The Qom Formation is composed of a vast outcrop in Central Iran and has attracted a lot of attention due to its high hydrocarbon reservoir. The facies diversity of this formation has resulted in noncompliance between its stratigraphic units in different areas. This study is primarily concerned with analyzing depositional facies, depositional environment and digenetic processes dominating the Qom Formation in Rameh section in northeastern Garmsar. Field and microscopic observations led to identification of facies, depositional environment and diagenetic processes of the Qom Formation in the studied section.
Geographical Setting of Rameh Section
Abdolabad-Rameh region is 214,000 square acre and located in northeastern Garmsar (Semnan province), 60 kilometers of Abdolabad village. It is formed by geographical coordinates of 52˚42'20"E and 35˚21'12"W. It is surrounded by Kabir River in the north, Kavir-e-Bozorg in the south, Sorkhe in the east and Ivanaki in the west (Figure 1 geographical setting, satellite image and access ways to Rameh section).
To identify the facies characteristics and the conditions dominating the depositional environment of the formation, the geological maps of Semnan province (at 1:250,000 scale)  were employed and then the related literature including textbooks, published articles and reports were closely examined. 113 samples of microscopic thin sections were collected and examined under polarizing microscope. These carbonate microscopic thin sections were used to reveal the characteristics of microscopic facies (namely, grain size and other textual features), diagenetic characteristics and microfossils. Dunham (1962)  classification system, refined by Embry & Klovan in 1971  is employed to describe the composition of the carbonate microfacies and name them. Additionally, an analysis of depositional environment and facies was undertaken based on vertical and lateral changes of microfacies, Walther Law  and a comparison between those microfacies and early and recent environments   . Also, identification and classification of the facies as well as recommendation of depositional models were done using information gathered from Carozzi  , and Carozzi and Lasemi  . Having collected data from thin sections, the depositional environment of this formation was interpreted and explained.
3. Stratigraphy of the Qom Formation
The Qom Formation has 420 m thickness and is located in Rameh section. It is consists mainly of Limestone, sandy Limestone, pebble-rich Limestone and Marl. This formation is unconformably overlaid and underlaid with Upper Red Formation and Lower Red formation. It hosts four Facies association is, namely A, B, C and D, as follow:
3.1. Facies Association A (Open Marine Facies)
This facies association contains four facies as follow:
Facies A1: Bioclast lime mudstone: This facies contains less than 10% fossils scattered as echinoid, benthic foraminifera, bivalves and red algae. Dolomitization process is also noted here. This facies is reported to have weak-sorted allochems (Figure 2(a)).
Facies A2: Sandy bioclast wackestone: This facies includes medium-to-thick bedded
Figure 1. Geographical setting, satellite image and access ways to Rameh section in the Qom formation.
Figure 2. Microscopic images of Facies association A. (a) Bioclast lime mudstone; (b) Sandy bioclast wackestone; (c) Sandy bioclast packstone; (d) Bioclast roadstone.
limestones, containing 10% red algae, 10% bivalves and 20% bryozoan, brachiopod, and a small portion of quartz, which are scattered within a lime matrix (Figure 2(b)).
Facies A3: Sandy bioclast packstone: This facies consists of skeletal fragments like 20% red algae, 10% brachiopod, 10% bryozoan, 10% tubularia and rotalia as well as 10% quartz (Figure 2(c)).
Facies A4: Bioclast roadstone: This facies is mostly covered by 30% to 60% bioclast fragments. These fragments are weak-sorted and varied in types, including 40% red algae, 10% echinoid and a small amount of benthic, miliolid, strachod and gastropod foraminifera (Figure 2(d)).
Abundant number of allochems like brachiopod, echinoderm, bivalves, sponge, clacisphere and bryozoan can be found in this open marine Facies association. These organisms are sensitive to salinity, and can only survive in an open marine environment   -  . Echinoderms are limited to marine environment although certain moving species may enter lagoon area and estuary in brackish water. They are found in habitats ranging from shallow intertidal areas to abyssal depths, sometimes about 1130 m  . Most facies which contain echinoderms are associated with open marine environment   . Photic zone is characterized by middle-ramp deposits, which typically host crinoids, bryozoan and bivalves  . Brachiopods are marine organisms, found in both soft and tough basins   . Non-skeletal allochems of this Facies association include ooid and intraclast, which were most likely transported by tidal currents to this facies. Horizontal lamination is one of the depositional structures in this facies, suggesting deposition in a relatively shallow water. The presence of limemud together with stenohalines indicates that this Facies association was formed in a normal-saline and low-energy environment like open marine  .
