent to the third-order sedimentary cycle comprising a large number of small-scale shallowing upward cycles or parasequences.
In general, the discontinuity surface divides younger layers from older ones, which is made up of two types: type 1 discontinuity: when the degree of falling of the sea level is more than the basin subsidence in the continental margin, and type 2 discontinuity: when the rate of falling of the sea level is slow, falling down to the middle part of the continental shelf.Each sedimentary sequence could be divided into the three systems tract, each associated with a specific part of the curve of sea level changes.The lower part of each sequence includes lows- tand systems tract (LST), mainly consisting of epicontinental facies.The middle part of each sequence includes transgressive systems tract (TST) formed during seawater transgression, and the upper part of each sequence has also been formed with a systems tract related to the relative static state of seawater or the beginning of the seawater falling stage (Highstand systems tract) (HST).
To study of sequence microbiostratigraphy of Jahrum and Asmari formations in study area the following steps are done.1) Detailed study of microfacies (lithofacies and biofacies) and Dunham classification, 1962 [6].A wide range of carbonate, clastic (Litofacies) and evaporite facies have been identified in petrographic and field studies of the studied formations.Type of skeletal and non-skeletal grains, grain size and their frequency percentage, Matrix and cement have been used to discriminate and identify carbonate facies from factors such as constituents of carbonate rocks including orthochem and allochem.Skeletal grains are mostly Nummulitidae, Alveolinidae, Soritidae, Peneropelidae and Miliolidae and Algae families.For determination of carbonate microfacies use of Flugel 2004 [7].Non-skeletal grains include pellet and intraclast, the frequency of which in some samples reaches to 50%.In general, having precisely carried out the study and nomenclature of microfacies, the depositional model of each of the formations under study has been brought out according to the relative changes curve of the sea level based on their time.[18] [19], Mirzaee Mahmoodabadi et al. (2013) [20] [21], and Mirzaee Mahmoodabadi (2014) [22], have been used.

Sequence Stratigraphy
The Beyza section with a thickness of 395 meter comprises the Sachun, Jahrum and Asmari formations, and with regard to sequence stratigraphy, includes two third-order depositional sequences.As for the small outcrop of Sachun formation in this section, only the sequence stratigraphy of the Jahrum and Asmari formations are investigated.Jahrum formation in the studied section consisted of a third-order depositional sequence (DS1).Its lower lithostratigraphic boundary is gradational and consistent with Sachun formation and its discontinuity is 2 type (SB2).The TST systems tract of this sequence, having a thickness of 100 meter, is comprised of dolomite, dolomitic limestone and thin-bedded limestone.HST facies with a thickness of 250 meter is composed of limestone and medium to thick-bedded dolomitic limestone.Maximum flooding surface (MFS) is identified with pelagic bioclast wackestone.This systems tract is ended to the first type discontinuity (SB1) under depositional sequence of the Asmari formation.This boundary is distinctive on the land due to having paleosol and red beds.On this boundary the transgression of thin to medium-bedded carbonates of Asmari formations (TST facies) is started, and ends with HST facies of weathered medium to thick bedded limestone under Razak formation.4 shows Sequence microbiostratigraphy of Jahrum and Asmari formation in Beyza section.

Sequence Microbiostratigraphy of Jahrum and Asmari Formation in Sarvestan
A Section

Sequence Stratigraphy
The Sarvestan A section with a thickness of 170 meter consists of only Jahrum formation.Jahrum formation in this section consisted of one third-order depositional sequence (DS1).Lower lithostratigraphic boundary is gradational and consistent with Sachun formation and its discontinuity is 2 type (SB2).The TST and HST systems tract of this sequence, are comprised of dolomite and dolomitic limestones.This systems tract is ended to the first type discontinuity (SB1) under Razak formation.Asmari formation in this section has been eroded.
Because for digenesis process such as dolomitization of carbonate Jahrum formation in this section about of 170 meter of dolomite and dolomitic limestone only a vague appearance of fossil remains and determination of microfossils is difficult .The fauna in this section only consists of miliolids and Algae.Figure 5 shows Sequence microbiostratigraphy of Jahrum and Asmari formations in Sarvestan A section.

