Lithostratigraphy, microfacies investigation and paleoenvironmental reconstruction of the Jahrum Formation in the west and north of the Bandar Abbass area, South Iran.
The limestone of the Jahrum Formation consists one of the reservoir intervals in the Zagros basin in Iran. In order to study the Jahrum Formation a surface section of Anguru of this formation in the Zagros Basin has been selected. Anguru Section in Tange Keshar is located in the folded Zagros Belt and follows its characteristics. The Zagros Basin with 1800km long is located in the middle parts of the Alp-Himalaya orogenic belt, and with a thick bound with northwest-southeast trend is start form Turkey and continued to Hormoz Straight in Iran. This mountain range is northeast is bounded with Taurus at west and Persian Gulf in south-southwest. In the Zagros Basin one of the area in which the Jahrum Formation is widely exposed is northwest and west of Bandar Abbass. Since the limestones of the Jahrum Formation are resistant against weathering, they formed highlands in the Zagros folded belt and the outer layer of different anticlines. Although Jahrum Formation is more resistant against weathering in compare with the Asmari one. It is noteworthy to say that climate has an important role in forming the geomorphology of the outer layers of the formation.
Review of the literature revealed that integrated studies (lithostratigraphy, biostratigraphy, depositional environment and sequence stratigraphy of the Jahrum Formation have not been carried out in the Bandar Abbass Area. The lack of data makes the correlation of this area to other parts of the Zagros area impossible. So, in this investigation, the Jahrum Formation in the Anguru Section in Bandar Abbass Area has been studied as a point of view of microfacies, depositional environments, and biozonation based on the foraminifera. The results have been used for reconstruction and analysis of the depositional environments.
MATERIAL AND METHODS
After the field study in the Bandar Abbass area, the Anguru Section has been selected. About 200 samples have been selected for thin section preparation. Thin sections were stained with alizarin red-s for differentiation of calcite and dolomite following Dickson . Based on the types and percentage of the allochems, texture, fossil contents and size of the fossils, 6 types of microfacies and 6subfacies have been identified. For limestone analysis, Dunham classification . and for microfacies classification Flugel standard facies belt . Buxton and Pedley classification . for depositional environment have been used. and. Based on these data a sedimentolgoical and petrographical log has been prepared. Finally a 3D depositional model has been proposed for the Jahrum Formation.
The selected section of Anguru is located in the Zagros basin. Anguru section (27[degrees] 16' N and 55[degrees] 50' E) is located in west of Bandar Abbass City, south Iran, Eastern part of the Zagros area (Fig. 1). The Anguru Anticline with 45km long and 12km wide is located in the west-northwest of Bandar Abbass and is 55km far from Bandar Abbass (16' and 27[degrees] N and 55[degrees] and 50' E. Based on the field studies
Review of literature indicates that many studies have been done on the Jahrum Formation of the Dezful Embayment because of oil field spreading. The Jahrum formation biostratigraphically has been studied by James, and Wynd, . Rahaghi, . Kalantari, . Sadegholvad, and Faghih, . Hottinger and Khosrotehrani et al., . sedimentology, depositional environments and sequence stratigraphy of this formation have been studied by Seyrafian, Vaziri-Moghaddam et al., . Nadjafi et al., . Taheri et al., . Parastoo, Khatibimehr and Moallemi, These studies indicated that the Jahrum Formation has been deposited in a shallow marine carbonate ramp. Based on the benthic foraminifera the age of the Dammam Formation (equivalent to Jahrum Formation) is Priabonian (Boukhari et al. These studies revealed that the Jahrum Formation of the Bandar Abbass area has not been studied integrately.
The Jahrum Formation is 278m thick, which is paleontologically identified that 104m is related to the age of Middle Eocene and 174m to Late Eocene. During the field trips, 280 samples have been selected of the studied interval. The stratigraphic column of the Jahrum Formation in the Anguru Section is presented in the figure 2.
