نشریه رخساره های رسوبی
سال دوازدهم شماره 2 (پیاپی 23، پاییز و زمستان 1398)

Alborz Mountain (with East-West trend) in North of Iran and South of Caspian Sea has been created by the collision of central Iran and South Caspian plates (Stocklin, 1974). Upper Cretaceous strata in the Alborz area are consist of marine carbonates and marls that are different in western and eastern regions. Tertiary conglomerates and sandstones strata disconformably overlie those. In the Northern Alborz, Cretaceous carbonate strata are continuously overlain by shallow marine lower Paleocene layers (Rezaeian, 2009). Cretaceous-Paleocene strata in the north flank of central Alborz are consist of 235 meters of carbonate, siltstone, sandstone, and evaporitic layers. For reconstruction of the sedimentary environment in this strata, one stratigraphic outcrop in the south of Sari city has been sampled and studied. In this study, benthic foraminifera, palynomorphs, and palynofacies have been examined.

Seventy rock samples have been processed using the standard palynological processing method (Traverse, 2007). In this method, carbonate and siliciclastic matters have been removed by HCL and HF (37%), then the residual matter has been sieved and centrifuged (using ZnCl2), and finally, palynological slides have been prepared. Also, seventy thin sections in order to the identification of benthic foraminifera have been prepared. Finally, all data have been plotted on diagrams using Corel Draw, Triplot, Adobe illustrator, and Excel software.

The studied section from base to top is consists of carbonate strata with siliciclastic intercalation layers, marl, argillaceous limestone, shale, and evaporate. To the identification of relative age and Cretaceous-Paleocene boundary in the studied section, fossils content of samples have been studied. Four species of dinoflagellates, ten species of spores, seven species of pollens, and two species of foraminifers have been identified. Based on stratigraphic distribution of Tricolpites phillipsii (Ocampo et al., 2006; Schulte et al., 2010; Vajda and Raine, 2003; Willumsen and Vajda, 2010), Palaeopridinium cretaceoum (Ogg et al., 2016) and Elphidiella multiscissurata, Campanian-early Paleocene age can be proposed for these strata. K-Pg boundary is identified in the sample (K-T 36) based on the first appearance of Tricolpites Phillipsii index species. Thin section studies lead to the identification of five sedimentary facies (evaporite, siltstone, litarenite, sublitarenite, and bioclast packstone/wackstone) in the studied samples that proposed shallow marine carbonate ramp as a sedimentary environment for these layers. Organic matter in palynological slides can be used as a paleontological factor to reconstruct the paleoecology and sea-level changes in the past (de Araujo Carvalho et al., 2006). High abundance of terrestrial palynomorphs (spore and pollens) and plants particles (phytoclasts) and low abundance of marine palynomorphs (dinoflagellate, acritarch, chitinozoans, …) and structureless organic matter (SOM) can prove landward environments and relative sea-level fall event and vice versa (de Araujo Carvalho et al., 2006; Handford and Loucks, 1993; Tyson, 1993). Palynological studies in this section show a relative sea-level rise trend from the base of the section to the middle parts, then a relative sea-level fall trend from the middle parts towards the end of the section. Moreover, there is a relative sea-level fall event across the K-T boundary the can be correlated with the global sea-level diagram (eustasy). Based on this fact, we can say that the Cretaceous-Paleocene basin in this area was connected to the open oceans.

Cretaceous-Paleocene strata in the South of Sari (Northern Iran) are composed of carbonate, evaporate, and siliciclastic sedimentary rocks. In this study, based on paleontological investigations and the appearance of Tricolpites phillipsii (index species of Paleocene age), Cretaceous-Paleocene boundary has been identified in K-T36 sample (133 meters from the base of section). Stratigraphical distribution of index fossils proposed a Campanian-early Paleocene age for the studied strata. Facies studies lead to suggest a shallow carbonate ramp as a sedimentary environment for these strata. Statistical analysis on palynological factors shows a deepening upward trend from the base of the section to the middle parts. Then there is a shallowing upward trend from the middle parts towards the end of the section. Moreover, one significant shallowing event can be detected across the K-T boundary that can be correlated with global sea-level fluctuations that show us the connection between this basins to the open oceans.AcknowledgmentThis study has been supported by the University of Zanjan and Khazar Exploration and Production Company.

