چكيده لاتين :
Introduction:
This paper deals with the Oligocene-Miocene sediments of the Asmari Formation located in southwest of Iran. The Asmari Formation comprises most of Iran’s recoverable oil reservoirs, which are trapped mainly in large anticlines in Zagros Mountain chain. The type section is located in Tang-e Gel-e Tursh (Valley of Sour Earth) on the southwestern flank of the Kuh-e Asmari and has an approximately 314 m of hard cream to brown limestone. Lithologically, the Asmari Formation in Fars sub-basin consists of limestone and sometimes of dolostone, but in some areas including Khozestan sub-basin lithic and limy sandstones (the Ahwaz Member) occur. Also, in Lorestan sub-basin some anhydrite deposits (the Kalhur Member) can be found within the Asmari Formation (Motiei, 2003). Biostratigraphy and microfaunal characteristics of the Asmari Formation were introduced by Wynd (1965) and later by Adams and Bourgeois (1967) Later, more researchers studied the biostratigraphy and microfacies of the Asmari Formation (Seyrafian and Hamedani, 1998; Seyrafian, 2000; Vaziri-Moghaddam et al., 2006; Amirshahkarami et al., 2007a, 2007b; Sadeghi et al., 2009, Sadeghi et al., 2011). Cahuzac and Poignant (1997) assemblage zones of Oligocene-Miocene of Europe were used for the Asmari Formation. Ehrenberg et al. (2007) revised the biostratigraphy of the Asmari Formation based upon Strontium isotopes Later, Laursen et al., (2009) used Strontium isotopes and suggested seven new biostratigraphic assemblage zone. In Dezful embayment the most recent biostratigraphic st dy was carried out on the Asmari Formation by van Buchem et al., (2010) that confirms the work of Laursen et al., (2009). The present research studies sedimentary succession of the Oligocene Asmari Formation at Tang-e Moshkan stratigraphic section aimed at evaluating lithostratigraphy, biostratigraphy and determining microfacies, microfacies class, sub-environments, and sedimentary environment. The study section is located in the southern flank of the Tamar anticline (Sub-coastal Fars sub-zone) in the Zagros folded belt. It was selected in boundary of Fars and Kohgiluyeh and Boyer-Ahmad provinces with a thickness over than 350m consisting of limestone and marly limestone strata. In this section, lower and upper boundaries of the Asmari Formation are continuously and graduality located over Pabdeh and under Razak formations as homoclinal ones, respectively. The Asmari Formation in Moshkan section can be found in the boundary between Fars and Kohkiluyeh-Boyer Ahmad provinces. The Asmari Formation has some similarities and differences to the Tang-e Gel-e Tursh type section, in Khuzestan province, which was first measured and elaborated by Richardson (1924) and later by Thomas (1948). In either the type section or in Moshkan section the Asmari Formation overlies the Pabdeh Formation. Whereas the upper layers of the Asmari Formation in the Moshkan section are overlain by Razak Formation, but in the type section they are overlain by the Gachsaran Formation.
Material and methods:
Geological maps, satellite images and field observations were used to find a good outcrop for the Asmari Formation in Tamar anticline (Sepidan area, Fars Province). The section is located 35 km Southeast of Yasuj and 111 km Northwest of Shiraz with these coordinates: N 30° 25? 55.60? and E 51° 47? 37.53?. The stratigraphic section was measured by Jacob and sampled bed by bed with an interval of 2 meters. 195 samples were collected and thin-sections were prepared for analysis. Biostratigraphic and age determination were carried out using Laursen et al. (2009). The classification of carbonate rocks is followed using the nomenclature of Dunham (1962) and Embry and Klovan (1971). Microfacies recognition and determination is based on Lasemi and Carozzi (1981). Sedimentary sub-environments and depositional environments were recognized according to the schemes porposed by Wilson (1975), Buxton and Pedley (1989), Geel (2000), Pomar (2001), and Flugel (2010).
