فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و چهارم شماره 4 (پیاپی 73، زمستان 1397)

A completely lithologic sequence of Permian strata including Doroud (mainly of sandstone and shale), Ruteh (mainly of limestone with interbed marl) and Nesen (mainly of limestone and shale) formations is exist in Central Alborz zone from Siyah-Bishe and Harijan in Chalous Road to Yoush, Baladeh, Zarin-Kamar and Varazan in south of Amol in Mazandaran Provence. Based on the field geology and petrography, these igneous rocks with basic alkaline nature are found as dike, sill and small diabasic, micro-gabbroic and lamprophyric intrusions in the Doroud Formation and as basaltic lava flows and related pyroclastics in the upper part of Ruteh Formation. In fact, low-depth microgabbroic and lamprophyric intrusions in the Doroud Formation were residual and solidified melt in feeding vents of the basaltic volcanism in the upper part of Ruteh Formation. Doroud Formation was deposited in a Fluvial- Deltaic environment in Early Permian, Ruteh and Nesen formations were deposited in carbonate platform (lagoon, barrier and open marine) in Middle–Late Permian respectively. Lack of continues reefal facies and turbiditic sediments indicate that the Routeh and Nesen carbonate formations deposited in a carbonate ramp platform. At this time, the central Alborz was as a passive margin in the south side of the Paleo-Tethys and its alkaline basic magmatic activity is interpretable with the Late Paleozoic extensional tectonics in the north side of the Gondwana and simultaneously with initial stages of Neo-Ttethys development in Zagros. According to Berberian and King (1981), Paleozoic magmatic activity has not been extended in Iran, but the study of these rocks is a very important key to better understanding the geological events of this period in Iran and adjacent countries. For example, the opening time of the Paleo-tethys and Neo-tethys oceans need to understand their geologic events (such as magmatism and metamorphism) in the Paleozoic era. One of the most widespread events is the magmatic activity of the Ordovician–Silurian in the Eastern Alborz (Sultan Meydan Complex), which can be certified on the occurrence of extensional tectonics in the early stages of opening of the Paleo-Tethys ocean (Ghasemi and Khanalizadeh 2012; Ghasemi and Kazemi 2013; Derakhshi and Ghasemi 2013, 2014; Derakhshi et al. 2014, 2015; 2017). Then, other magmatic phases have been occurred in the Devonian (Ghasemi and Dayhimi 2015; Dayhimi 2012; Derakhshi and Ghasemi 2013, 2014), Carboniferous (Naderi et al. 2018a,b) and especially in Permian (Berberian and King 1981; Vahdati Daneshmand 1991,1999; Saidi and Ghasemi 1991; Gaetani et al. 2009; Delavari et al. 2017; Rostami et al. 2018). The discussions and views on the place of Iran during Permian and the closure time of the Paleo-Tethys ocean and opening of the Neo-Tethys ocean reveal the importance of the Permian magmatism. After Permian, with developing of the Neo-Tethys ocean, some parts of Iran (such as Central Iran and Alborz), as the Cimerian territories, were separated from the north of Gondwana and moved to the northern Eurasia super-continent (Gaetani et al. 2009). With the closure of the Paleo-Tethys ocean at the Early Jurassic (Boulin 1988) or the Late Triassic (Stöcklin 1974; Alavi et al. 1997; Stampfli and Borel 2002; Horton et al. 2008; Wilmsen et al. 2009; Zanchi et al. 2009), which is characterized by the Eocimerian discontinuity in Alborz, the connection of the cimerian microcontinent to southern Eurasia have been occurred. Thus, over the Middle to the Late Permian, the cimerian territories embedded between the two oceanic zones, the newly emerging Neo-Tethys in the south (Zagros) and the destroying Paleo-Tethys in the north becoming closer to Eurasia (Stampfli et al. 2002; Nikishin et al. 2002; Gaetani et al. 2009; Berra and Angiolini. 