The Kuh-e-Bande-Abdol-Hossein section, which is located southeast of Anarak, was first mentioned by Reyer and Mohafez (1970) and later examined in more detail by Sharkovski et al. (1984) and Wendt et al. (2005). We re-examined this section because it is mainly composed of sedimentary rocks ranging from Ordovician to Permian age (Hairapetian et al. 2015; Lensch and Davoudzadeh 1982). Herein we focus on the Middle to Upper Devonian strata in order to establish the biostratigraphic framework of this section by means of conodonts. We also briefly discuss the conodont biofacies of the Kuh-e-Bande-Abdol-Hossein.

The measured profile is located approximately 32 km southeast of Anarak and 180 km northeast of Isfahan (E 53° 52′ 55″ and N 33° 10′ 90″ WGS coordinates). The entire section has a thickness of approximately 1200 m. In order to improve the biostratigraphy of the Kuh-e- Bande-Abdol-Hossein section, 78 conodont samples of roughly 2 to 3 kg each were taken from the 366m of carbonates and processed by conventional methods using 10% formic acid. Washed residues were sieved and separated into three fractions, and conodonts were handpicked utilizing a microscope. Depending on the depositional facies setting, the number of conodonts per sample is highly variable, e.g., in dolostones, no conodonts were found. In contrast, in shallow-water limestones, a good number of species occurred in separate beds. A total number of 1917 conodonts were obtained from the residues, which led to the identification of 41 species and subspecies within six genera.

Based on the revealed conodont data, six conodont zones were discriminated as follow: expansus zone, subterminus zone, Upper falsiovalis to transitans zones, transitans to lower rhenana zones,upper rhenana to linguiformis zones, triangularis to termini zones. Bahram Formation at the studied profile spans late Givetian (expansus zone) to Early Famennian (triangularis to termini zones. Biofacies interpretation and conodont frequency reveal the Icriodid-Polygnathis to Polygnathid-Icriodid biofacies due to the deepening of the depositional basin. CAI interpretation also shows the variation from CAI=1/5-2 to CAI=4-4/5. The studied interval is composed of an overall shallow-water, nearshore to open marine facies setting.

This study is undertaken at the Department of Geology, Faculty of Sciences, University of Isfahan. The financial supports by the Vice-Chancellor for Research and Technology, University of Isfahan, was highly appreciated.

Permian sequences of Zagros and Arabiab peninsula including Dalan Formation and its equivalent, Khuff Formation, are precisely investigated in order to hydrocarbon exploration. Type section of Dalan Formation is described by Szabo & Kheradpir (1978) and composed of three members; Lower and Upper Dalan and Nar evaporite Member between them. Studied succession in the present paper is located in Gahkum anticline, southeast of High Zagros Zone, SE Iran. We should pass 134 km northward from Bandar Abbas to attain Sarchahan village and then 7.7 km eastward in order to access the section. Coordinates of section are: 28° 5' 40.5"N, 55° 55' 18.19"E.

Numerous researches have been conducted on the Dalan and Khuff formations in the Middle-East. For example, Alsharhan & Narin (1995) studied the sedimentary environment and stratigraphy of Permian sequences in the Arabian Peninsula. Angiolini et al. (1998; 2003) and Henderson & Mei (2003) studied the paleogeography and paleoclimatology of North and southeast of Oman by using conodonts. Kolodka et al. (2011) studied the biostratigraphy and microfacies of Dalan Formation. Dalan Formation in the Gahkum section with 685m thickness conformably overlie the marine uppermost beds of Faraghan Formation and can be divided to Lower and Upper Dalan Members. No sign of Nar evaporate Member is seen between them. The absence of this member may be related to lateral facies changes of evaporite to carbonate. The Upper Dalan Member is disconfomably overlain by Middle to Late Triassic dolomites of Khaneh-Kat Formation (Fakhari, 1995, Sabzehei, 1993). On the basis of foraminifer stratigraphy, Kolodka et al. (2011) proposed the age of Wordian for the base of Lower Dalan Member, and the present conodont species; Sweetognathus iranicus, Hindeodus wordensis and Merrillina divergens prove this age. On the other hand, the base of Khuff Formation (equivalent of Dalan Fm.) in the southeast and interior Oman has the age of Wordian (Angiolini et al., 1998; 2003). Conodonts are associated with microvertebrate remains such as skeletal particles, scales and teeth of paleoniscoids, hybodontiforms and osteichthyan fishes.