3.2. Facies Association B (Barrier Facies)
This facies association consists of four facies as follow:
Facies B1: Ooid grainstone: This facies is composed of 60% ooids. The skeletal allochems like miliolid and bivalves can be found here as well. Compound ooids also exist there, in which two or three ooids form the core of the larger ooids. The core of most ooids is composed of the bioclasts like brachiopod and echinoid. The pore space between allochems is filled by cement (Figure 3(a)).
Facies B2: Bioclast grainstone: The proportion of bioclasts is more than 50% Skeletal allochems are composed of 50% bivalves, 20% milioids, 10% red algae, 7% gastropod, and less than 5% strachod, echinoid, peloid and green algae. The pore space between allochems is filled by cement (Figure 3(b)).
Facies B3: Coral boundstone: This facies consists mostly of coral, which is located between dark, thin-to-medium bedded limestones. Other allochems include 30% crinoid, 25% brachiopod and small amount of bivalves, bryozoan and benthic foraminifera. Some of these allochems are greater than 2.5 cm in size. The matrix of this facies is covered by limemud, small amount of calcite cement and considerable amount of organic matter, making it dark (Figure 3(c) & Figure 3(d)).
Lack of a lime-mud matrix is the most salient characteristics of this facies. The presence of grainstones suggests a high-energy environment like shoal. The abundance of ooids and paucity of oncoids indicate deposition in shallow and high-energy environment like shoal  . The presence of the fossils associated with the upper parts of penetration depth together with the facies setting point to the deposition of the facies association in the seaward shoal environment  . The presence of ooids, skeletal fragments such as bryozoan, echinoderm and brachiopod along with cross-bedding and cross-lamination, and lack of limemud all indicate that this facies association is formed
Figure 3. Microscopic images of facies association B. (a) Ooid grainstone; (b) Bioclast grainstone; (c) & (d) Coral boundstone.
in shoal environment and above the wave effect line   . This Facies association is also characterized by the occurrence of cross-bedding in ooid-rich facies. Ooid-rich facies with cross-bedding are formed within warm, shallow and turbulent water, saturated or supersaturated in calcium carbonate. Ooids and other bioclasts associated with depositional structures like cross-bedding are formed in high-energy shoal environments  . The presence of stenohalines like echinoderm, bryozoan, brachiopod and bivalves in seaward shoal environment is indicative of their formation in high-energy environments, transported by tidal currents  . Ooid-dominated facies are associated with high-energy shoal environment  . According to the comparisons made with similar recent facies, grainstones with bivalve shells, echinoderm and bryozoan are likely to be formed at the depth of 30 to 100 meters   . Generally speaking, shoal facies, which are deposited in platform margin sub-environment, separate open marine from restricted lagoon environment  .
3.3. Facies Association C (Lagoonal Facies)
This facies association is mainly composed of three facies, as follow:
Facies C1: Dolomitized mudstone
This massive-to-thick bedded facies is often made up of bivalve-rich limemud, and its matrix is dolomitized. These dolomitized mudstones contain intraclasts and bioclasts (Figure 4(a)).
Facies C2: Bioclast intraclast wackestone
This facies contains 14% intraclast. Other allochems included in this facies are 10% bivalves together with scant amount of miliolid, benthic foraminifera and gastropod. The skeletal fragments do not have high density and floats in a matrix of limemud (Figure 4(b)).
Facies C3: Bioclast packstone
This facies contains 20% miliolid, 10% red algae, 10% green algae, 10% peloid, 10%
Figure 4. Microscopic images of facies association C. (a) Dolomitized mudstone; (b) Intraclast bioclast wackestone; (c) Bioclast packstone; (d) Miliolid bioclast packstone.
gastropod, 10% bivalves and a small portion of intraclast in limemud (micritic) matrix (Figure 4(c)).
Microfacies C3a: Bioclast packstone miliolid: This facies hosts 25% miliolid, 25% benthic foraminifera, 15% red algae, 10% bivalves and strochod (Figure 4(d)).