2014; Domeier and Torsvik 2014). The Permian–Triassic extentional phase is one of the most important rifting phases in Iran, indicating the opening of the Neo-Ttethys oceanic basin whose signs are found as basaltic magmatism in Alborz and other parts of Iran (such as Central Iran and Sanandaj–Sirjan) (Berberian and King 1981; Ghaesmi and Jamshidi 2012; 2013; Ghasemi et al. 2018). The studied area is located in northeastern Baladeh (Central Alborz) from Siyah-Bishe and Harijan on Chalous road to Yoush, Baldeh, Zarrin Kamar and Varazan in south of Amol in Mazandaran province. It contains sedimentary units of Permian (Doroud, Ruteh and Nessen formations), along with igneous basic alkaline rocks (volcanic and intrusive). Main spreading of volcanic rocks in the Baladeh area has been reported between the Ruteh and Nessen formations (Vahdati Daneshmand 1991,1999; Saidi and Ghasemi 1991; Delavari et al. 2017; Rostami et al. 2018), but based on the findings of this research and in according to Gaetani et al. (2009), these magmatic rocks are located not between the Ruteh and Nesen Formations, but in the form of intrusive bodies in the Doroud and the lower part of the Ruteh formations, and in the form of extrusive and pyroclastic rocks in the upper part of the Ruteh Formation. The intrusive bodies that were originally solidified magma in the feeding vents of volcanic rocks widely cropped out in the form of dikes, sills and small diabasic, microgabbroic and lamprophyric intrusions in the Doroud Formation and in the lower part of the Ruteh Formation in Siyah-Bishe and Harijan on the Chalous road and are not mentioned in any previous studies. In addition, lamprophyric dikes are also reported in the region for the first time in this study.  The systematic study of the Paleozoic igneous events of the Alborz region (Ghasemi and Khanalizadeh 2012; Ghasemi and Kazemi 2013; Derakhshi and Ghasemi 2013; 2014; Derakhshi et al. 2014, 2015; 2017; Dayhimi 2012; Naderi et al. 2018a; b) and northeastern Iran around Mashhad (Li et al. 2018; Mobasheri et al. in press) and their use to study the evolution of the Paleo-Tethys ocean needs to studying and precision field surveying, petrography, mineral chemistry, as well as precise chemical analysis of major, minor, trace and rare earth elements of whole rocks, and even isotopic analysis and age dating from the Permian igneous specimens of Central Alborz. But, due to the variety of topics, the large amount of the data and processing and interpretation, this paper only refers to the precise stratigraphic position, diversity occurrences of igneous rocks, geological relations and their geomodynamic significance in the analysis of the evolutionary trend of the Alborz Basin during the Permian period. Studies and field sampling were carried out in all Permian outcrops of the region, using previous research data and tracking these rocks in high-resolution satellite images of the area, as well as performing via precise cross-sectional surveys in the available outcrops in a systematic and selective manner. Preparation of thin sections and their petrographic studies have been carried out in the laboratories of the Faculty of Earth Sciences, Shahrood University of Technology.  A completely lithologic sequence of Permian strata including Doroud (mainly of sandstone and shale), Ruteh (mainly of limestone with interbed marl) and Nesen (mainly of limestone and shale) formations is exist in Central Alborz zone from Siyah-Bishe and Harijan in Chalous Road to Yoush, Baladeh, Zarin-Kamar and Varazan in south of Amol in Mazandaran Provence. Based on the field geology and petrography, these igneous rocks with basic alkaline nature are found as dike, sill and small diabasic, micro-gabbroic and lamprophyric intrusions in the Doroud Formation and as basaltic lava flows and related pyroclastics in the upper part of Ruteh Formation. The intrusive rocks show ophitic, subophitic, poikilitic, poikilophitic, microlitic porphyry, intergranular