Conodont studies in addition to the stratigraphic situation of the base of Dalan Formation in the Gahkum section show the age of Wordian. It seems that the base of Dalan Formation is diachronous and becomes younger from the northwest to the southeast of High Zagros. The absence of Nar evaporite Member between Lower and Upper Dalan carbonate members, may be related to the lateral facies changes. This is the first report of paleoniscoid, hybodontid and osteichthyan fish remains from the Permian strata of Zagros Mountains.Acknowledgment This paper is the result of project no. 95/200/168, so we appreciate Deputy of Research and Technology, University of Hormozgan for financial supports. Authors thank M.K. Kamali and F. Amin-Nezhad who helped us in field and laboratory works.

The Binalud Mountains in the northeast of Iran is a limited structural unit between the central Iran, Kopet-Dagh, and Alborz Mountains, mainly considered as a southeastern extension of Alborz Mountains (Lammerer et al., 1983; Eftekhar-Nezhad & Behroozi, 1991; Alavi, 1992; Seyed-Emami & Schairer, 2011). Jurassic successions, which have very well extended outcrops in the eastern Alborz and Binalud Mountains, were the main topic of the publications by Seyed-Emami & Schairer (2010), Wilmsen et al. (2009a,b), Raoufian et al. (2011, 2014), Seyed-Emami and Raoufian (2017), Seyed-Emami et al. (2018). Dalichai Formation in the eastern Alborz and Binalud Mountains comprises a thickness of marl and marly limestone, with an abundance of ammonite fauna which is the most important characteristic of the formation in the Binalud Mountains. This fauna in the Alborz and Binalud has long been considered in different papers (e.g., Shafeizad et al., 2005; Seyed-Emami et al., 2008, 2013, 2015, 2018; Raoufian et al., 2011, 2014). In this research, an outcrop of Dalichai Formation in the Bāghi stratigraphic section, northwest of Esfarayen city, was lithostratigraphycally and biostratigraphically studied.

The base of the measured stratigraphic section is located in 37°11′45.34″ and 57°16′21.89″, 29 km northwest of Esfarayen city (Esfarayen to Jajarm route). After field studies, reviews of lithofacies in the field, and drawing the log, 540 in situ ammonite specimens were obtained from the section. They are kept in the Museum of the Geology Department at the Ferdowsi University of Mashhad. The biometric details of ammonite specimens, including the dimensions of the specimens, size, depth, and diameter of the umbilical area, the number, arrangement, and shape of ribs, marginal keels, nodes, suture lines, and apertural shape, have been studied. The studied ammonitic fauna are typically Northwest Tethyan and belongs to the Submediterranean Province with episodic Mediterranean and Subboreal affinities (Seyed-Emami et al., 2013). It reflects the relationship between these regions with other parts of the Paleotethys sidelines during the Jurassic time.

In the Baghi section, the Dalichai Formation unconformably overlies the siliciclastic Shemshak Formation and gradationally underlies the cliff-forming carbonate rocks of Lar Formation. The Dalichai Formation, with a thickness of 655 meters, is divided into ten separate informal members and is mainly composed of marls, limestone, and marly limestone. One of the most important features of the Dalichai Formation in this section is the abundance of ammonite fauna, especially in the lower half of the section. Due to the specific color, lithology, and abundance of ammonite fauna, the second member is a key member.Paleontological studies in this research is led to the determination of 96 species, 26 subgenera, 39 genera, 20 subfamilies, 14 families, and 6 superfamilies belonging to 3 suborders of Ammonitida. According to the mentioned assemblage, 17 standard biozones are recognized, consisting of Grantiana, Parkinsoni, Zigzag, Retrocostatum, Bullatus, Gracilis, Anceps, Coronatum, Athleta, Lamberti, Transversarium, Bifurcatus, Bimammatum, Planula, Platynota, Hypselocyclum, and Acanthicum. These biozones show the age of late Bajocian – late Kimmeridgian for the Dalichai Formation. The studied ammonitic assemblages are closely related to the other Middle-Late Jurassic fauna from Iran and further peri-Mediterranean regions, which correlates with the Sub-Mediterranean province.