The lower contact of the Jahrum Formation with the Pabdeh Formation is gradational (Fig. 3a). The Pabdeh Formation consists of pelagic microfacies and planktonic fauna. The Jahrum Formation in this section lithologically consists of fossiliferous limestone and beige to gray medium to thick layered dolomitic limestone. The base of the Jahrum Formation consists of argillaceous limestone and medium bedded dolomitic limestone with abundant benthonic (Operculina spp.) and planctonic foraminifera and with echinoid debris, bivalve and bryozoans. In the middle part of the formation in spite of Operculina spp. large benthic foraminifera like Nummulites sp., Assilina sp., and Discocyclina sp along with bivalve and echinoid debris are present (Fig. 3a). The upper part of the Jahrum Formation consists of thick bedded fossiliferous limestone of shallow marine water and lagoonal environment like Dacycladacea, Gastropoda and Bivalve (Oyster), along with benthonic foraminifera like Orbitolites complanatus 'Nummulites sp. cf. N. striatus ' Nummulites sp. cf. N. fabbianii ' Alveolina spp .Baculogypsinoides sp. (Fig. 3d).
The Jahrum Formation is widely exposed in the studied area and because of high resistance against withering, forms the cliffs especially in Tange Keshar. High- karstification and well-developed joint formation caused to form a good hydrocarbon reservoir. Based on paleontological studies the lower contacts of the Jahrum Formation with the Asmari Formation is paraconformable (Fig. 3d).
Based on the field studies and petrographic investigation of the Jahrum Formation 6 types of microfacies have been identified. These microfacies have been deposited in the lagoon and open marine environments. These microfacies have been described subsequently.
Lagoon Microfacies (JA):
Lagoon microfacies have been developed in the middle to upper parts of the Jahrum Formation. This facies belt is observed with the texture of grain-supported texture, especially packstone.
JA1: Benthic foraminifera bioclastpelloidal packstone:
This microfacies is characterized with grain-supported texture with abundant and variable porcelaneous bioclasts. Foraminifera are including Textularia spp., Quinqueloculina spp., Elphidium spp., Nummulites sp., Triloculina trigonula, Spiroloculina spp., Pyrgo spp., Spirolina spp., Bigenerina sp., Triloculina sp., Nummulites sp. cf. N. fabbianii, Orbitolites complanatus, Orbtolites sp., Penarchaias sp. cf. P. glynnjonesi, Triloculina tricarinata. Nonforaminifera microfossils are Lithophyllum sp., Lithothamnium sp. Lithoporella sp., Bryozoan, Echinoid and bivalve debris. The amount of pelloid varies between 15-40%. Besides pelloid, wide variety of lagoonal microfaun (Miliolid, micritized red algae) and open marine microfauna (Nummulites, Echinoid debris) are present in this microfacies.
This microfacies is observed in the upper parts of the Jahrum Formation in Anguru Section. mixing of the porcelaneous and hyaline test of marine fauna indicates that the lagoon is attached with the open marine environment (Fig. 4a).in this microfacies a subfacies has been identified, namely Nummulites /Orbitolites bioclast pelloidal packstone. The main allochems of this microfacies is pelloid, Orbtolites sp., Orbitolites complanatus, Nummulites sp. cf. N. fabbianii, and Nummulites sp.. the other bioclasts are Textularia spp., Quinqueloculina spp., Elphidium spp., Triloculina trigonula, Pyrgo spp., Russella spp., Spirolina spp., Bigenerina sp., Triloculina sp., Rhabdorites malatyaensis, Rhabdorites sp., Penarchaias sp. cf. P. glynnjonesi, Neotaberina neaniconica n. gen. n. sp., Alveolina spp., Praerhapydionina delicata, Asterigerina rotula, Lithophyllum sp., Bryozoan debris, Bivalve debris.
This microfacies is grain-supported and well sorted. Presence of micrite indicates that this microfacies has been deposited in low energy environment of lagoon. The comprises bioclast of this subfacies such as porcelaneous benthonic foraminifera and pelloid is indicate the deposition in shallow marine. Stratigraphic position are also indicate the deposition in the lagoon (Fig. 4b). dolomitization is one of diagenetic feature has been affected on this microfacies. the amount of dolomite is between 5 to 25%. Dolomitization is fabric destructive and obliterated the precursor fabric of the allochems. This microfacies is observed in the middle part of the Jahrum Fonnation of the Anguru Section.