and microgranular textures composed of olivine, clinopyroxene, plagioclase and amphiboles as the main minerals. The extrusive rocks show amygdaloidal, hyalomicrolitic porphyry, microlitic porphyry, hyaloporphyry, intersertal and trachytic textures composed of olivine, clinopyroxene and plagioclase as the main minerals. The amigdals filled by secondary minerals such as calcite, chlorite and quartz. Compositional zoning and sieve texture in clinopyroxene and plagioclase and skeletal texture and corrosion of crystal margins in olivine and clinopyroxene are widely seen. In fact, low-depth microgabbroic and lamprophyric intrusions in the Doroud and the lower part of the Ruteh formations were residual and solidified melt in feeding vents of the basaltic volcanism in the upper part of Ruteh Formation. According to Delavari et al. (2017) and Rostami et al. (2018) Permian magmatic rocks of the Baladeh area have sodic alkaline nature and are derived from crystallization of a basaltic melt originated from partial melting of an Oceanic Island Basalt (OIB) source in a within plate tectonic setting (a deep garnet bearing mantle source of HIMU type). This tectonic setting can be the result of the extensional tectonics prevailing of the Middle–Late Permian, which is accompanied by pressure reduction on the mantle, rising the deep mantle plume, melting it at high depths, and formation of the basaltic magma. This extensional regime coincided with the early stages of crust uplifting and formation of the rift basins as a prelude to the formation and evolution of the Neo-Tethys Ocean in the southern part of the Paleo-Tethys. The geodynamic rearrangement of the Tethys realm during the Late Paleozoic–Early Mesozoic, has been accompanied by magmatic activity along the northern margin of Gondwana from the east of the Himalaya to Tibet, Oman, Iran and Turkey (Zhu et al. 2010). From the petrographic data and the chemical analysis of igneous rocks, along with the evidence of stratigraphy, facies and sedimentary environments can be used to determine the tectonomagmatic setting and the paleogeography of the Alborz land during Permian. Accordingly, Doroud Formation was deposited in a Fluvial-Deltaic environment in Early Permian, Ruteh and Nesen formations were deposited in a carbonate platform (lagoon, barrier and open marine) in Middle–Late Permian respectively. Lack of continues reefal facies and turbiditic sediments indicate that the Routeh and Nesen carbonate formations deposited in a carbonate ramp platform. At this time, the central Alborz was as a passive margin in the south side of the Paleo-Tethys and its alkaline basic magmatic activity is interpretable with the Late Paleozoic extensional tectonics in the north side of the Gondwana and simultaneously with initial stages of Neo-Tethys development in Zagros.

The Dalichai Formation in Darjazin section in NE Semnan with a thickness of 631 meters consisting of an alternation of bulish-gray was studied.This formation overlies the Shemshak formation disconformity with conglomerate bed and it has gradually been covered by the thick-beded limestones of the Lar formation. Based on the presence and stratigraphic distribution of the most dominant miospore, Klukisporites variegatus and Callialasporites dampieri of pollen areidentified in the Dalichai (late Bajocian - Callovian) and Presence of some index dinoflagellate species led to identification of three biozones in the Dalichai Formation. These include in ascending order, Cribroperidinium crispum Total Range Biozone (late Bajocian), Dichadogonyaulax sellwoodii Interval Biozone (Bathonian - early Callovian), Ctenididinium continuum Interval Biozone (early to middle Callovian). This biozonation corresponds largely to those established in Northwest Europe and Northwestern Tethys and reveals the marine connection between North of Iran with Northwest Europe and the Northwestern Tethys during the Middle Jurassic.