The limemud matrix of the facies association C, which is neomorphized in some facies, is the sign of a low-energy environment. Distribution of benthic foraminifera in recent environments is governed by various factors like temperature, salinity, water turbulence, light penetration, sedimentation rate and water depth   . Benthic foraminifera are relatively abundant in this facies association is. They are usually the habitats of shallow and low-energy environments like lagoon and reef shoal as well as shallow coastal environments, which are below the wave effect line   . The presence of foraminifera like miliolids, which are typical of shallow, restricted, low-energy marine environment, suggests that this facies association is formed in a shallow and low-energy environment like lagoon    . Gastropods also point to the limited seawater circulation in this Facies association is. Besides skeletal fragments, peloid, as a well-sorted allochems, can be found in this facies. Peloids are indicative of limited-cir- culation, low-energy and warm water, which is supersaturated in calcium carbonate     . The abundance of peloids and the presence of tidal facies suggest a low-energy, shallow and restricted lagoon environment. The presence of enchoids, as the indicator of calm, shallow and semi-saline water, together with algal activities suggests a carbonate lagoon environment of inner ramp  . Lagoon environments are characterized by fine-grain sediments which contain the typical organisms of restricted environments. The variety of the organisms and sediments in these environments depends on the amount of energy and water circulation in lagoon environment, which is controlled by the abundance of tidal channels and climatic conditions. The low diversity of animals in lagoon environments reflects deposition under hypersaline conditions, low water circulation and limited relationship to marine environments  . In addition to the organisms living in environments with limited water circulation, 10% to 15% open-marine fossils like bryozoan, brachiopod and echinoderm are observed there. Generally speaking, lagoon environment is characterized by low diversity of stenohalines  , and these fossils are transported there by potential thunderstorm activities  . Marine bioclasts and lagoonal fossils accompanied by intraclasts suggest deposition in lagoon environments and at the end of the platform margin.
3.4. Facies Association D (Tidal Flat Environment)
This facies association is composed of two facies as follow:
Facies D1: Sandy dolomudstone: This facies contains microcrystalline, equant and euhedral dolomites, with 20 to 50 micron in size. In some facies, dolomites have transparent margins while their central part is dim. Also, this facies contains 15% fine-to- medium-grained, poorly sorted angular quartz. It also has parallel lamination (Figure 5(a)).
Figure 5. Microscopic images of facies association D. (a) Sandy dolomudstone; (b) Stromatolite boundstone.
Facies D2: Stromatolite boundstone: They are the simplest forms of stromatolites, usually found in restricted tidal flat environments. They show desiccation polygons and have laminoid fenestrate or elongated cavities (Figure 5(b)).
This facies association hosts abundant limemud and sand-sized clastic particles and silt. The deposition of micritic facies together with fine-grained skeletal and non-skeletal fragments has occurred in this facies association is, suggesting low-energy environments and calm waters. The presence of dolomicritic facies in this facies association points to its formation in a shallow environment dominated by evaporation conditions. The paucity of fossils in this facies association indicates lack of suitable environmental conditions and limited water circulation   . The limemud mixed with abundant clastic particles of low fauna group and close association with clastic interlayers near the sea point out that limemud is formed in a low-energy, marine shelf (upper tidal) setting due to the sea level fall  . The morphology of the stromatolites points to their formation in low-energy to high-energy parts of tidal flat environments   . According to the studies conducted on stromatolites  -  , the height and width of stromatolies increases with increasing depth of the basin. Flat stramatolies are mostly observed in low-energy environments like supratidal and upper intertidal, whereas fan-like, dome-shaped and columnar stramatolites and thrombolites were formed in deeper environments such as lower intertidal and shallow subtidal. Shallow subtidal was developed within ooid-enchoid shoals. Various evidence like expansion of microcrystalline dolomites, lack of Bird’s eye fabric and dominance of limemud in this facies association suggest that tidal flat facies association has moved out of water, and intertidal to supratidal environmental conditions were dominant.