Sequence Stratigraphy
The Sarvestan B section, with a thickness of 310 meter includes Jahrum and Asmari formations.Sarvestan section in the region under study area is composed of 3 third-order sequences.The first sedimentary sequence is related to Jahrum Formation and sequences number 2 and 3 are related to Asmari Formation.6 shows Sequence microbiostratigraphy of Jahrum and Asmari formations in Sarvestan B section.

Sequence Stratigraphy
The Estahban section, with thickness of 180 meter includes Jahrum and Asmari formations.This section includes three third order depositional sequences.One depositional sequence DS1 is related to Jahrum Formation.tone and HST facies consisted of dolomite and dolomitic limestone.This depositional sequence with erosional boundary (SB1) has been under Razak clastic formation.7 shows Sequence Microbiostratigraphy of Jahrum and Asmari formation in Estahban section.Figure 8 and Figure 9 show some identified microfossils in study area.

Sequence Microbiostratigraphy of Study Area
Based on lithofacies, biofacies and sequence stratigraphy evidences recognized relationships between sequence  strtigraphy and microbiostratigraphy in study area.Relationship between major parameters of identified depositional sequence and microbiostratigraphy in study area below is separately stated.
SB1.The effect of Pyrenean orogeny phase in upper Eocene study area have been exposed and SB1 formed between Jahrum and Asmari formations, this boundary recognized with conglomerate, micro conglomerate, lime sandstone and sandy limestone, in these facies not found fauna and flora, although it is possible existence some reworked microfossils, furthermore we can't suggest biozonation, in study area for SB1 between Jahrum, Asmari and Razak formation.Also some parts of depositional sequence and biozons eroded and eliminated, for example in all sections in study area upper part of HST of DS1 (Jahrum formation) and some biozones related to Upper Eocene have been eliminated, for example in Estahban, Sarvestan B and Beyza sections all biozones related to Upper Paleocene to Middle Eocene and eliminated biozones of Upper Eocene.
SB2.In study area SB2 exist in lower boundary of DS1 (between Sachun and Jahrum formations) and between DS2 and DS3 of Asmari formation.Some parts of biozones in study area could correlate with SB2.For example upper limit of biozones No 4 in Estahban section overlying to SB2 of DS2 in Asmari formation.
LST.This systems tract includes deposits that accumulate after the onset of relative a sea-level rise.when the sea level is at the lowest level or sea level start to transgressive lowstand systems tract are formed and often conglomerate, micro conglomerate and paleosol are formed, for example between Jahrum and Asmari formations in all studied sections in over SB1 and beginning of DS2 (Asmari formation) LST have been.LST paleoenvironment and paleoecological isn't suitable for grow and reproduce foraminifera and theses systems tract don't have any index micro fauna and flora.
TST.This systems tract is comprises of accumulated coastal onlapping deposits and MFS microfacies.Stack- ing patterns exhibit backstepping onlapping retrogradational parasequences.In cases where there is a high sediment supply the parasequences may be aggradational.In study area this systems tract include marine deposits such as thin to medium limestone and dolomitic limestone (Estahban, Sarvestan B and Beyza sections) and dolomite (Sarvestan A).Paleoecological situations (salinity, temperature, photic zone, depth, •••) are suitable for reproduction and growth large foraminifera.For example in Beyza and Estahban sections in biozone 1 (Jahrum formation) and biozone 2 (Asmari formation test of large foraminifera often formed hyaline microcrystalline calcite.In deposits of this systems tract in all studied sections large foraminifera related to open marine and lagoon sub sedimentary environments, for example Nummulites sp.Nummulites intermedius, Nummulites fichtelli, Operculina sp and other microfossils, especially in biozones of Asmari formation.MFS.This surface marks by the time of maximum flooding or transgression of the shelf and it separates the transgressive and highstand systems tract.Seismically, it is often expressed as a downlap surface.Marine shelf and basinal sediments associated with this surface are the result of slow rates of deposition by pelagic-hemipelagic sediments and they are usually thin and fine grained.These fine sediments make up the condensed section.In study area accurate identification mfs is difficult and in much depositional sequence instead MFS considered maximum flooding zone (MFZ).MFS and MFZ in all section recognized by mixed pelagic and benthonic foraminifera wackestone in Jahrum and Asmari formations.There are commonly widespread thin bedded concentrations of fauna (condensed sections) with high abundant and diversity.
HST.This systems tract formed during the late stage of base level rise the rate of sea level rise drops below the sedimentation rate.In this period of sea level Highstand System Tract is formed.It is bounded by maximum flooding surface at the base and sequence boundary type one (SB1), type two (SB2) and correlative conformity (C.C) on the top.paleoecological parameters are suitable for growth and expanding large foraminifera in HST.In study area HST have been formed of thick to massive limestone, dolomithic limestone and limestone and commonly weathered.
In study area more identified biozones existent in HST.For example in Beyza section biozones 2 and 3 (Jahrum formation) and biozone 4 (Asmari formation) are in HST.In Saevestan B section biozones 3 and (Jahrum formation) and biozones 5 and 6 (Asmari formation) are in this systems tract.In Estahban section biozones 3 and 4 (Jahrum formation) and biozones 4 and 5 (Asmari formation have been in HST.
More tests of large benthonic foraminifera in HST systems tract are porcelaneous calcite, such as Alveolona sp, Alveolina aragonensis, Austrotrilina sp, Austrotrilina asmaricus and Archias sp.Figures 4-7 shows relationship between sequence stratigraphy evidences with identified biozones and sequence microbiostratigraphy models in study area.