The Mozduran gas reservoir in the Khangiran gas-field is formed in the Kopet-Dagh sedimentary basin in the form of sequential sequences of clastic and carbonate sediments of the old sea water. The reservoir is located in the formations of Shurijeh and Mozduran, consisting of limestone, dolomite, sandstone and shale. The Mozduran Formation was deposited in a carbonate platform and Shurijeh Formation in the river systems to coastal deltas during the Jurassic. The origin of brine formation waters was the old evaporated seawater. The old evaporated seawater has been in contact with various formations since the burial time, and their initial composition has altered. Ion concentration of major, minor and trace elements in the produced waters and two deep brine samples below the Shurijeh reservoir were measured to investigate the geochemical evolution of the brine formation waters. The concentration of all ions has increased to saturation in brine due to the evaporation of sea water. However, over the time, the concentrations of Ca, Li, Sr, B and I ions have been increased compared to the original source and the concentration of Na, Mg and SO4 ions have been decreased. The geochemical evolution of this reservoir has been affected by evaporation, water-gas and water-rock reactions such as dolomitization, albitization of plagioclase, ilitization of Smectite, sedimentation or dissolution of sulfate minerals, magnesium carbonate precipitation. Concentration of potassium and chloride ions was mainly influenced by the process of evaporating the old sea water. The results of this research are used to identify the history of sedimentation, secondary geochemical processes in the reservoir, determination the origin and the salinization mechanism of produced water from the gas reservoirs area to achieve sustainable management of the reservoir. The concentrations of brine formation waters reflect both the chemical characteristics of the sources from which they originated and the diagenetic reactions that had taken place. The modified original seawater, locked in during the time of sedimentation, is proposed as an origin of most deep saline formation waters. The evaporated seawater has been in contact with various formations in oil/gas fields since the time of burial. The chemistry of produced water in oil and gas reservoirs can be changed over the time by various processes and secondary reactions such as water-rock and/or water-gas interactions. The study area, Khangiran gas field, is located northwest of Iran. This gas field composed from three reservoirs of Mozduran, Shurijeh B and Shurijeh D. Mozduran reservoir consists of thick layer of carbonate rocks and interlayers of marl and shale. According to geological, hydrochemical and isotopic studies, the old evaporated sea water is proposed as main origin of the produced water and salinity. Over the time, the effect of various secondary processes inside the reservoir has caused a change in the initial concentration of the main ions of the brine relative to its original source (evaporated sea water). The main agent of this research is to investigate the geochemical evolution of the brine below the Mozduran gas reservoir and also to identify the secondary processes that affect its chemical composition during the geological times. Two brine water samples were taken from observation well no. 13 and 17 which drilled to Shurijeh B and D reservoir. The water content in the wellbore was drained by gas flow and after that, the brine sample was brought to the surface from a depth of 3000 m below sea level using a bottom-hole sampling device. In this study, the 6 samples are also taken from saline and fresh produced waters of the Mozduran gas wells to measure the concentration of major ions and trace elements. The trace elements, and Br and I anion concentrations were analyzed by ICP-MS, ICPOES and IC techniques. Various methods have been used to investigate the geochemistry evolution of the Mozduran reservoir. The water chemistry of brine formation waters can be altered by evaporation or water–rock interactions during burial. Br is conservative ions and cannot be easily removed from solution and used to study for geochemical evolution; therefore, Br ion concentration of Mozduran reservoir waters is most probably equal to the original Br concentration of its source, evaporated seawater. The brine and produced waters in the study area are enriched in Ca, Sr, Li, I and Rb, and depleted in SO4, Na and Mg ions, with respect to the initial evaporated seawater. Therefore, the seawater evaporation is not the only process controlling the geochemical composition of the brines in the area. The ion concentration variations are due to processes such as sulfate reduction, anhydrite precipitation, organic material-water reactions, limestone dolomitization, and albitization of plagioclase feldspar.  Based on the chemical evolution after seawater evaporation, the KA waters are classified into four groups: (1) no evolution (Cl, K ions), (2) water-rock interaction (Na, Ca, Mg, Li and Sr ions), (3) water-gas interaction (SO4 and I ions) and (4) both water-rock and water-gas interactions (Mn and B ions). The chemical evolution processes of the KA waters include dolomitization, precipitation, ion exchange and recrystallization in water-rock interaction. Bacterial reduction and diagenesis of organic material in water-gas interaction also occur. Therefore, the concentration of all ions increases due to the evaporation of seawater up to saturation, but some of the ions’ concentrations have changed under the influence of rock/gas-water interactions. The chemical evolution of the brine waters is classified into four groups: (1) only evaporation, (2) water-rock interaction, (3) water-gas interaction and (4) the interaction of both rock and gas with water.

