The lithofacies and paleo-environments were identified based on occurrence of skeletal and non-skeletal grains, size, roundness, sorting, orientation of the grains, and type of matrix. Lithofacies were determined based on Dunham (1962). The skeletal grains are comprised of benthic foraminifers, echinoderms, bivalves, microgastropods, and coral remains, and the green algae are recognizable as organic constituents. Specifically, the non-skeletal grains are composed of intraclasts and extraclasts. The stratigraphic distribution of both skeletal and non-skeletal grains (bioclast, intraclast, and extraclast) is presented in Fig. 3. The presence of Dasycladacean algae at the studied area indicates an intertidal environment (Khosrow Tehrani, 2011).
The presence of corals and echinoids and microgastropods at the studied section indicates the lagoon sedimentary environment or the reef depositional system in the uppermost open marine section (Khosrow Tehrani, 2011). Microscopic investigation of the Nayband Formation supported the presence of six major lithofacies, namely: (Fig. 8,9,10)
7.2. Inner ramp facies belt (biostructural carbonate reef or bioclast ridge)
Lithofacies B. Sandy Bioclastic Boundstone / Grainstone
The main components of this microscopic facies are 40% bioclasts or different skeletal grains along with microsparitic to asparaitic cement. Among the sub-elements of this microscopic facies, we can mention the destructive quartz grains up to about 25% (microfacies B). Skeletal grains include sponges, worm trace, algae and many crinoids, bryozoa, bivalves, mollusks, microgastropods and corals and with about 5% benthic foraminifera which are found in the Sh8 with a thickness of up to 2 meters and exposed in the rock unit No. 11 of Shahdad.
Because quartz degradation grains and skeletal grains with medium to fairly good sorting and roundness are seen in a grainstone texture with a sparite background without the presence of a micrite matrix (representing a high-energy environment). Moreover, because of the shallow depth, warm to moderate climate, normal water turbulence and salinity, this part of the carbonate platform, which is a type of carbonate ramp with low subsidence along with slight entry of destructive and debris materials, causes the formation and deposition of carbonate sediments in the grainstone facies, along with open sea and lagoon bioclasts, indicates deposition in the high-energy conditions of tidal channels on the edge of the reef. Greenstone in this facies mainly lacks porosity, because all spaces between the particles, inside the particles, and mold porosity are filled by different generations of spar calcite cements. Its formation environment can be compared to 12–14 RMF Flugel (2010) and belongs to the reef environment to shallow reef range.
Lithofacies C. Sandy Bioclastic Grainstone
The main components of this microscopic facies are approximately 20–30% bioclasts or skeletal grains. Destructive quartz grains are the only minor element of this microfacies (microfacies C) at up to about 40%.
Skeletal grains include fragments of bivalves, mollusks, corals, crinoids, echinoids, dacycladaceae algae, shell fragments, and about 5% benthic foraminifera, which are outcropped in samples No. 21 and 22 of Shahdad with thicknesses of up to 2 meters in the rock unit of Shahdad 11.
The absence of a micrite matrix along with open sea and lagoon bioclasts and the abundance of coral, sponge, and microgastropod bioclasts in this microfacies indicate energetic environmental conditions, especially sedimentation in biostructural reef environments formed in bioclast stacks (bio-constructed) and comparable to 14 RMF Flugel (2010), and belong to the open marine environment of the inner ramp. Furthermore, its environment can be considered because of the presence of lithoclasts, energetic tidal channels in front of the reef. The reef was considered towards the open sea, which is exposed in sh21 with a thickness of up to 4 meters in rock unit number 24 of Shahdad. Nevertheless, it seems that the existence of a coral reef bandstone belt or bio constructed patches of sample no. sh 6 with greenstone facies (12 RMF), which is a low expansion and without coral dominant facies, actually existed at the edge of the platform and the environment of the lagoon to some extent. It is separated from the sedimentary environment of the open sea, and as a result, because of its low thickness as well as the density in some places, it is in the form of scattered single coral accumulations or separate and isolated communities (come reefs/ or bio-constructed patches). It can partially represent a reef environment in the shallow open sea, which includes the open marine to the shallower mid-ramp environments.
Therefore, in general, the absence of a significant presence of framework-forming organisms in the euphotic zone (reef-building corals) and the lack of effect of reefs and the type of carbonate sediments produced, as well as the paleoecology of foraminifera in the study area, the sedimentation environment of these deposits, and the presence of four sedimentary environments (i.e. open sea, reef, behind the reef or lagoon, and coastal or tidal), indicate it is a carbonate platform ramp or carbonate ramp.
7.3. The facies belt of the internal ramp (in front of the reef to the open sea)
Lithofacies D. Sandy Bioclastic Packstone
The main components of this facies are often skeletal grains or bioclasts (between 20% and 40% in decreases, and according to the percent abundance of
allocems, towards the open sea, they are separated into three groups: ooid packstone bioclasts, wackestone bioclasts, and fossilized mudstone.
Lithofacies E. Sandy oolitic Biointraclastic Packstone
The main components of this facies are skeletal different samples along with a micrite to microsparite background. Skeletal grains mostly consist of fragments of sponges, shells, bivalves, mollusks, crinoids, echinoids, microgastropods, dasycladaceae algae, and sometimes some bryozoa, sponge nodules and perfect corals individually with a smaller smaller percentage of benthic foraminifera.