This study was aimed at the biostratigraphy and sedimentary environments reconstruction in one of the wells in Maroun oilfield at Dezful embayment zone. We also compared the studied sub-surface section with the equal intervals in Mala-Kuh, Ghale-Nar oilfield, Moshkan section, and Firouzabad section. The thickness of Asmari Formation in the studied well is 370.5 m and consists mainly of limestone, dolomite, dolomitic limestone, sandstone and argillaceous limestone. In this well, the lower boundary of the Asmari Formation with Pabdeh Formation and its upper boundary with Gachsaran Formation is continuous.

For this study, 150 thin sections (mainly drilled cores) were studied in detail for paleontology, biostratigraphy, allochems identification, and microfacies determination. References such as Loeblich and Tappan (1980), Boudagher-Fadel, (2008), Adams & Bourgeois (1967), and other related articles were used to identify the available microfossils. We used the biozonation scheme of Laursen et al. (2009) and Van Buchem et al. (2010) to define and correlate the biozones. Classification of rock types was done according to Dunham (1962) and Embry & Klovan (1971), and microfacies identification and interpretation were carried out based on Wilson (1975), Buxton & Pedley (1989), Geel (2000), and Flugel (2010),

Based on paleontological studies, 43 genera and 61 species of benthic and planktonic foraminifera were identified. They have been classified into six zones (five assemblage zones and one indeterminate zone) as follows:1- Globigerina spp.-Turborotalia cerroazulensis-Hantkenina Assemblage ZoneThis biozone is 76 meters thick and lies between the depths of 3706.5 to 3630.5 m. It corresponds to the upper part of the Pabdeh Formation. In this biozone with Oligocene (Rupelian) age, Globigerina spp. are abundant, and the extinction of Turborotalia cerroazulensis occurs in this biozone.2- Lepidocyclina-Operculina-Ditrupa Assemblage ZoneThis biozone is 129.5 meters thick and lies between 3630.5 to 3501 m. The beginning of this biozone is based on the first occurrence of the Lepidocyclina sp. and Operculina complanata. Its end is marked by the first appearance of the Miogypsinoides compalanatus and Spiroclypeous blankenhorni. This biozone belongs to the Chattian-Rupellian age.3- Archaias asmaricus-Archaias hensoni-Miogypsinoides compalanatus Assemblage ZoneThe thickness of this biozone is 80 meters and lies between 3501 to 3421 meters deep. The onset of this biozone is marked by the first occurrence of Miogypsinoides compalanatus and Spiroclypeous blankenhorni. The first occurrence of Elphidium sp.14, Peneroplis farsensis and disappearance of Archaias distinguish the end of the biozone. The age of this biozone is Chattian.4- Elphidium sp.14 – Peneroplis farsensis Assemblage ZoneThis Assemblage zone is 83 meters thick and lies between 3421 to 3338 meters deep. The age of this biozone is Aquitanian. The onset of this biozone is determined by the occurrence of the Elphidium sp.14, Peneroplis farsensis, and the end of this biozone is characterized by a decrease in the fossil fauna assemblage known as the Ambiguous Zone.5-Indeterminate ZoneWith 70.44 meters thickness, this zone lies between the depths of 3338 to 3267.57 meters. A decrease in fossil assemblages characterizes this interval. According to its stratigraphic locality, this indeterminate zone is related to Aquitanian (Laursen et al., 2009). The fossils present in this biozone are very sparse and consist mainly of porcelaneous and indeterminate milliolids.6- Borelis melo curdica-Borelis melo melo Assemblage Zone?This biozone is related to the uppermost part of the Asmari Formation. The beginning of this biozone is defined based on the occurrence of the Borelis spp., continues to the end of the Asmari Formation. This biozone is 7.56 meters thick and lies between 3267.56 to 3260 meters deep. The age of this biozone is cautiously Burdigalian.Based on the mentioned biozones, the age of Asmari Formation in the studied well is Rupelian – Chattian –?Burdigalian.According to sedimentary texture, sedimentary structures, skeletal and non-skeletal elements in the microscopic studies if the provided thin section, 12 microfacies related to the outer ramp, middle ramp (distal and proximal parts), shoal and inner ramp environments (patch reef, lagoon (semi-restricted and restricted( and tidal flat were identified in the studied well. Gradual changes of facies to each other, the lack of coated structures, and the absence of coated grains are the main evidence suggesting deposition in a homoclinic carbonate ramp. Microfacies analysis shows that planktonic foraminifera in the upper part of the Pabdeh Formation belongs to the outer ramp. Due to the presence of large hyaline foraminifera, the lower part of the Asmari Formation is deposited in the distal part of the middle ramp, while the presence of lenticular foraminifera demonstrates the proximal ramp environment. A high-energy, shallow-shoal environment with the presence of thick-walled lenticular foraminifera, cementation, and grainstone textures characterize the boundary between the middle and the inner ramp. Inner ramp environment is considered by the presence of patchy corals (patch reef) and imperforate foraminifera (Archaias, Peneroplis, milliolids) of the lagoon (semi-restricted and restricted lagoon) and tidal flat zones.

