Microbiostratigraphy of Miocene Guri Member of the Mishan Formation in the Bandar Abbas area, SE Zagros, Iran

Three sections of Guri Member of the Mishan Formation in the Bandar Abbas area are investigated for foraminifera biozonation and lithostratigraphic correlation. The base of Guri Member conformably overlies Gachsaran Formation (or Razak Formation in some sections), whereas the top of the member is overlain by marls. Thirty-four foraminifer species are identified and prove the age of Burdigalian for the Guri Member in the Gery Sheikh and Anguran sections and Burdigalian–Langhan range in the Bāz section. Two assemblage zones are recognized based on the distribution range of species. Correlation to other sections shows the eastward marine progression and age change from the Burdigalian in the west to the Burdigalian–Langhian in the east of Bandar Abbas area.


Introduction
Neogene sequences of the Zagros basin (especially Asmari and Gachsaran formations) in the west and southwest of Iran have been investigated for hydrocarbon exploration for more than a century (Motiei 1995). Many of the studies on Neogene strata in the west and southwest of Iran are focused on the prolific Asmari Formation (e.g., Adames and Bourgeois 1967;Amirshahkarami et al. 2007;Amirshahkarami 2013;Sahraeyan et al. 2014;Taheri et al. 2017). On the other hand, Mishan Formation (especially Guri Member) in the west and southwest of Iran is barren, so it has rarely been studied in these areas. The Guri Member of the Mishan Formation is a prolific hydrocarbon reservoir in the Bandar Abbas Hinterland, southeast of Zagros (Kashfi 1982). So, more palaeontologic studies are needed to understand its stratigraphic situation.
The Mishan Formation as the middle unit of the Fars Group is composed of two parts: the Guri Member (limestones, marly limestones and marls) and the upper unnamed marly member. James and Wynd (1965) introduced the Mishan Formation and measured its type section in the Gachsaran oilfield. The total thickness of the Mishan Formation in the type section reaches 710 m with 61 m basal fossiliferous limestone named "worm beds" (Guri Member). Then, they measured the type section of Guri Member with 112.8 m thickness in the Tange Guri valley, near Lar City in the south of Fars Province.
Besides the general geological studies by Fakhari (1995), there have been few researches on the microfacies, sedimentary environments and paleontology of the Miocene strata of the Bandar Abbas area (SE Zagros). For example, Heidari et al. (2014) and Daneshian et al. (2016aDaneshian et al. ( , b, 2017 studied the biostratigraphy of these strata in some sections. Fanati-Rashidi et al. (2015) and Hassani et al. (2018) assessed the microfacies and sedimentary environments of the Mishan Formation in this area. Heidari et al. (2012) and Vega et al. (2010Vega et al. ( , 2012 described decapod crabs and introduced new species from the Miocene Mishan Formation beds of some sections. Kroh et al. (2011) and Gholamalian et al. (2015) reported bivalves and echinoids of these sequences. Gholamalian et al. (2020) described some octopods from the Miocene Mishan Formation of Bandar Abbas area.
Biostratigraphy of the Guri Member in the Gery Sheikh, Anguran and Bāz sections in addition to the correlation of them to other sections and discussions on the lateral stratigraphic changes are the main aims of the present research.

Geographic position
Three sections (Gery Sheikh, Anguran and Bāz) were selected for the present study in the west, north-west and north of Bandar Abbas, southeast Zagros basin (Fig. 1A). The Gery Sheikh section is accessible by passing 77 km in the Bandar Abbas-Lar road. The Bāz section is located 94 km north of Bandar Abbas, and 24 km west of Gohreh village. This section is located on the southern flank of the Bāz anticline. The Anguran section is 57 km NW of Bandar Abbas, near a salt diapir (Fig. 1B).

Materials and methods
One hundred forty-one limestone and marly limestone samples from three sections (66 from Bāz, 40 from Anguran and 35 from Gery Sheikh) were collected and thin sections obtained. Thin sections were observed by stereomicroscope in the laboratory of paleontology, University of Hormozgan. Then the best samples were selected and photomicrographs taken by a digital microscope. Thirty-four foraminifer and one algae species enabled us to use zonations of Wynd (1965), Adames andBourgeois (1967), Blom (1969), Cahuzac and Poignant (1997) and Boudagher-Fadel (2012).
All samples and data are stored and freely available in the Department of Geology, University of Hormozgan, Bandar Abbas.

Regional geology
Studied sections are located in the Bandar Abbas Hinterland, southeastern part of the Zagros folded belt (Fig. 1A). The presence of large anticlines and synclines, numerous salt diapirs and high thickness of Neogene strata are important index characters of this zone (Motiei 1993). Ediacaran to Middle Cambrian evaporites and volcanic rocks of Hormuz Series are the oldest stratigraphic unit in the area. These strata crop out as salt diapirs (Stöcklin and Setudehnia 1991;Aghanabati 2004). Salt diapirs have deformed many of the Mesozoic and Cenozoic lithostratigraphic units. Cretaceous to Early Miocene strata can be observed in the cores of many anticlines. Early Miocene to Pleistocene Fars Group (Gachsaran, Mishan, Aghajari, and Bakhtiari formations) composes the uppermost strata of the anticlines and forms the plains.