Abundant pelloid along with Orbitolites and Miliolids and with a lower amount Nummulites and mudsupported facies, proposed the deposition in the lagoonal environments. Based on the Buxton and Pedley, it is equivalent to RMF-20 of Flugel facies belt No. 2. The condition of this environment is shallow, with medium to low energy and presence in the photic zone below the fair-weather wave base. The lagoon is well connected to open marine environments and is temperature and salinity of this environment is close to the open marine.
JA2: Pelloidal benthic foraminifera packstone to grainstone:
This microfacies is characterized with grain-supported texture. The main allochems of this microfacies are pelloid and porcelanous benthic foraminifera including Quinqueloculina spp., Elphidium spp., Orbitolites sp. Praerhapydionina delicate, Textularia spp., Neorhipidionina spiralis, Rhabdorites malatyaensis and nonforaminiferal fossils like bivalve debris, red algae (Lithothamnium sp. .Lithophyllum sp.). The AJ2 is present in the upper parts of the Jahrum Formation in the Anguru Section. Bioturbation and micritization are the main diagenetic features affected on this microfacies. About 3-5% sand sized quartz grains are scattered in the matrix (Figure 5a and b).
This microfacies is equivalent to RMF-20 of Flugel and facies No. 2 (Buxton and Pedley, 1989). Abundant porcelanous foraminifera along with the existence of detrital quartz grains indicate the deposition in the shallow, medium to low energy environment of the lagoon. Wide variety of euryhaline macrofauna reveals that the lagoon is attached to open marine by tidal channel. This lagoon environment is characterized by shallow, medium to low energy and presence in the photic zone below the fair-weather wave base.
Open marine Facies Association:
4 microfacies have been identified in the open marine facies belt. These microfacies have been described below.
JB1: Nummulites wackestone to packstone:
Nummulites are the main allochems of this microfacies. In outcrop view, large Nummulites with the size of about 20mm is observable (Figure 6). Size and form of the Nummulites is function of depositional environment. The elongate Nummulites is related to the offshore deeper water environment and shows the increase in the accommodation space (Jorry, 2004/ Nummulites are the major constituents of the Paleogene deposits especially in Middle Eocene. The size of them is about 160mm (Rasser et al., 205). Pelloid up to 45% is also observed. According to Hottinger (1983), Nummulites indicates the deeper parts of the basin. Large and abundant allochems in the micrite indicates that the energy is low and only occasionally is storm. So the Nummulites microfacies has been deposited below Fair weather wave base. Based on the predominant allochems and texture 3 subfacies have been identified in this microfacies:
1). Nummulites packstone to wackestone (Fig. 7a)
2) Peloid Nummulites packstone (Fig. 7b and c)
3) Nummulites bioclast pelloidal packstone (Fig. 7d)
A large Nummulites Belt in the northern and southern parts of the Tethys ocean in the carbonate Eocene is observed (Figure 8). Presence of micrite and high degree of fragmentation indicates the inversion texture and reveal that a low energy environment is occasionally is affected by storm waves. Waves energy is high enough to break the allochems, but not high enough to remove the micrite. Large size of Nummulites, Autochthonous Nummulites, well sorting in Nummulites test and grain-supported texture suggest that this facies is deposited in Nummulites Bank. This facies is formed in the mid ramp below wave base. Such a Nummulites Bank is reported in shallow marine environment of Eocene of Tethys Ocean. Distribution and formation of this facies caused to form bioclast and Nummulites Band in the mid ramp part of the platform. It is noteworthy to say that orientation of Nummulites is an indication of paleocurrent.