One of the most extensive Tertiary succession in Central Iran is the marine strata of the Qom Formation, which was studied based on paleontology studies. The type section of the Qom Formation has been studied in the areas around Qom city (Bozorgnia, 1965). One of the most important achievements obtained from the formation is evaluating stage boundary (Oligocene-Miocene interval) and biostratigraphy of it. In this section the Qom Formation consists of 346 m thick which mainly consists of marl, marly limestone, and limestones.

In this study, 78 samples from the Gurpi Formation interval have been studied. The samples were prepared following the standard smear slide method (Bown and Young 1998). All slides were examined under the polarized light microscope at ×1000 magnification. The nomenclature of calcareous nannofossil follows the taxonomic schemes of Perch-Nielsen (1985), Martini (1971), and Okada & Bukry (1980).

Calcareous nannofossils are among the most important fossil indicators for determining relative age, biostratigraphy studies, paleoecology studies, and correlation of sedimentary strata of various geological periods in different parts of the world (Hewaidy et al. 2014; Faris et al. 2016). A succession of the Qom Formation in southwestern flank Navab anticline was selected to study biostratigraphy based on calcareous nannofossils. In this section, the Qom Formation with 346 m thickness mainly consists of marl and marly limestones. For introducing index species, calcareous nannofossil assemblages, and biozones, slides have been studied at the Qom Formation, which led to the recognition of 29 species of calcareous nannofossils. In this study, four biozones were detected in the formation. Bio zones including Triquetrorhabdulus carinatus Zone (CN1a-b), Discoaster druggii Zone (CN1c), Sphenolithus belemnos Zone (CN2), and Helicosphaera ampliaperta Zone (CN3) with late Oligocene (late Chattian) to early/middle Miocene (Burdigalian-Langian) in age. According to the first and last occurrence of index species, the following biozones are identified:1. Triquetrorhabdulus carinatus Zone (CN1a-b)This biozone is recorded from the LO Helicosphaera recta or Sphenolithus ciperoensis to the FO of Discoaster druggii. The age of this zone is late Oligocene (Chattian) to early Miocene. The thickness of this zone has been measured at approximately 14 m. In this zone, some associated fossil assemblages are Coccolithus miopelagicus, Discoaster deflandrei, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Pontosphaera multipora, Pyrocyclus orangensis, Reticulofenestra bisecta, Reticulofenestra locker, Sphenolithus capricornutus, Sphenolithus calyculus, Sphenolithus delphix, Sphenolithus conicus, Sphenolithus compactus.2. Discoaster druggii Zone (CN1c)The second zone is recorded from the FO of Discoaster druggii to the LO of Triquetrorhabdulus carinatus. The age of this zone is the early Miocene. The thickness of this zone is 187.68 m. Some fossil assemblages in this zone include Braarudosphaera bigelowii, Cyclicargolithus floridanus, Coccolithus pelagicus, Coccolithus miopelagicus, Discoaster druggii, Discoaster deflandrei, Helicosphaera ampliaperta, Helicosphaera granulate, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Helicosphaera scissura, Pontosphaera multipora, Pyrocyclus orangensis, Sphenolithus conicus, Sphenolithus compactus, Sphenolithus dissimilis, Sphenolithus moriformis, Triquetrorhabdulus carinatus.3. Sphenolithus