The Pabdeh Formation (Paleocene–Oligocene) is located in the Zagros foreland basin. This formation was well appreciated as a source rock and somehow a potential reservoir particularly in the Dezful Embayment. Although it has been less appreciated due to overwhelming fine grained lithology in the majority of the Zagros basin. Microfacies analysis and depositional environment of the Pabdeh Formation was studied by examining 500 samples collected from three surface sections namely Malekshahi, Gandab and Pirmohammad, which are located around the Malekshahi city. Based on petrography, proportional abundance of foraminifera and other constituents, texture and microfacies characterizes, eight facies associations were recognized which were deposited in three facies belts on a ramp. As a whole, the Pabdeh Formation comprises marl and limestone beds. Based on geochemical analysis (calcimetry and XRF analysis), the marl lithofacies was established, although it was literally described as shale. Wet chemical oxidation method was employed to determine the organic matter content of some selected samples, which revealed relatively higher organic matter content in marlstones. A mixed carbonate-clastic system may develop, once clastic input exerted into a ramp environment. Such intervals are defined with shale, sandstone and carbonate beds (Tucker 2003). Marl could deposited instead of shale, if the clastic input are limited (Holland 1993). Periods of clastic input in such environments are probably linked to sea-level changes. In present research, the Pabdeh Formation is proposed as a typical carbonate-clastic sedimentation, which was deposited during Paleocene–Oligocene in the Zagros foreland basin. The Zagros basin comprises a thick sedimentary succession over the Precambrian basement (Alavi 2004). The basin was a part of Gondwana supercontinent during Paleozoic which was bordered by Paleotethys in the north, a passive margin during Mesozoic and a convergent orogenic belt in Cenozoic (Bahroudi and Koyi 2004; Sepehr and Cosgrove 2004). The Neotethys was initiated since Late Triassic between Iran and the Arabian plate (Berberian and King 1981; Sepehr and Cosgrove 2004), and ceased during late Cretaceous due converging movement of subducting Arabian Plate beneath the Iran sub-plate (Berberian and King 1981; Stoneley 1981; Berberian 1995). Continental collision in Cenozoic led to the formation a Zagros fold-thrust belt and foreland basin where since Late Cretaceous onward, sedimentation continued without any major break. The succession includes the Gurpi (shale and marl), Amiran (siltstone, sandstone, limestone and conglomerate), Pabdeh (argillaceous limestone, shale and marl), Taleh Zang (limestone), Kashkan (siltstone, sandstone and conglomerate), Shahbazan (dolomitic limestone and dolostone), Asmari (limestone), Gachsaran (anhydrite, marl and limestone) and Aghajari formations (calcareous sandstone, siltstone and sandstone) (James and Wynd 1965). During Paleocene–Eocene the argillaceous limestone and pelagic marls of the Pabdeh Formation were deposited in the middle part of the Zagros basin in a ramp setting (intrashelf basin) (Motiei 1993; Mohseni et al. 2011). The Pabdeh Formation was subject of numerous researches since it is considered as a potential source interval. For example Mohseni et al. (2011) untilized trace fossils for interpreting the depositional environment and recognizing turbidity currents (Mohseni and Al-Asam 2004; Khodabakhsh et al. 2009; Mohseni et al. 2011). Khavari et al. (2014) and Senemari (2018) examined the nanostratigraphy and paleoechology of the Pabdeh Formation. This formation is well defined by the purple shale member at the base with Gurpi Formation and conformably underlies by the Asmari Formation (Motiei 1993) and laterally passes into the Kashkan and Shahbazan formations (Alavi 2004). Although James and Wynd (1965) described the Pabdeh Formation as shale interval with subordinate limestone beds, recent studies revealed a major part of this formation includes limestones which were deposited in ramp setting (Mohseni 2003; Behbahani 2006). Since a major part of the so called shales beds are recognized here as marls, the main goal of this research is examining microfacies, depositional environment and petrographic reappraisal of the formation in three selected surface sections around the Malekshahi County using geochemical approaches.    