One hundred forty one samples from three sections (66 from Bāz, 40 from Anguran and 35 from Gery Sheikh) are collected and thin sections provided. Thirty four foraminifer and one algae species enabled us to use zonations of Wynd (1965), Adams and Bourgeois (1967), Blom (1969), Cahuzac and Poignant (1997) and Bodagher-Fadel (2012).
All samples and data are stored and freely available in the Department of Geology, University of Hormozgan, Bandar Abbas.
Regional geology
Three studied sections are located in the Bandar Abbas Hinterland, southeastern part of 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 those croup out as salt diapirs (Stöcklin and Setudehnia 1991, Aghanabati 2004). These 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 644m thickness conformably overlies the top of Razak Formation (Fig. 2A). The succession begins with 5.5m massive limestone and 36m alternation of thick bedded limestones and marls. The succession continues with 498.5m thick bedded limestones, marly limestones and marls. Nine bryozoan and coral reefs are present here. 136m marl and 9m thick bedded marly limestone form the upper part of sections. The marly member conformably overlies the top of Guri Member (Fig. 2B).
The Guri Member of Mishan Formation in the Anguran section with 329m thickness conformably overlies the gypsiferous limestones of the top of the Mol Member of Gachsaran Formation (Fig. 2C). The Guri Member starts with 49m massive limestone and the succession continues with 179.5m thin bedded and massive limestones and marls. This part contains five bryozoan barriers. The upper part of section is composed of 154m marl and marly limestones including the last 2m of marly limestone bed as the top of Guri Member (Fig. 2D).
The Guri Member in the Gery Sheikh section conformably overlies the gypsiferous limestones of the top of the Mol Member of Gachsaran Formation (Fig. 2E). The base of Guri Member can be recognized by 2m marly limestone and continues with 73.5m massive and thin to medium bedded limestones. Three meters of bivalve-bryozoan reefal limestone is present in the middle part of section. The succession continues with 32m thin and thick bedded limestones, marly limestones and marls. 0.5m bed of limestone marks the top of Guri Member (Fig. 2F).
Biostratigraphy
Thirty four foraminifer species are identified and helped us to use zonations of Wynd (1965), Adams and Bougeois (1967) and Cahuzac and Poignant (1997) for benthic fauna and Blom (1969) and Bodagher-Fadel (2012) for planktons.
11.9 meters of the base of Guri Member in the Bāz section section (samples GA1 and GA2) were barren; so the age of Burdigalian for this part is tentatively estimated based on the stratigraphic position.
Zone 1: Assemblage fauna of this zone in the Bāz section contains Miogypsina globulina, Sphaeogypsina globulus, Pseudotaberina malabarica, Borelis melo cordica, Borelis pygmaea, Flosculinella bontangensis, Dendritina rangi, Triloculina trigonula, Triloculina tricarinata, Quinqueloculina sp., Pyrgo sp., Schlombergerina sp., Neorotalia viennoti, Operculina complanata, Oper. sp., Elphidium sp., Amphistegina lessoni, Amph. bohdanowiczi, Lepidocyclina sp., Praerhapydionina delicata, Peneroplis thomasi, Pen. farsensis, Globigerina praebulloides, Globigerina subquadrata, Globorotalia archaeomenardii, Bigenerina sp., Lithothamnium sp. (Figs 3 & 4). This assemblage is equivalent to Zone 64 of Wynd (1965) and has the age of Burdigalian.
According to Boudagher-Fadel (2008) and Boudagher-Fadel and Price (2013), Miogypsina globulina has the age of Burdigalian, whereas Brandano et al. (2007) reported it from the Early Burdigalian age. Yazdi-Moghaddam 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 Burdigalian strata of Zagros basin (Adames and Bourgeois 1967, Kalantari 1992). In addition, Rahaghi (1973) recorded this species from the Burdigalian strata of the Qom Formation in the 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.