belemnos Zone (CN2)This zone spans the interval from the LO of Triquetrorhabdulus carinatus to the last occurrence of Sphenolithus belemnos. The thickness of this zone is 81/32 m. The age of this zone is the early Miocene. Some fossil assemblages in this zone include Braarudosphaera bigelowii, Cyclicargolithus floridanus, Coccolithus pelagicus, Coccolithus miopelagicus, Discoaster druggii, Discoaster deflandrei, Helicosphaera ampliaperta, Helicosphaera granulata, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Helicosphaera scissura, Pontosphaera multipora, Sphenolithus belemnos, Sphenolithus conicus, Sphenolithus compactus, Sphenolithus moriformis.4. Helicosphaera ampliaperta Zone (CN3)The last nannofossil unit recorded in this study is the CN3. This zone spans the interval from the LO of Sphenolithus belemnos to the LO of Helicosphaera ampliaperta. The thickness of this zone is 63 m. The age of this zone is early/middle Miocene (Burdigalian-Langian). Some fossil assemblages in this zone include: Braarudosphaera bigelowii, Cyclicargolithus floridanus, Coccolithus pelagicus, Discoaster deflandrei, Discoaster druggii, Helicosphaera ampliaperta, Helicosphaera obliqua, Helicosphaera euphratis, Helicosphaera intermedia, Helicosphaera scissura, Pontosphaera multipora, Sphenolithus compactus, Sphenolithus moriformis.Consequently, according to the identified biozones, the age of the section is late Oligocene (Late Chattian) to early/middle Miocene (Burdigalian-Langian).

The Kopet-Dagh Basin is located in the northeast of Iran, extended over Iran, Turkmenistan, and Afghanistan with a west-northwest to east-southeast trend. The Iranian part of the basin is located between 61°14ˊ and 54°00ˊE and 38°16ˊ and 36°00ˊ N. Cretaceous successions are thick and well exposed in the northeast of Iran. The Cretaceous Ammonites are studied by Seyed-Emami (1980), Seyed-Emami & Aryai (1981), Seyed-Emami et al. (1984), Immel et al. (1997), Raisossadat (2004, 2006), Mosavinia et al. (2007, 2014), and Mosavinia & Wilmsen (2011). Calcareous nannoplankton biostratigraphy of Aitamir formation is studied by Susani (2005) and Hadavi & Musazadeh (2005).

The studied section, the Amirabad section, is located at a distance of 65 km to the northeast of Mashhad city, on the Mashhad-Kalat road (coordinates: N36°34ˊ59˝ and E60°09ˊ24˝; altitude: 720 m). Aitamir Formation is measured 668 m in this section and comprised siltstones, fossiliferous (bivalve and ammonite) sandstones, glauconitic sandstones, dark gray to greenish-gray silty shales, and dark gray shales. It conformably overlies the Sanganeh Formation and is overlain by the Abderaz Formation. A total of 139 samples were collected from the section at intervals 3-5 m to study the calcareous nannofossils. In addition, 40 m part of the topmost of the underlying Sanganeh Formation as well as the lowermost of the overlying Abderaz Formation were also measured and sampled. For nannofossil biostratigraphic investigations, all samples were processed using the gravity settling technique (Bown & Young, 1998). The prepared slides were observed under a Nikon Optiphot II Pol light microscope with a magnification ×1000. Bibliographic references for the calcareous nannofossils are given in Perch-Nielsen (1985), Varol (1992), and Burnett (1998). Ammonite samples were collected across the section and named and photographed at the paleontology laboratory of the Payam-e-Noor University of Mashhad. 

