The Asmari Formation in the studied sections consists of both skeletal and non-skeletal carbonates. Skeletal grains mainly correspond to benthic and some pelagic foraminifera, red algae, bivalves, crinoids, bryozoa and some coral fragments. Benthic foraminifera are the main skeletal component and display a broad diversity range (e.g. Operculina sp., Miliolids, Elphidum sp. Archaias sp, Dendritina rangi (Fig. 4A to F). Planktonic foraminifera consist mainly of Globigerinids (Fig. 4G). Red algae is the second most important skeletal component representing variable types like Peyssoneliacean, Subterraniphyllum and Lithophyllum (Fig. 4H to K). Non-skeletal grains consist mainly of ooids, intraclasts and peloids (Fig. 4L). Based on lithology, texture and sedimentary characteristics, 12 marine microfacies could be differentiated in the studied Asmari Formation. These microfacies reflect a succession that was deposited upon a vast extended carbonate ramp consisting of an outer, middle and inner carbonate ramp. The latter can be further subdivided into different sub-environments including shoal, open lagoon, restricted lagoon and intertidal setting. In the following discussion, the different microfacies types are described and interpreted in terms of paleoenvironment.
I) Outer ramp
Planktonic foraminifera wackestone (MF1)
This microfacies is characterized by grains floating in a micrite matrix with loose packing (Fig. 5A). Clastic carbonate grains are uncommon. The constituents of these wackestones consist dominantly of planktonic foraminifera (globigerinids and globorotalids), and rarely echinoid fragments, without benthic foraminifera and any features characteristic of shallow-water and/or high-energy sedimentation. Sand-sized planktonic foraminifera are the main components, which mixed with minor thin-shelled bivalves, echinoderm and ostracod shell fragments in the
context. They show intergranular and moldic porosity filled with a cement composed of calcite and iron oxide. Furthermore, replacement of rhombohedral dolomite is common.
The depositional environment is interpreted to be situated just below wave base within the photic zone of an outer ramp based on sedimentological characteristics and faunal content (cf. Buxton and Pedley 1989; Pedley 1998). Abundant planktonic foraminifera within a micritic matrix indicate calm and aphotic sedimentation conditions, below storm wave base(SWB). This interpretation is based on co-occurrence of planktonic foraminifera which can be used as index for open marine conditions (Geel, 2000) and by the fine-grained matrix. Moreover, according to Ćosović et al. (2004) the lack of benthic foraminifera and red algae indicates the lower limit of the photic zone. This microfacies occurs at the bottom of the lower Asmari.
Bioclastic wackestone/mudstone (MF2)
This microfacies is made up by bioclasts such as echinoids, bryozoans, ostracods, echinoderms and pelagic foraminifera that float in a micritic to calci-siltitic matrix (Figs. 4, 5B). In this light-yellow clayey wackestone/mudstone main planktonic fauna consists of dispersed Globigerina. Furthermore, rarely bentic large foraminifera such as Amphistegina and Lenticulina are recognized in this facies. Bioturbation and burrowing are widespread and locally the matrix is neomorphosed to microsparite. The decrease in the proportion of planktonic foraminifera and common bioturbation differentiates this microfacies from MF1.
The depositional environment of MF2 is placed below wave base in the photic zone. This interpretation is based on accumulation of planktonic foraminifera with bioclasts such as bryozoans, as well as bioturbation and muddy matrix (Wilson, 1975; Geel, 2000; Flügel, 2010).
II) Mid ramp
Operculina algal packstone/rudstone (MF3)
This microfacies is dominated by large planktonic foraminifera, especially Operculina, Miogypsina and coralline and Peyssoneliacean red algae (Fig. 5C). Additional components are fragments of corals, echinoderms and bryozoans. The packstone is composed of a variety of allochems in terms of type and size, including mainly sand to silt-sized fragments of planktonic foraminifera. The fabric is grain-supported with slightly tight packing. Foraminifera and algae display co-existence and in some cases are encrusted (see Fig. 4K). Coralline algae rudstone to packstone is a common Miocene ramp microfacies (Buxton and Pedley, 1989) comprising Echinoderm, rotalia and red algae. Both moldic and framework porosity are well developed and partially filled with cements.