According to Racey,  Nummulites along with Assilina, Discocyclina and Alveolina have been occurred in the mid ramp. Based on Bassy presence of Nummulites along with Assilina proposed the deposition in the mid and proximal outer ramp. Based on the foraminifera depositonal environement of the Nummulites varies from mid to outer ramp, so Nummulites along with Discocyclina and Operculina is related to outer ramp and without them is related to mid ramp. The major distribution of this microfacies is related to mid ramp of the Eocene in the Zagros Basin. The same microfacies is introduced by Khatibimehr and Moallemi and Sadeqi et al. in middle ramp. This microfacies is equivalent to RMF-13 and above fair-weather and Buxton and Pedley, is equivalent to facies belt No. 5 related to mid ramp.
JC1: Planktonic foraminifer Discocyclina/ Nummulites/ Operculina pelloidal wackestone to packstone:
The texture of JC1 varies from wackestone to packstone. This microfacies consist of Discocyclina, Nummulites and Operculina and with a lower amount planktonic foraminifera like Globigerina spp., Globigerina yeguaensis. Large, planar Discocyclina indicates the deposition in nearly deep marine (Geel, 2000). Bioturbation and burrowing is common (Fig. 9a and b). Based on the bioclasts and texture 2 subfacies have been identified including:
1) Nummulites/ Discocyclina wackestone to packstone (Fig. 9c)
2) Nummulites/ Assilina packstone (Fig. 9d)
Dolomitization is a common diagenetic process in this facies. This facies is an observant of deeper water in west-northwest of Bandar Abbass in the Eocene.
Association of Discocyclina and Nummulites indicates the deposition in the fore reef facies with the depth of about 50-80m (Vaziri-Moghaddam et al., 2002). Based on the Racey, (2001) elongate Nummulites and Discocyclina is related to nearly deep water and thick lenticular Nummulites is related to shallow parts (mid ramp). This microfacies is equivalent to RMF-3 of Flugel (2010) and facies belt No.5 of Buxton and Pedley (1989) and formed in the outer ramp.
JC2: Operculina/(small) Nummulites pelloidal packstone:
The main allochems of this microfacies are benthonic foraminifera and Nummulites. Other fossils are Textularia spp., Planorbulina spp., Quinqueloculina spp., Elphidium spp., Bolivina sp., Triloculina trigonula, Spiroloculina spp., Pyrgo spp., Nummulites sp.cf N. Globules, Cibicides sp., Assilina sp., Assilina sp.cf. A. spira, Spirolina spp., Bigenerina sp., Triloculina sp., Globorotalia spp., Valvulina spp. and Bryozoan and Echinoid debris. Bioturbation and burrowing are common. Pelloid is subordinate allochems (Fig. 10 a and b). Based on the present fauna this microfacies is related to open marine environment. This microfacies is observed in the lower part of the Anguru Section.
The assemblage of Operculina and Nummulites indicates the deposition in the deep water condition (Moallemi and Khatibimehr, 2010). The difference in this microfacies with JC1 is in the lack of Discocyclina and presence of Operculina. This microfacies is equivalent to facies belt No. 5 of Buxton and Pedley (1989) and was deposited in the outer ramp.
Frequency Distribution of Microfacies:
The identified microfacies in the Jahrum Formation in the Anguru Section have been summarized in table 1. Petrographic analysis of thin sections revealed that the microfacies of the lagoon environment are more abundant than other facies belt. Open marine and Nummulites bank facies belt come next regarding their abundance.
Based on the field study and by the use of the lateral and vertical change of microfacies and based on the Walter's law, the Jahrum Formation has been deposited in 6 microfacies association. The comparison of this microfacies with the standard facies belt of Flugel (2010) and Buxton and Pedley, (1989) the Jahrum Formation has been deposited in the distally steepen ramp. Gradual change of microfacies, the lack of main barrier reef and turbidite deposit in the inner parts of the platform, and the presence of redeposited sediment in the distal part of the platform suggested the distally steepened ramp environment. Generally 6 types of microfacies have been identified which have been deposited in the inner and mid parts of the platform.