Discussion of Results & Conclusions:
Lithostratigraphy of Asmari Formation in Moshkan section
In Moshkan section, southern flank of the Tamar anticline, 600 m of Pabdeh, Asmari and Razak Formations were measured. The Asmari Formation in the study area is 350 m in thickness and is composed of limestone and marly limestone layers. From base to top the Asmari Formation is lithostratigraphically composed of the following units:
A1 unit: 149 m (241-390 m) of marly limestone, limestone and covered units with cream, yellow, grey and white thin to medium bedding.
A2 unit: 46 m (390-436 m) with medium to very thick layers of grey, cream, orange and green limestone and marly limestone.
A3 unit: 50 m (436-486 m) of medium to thick limestone layers which are grey, cream and white.
A4 unit: 34 m (486-520 m) of limestone with interbeds of medium to thick grey, cream, white and orange marly limestone.
A5 unit: 46 m (520-566 m) of grey, cream, yellow, green marly limestone and limestone with medium to very thick layers.
A6 unit: 25 m (566-591 m) of thin to thick layers of cream, green to grey marly limestone.
Biostratigraphy of Asmari Formation in Moshkan section
Old to young biozones from Asmari Formation in Moshkan section are determined as follows:
1- Nummulites vascus - Nummulites fichteli Assemblage Zone
This assemblage zone is 345 m in thickness (241-586 m). The lower boundry is defined by the first occurance of Nummulites vascus and Nummulites fichteli. The upper limit is recognized by the extinction of the above mentioned species and the appearance of Archaias spp. This assemblage zone is correlated to the assemblage zone no. 2 of Laursen et al., (2009) that represent Rupelian for the Asmari Formation.
2- Archaias asmaricus / hensoni - Miogypsinoides complanatus Assemblage Zone
The thickness is 14 m (586-600 m). The lower boundry corresponds to the last occurance of Nummulites spp. and the occurance of Archaias spp. This assemblage zone corresponds to the assemblage zone no. 4 of Laursen et al., (2009) that indicate Chattian. Therefore, Biostratigraphy studies conducted on 195 thin sections resulted in identification of 37 genera and 44 species of benthic foraminifera, 4 genera and 4 species of planktonic foraminifera along with some non-foraminifera, determination of Oligocene (Rupelian - Chattian) age, according to the biozonation of Laursen et al., (2009).
Microfacies and sedimentary environment of the Asmari Formation in Moshkan section
Microfacies characteristics were described in thin sections from 195 samples according to Dunham (1962), Embry and Klovan (1971), Lasemi and Carozzi (1981). The recognized microfacies, foraminiferal distribution and sedimentological analysis allowed the recognition of eight microfacies class of open marine, shoal and lagoon sub-environments that microfacies class 1-1 belong to Pabdeh Formation, microfacies class 1-2, 1-3, 1-4, 2-1 & 3-1 belong to Asmari Formation and microfacies class 3-2 & 3-3 belong to Asmari and Razak Formations.
Open marine Sub-environment
Planktonic Foraminifera Bioclast Wackestone - Packstone
Abundance of planktonic foraminifera and lack of large benthic foraminifera represents outer slope to deep part of the basin (Schmidt et al., 2004). The biota are located in the aphotic part of the outer ramp below the storm wave base (Geel, 2000; Romero et al., 2002; Pomar, 2004).
Bioclastic Lepidocyclinidae Wackestone - Packstone
the presence of abundant lepidocyclinids and fragments of coralline red algae indicate oligophotoic zone of open marine in lower foreslope (deep or distal middle ramp) between SWB and FWWB with low to medium energy (Hottinger, 1983; Reiss and Hottinger, 1984; Leutenegger, 1984; Hottinger, 1997; Hallock, 1999; Hohengger et al., 1999).
