Biostratigraphy of Early Oligocene - Early Miocene Asmari Formation at the Southern Flank of Mish Anticline, Zone,

In this research, biostratigraphy related to the Asmari Formation at the southern flank of Mish anticline, Izeh Zone, is discussed. In the studied stratigraphic section, the Asmari Formation overlies the Pabdeh Formation and underlies Quaternary sediments. A study of 192 samples of the 328 m-thick Asmari Formation led to identification of 16 species and 26 genera of foraminifera taxa and identification of four Oligocene biozones. The Globigerina spp . and Nummulites intermedius- vascus Assemblages Zones represent the Rupelian age. The Ruplian- Chattian age is also defined by Lepidocyclina- Operculina- Ditrupa Assemblages Zone and Archias asmaricus- Miogyosinoides spp. Assemblages Zones that indicates a Chattian age.

The Zagros Mountain is southern part of an Alpine oroganic (Alavi, 2004). It extends from southeastern Turkey through the northern Syria and Iraq to western and southern Iran (Ghazban, 2007). Post tectonic and sedimentary events in Zagros resulted in formation of several deinable basins ( Figure 1B): Thrust Zone, Lorestan, Izeh, Dezful Embayment, Abadan plain, Fars, Bandar Abbas Hinterland (Falcon, 1974). The Oligocene-Miocene marine deposits of Iran, Qom and Asmari Formations, are one of the best known carbonate reservoirs in the world. The Asmari Formation is present in most of the Zagros foreland basin. Biostratigraphy of the Asmari Formation has been the subject of detailed study ever since the irst petroleum reservoirs were discovered, in Masjed Soleyman area. The Oligo-Miocene reservoirs are currently being utilized proliically not only in Iran but also in other parts of the Middle East e.g. Kirkuk Field in Iraq (Murris, 1980). Because of the economic importance, it pays to concentrate on the diferent properties of the oil-bearing Asmari Formation. Van Buchem et al. (2010) proposed that lateral facies changes may cause stratigraphic traps to appear in the Asmari Formation.This paper deals with biostratigraphy study of Asmari Formationm outcrop at southern lank of Mish anticline (Tang-E Malaghon in the Izeh Zone), whose results could contribute to a better understanding of the subsurface Asmari Formation in adjacent oilield areas. The studied stratigraphic section is located at southern lank of Mish anticline ( Figure 3c) with geographic coordinates 50º 56′ 22″ E and 30º 18′ 11″ N. The Asmari Formation is 336 m thick in study section and overlies the Pabdeh Formation and underlies the Quaternary sediments.

Previous Studies
The Asmari Formation was named as a Cretaceous-Eocene interval by Busk and Mayo (1918). It was deined as an Oligocene Nummulitic limestone and described as an Oligocene-Miocene carbonate sequence (Taheri et al., 2015). The biostratigraphic framework of the Asmari Formation was established by Wynd (1965) and revised by Adams & Bourgeois (1967). The age of the Asmari Formation was deined from Rupelian to Burdigalian. It is divided into lower (Oligocene), middle (Aquitanian) and upper (Burdigalian) parts. Van Buchem et al. (1965) applied strontium isotope dating to the Asmari Formation and established biozonation of wynd (1965) and Adams & Bourgeois (1967). This also enabled the introduction of new biozonation for the Asmari Formation and clearly diferentiated Rupelian from Chattian intervals ( Figure 2).
More recent studies of the Asmari Formation have been conducted on biostratigraphic, depositional environment and sequence stratigraphy criteria.  Taheri et al. (2015) and Vaziri-Moghaddam et al. (2006.