The evidence like wide variety of lagoonal bioclast (e.g. Miliolids), porcelanous test benthonic foraminifera (e.g. Coskinolina, Dictioconus and Valvulinid) and pelloid and intraclast along with marine fauna (Echinoids), sorting and grain -supported texture proposed the deposition of JA1 and JA2 in the shallow open lagoon environment of the inner ramp. The presence of abundant Nummulites with distinct orientation in JB1, large size and autochthonous of Nummulites, well sorted Nummulites tests, and grain supported texture indicate the deposition in the Nummulites Bank in the mid part of the ramp, below faiweather wave base. Such Nummulites banks have highly been reported in shallow marine deposits of Eocene (Racey, 2001). Paleocurrent reconstruction can be done by the consideration of Nummulites orientation.
The open marine environment starts with Nummulites bank. By increasing depth Operculina and then Discocyclina appears. Deepening of the water is indicated by the presence of the majority of planctonic foraminifera and with a lower amount Operculina and small Nummulites (JC1 and JC2). The 3D schematic block diagram of the Jahrum Formation is presented in figure 11.
* Based on the field study and petrographic investigation of the Jahrum Formation in the Anguru Section the following can be concluded.
* In the Anguru Section, the lower contact of the Jahrum Formation with the Pabdeh Formation is gradational and the upper contact with the Asmari Formation is paraconformable and is characterized with a paleosol horizon.
* The thickness of this formation in the studied section is about 278 m and lithologically consists of limestone and dolomite. Based on the paleontological studies the age of the Jahrum Formation is Middle Eocene (Lutetian to Bartonian) to Late Eocene (Priabonian).
* 3 faces belt have been identified including lagoon (JA), Nummulites Bank (JB) and open marine (JC).
* Gradual change of microfacies, the lack of main barrier reef and turbidite deposit in the inner parts of the platform, and the presence of redeposited sediment in the distal part of the platform suggested the distally steepened ramp environment.
* The identified microfacies in the Jahrum Formation in the Anguru Section have been summarized in table 1. Petrographic analysis of thin sections revealed that the microfacies of the lagoon environment is more abundant than other facies belt. Open marine and Nummulites Bank facies belt come next regarding their abundance.
Received 11 Feb 2014
Received in revised form 24 February 2014
Accepted 22 April 2014
Available online 30 April 2014
The authors would like to thank Research Institute of Petroleum Industry for permission of publishing the paper.
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(1) Moallemi, S.A., (2) Daneshian, J., (3) Hosseinzadeh, M.
(1) Professor Assistant, Research Institute of Petroleum Industry, Tehran, Iran,
(2) Pesearch Assistant, College of Basic Science, Kharazmi University Tehran, Iran,
(3) Department of Geology, College of Basic Science, Tehran Science and Research Branch, Islamic Azad University, Tehran, Iran.
Corresponding Author: Moallemi, S.A., Professor Assistant, Research Institute of Petroleum Industry, Tehran, Iran,
Table 1: the identified microfacies of the Jahrum Formation in the Anguru section of the Bandar Abbass Area. 0.1 Facies Facies Name Subfacies Code JA JA1 Benthic foraminifera Nummulites /Orbitolites bioclast pelloidal bioclast pelloidal packstone packstone JA2 Pelloidal benthic -- foraminifera packstone to grainstone JB JB1 Nummulites wackestone Nummulites packstone to packstone to wackestone Peloid Nummulites packstone Nummulites bioclast pelloidal packstone JC JC1 Planktonic foraminiferal Nummulites/ Discocyclina/ Nummulites/ Discocyclina Operculina wackestone pelloidal wackestone to packstone to packstone Nummulites/ Assilina packstone JC2 Operculina/(smaW) -- Nummulites pelloidal packstone 0.1 Facies Depositional Environment Facies Frequency Code of the Anguru Section JA JA1 Lagoon Inner Ramp Abundance JA2 Abundance JB JB1 Nummulite Inner to Common Bank/shoal Mid Raap Common Abundance JC JC1 Open marine Mid Ramp Abundance Common JC2 Abundance
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|Author:||Moallemi, S.A.; Daneshian, J.; Hosseinzadeh, M.|
|Publication:||Advances in Environmental Biology|
|Date:||Feb 14, 2014|
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