Microfacies analysis and depositional environment of the Pabdeh Formation was studied by examining 500 samples collected from three surface sections namely Malekshahi, Gandab and Pirmohammad, which are located around the Malekshahi city (southeast Ilam Province, the Zagros basin; Iran). Petrographic investigations of the rhythmites was performed by using polarizing microscope (Zeiss, Axioscope, 40) on three intervals comprising pronounced rhythms. The nomenclatures of lithofacies are based on Dunham (1962) and Pettijohn (1987). Element composition was measured by S4 Explorer/ x-ray Spectrometry-Bruker (WD XRF) in XRF analysis laboratory at Yazd University. The carbonate content (= CaCO3%) were determined by titration method (Carver, 1971). The organic matter content was determined via wet chemical oxidation (by hydrogen peroxide) (Lewis and McConchie 1994).    As a whole, the Pabdeh Formation comprises marl and limestone beds. Distinction between marl/limy marl and limestone beds was done based on geochemical analysis (calcimetry and XRF analysis). Although marl lithofacies was literally described as shale. Wet chemical oxidation method was employed to determine the organic matter content of some selected samples, which revealed relatively higher organic matter content in marlstones. Based on petrography, proportional abundance of foraminifera and other constituents, texture and microfacies characterizes, three micro/lithofacies associations were recognized within the selected rhythms which were deposited in three facies belts on a carbonate ramp environment: A)            Inner Ramp, include the following microfacies (MF): MF A1- Bioclastic mudstone with extraclast and quartz MF A2- Oncoid bioclastic wackestone-packstone MF A3- Ooid packstone B)            Middle Ramp, include the following microfacies and lithofacies (LF): MF B1- Algal bioclastic packstone MF B2- Bioclastic mudstone-packstone with benthic foraminifera and echinoderm LF B3- Marl/calcareous marl lithofacies C)            Outer Ramp, include the following microfacies and lithofacies (LF): MF C1- Bioclastic mudstone-packstone with planktonic foraminifera and glauconite LF C2- Marl/calcareous marl lithofacies Two lithofacies (LF C2 and B3) alternate with the recognized limestone microfacies (as described above) resulting two cycle types: 1-             Alternation of LFC2 with microfacies type C (LF C-MF C cycle) 2-             Alternation of LFB3 with microfacies type B (LF B-MF B cycle)  Sedimentation in the ramp environment may be affected by clastic input (mainly as event deposits). In some cases, more than 90% of basins was filled by these evenest (i.e. tempestite/turbidites; Colombie et al., 2012). The cyclic (=rhythmite) deposition of micro/lithofacies in the study area indicate two depositional mechanisms: 1) background in-situ sedimentation (carbonate facies), 2) fine clastics transported by traction currents from shallow-marine that dumped siliciclastics into the deep-marine. The latter mechanism show characters of event deposits as it was reported by Khodabakhsh et al. (2009) and Mohseni et al. (2011). Limestone-marl alternations in the Pabdeh Formation was possibly controlled by periodic fluctuations in siliciclastic input by shallow-water derived currents (probably turbidity currents). The trace fossils of Zoophycos group are normally associated with turbidites and debris flows in ramp environments. Frequent occurrences of such trace fossils in the study area could be assigned to event deposits. This study revealed the Pabdeh Formation comprise 8 microfacies deposited in a carbonate ramp. The Zoophycos trace fossil displaying an evolutionary change in acceding order which suggest shift from more calm water setting toward relatively agitated condition upward throughout the formation. Marlstone lithofacies is consistently distributed throughout the whole section, which was described literally as shale beds. Relatively more Al2O3 content of marlstones in compare to limestone beds could imply more clastic input during deposition of these beds.