Geological Setting
By the end of Mesozoic time, the principle palaeogeographic features of southwestern Iran were the main trough of the Tethys to the north and Zonation in Asmari Formation (Adams & Burgeous, 1967;Van Buchem et al., 2010;Wynd, 1965 (Murris, 1980). In the center of troughs (Foredeep of Zagros foreland) the Pabdeh Formation was deposited ( Figure 3). By the end of Eocene time, the widespread regression caused the greater portion of the region except the central parts of the troughs to emerge. The resulting disconformity is present over the entire area where Jahrum and Shahbazan formations are developed.
Zagros Basin gradually shallowed in Oligocene-Early Miocene time and was covered by the carbonate of the Asmari Formation (James & Wynd, 1965). The local variations in lithology within the Asmari Formation are mainly recognized as members such as the Kalhur anhydrite and the Ahwaz sandstone. Locally, favorable conditions for deposition of evaporate had developed during this time as evidenced by the Kalhur Member of Asmari Formation and other thin evaporate Tongues (Motiei, 1993).
The Calcareous sandstones and sandy limestone of the Ahwaz Member unconformably overlies the Eocene-Oligocene Jahrum Formation in the Dezful Embayment. They record near-shore deltaic environments which passed eastward into shallow carbonates (Motiei, 1993).

Methods
For this research, 192 samples from the Asmari Formation in the selected stratigraphic section were studied.  (1988) and Rahaghi (1980).

Biostratigraphy of Study Section
Benthic foraminifera are abundance and diverse in most samples of the Asmari Formation at the study section. The zonal scheme presented here consists of 4 zones on the basis of the stratigraphical distribution of benthic foraminifera recognized in thin section ( Figure 4). They are described in ascending order, following the stratigraphic development; 26 genera and 16 species of benthic foraminifera were recognized ( Figures 5, 6 and 7).
Deinition: This biozone is characterized by the rich small planktonic foraminifera such as Globigerina spp. The lower limited of this zone is marked by last occurrence of Turborotalia cerroazulensis and Hantkenia sp, and an abundance of Globigerina sp., and the upper limit by the irst In northern Dezful Embayment and adjusting are, it is placed in transitional beds between Pabdeh and Asmari Formations but to the south on the evaporative layers, as called basal anhydrite , is placed. To the northeast, with decreasing depth, planktonic frequency decreases instead the number and diversity of benthic fauna increases. In this area, they are called by the name of Ditrupa sp., Eouvigerina sp., Haplophragmium slingeri Assembalge Subzone (Wynd, 1965).
In the northern Lorestan and along Main Thrust Zagros fault, there is a red clastic limestone bed that contains plenty of planktonic foraminifera. It is belonging to Razak Formation. The Razak Formation is interpreted as the distal facies of a clastic wedge whose proximal facies presumably extended to the northeast into the Zagros thrust zone (Ghazban, 2007). The age of the formation is generally Early Miocene.
Characterization: This interval also characterized by the presence of Ditrupa spp. (Worm Tube).

Correlations
The lowermost part of Asmari Formation in the study section coincides with Globigerina ssp.  Sadeghi et al., 2009) . Therefore, in this area, the age of upper part of the

Ditrupa Assemblage Zones to late Chattian
Archaias asmaricus-Miogypsinoides spp. Assemblages Zones comprises of Archias asmaricus, A. kirkukensis which were regarded as indicator of Chattian time ( Adams & Bourgeois, 1967). According to Eames and Banner (1962), Archaias asmaricus and A. hensoni range from the Early to Late Chattian. Their last occurrence has been considered near or just after the base Miocene based on Sr isotope dating by Ehrenberg et al. (2007). Therefore, in this area, the Lepidocyclina-Operculina-Ditrupa Assemblage Zone based on its stratigraphical position is considered to be Late Rupelian-Early Chattian in age.
The Archaias asmaricus-Miogypsinoides spp. Assemblages Zones has been identiied from the shallow-marine carbonate sequence while At the study section, upper part of Asmari Formation is not exposed; therefore we are not able to describe its biostratigraphical criteria.

Conclusions
The Asmari Formation in the Mish anticline, Izeh Zone, overlies on the Pabdeh Formation and underlain unconformably by the Gachsaran Formation. According to distribution of the foraminiferal assemblages, the age of the Asmari Formation in the study section is deined as Early Oligocene (Ruplian) -Early Miocene (Aquitanian