Pseudotaberina malabarica was previously reported by Berggren et al. (1985) and Banner and Highton (1989) from the Late Burdigalian – Langhian (N6 to N9 planktonic zones), but Kalantari (1992) recorded this species from the Early to Late Burdigalian strata of the Gachsaran Formation and the Guri Member of Mishan Formation. By attention to the associated fauna; we can accept the range of Burdigalian to Langhian for this species.
Zone 1 in the Anguran section contains Sphaeogypsina globulus, Pseudotaberina malabarica, Borelis melo cordica, Flosculinella bontangensis, Dendritina rangi, Triloculina trigonula, Triloculina tricarinata, Quinqueloculina sp., Pyrgo sp., Schlombergerina sp., Ammonia beccari, Neorotalia viennoti, Operculina complanata, Oper. sp., Elphidium sp., Amphistegina lessoni, Amph. bohdanowiczi, Amph. canaensis, Amph. bowdensis, Peneroplis thomasi, Pen. farsensis, Praerhapydionina delicata, Goloborotalia archaeomenardii, Reussella sp., Bigenerina sp., Textularia sp., Lithothamnium sp. (Figs 3 & 5).
According to Adames and Bourgeois (1967), Borelis melo cordica has the age of Burdigalian. Also, Amphistegina bohdanowiczi has the age of Burdigalian (Yazdi-Moghadam et al. 2021). Kalantari (1992) reported Peneroplis thomasi from the Early Miocene strata, whereas Henson (1950) believed the range of Late Oligocene – Early Miocene for this species in Iran, Iraq and Turkey. Peneroplis farsensis ranges from the Late Aquitanian to Burdigalian (Adames and Bourgeois 1967).
Zone 1 of Gery sheikh section contains Pseudotaberina malabarica, Dendritina rangi, Triloculina trigonula, Triloculina tricarinata, Quinqueloculina sp., Pyrgo sp., Schlombergerina sp., Ammonia beccari, Neorotalia viennoti, Operculina complanata, Oper. sp., Elphidium sp., Amphistegina lesson, Amph. bohdanowiczi, Praerhapydionina delicata, Peneroplis thomasi, Pen. farsensis, Goloborotalia archaeomenardii, Reussella sp., Spirolina cylindrica, Bigenerina sp., Textularia sp., Lithothamnium sp.. This assemblage is also equivalent to Zone 64 of Wynd (1965) and has the age of Burdigalian.
Amphistegina bohdanowiczi which is present in the most parts of the Gery Sheikh secion, notably at the base of it and proves the age of Burdigalian. Pseudotaberina malabarica occurs along all of 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).
Zone 2: The assemblage fauna of this zone contains Orbulina universa, Globorotalia archaeomenardii, Globigerina praebulloides, Globigerina subquadrata, Pseudotaberina malabarica, Triloculina tricarinata, Triloculina trigonula, Amphistegina lessoni, Operculina complanata, Assilina, sp., Neorotalia viennoti, Dendritina rangi, Textularia sp., Lithothamnium sp. (Fig. 4).
According to Kennet and Srinivasan (1983) and Boudagher-Fadel (2012), the first occurrence of Orbulina universa is the index of N9B Zone of the Langhian. Also, the present association is equivalent to the Zone 65 of Wynd (1965). This zone is restricted to the Bāz section (Fig. 4).
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 other sections in this area. So, the Hunguyeh and Khorgu sections are selected for correlation (Fig. 7). Vega et al. (2010; 2012) in the outskirt of 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 stage. The Guri Member in the Hunguyeh section with 126m thickness is composed of massive limestones. The total thickness of the Guri Member in the Khorgu section reaches 749m (Vega et al. 2012). According to Hassani and Hosseinipour (2017), uppermost 106m of this section is composed of open marine pelagic limestones and marls.
The correlation of sections shows obvious changes in the strata underlying the Guri Member. Gypsum beds and gypsiferous marls of Mol Member of 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 the result of Main Zagros Fault movements.
The thickness of Guri Member dramatically increases eastward; from 126m in the west (Hunguyeh section) to 749m in the Khorgu section. This 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 & 7).