This study focuses on fining-upward sequences of carbonate-evaporite series of the Kangan Formation in the central Persian Gulf area. The studied sequences shows a typical succession of tempestite deposits. Comparison of these units with sedimentary deposits of other graded bedding forming mechanisms, showed that these strata have been formed during the occurrence of storms at the time of Kangan deposition in this region. Formation of these units in shallow depths of gently sloped ramp setting, well sorting especially at the middle of the unit, absence of slumping related structures in these sediments, centimeter to decimeter scale, lack of terrigenous sediments and their considerable repetition in middle to upper parts of the Kangan Formation confirms the role of storms in their formation during lower to middle Triassic in the Persian Gulf Basin. Study of these strata could be a great help for understanding the climate change at the Triassic time in Persian Gulf Basin.    Event stratigraphy have been considered in many studies and many models have been proposed for their sediments deposition. The Bouma sequence is the most well-known model for these deposits which is the result of turbidity currents. There are not many studies about such deposits in carbonate formations. A total of four models have been proposed for deposition of fining-upward sequences in carbonates. They include 1) turbidity currents by rivers which could not be used for carbonates as they are not affected by runoffs. 2) Tempestites in carbonate formations. The term introduced by Aigner in 1982 and now has a wide application in carbonate storm deposits. 3) Slumping and their resulted local fining-upward sequences. 4) Internalites which are the result of wave breakdown in mid to outer ramp settings. The former have been introduced recently but their use in interpretation of fining-upward sequences is increasing in recent years. With respect to each model mechanism, the effective factors and so the depositional situations could be reconstructed. Tempestites have been frequently formed in the Triassic due to the warm and arid climate. The Persian Gulf basin was in 10 degree south in that time. So, the Kangan Formation was selected for more considerations to find possible tempestite deposits. This study focuses on the event deposits in this formation using cores and thin sections’ data. With respect to the reservoir quality of this formation in our country, determination of its sedimentary environment and processes are so important.  Macroscopic studies was carried out on 540 m cores of the Kangan Formation in three wells. A total of four plugs prepared from each meter of core. Thin sections were prepared from trims of plugs. One third of each thin section stained with Alizarin Red-S to distinguish calcite from dolomite (Dickson 1965). Facies named according to Dunham classification (Dunham 1962).  Fenestral mudstone, stromatolite boundstone and anhydrite have been deposited in peritidal environment. These deposits have unique structures such as bird’s eye or anhydrite nodules. Massive to layered anhydrites shows deposition in warm and dry climate sabkha environment. Mudstones associated with anhydrites have no allochems. The facies have been dolomitized in most cases. Dolomites are fine and xenotopic. Stromatolite boundstone exhibit fine lamination and fenestral voids. Thrombolite has been formed in the base of the formation after the Permian–Triassic mass extinction. The lagoon environment has low energy and limited water circulation. Low diversity of fauna, lack of exposure related structures, frequency of micrite and peloids denote to lagoon environment. Both small and large allochems are visible. Bioturbation and micritization are obvious. Ooid and peloid grainstones build the main part of the shoal depositional setting. These facies have been deposited in high energy environment which is obvious from sand size ooids, lack of lime mud (micrite) and well sorting. Cross-bedding could be seen on core samples. Intraclast/bioclast grainstone is related to sea-ward shoal sub-environment. Gradual change in facies and lack of a complete build-up reveal that these facies have been deposited in a ramp setting. There are also no slumping related structures. Core and thin sections’ studies show that the studied sequences begin with an erosional surface. Erosional related evidence including rip-up clasts and flame structures are evident. The basal facies show clear fining-upward sequence. Toward the top, cross-laminations are visible which have been developed within the well sorted grainstones. There are no silt or fine sand size particles in this sequences and so the parallel lamination never developed at the topmost intervals. The uppermost facies are consist of massive micrites with anhydrite cement or nodules. Tempestite deposits are developed in shallow to deep carbonate and siliciclastic environments. They are thicker with larger size particles in shallow settings. They are also more mature with basal erosional surface in such environments. The fining-upward sequence and erosional surface show that the environmental energy decreases after a sudden increase. Presence of anhydrite shows that the environment was arid and warm during deposition of these units. Results of this study show that there have been many storms in present Persian Gulf location at Early and Middle Triassic time. The resulted sequence starts with an erosional surface and continues with parallel lamination in high current regime. The uppermost units could not be distinguished and have a massive structure due to high micrite content. The main difference of these sequences with turbidites lies in their mechanism. Their high frequency and shallow environment, lack of slumping related structures and terrigenous sediments as well as low clay content and high frequency of intraclasts and pleoids demonstrate a storm mechanism for their deposition. The result is tempestite deposits in the Kangan Formation.