4. The Depositional Environment of the Qom Formation in Rameh Section
The analysis of depositional environment, as the best way in determining the conditions and quality of deposition during their formation, is used to examine factors affecting depositional environment. The results of petrographic and geochemical studies were employed to present the facies or depositional models to understand depositional environments easily  . Regarding the comparison made between old  and recent environments and using law of correlation of facies  as well as various depositional models introduced by different scholars like Ervin  , Wilson  , Flugel  , Read  , Carozzi  , Tucker and Write  and Incell   , the sediments of the studied area were analyzed and interpreted and finally the depositional model was proposed. According to the type and abundance of allochems and other textural and structural properties obtained from field and laboratory observations, and considering the comparison between lateral and vertical changes of various facies in the studied sequence, the depositional model of the studied area was presented. Lack of the gradual transformation of facies into each other and the presence of intraclasts in carbonate shelf or carbonate ramps as well as the high variety of reef facies  has caused the geologists to recognize the Qom Formation as a rimmed shelf carbonate platform (Figure 6). The stratigraphy column and depositional environment of the Qom Formation in Rameh section in Figure 7.
5. Diagenetic Processes
Carbonate rocks of the Qom Formation in Rameh section have undergone various
Figure 6. The depositional model of the carbonate facies of the Qom Formation, displaying the formation of the above facies in open marine, shoal, lagoon and tidal flat in carbonate shelf platform.
Figure 7. Stratigraphy column and depositional environment of the Qom Formation in Rameh section.
diagenetic processes including dissolution, porosity, cementation, compaction, micriti- zation and dolomitization (Table 1 diagenetic sequence of the Qom Formation in Rameh section).
5.1. Dissolution and Porosity
Various types of cements are contained in the Qom Formation, as follow:
- Blocky cement: This cement has coarse grains and fills the intergranular pores and the cavities of the Qom Formation (Figure 8(a)).
- Clear syntaxial cement: Clear syntaxial overgrowth cement commonly grows in optical continuity with crinoid grains (Figure 8(d)).
- Drusy cement: This cement mostly fills the cavities resulting from dissolution of skeletal fragments (Figure 8(e)).
In the Qom Formation, the micritization often influences bivalve shells and brachiopods (Figure 8(c)).
Physical compaction is characterized by the fracture of bivalve shell and the subsequent interparticle porosity (Figure 9(a)).
Dolomitic cement is one of the most important kind of cement in the Qom Formation.
Table 1. Diagenetic sequence of the Qom formation in Rameh section.
Figure 8. (a) Intergranular porosity occurring in the pore space between non-skeletal fragments is filled by Blocky cement; (b) Bird’s eye porosity that is developed in the algal coatings of upper tidal flat areas; (c) The micritization process on a bivalve fossil. This process has destroyed the internal structure of this fossil; (d) Syntaxial cement that is formed in the vicinity of and in optical continuity with a crinoid grain, usually observed extinct; (e) Drusy cement, crystals size increases toward the center of the cavity; (f) Mechanical compaction in the Qom Formation is displayed, where an increase in the compaction accounts for the longitudinal contact between grains.
Figure 9. (a) Increasing compaction results in the fracture of bivalve shell and the subsequent interparticle porosity; (b) Very fine-grained dolomites (dolomicrites) of approximately the same size as subhedral boundaries (Planar-s). In this type of dolomites, the early sedimentary textures are found in terms of algal coating and Bird’s eye porosity; (c) Coarse-grained and pore-filling dolomites along with clear and euhedral crystals of dolomite have filled the pore space within a dolomicrosparite matrix; (d) Boring process affecting brachiopod shell.
Dolomite crystals are dark, anhedral and compact. They lack fossils, yet have Bird’s eye porosity. In this type of dolomites, the early sedimentary textures are found in terms of green or blue algal coatings, which may result in Bird’s eye porosity (Figure 9(b) & Figure 9(c)).
6. Biological Process in the Qom Formation
Boring: Boring process usually affects bivalve shells in the Qom Formation (Figure 9(d)).
The Qom Formation in Rameh section hosts 420 m thin-to-massive limestone, pebble-rich to sandy limestone and marl limestone. This formation is unconformably overlaid and underlaid with siliciclastic deposits of Upper Red Formation and Lower Red Formation. According to field observations and laboratory investigations, the Qom Formation in Rameh section contains 12 facies, occurring in tidal flat, lagoon, shoal and open marine environments in a rimmed shelf carbonate ramp. This formation undergoes various diagenetic processes including dissolution, porosity, cementation, micritization, compaction and dolomitization.