Conclusions
Based on field and laboratory studies such as microfossils, microbiostratigraphy, microfaceis, sedimentary and sequence stratigraphy evidences recognized 3 depositional models based on relative sea level change that themes presented during Paleocene-Oligomiocene in study area.
Jahrum formation consists of one depositional sequence and Asmari formation consists of two depositional sequences.
Microfossils and microbiostratigraphy studies in study area will lead to the identification of 6 biozones in studied area.
In Beyza and Estahban sections biozones 1, 2 and 3 are related to Jahrum formation and suggest Paleocene to Middle Eocene age for this formation and biozone 4 is related to Asmari formation and suggests Oligocene age for this formation.
In Sarvestan section biozones 1, 2 and 3 are related to Jahrum formation and suggest Paleocene to middle Eocene age for this formation and biozones 4 and 5 are related to Asmari formation and these biozones indicate a Oligocene-Miocene (Aquitanian) age for Asmari formation in this section.
Sequence microbiostratigraphy studies in study area indicate that biozones of Jahrum formation are formed in DS1 and biozones of Asmari formation are formed in DS2 and DS3.
Sequence microbiostratigraphy studies indicated some relationship between depositional sequences parameters and distribution of microfossils and biozones in study area as in the case of: SB1.LST facies are often excessive in SB1 and in these facies, fauna and flora are not found, although it is possible that some reworked microfossils exist, furthermore we can't suggest biozonation.Also some parts of depositional sequence and biozons are eroded and eliminated.For example in all sections in study area upper part of HST of DS1 (Jahrum formation) and some biozones related to upper Eocene have been eliminated, for example in Estahban, Sarvestan B and Beyza sections all biozones related to Upper Paleocene to Middle Eocene and eliminated biozones of upper Eocene.
SB2.In study area SB2 existed in lower boundary of DS1 (between Sachun and Jahrum formations) and between DS2 and DS3 of Asmari formation.Some parts of biozones in study area could correlate with SB2.For example upper limit of biozones no 4 in Estahban section overlies to SB2 of DS2 in Asmari formation.
LST.This system tract is located between Jahrum and Asmari formations in all studied sections in over SB1 and beginning of DS2 (Asmari formation).Paleo ecological situation in LST is not suitable for foraminifer's growth and reproduction and this system tract doesn't have any index micro fauna and flora.
TST.In studied area this system tract includes marine deposits such as thin to medium limestone and dolomitic limestone in this system tract Paleoecological situation which is suitable for reproduction and growth large foraminifera.For example in Beyza and Estahban sections in biozone 1 (Jahrum formation) and biozone 2 (Asmari formation test of large foraminifera often formed hyaline micro crystalline calcite. MFS. MFS and MFZ in all sections are recognized by mixed pelagic and benthonic foraminifera wackestone in Jahrum and Asmari formations.These microfacies formed in deep lagoon and open marine sub-sedimentary environments.There are commonly widespread thin bedded concentrations of fauna (condensed sections) with high abundant and diversity.
HST.In studied area more biozones have been identified in this system tract.For example in Beyza section biozones 2 and 3 (Jahrum formation) and biozone 4 (Asmari formation) are in HST.In Saevestan B section biozones 3 and 4 (Jahrum formation) and biozones 5 and 6 (Asmari formation) are in this system tract.In Estahban section biozones 3 and 4 (Jahrum formation) and biozones 4 and 5 (Asmari formations) have been in HST.
Tests of most identified large foraminifera in HST such as Alveolona sp, Alveolina aragonensis, Austrotrilina sp, Austrotrilina asmaricus and Archias sp.microfossils in HST are made of porcelaneous calcite.
Tests of most identified large foraminifera in TST such as Nummulites sp.Nummulites intermedius, Nummulites fichtelli and Operculina sp in TST are made of hyaline calcite.