Middle Albian to Cenomanian strata of the Sarvak Formation indicate pelagic facies in Tang-e-Chenarbashi stratigraphic section. These strata deposited in five sequences of the third order and nine sequences of the fourth order. These sequences were determinated in a succession including six microfacies mudstone, sponge spicule packstone, peloidal packstone, radiolarian packstone, Oligosteginid, planktonic foraminifera wackestone - packstone, benthic foraminifera echinoidal wackestone - packstone. Study of the third order sequences show that in some of them exist paraconformity which related to sequence boundary of the fourth order sequences. Investigating these sequence boundaries and recognition the sequences were done based on planktonic foraminifera biozones. In this study, separating the fourth order sequences is generally based on fossil fauna. In the fourth order sequences, maximum flooding surfaces (mfs) were recognized based on maximum diversity and abundance of planktonic taxa and decreasing of benthic foraminifera. The Sarvak Formation, one of the four formations of the Bangestan Group in Zagros basin, generally includes two facies, deep facies in the Lurestan province and shallow facies in the Khuzestan and Fars provinces. The Sarvak Formation deposited during Middle Albian to Early Turonian which the two main orogenic phases, Austrian (Albian / Cenomanian) and Sub-Hercynian (Cenomanian / Turonian) occurred. The warmest climate and the highest sea level took place along the Earth history in Albian to Cenomanian, and this could be a good and unique objective to investigation the effects of many interesting phenomena due to the amazing and unrepeated events. In addition, depositions of this time span could be included interesting depositional sequences. The aim of this research is recognition of depositional sequences of the Sarvak sediments by using of fossils. One stratigraphic section with 712 meters thickness was selected for studying of deep facies of the Sarvak Formation in Lurestan province. Totally, 148 thin sections were studied for recognizing microfacies characteristics and paleoenvironment based on Flugel (2010) and Wilson (1975). Study of sequence stratigraphy of the Sarvak Formation in the examined section was carried out based on Hunt and Tucker (1992, 1995) by using microfacies with fossil content. A critical and detail biostratigraphic study with relay on planktonic foraminifera and microfacies is used here is based on Daneshian et al.(2011, 2013). On the basis of the mentioned references, age determination clears to us a Middle Albian to Cenomanian age for the Sarvak Formation in the selected section. Also, 6 microfacies including mudstone, sponge spicule packstone, peloidal packstone, radiolarian packstone, Oligosteginid, planktonic foraminiferal wackestone - packstone, benthic foraminiferal echinoidal wackestone – packstone, were determinated. Study of depositional sequences in the pelagic and hemipelagic sediments is complicated and confusing. In fact, lack of exposures and also similarity and monotonous between the facies constructing the depositional sequences, make it difficult to differentiate systems tracts. Geological field studies do not play a critical role in sequence stratigraphic studies of the Sarvak deposits in Tang-e-chenarbashi section. thus, we focused on microfacies data to find the system tracts. The microfacies show that the strata deposited in five third-order sequences. The problem displayed when we faced to some paraconformity existed in the third order sequences. The paraconformities were recognized by biostratigraphic studies. Since these paraconformities are belong to the fourth order sequences, we persuade to elicit the fourth order sequences for the studied section, but when we know this fact the cause of fourth and lower order sequences is Milankovitch cycles, we should draw our attention to fossils than microfacies. The purpose of this paper is not Cyclostratigraphy but is to clear role of fossils in separating depositional sequences. In this research two functions of fossil data are considered: 1) dating the depositional sequences using global biozones, and 2) investigation of system tracts where microfacies are ineffective tools. In this study the abundance of four main fossil groups including planktonic foraminifera, benthic foraminifera, oligosteginids and echinoid fragments illustrated against lithostratigraphic column, sea level changes and investigated sequences. Many features such as amount of planktonic foraminifera, broken fossil fragments, dramatically the increase or decrease in r-selected or k-selected taxa and many other similar to these, coincide with ts surface or mfs which help us to demonstrate the forth-order depositional sequences. This study tries to establish relation between classic usage of fossils just for age determining and use it as main allochem that is able to reflect many small scale events which tracking them in the deposit is impossible.
Generally, five third-order sequences in a succession including six microfacies were recognized. Biostratigraphic data helped us to know the age of depositional sequences to attribute them to correct order with more confidence. Also, on the basis of extracted biozones, some paraconformities have been identified.