Nannofossil bioevents have been utilized to biostratigraphically classify the exposed sedimentary succession based on the CC biozonation scheme of Sissingh (1977, modified by Perch-Nielsen, 1985) and the NC biozonation scheme of Roth (1978, modified by Bralower et al., 1995). The biozones CC8 to CC10 and NC9 to NC13 were determined, and accordingly, the Aitamir Formation is attributed to Albian to late Cenomanian time interval. The lithology of sandstones and siltstones at the lower part of the Aitamir Formation influenced the preservation of nannofossil taxa. In this part, the Ammonite biozones have covered the gap of nannofossil biozones. The upper part of Leymeriella tardefurcata, Hoplites dentatus, Euhoplites loricatus, Euhoplites lautus Mortoniceras (M.) inflatum, Stoliczkaia dispar, Mantelliceras mantelli, Mantelliceras dixoni, Cunningtoniceras inerme and Acanthoceras rhotomagense biozones were determined at the Aitamir Formation except for the upper 20 meters, which contains no ammonite sample. Based on Ammonite biozones, the age of Late Early Albian to Middle Cenomanian is attributed to the Aitamir Formation.

The Binaloud Mountains, with an almost eastern-western trend, is extended from the Jajarm area in the Northern Khorasan to the Torbat-e-Jam region. In the Binalud zone, Jurassic succession in many places covered Paleozoic rocks and metamorphic assemblages, known as Mashhad Phyllite. Chaman-Bid Formation is one of the most famous Jurassic deposits in the northeastern. The studied sequence in the Ghroneh section is located at 43 kilometers north of Neyshabur and northeast of Ghroneh village (580 45' 36'', 360 45' 05''), respectively contains siliciclastic rocks of Kashafrud Formation, shale, marl, limestone, and sandstone beds of Chaman-Bid Formation, and thick to massif limestones of the Mozduran Formation. Most paleontological studies on the Chaman-Bid Formation are done based on ammonites (Majidifard, 2003, 2008; Raoufian et al., 2011, 2014; Seyed- Emami et al., 2013, 2018), palynomorphs (Mafi et al., 2014a,b), and more recently nannofossils by Amani et al. (2016). Based on calcareous nannofossils, the Early Bajocian–Early Berriasian age was suggested for the mentioned deposits in the Ghroneh section. In the present study, for the first time, ostracods of the Chaman-Bid Formation have been investigated. The present study introduces the Chaman-Bid Formation ostracods and compares this fauna within the calcareous nannofossils biozones.

  The thickness of the Chaman-Bid Formation in the studied section is 715 meters and consists of shale, marl, and limestone, interbedded with some sandstone beds. Sixty-four samples of the Chaman-Bid Formation with intervals of 10 to 15 meters were collected. 300 grams of each sample were soaked in water and hydrogen peroxide for 24 hours and washed in 30 and 60 mesh sieves. The residue was dried, and all ostracods in each sample were separated. The ostracods were studied with a Zeiss ZH55 microscope at 40x magnification and photographed in scanning electron microscopy (SEM) at the central laboratory of the Ferdowsi University of Mashhad.

Ostracoda is one of the most valuable crustaceans in paleontological and paleoecological studies. The remains of these small and microscopic crustaceans are widely distributed through Phanerozoic, and individuals belonging to this subgroup are characterized by their particular morphological and physical characteristics (Van Morkhoven, 1963). In the present study, 45 genera and 59 species of Ostracoda have been identified in the Chaman-Bid Formation in the studied section. Previous studies of Ostracoda have been compared with ammonite zones. The current study is important since there are no reports on Iran's Jurassic ostracods. Therefore, introducing ostracods and determining their age range is a significant objective of this study; however, their preservation is relatively poor. The study of the Chaman-Bid Formation ostracods in the Ghroneh section led to identifying of 59 species belonging to 45 genera. For relative age dating in the present study, calcareous nannofossils have been used and identified ostracods correlated with them. These assemblages have been compared with 12 previously reported nannofossil zones from the Ghroneh section. Nannofossil zones (Amani et al., 2016) belong to NJ9 to Early CC1 and confirm the age of Early Bajocian to Berriasian for the discussed ostracod assemblages.