Figure 3 .
Figure 3.Some field photos of study area.

Figure 5 . 2 : 3 : 4 :
Figure 5. Sequence Microbiostratigraphy of Jahrum and Asmari formations in Sarvestan A section.ter includes TST and HST facies.Lower boundary of this sequence is SB2 and upper boundary with Razak Formation is SB1 evidence.The upper parts of the third depositional sequence have been eroded.

Biozone 1 :Biozone 2 : 4 :
Somalina sp, Somalina Stefani Assemblage subzone.This assemblage is determined by the presence of Somalina sp, Somalina stefani, Orbitolites sp, Orbitolites complanatous, Miliolids, Dictyoconus sp and other microfossils.The faunal assemblage of this zone corresponds to the zones no 44 and 45 Wynd 1965 and represents a lower to middle Eocene age for Middle part of Jahrum formation.Operorbitolites Acro zone.This zone represents of the appearance and disappearance of Operorbitolites sp.Some fauna that seen in this zone, are Litunella sp, Dictyoconus sp, Nummulites sp and Miliolids.This acrozone corresponds to biozone no 44 Wynd 1965 and suggests Lower Eocene age for Middle part of Jahrum formation in this section.Biozone 3: Dictyoconus, Coskinolina, Orbitolites complanatous Assemblage zone.This assemblage is determined by the presence of Dictyoconus sp, Coskinolina sp, Orbitolites camplanatous, Orbitolites sp, Alveolina sp, Miliolids, and other microfossils.The faunal assemblage of this zone corresponds to the zone No 50 Wynd 1965 and represents Middle Eocene age for upper part of Jahrum formation.Biozone Nummulites intermedius, Nummulites fichtelli, Nummulites vascus Assemblage zone.This assemblage zone is recognized in the lower part of the Asmari Formation and is marked by the first appearance of Nummulites intermedius, Nummulites fichtelli, Nummulites vascus.Other microfossils in this zone are such as Operculina sp, Valvulinid sp and Miliolids.These microfauna correspond to Eulepidina, Nephrolepidina, Nummulites biozones of Adams and Bourgeois1967 and Nummulites vascus-Nummulites fichtelli biozone of Laursen et al. 2009 and indicate a Oligocene age.