The depositional environment of this microfacies corresponds to the deepest part of a mid ramp just above the lower limit of the photic zone. This interpretation is supported by occurrence of large foraminifera such as vast range of Operculina which is often encountered in the latter environmental setting (Geel, 2000; Romero et al., 2002). Moreover, the rare occurrence of planktonic foraminifera and absence of lagoonal foraminifera like miliolids are additional arguments in support of a mid ramp setting.
Bioclastic bindstone (MF4)
This microfacies is characterized by a bindstone fabric with Peyssoneliacean red algae which trap and bind allochems and fine carbonate particles (Fig. 5D). Principal constituents which were trapped are fragments of corals, gastropods, bivalves, echinoids and benthic foraminifera. The fabric displays a tight packing with rare intergranular porosity, which is locally filled with sparitic equant cement.
The depositional environment of this microfacies corresponds to the upper part of a mid ramp situated between an euphotic and mesophotic zone below the fair-weather wave base. Although the environment of this microfacies is interpreted as low energy lagoonal environment by Rasser (2000, 2003), the limited presence of skeletal grains representative of low energy environment, like millolids, point toward sedimentation under relatively lower energy conditions. This suggests sedimentation at a lower energy level and deeper setting compared to the former microfacies MF3.
Oncoidal peloidal packstone to grainstone (MF5)
This microfacies is characterized mainly by oncoids (Fig. 5E) which formed by encrusting and binding of sedimentary grains. Well-sorted peloids are the second important component. The other grains consist of some bioclasts such as echinoids, benthic foraminifera, ostracods and also ooids. The fabric is grain supported with intergranular pores with sparitic infill which locally changes to a micritic matrix. The pores are partially filled by blocky calcite.
The depositional environment of this microfacies corresponds to the border of mid and distal inner ramp in the euphotic zone around fair-weather wave base. This interpretation is supported by the presence of well sorted peloids and rare ooids reflecting a hydrodynamic shoal environment of with oncoids and rare large bentic foraminifera support a moderately high-energy setting as encountered at the transition of inner to mid ramp.
III) Inner ramp
Bioclastic and ooid grainstone (MF6)
This microfacies is made up by the presence of ooids displaying an oval, circular or elongate outline and concentric internal structure (Fig. 5F) showing several cortical layers around a core which is usually dissolved. They are well-sorted and densely packed. In some cases, they are micritized. Other components consist of a broad range of peloids, benthic foraminifera and fragments of bivalves and gastropods. Intergranular and/or moldic are common pore types of this microfacies (Gharechelou et al. 2015).
The depositional environment of this microfacies corresponds to a distal inner ramp near-shoal to lagoon with deposition above fair-weather wave base. This interpretation is supported by the high frequency and well sorting and rounding of ooids and peloids making-up a grainstone texture indicating that this facies was deposited in a high energy shoal/barrier environment (Tucker and Wright, 1990; Tucker et al., 2009). However, co-occurrence of ooids with porcelaneous foraminifera (miliolids and dendritina rangi) within a fine-grained matrix point to a relatively low energetic environment, indicating a textural inversion and redisposition. This means that after forming ooids in an energetic shoal environment, components were transferred to a low lagoonal setting (Warren, 2006; Flügel, 2010).
Reefal rudstone to boundstone (MF7)
This microfacies is characterized by coral of scleractinia or hexacorals together with red algae and large foraminifera especially acervulina sp. (Fig. 5G). Other components consist of very small skeletal debris and intraclasts occurring in a micritic matrix. They show a tightly packed fabric with poor sorting, as well as some framework porosity. These pores are usually filled by both calcite and evaporitic cement.
The depositional environment of this microfacies corresponds to the distal inner ramp that is situated above fair-weather wave base (FWWB). This interpretation is supported by the occurrence of coral rudstone (see Wilson, 1975 and Flügel, 2010). They are restricted to two samples which point to limited and a discontinuous distribution, likely forming patch reefs.
Bioclastic miliolid packstone (MF8)
This microfacies consists of a vast range of small benthic foraminifera especially miliolids (Fig. 5H), making up a packstone. Calcareous algae like halimeda and dasycladacea as well as dendrita rangi foraminifera are poorly represented. Miliolid, dendritina packstone is recognized with beige to gray color and grainy texture. Skeletal allochems, porcelaneous foraminifers, specially miliolids and denderitina, are the main component of MF8. Additionally, echinoderms, bivalves and gastropods can be found. The fabric is grain-supported with relatively tight packing.