Destructive quartz grains (about 10–40% in different samples) form the sub-elements of this microscopic facies (microfacies D) which is outcropped with a thickness of up to 2 meters in sample number 1 of Shahdad / sh 1 in rock unit 11 of Shahdad. This microfacies is comparable to 14 RMF Flugel (2010) and belongs to the open marine environment of the inner ramp. Because of the low roundness, its formation environment is a little far from the bio-structural reef, and the presence of microsparite to micrite matrix indicates medium to low wave energy.
Sandy Lithobioclastic Packstone
The background in this microfacies consists of micrite to microsparite. Allochems account for 60% of this microfacies, which includes 25% bioclasts, mostly rounded and transported fragments of bivalves, mollusks, bryozoa, crinoids, dacycladaceae algae, sponge nodules, and up to 5% benthic foraminifera. Also seen in this microfacies are 20% of medium-sized angular to slightly rounded quartz and 10% of intraclasts, including micrite rock pieces.
The environment of the formation of this microscopic facies with the presence of a micrite background indicates a low to medium energy environment with the dominant presence of open sea bioclasts such as corals, sponges, bivalves, brachiopods, and gastropods. The lack of lagoon bioclasts (benthic foraminifera and algae) with the presence of lithoclasts and the entry of destructive materials and debris from the coastal belt to the basin represents the inner part of the carbonate ramp, especially the open sea, which is comparable to RMF 14 Flugel (2010) and belongs to the open marine environment of the inner ramp.
In fact, the sediments of such facies are mainly in the carbonate ramp platform as the most extreme facies belt, which generally originated from the shallower or lagoonal part and were transported to this environment (open sea) by waves, currents, and storms. By moving away from the coastline, the percent abundance of the constituent components
grains (between 10% and 30% in different samples) along with micrite to microsparite cement. Skeletal grains are different and mostly comprise pieces of coral, sponge, shells, bivalve, mollusk, crinoid, echinoid, microgastropod, dasycladaceae algae, and sometimes a little bryozoa, brachiopod, and sponge and perfect corals individually with a smaller percentage of benthic foraminifera.
Among the sub-elements of this microscopic facies, we can also mentionthe destructive quartz grains (about 10–30% in different samples; lithoclasts [10–30% percentage of igneous rock fragments, micrite, etc.] in the form of extraclasts) sometimes together with a small percentage (less than 5%) of the ooids, indicating that the ooids are single-layered (surficial), and the central core in most of them is an intraclast (microfacies E) which is outcropped with a thickness of up to 2 meters in samples No. 22 and 21 of Shahdad / sh 21, 22 stone unit 11 of Shahdad.
The existence of a few single-layered and thin radial ooids in this facies indicates the location of its depositional environment in the vicinity of the reef, which is usually associated with lithoclasts of different types and amounts. The study of this carbonate facies, which has a micrite to slightly microsparic background, shows that these deposits were left in a quiet environment with low to medium energy. The fauna in the sediments of this section, which consists of groups of corals, sponges, and brachiopods, algae, gastropods and benthic foraminifers, are indicative of shallow marine environments and light-influenced areas, and their distribution depends on changes in water salinity and type of substrate.
The formation environment of this microscopic facies has been determined to be in front of the reef or near the reef towards the open sea, which is comparable to RMF 14 Flugel (2010) and belongs to the open marine environment of the inner ramp.
7.4. Middle ramp facies belt
Lithofacies F. Silty Bioclastic Wackestone / Mudstone
The main components of this microscopic facies are skeletal grains (between 5% and 20% in different samples) along with micrite background. One of the characteristics of this facies is the presence of a mixture of open sea benthic and pelagic fossils, which in the studied samples of benthic fossils included fragments of sponges, echinoids,
dacycladaceae algae (especially Diplopora), and some crushed benthic foraminifera along with the predominant presence of pelagic bivalves. The destructive quartz grains are also seen in small amounts (5–10%) and scattered in the micrite field (microfacies F) which is exposed in most Shahdad samples numbers 13–16, 23–25 with different thicknesses.
The sedimentary environment of this facies is based on the micrite context and the presence of pelagic bivalves and the low abundance and small size of its allocems with grains less than 1 mm in diameter, in the deeper area of the open sea and in accordance with the standard facies RMF 5 Flogel, 2010, and it belongs to the shallower environments above the mid-ramp, after which the facies of the deeper part can be seen as dark shale, which sometimes becomes shallower due to the influence of river or deltaic surface currents, alternating with more Grivaki sandstone. Sometimes it is deepened with a thin layer of limestone, and sometimes it is marly. Therefore, when the conditions for calcium carbonate sedimentation are dominant, the sediments are fine-grained limestone (micrite), and in the absence of the mentioned conditions, sedimentation of the type. The fine-grained sediments were from Chale.
One of the most important components of this facies is the abundance of carbonate mud along with small bioclasts such as benthic foraminifera and pelagic bivalves, which indicates sedimentation in a low-energy environment as well as the lack of biological disturbances, which indicates a lack of oxygen production.