Table 4 .
The geographic coordinates and lithostratigraphy of he studied sections.
This microfauna corresponds to the biozone no 43 Wynd 1965 and suggests a Upper Paleocene age for the Lower part of the Jahrum Formation.The faunal assemblage of this biozone corresponds to the zones no 44 and 45 Wynd 1965 and represents a Lower to Middle Eocene age for Middle part of Jahrum formation.The faunal assemblage of this zone corresponds to the biozone No 51 Wynd 1965 and represents middle Eocene age for upper part of Jahrum formation.Assemblage zone.This assemblage zone is recognized in the lower part of the Asmari Formation and is marked by the first appearance of Nummulites intermedius and Nummulites fichtelli, other microfossils in this zone are Operculina sp, Valvulinid sp and Miliolids.These microfauna correspond to Eulepidina, Nephrolepidina, Nummulites biozone of Adams and Bourgeois1967 and Nummulites vascus-Nummulites fichtelli biozone of Laursen et al. 2009.This biozone indicate an Oligocene age for Asmari formation for this section.Figure Biozone 1: Katina sp, Lokhartia diversa, Assemblage zone.Assemblages biozone 1 characterized by an association of Katina sp, Rotalia sp, Lokhartia diversa, Dictyoconus sp, Fallotella sp, Miliolids and Algae.Biozone 2: Somalina sp, Orbitolites complanatus, Assemblage zone.This assemblage is determined by the presence of Somalina sp, Somalina Stefani, Orbitolites sp, Orbitolites complanatous, Miliolids, Dictyoconus sp and other microfossils.Biozone 3: Nummulites sp, Alveololina sp, Assemblage subzone.This assemblage is determined by the presence of Nummulites sp, Alveolina sp, Orbitolites sp, Orbitolites complanatous, Miliolids, Dictyoconus sp and other microfossils.Biozone 4: Nummulites intermedius, Nummulites fichtelli, This sequence is 250 meters thick in which TST facies are approximately 80 meters thick and are made mostly of lagoonal facies.Thickness of HST facies of the first sedimentary sequences is approximately 170 meters.These facies are made mainly of alternative lagoonal and bar facies which are characterized by increasing the thickness of layers.The lower boundary of the first sedimentary sequence is a gradual SB2 boundary with Sachun Formation and its upper boundary is an erosional SB1 evidence with Asmari Formations.The second sedimentary sequence with a thickness of 32 meters is composed of LST, TST, and HST facies in which LST facies are less than 1 meter thick and are made of red weathered sandy limes, sandstone, conglomerate and microconglomerate.TST facies of the second sequence are made of open marine facies that contain microfossils from Nummlitidae family and deposited open marine sub sedimentary environment.HST facies with 18 meters.This sequence is composed of severely weathered dolomitic limestone and dolomite.The lower boundary of the second sedimentary sequence is SB1 and its upper boundary is SB2 evidence.The third sedimentary sequence (DS3) with a thickness of 28 me- This assemblage zone is recognized in the lower part of the Asmari Formation and is marked by the first appearance of Miogypsinioiedes, Archias sp, Valvulinid and Miliolids.These microfauna correspond to Eulepidina, Nephrolepidina, Nummulites biozones of Adams and Bourgeois1967 and Archias asmaricus, Archias hensoni, Miogypsinoides camplanatus biozone of Laursen et al. 2009.This biozone indicate an Oligocene age for Asmari formation.Biozone 6: Austrotrilina Hochini, Austrotrilina Asmaricus, Archias, Assemblage zone.This assemblage zone is recognized in the middle part of the Asmari Formation and is marked by the first appearance of Austrotrilina Hochini, Austrotrilina Asmaricus, Archias sp, Valvulinid and Miliolids.These microfauna correspond to Miogypsinoides, Archias, Valvulinid biozones of Adams and Bourgeois 1967 and Miogepsina, elphidium, peneropelis farsennensis biozone of Laursen et al. 2009.This biozone indicate a Miocene (Aquitanian) age for Asmari formation.Figure