The depositional environment of this microfacies corresponds to the mid part of an inner ramp in the euphotic zone above fair-weather wave base. The occurrence of small to medium-sized porcelaneous foraminifera (miliolids) suggests an inner shelf environment (Geel 2000; Brandano and Corda 2003). Abundant allochems reflect a restricted lagoonal environment while gastropods indicate an open lagoon. The diversity and grain-supported fabric suggest higher hydrodynamic conditions than present in a restricted lagoon pinpointing to an open lagoonal environment.
Bioclastic echinoid wackestone (MF9)
This microfacies is characterized by abundant echinoid fragments with intraclasts and rarely benthic foraminifera (Fig. 5I). It shows a poor sorting and loose packing. It is micrite matrix supported. In this microfacies vuggy pores are common and partially filled by cement.
The depositional environment of this microfacies corresponds to a lagoon within an inner ramp setting that is situated above fair-weather wave base. This interpretation is supported by the co-occurrence of echinoids evidencing open marine conditions with the small benthic foraminifera pointing to a more restricted environment (Flügel 2010). This microfacies differs from the former by its low diversity in skeletal grains.
Evaporites (MF10)
Evaporite phases are made up by laminated gypsum (Fig. 5K) displaying two main textures, namely alabastrine consisting of fine crystalline anhedral to subhedral crystals and porphyroblasts displaying coarse crystalline subhedral to euhedral crystals. Another texture that is infrequently observed, associated with the former, consists of daisywheel gypsum displaying rosette structures with needle gypsum crystals at their margins. There are some lath-shaped phases that usually correspond to pseudomorphs of anhydrite, especially occurring in the alabastrine textures.
According to Warren, (2006) the main evaporite settings are 1) mud flat evaporites with sabkha, saline pans and salinas, 2) salteran evaporites and 3) deep water evaporites. Absence of textures like nodular and chicken-wire evaporites that relate to mud flat evaporites within sabkha, saline pans and salinas suggest that such a setting did not existed in the study area. The first study thin layer of the evaporite called basal evaporite possibly precipitated in a deep water setting (Kavoosi and Sherkati, 2012; Rafiei and Rahmani, 2017). This explanation is based on the continues position of this evaporitic layer between deep water sediment of Pabdeh and Lower Asmari Formations. However, the main evaporite unit followed directly upon mid ramp deposits, intercalated with carbonate wackestones and mudstones that reflect a restricted environment. Furthermore, the presence of elongated nodular anhydrite (Fig. 8I) together with the lack of microfossils indicate a saltern environment within a shallow and restricted lagoon, an interpretation which is in line with Daraei et al. (2015).
Dolomitized mollusk wackestone with evaporites (MF11)
This microfacies is characterized by abundant micritized bivalves and high degree of matrix dolomitization (Fig. 5J). Other components are represented by micritic agglutinated foraminifera, ostracods, brachiopods and calcareous algae. It corresponds to a matrix-supported microfacies with tight packing. Vugs, intergranular and microfracture pore types are common. They are filled with dolomite and evaporite cements. Replacement of micrite by rhombohedral dolomite is the most common diagenetic feature in this microfacies.
The depositional environment of this microfacies corresponds to a restricted lagoon above fair weather wave base with poor connection to the open basin. Evidence for this interpretation relates to the low diversity in skeletal grains (Gadzicki, 1983; Wilson and Evans, 2002). In addition, the micritic matrix, the poorly sorting of intraclasts and good taphonomic preservation of brachiopods point to an environment with low-energy conditions.
Mudstone (MF12)
This microfacies is characterized by a mud-supported fabric containing irregular and amalgamated discontinuous fenestral pores and solution vugs (Fig. 5L). Partially to entirely pore-filling by blocky to granular evaporitic and dolomitic cement has been observed. Dolomitization is common forming rhombohedral crystals in the matrix.
The depositional environment of this microfacies corresponds to an intertidal flat and/or restricted lagoon. This interpretation is supported by well-preserved and widespread fenestral pore types which according to Hardie and Shinn (1986) originated from desiccation-shrinkage in an intertidal flat. Absence of skeletal grains also supports this interpretation.