Bioclastic Lepidocyclinidae Nummulitidae Packstone-Grainstone (Rudstone)
abundance of large and flat tests of benthic foraminifera (nummulitids and lepidocyclinids) associated with fragments of bryozoans and echinoids represent open marine environment (Romero et al., 2002) in the oligophotic zone (Renema, 2006). This association is located in lower foreslope (deep or distal middle ramp) between SWB and FWWB (Pedley, 1996; Brandano and Corda, 2002). microfacies class lies in a shallower water in respect to the previous microfacies class (microfacies class 3-3-1-2).
Neorotalia Nummulitidae Bioclast Wackestone - Packstone
the presence of large hyaline benthic foraminifera such as nummulitids, lentile shaped rotaliids such as Neorotalia together with coralline red algae, corals and lack of lagoonal microfossils can represent the shallowest part of the open marine toward the shoal sub-environment. This microfacies is below and closer to the fair weather wave base (FWWB). This area is located in the oligo-mesophotic part of the upper foreslope (shallow or proximal middle ramp) (Geel, 2000; Pomar, 2001a, 2001b; Brandano and Corda, 2002; Corda and Brandano, 2003; Cosovic et al., 2004).
Shoal/ Bar Sub-environment
Bioclastic Nummulitidae Packstone - Grainstone ( Rudstone)
abundance of large symbiont bearing benthic foraminifera and coralline red algae, show shoal sub-environment in the euphotic inner ramp. This microfacies class is located above FWWB and it is affected by marine currents and waves. In some of this microfacies class high energy water washes away micrite (packstone-grainstone texture) and causes medium to high sorting and rounding. (Brandano & Corda, 2002; Corda & Brandano, 2003; Renema, 2006).
Lagoon Sub-environment
Lepidocyclinidae-Nummulitidae Imperforate Foraminifera Bioclast Packstone -Grainstone (Rudstone)
abundance of corallinacean, corals, imperforate and perforate foraminifera show high energy of lagoon sub-environment toward the shoal in the euphotic inner ramp. This microfacies class is located above FWWB and it is affected by marine currents and waves (Flugel, 2010).
Bioclastic Imperforate Forminifera Packstone - Grainstone
Abundance of peneroplids associated with miliolids indicate shallow hypersaline water in a protected lagoon (Romero et al., 2002). This biofacies class can change from open to restricted lagoon in the euphotic zone of shallow inner ramp above fair weather wave base (FWWB) (Leutenegger, 1984; Hottinger, 1997; Flugel, 2010).
Neorotalia Miliolids Bioclast Wackestone - Packstone
This microfacies contains porcelaneous, agglutinate and hyaline small foraminifera that represent open to restricted lagoon. The biota lived in the euphotic zone above the FWWB and inhabited the shallow inner ramp sub-environment (Sadeghi et al., 2011).
Results
The Asmari Formation in Moshkan section is 350 m in thickness with limestone and marly limestone layers. Based upon field observation and lithostratigraphic content this formation is divided into six units. In the study area it is underlain by the Pabdeh Formation and is overlain by the Razak Formation. Biostratigraphic studies on 195 thin sections resulted in the determination of 37 genera and 44 species of benthic foraminifera. Also, 4 genera and 4 species of planktonic foraminifera and some few non-foraminifera were recorded. According to the biozonation of Laursen et al. (2009) two assemblage zones were recognized. They include: 1- Nummulites vascus - Nummulites fichteli Assemblage Zone (Rupelian) and 2- Archaias asmaricus/hensoni - Miogypsinoides complanatus Assemblage Zone (Chattian). This biozonation coincides with the assemblage zones 2 and 4 of Laursen et al. (2009). Therefore, the age of the Asmari Formation in Moshkan section is Oligocene (Rupelian-Chattian). According to laboratory observations and analysis on thin sections the Asmari Formation in Moshkan section has eight microfacies classes relating to lagoon, shoal and open marine sub-environments. This represents that the Asmari Formation was deposited on a carbonate platform of homoclinal ramp.
Keywords: Folded Zagros, Sub-coastal Fars, Asmari Formation, Litho-biostratigraphy, Sedimentary environment, Oligo-Miocene, large benthic foraminifera