Lithostratigraphy
The Guri Member in the Bāz section with 644 m thickness conformably overlies the top of the Razak Formation ( Fig. 2A). The succession begins with 5.5 m massive limestone and 36 m alternation of thick bedded limestones and marls. The succession continues with 498.5 m thick bedded limestones, marly limestones and marls. Nine bryozoan and coral reefs are present here. 136 m marl and 9 m thick bedded marly limestone form the upper part of the sections. The marly member conformably overlies the top of the Guri Member (Fig. 2B). The Guri Member in the Gery Sheikh section conformably overlies the gypsiferous limestones of the top of the Mol Member of the Gachsaran Formation (Fig. 2E). The base of the Guri Member can be recognized by 2 m marly limestone and continues with 73.5 m massive and thin to medium bedded limestones. Three meters of bivalve-bryozoan reefal limestone is present in the middle part of the section. The succession continues with 32 m thin and thick bedded limestones, marly limestones and marls. Fifty centimeter bed of limestone marks the top of the Guri Member (Fig. 2F).
11.9 m of the base of the Guri Member in the Bāz section (samples GA1 and GA2) were barren, so the age of Burdigalian for this part is tentatively estimated based on the stratigraphic position.  (Figs. 3 and 4). This assemblage is equivalent to Zone 64 of Wynd (1965) and is of the Burdigalian age.
According to Boudagher-Fadel (2008) and Boudagher-Fadel and Price (2013), Miogypsina globulina is of the Burdigalian age, whereas Brandano et al. (2007) reported it from the Early Burdigalian age. Yazdi-Moghadam et al. (2021) reported Miogypsina globulina from the SBZ25 (Burdigalian) of Qom Formation in the Hamedan and Azarbaijan areas, north-west of Iran. Also, Borelis melo cordica is the index of the Burdigalian strata of the Zagros basin (Adames and Bourgeois 1967;Kalantari 1992). In addition, Rahaghi (1973) recorded this species from the Burdigalian strata of the Qom Formation in Central Iran. Kalantari (1992) reported Borelis pygmaea and Flosculinella bontangensis from the Early Miocene (Burdigalian) strata of the type section of Guri Member in the Zagros basin.
Amphistegina bohdanowiczi is present in most parts of the Gery Sheikh section (notably at the base of it) and proven to be of the Burdigalian age. Pseudotaberina malabarica occurs along all of the section (Fig. 6).
The association of Zone 1 is equivalent to SB Zone 25 of Cahuzac and Poignant (1997) and N5 to N7 zones of Blow (1969) in all of three sections. On the other hand, the present zone can be correlated to the Borelis melo group Meandropsina iranica Zone of Adames and Bourgeois (1967), Zone 64 of Wynd (1965) and Borelis melo group Zone of Nikfard Dill et al. (2020).

Correlation to other sections
Lateral lithologic and age changes of Guri Member in the southeastern Zagros encourage us to correlate the present sections together and to some others in this area. So, the Hunguyeh and Khorgu sections are selected for correlation (Fig. 7). Vega et al. (2010;2012), in their studies on the decapods from the Mishan Formation in the Hunguyeh and Khorgu sections, described the Guri Member and attributed it to the Burdigalian and Burdigalian-Langhian stages, respectively. The Guri Member in the Hunguyeh section with 126 m thickness is composed of massive limestones. The total thickness of the Guri Member in the Khorgu section reaches 749 m (Vega et al. 2012). According to Hassani and Hosseinipour (2017), the uppermost 106 m of this section is composed of open marine pelagic limestones and marls.
The correlation of the sections shows obvious changes in the strata underlying the Guri Member. Gypsum beds and gypsiferous marls of Mol Member of the Gachsaran Formation of the western sections (Hunguyeh, Gery Sheikh and Anguran) change to the sandstones of the Razak Formation in the Bāz and Khorgu sections. Alavi (2004, Fig. 2), confirms the eastward change of Gachsaran Formation evaporites to the Razak Formation sandstones as a result of the Main Zagros Fault movements.
The thickness of the Guri Member dramatically increases from 126 m in the west (Hunguyeh section) to 749 m in the Khorgu section in the east. The Guri Member is diachronous, changing from Burdigalian in the west to Burdigalian-Langhian in the east. On the other hand, these data show the eastward marine progression that may be the other effect of the Main Zagros Fault movements (Alavi 2004). In fact, these eastward changes can easily be observed in the southern Fars and Bandar Abbas Hinterland (Figs. 1 and 7).

Conclusion
Thirty-four foraminifer species proved the age of Burdigalian for the Guri Member of Mishan Formation in the Gery Sheikh and Anguran sections and Burdigalian-Langhian for the Bāz section. Two biozones are recognized: zone 1 is present in all sections and is of the Burdigalian age. Zone 2 is restricted to the Bāz section and is of the Langhian age. The correlation of the Guri Member sections in the southeastern Zagros shows the age change from the Burdigalian in the west to the Burdigalian-Langhian in the east of Bandar Abbas Hinterland. In addition, dramatic increase of the Guri Member thickness proves eastward marine progression during the Early to Middle Miocene in the southeastern Zagros area.