دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 40 2 2024 06 21 Facies, sedimentary environment and log facies of the Hojedk Formation (Middle Jurassic) in Parvadeh Tabas Coal Mine, South of Tabas رخساره‎‍ها، محیط رسوبی و رخساره‌های الکتریکی سازند هجدک (ژوراسیک میانی) در معدن زغال‌سنگ پرودۀ طبس، جنوب طبس 1 26 28549 10.22108/jssr.2024.141278.1284 FA نجمه مهدیه نجف آبادی کارشناسی ارشد گروه زمین‌شناسی، دانشکده علوم، دانشگاه اصفهان، اصفهان، ایران 0000-0001-8670-5973 محمدعلی صالحی دانشیار، گروه زمین‌شناسی، دانشکده علوم، دانشگاه اصفهان، اصفهان، ایران 0000-0001-8670-5973 وصال یحیی شیبانی استادیار، دانشگاه پیام نور مرکز طبس، خراسان جنوبی، ایران Journal Article 2024 04 26 The Middle Jurassic Hojedk Formation in the Parvadeh Coal Mine (90 km south of Tabas) has been studied. The study area is structurally located on the northern Tabas Block in the north of the Parvadeh–Nayband coal-bearing basin, east-central Iran. The Hojedk Formation has a thickness of 40 meters in the studied outcrop section. Five lithostratigraphic units have been identified, which mainly consist of sandstones, shales, conglomerates, siltstones and thin bioclastic sandy limestone. Based on field and petrographic studies, four groups of lithofacies including coarse- (Gcm and Gt), medium- (St, Sp, Sh and Sr) and fine-grained clastic (Fl and Fm) as well as carbonate facies (echinoid brachiopod grainstone-packstone) have been identified in the Hojedk Formation. The architectural elements of channels and point bars of a meandering tidal creek and coastal plain swamp have been identified in the Hojedk Formation and its palaeodepositional conditions can be attributed to the tidal flat of a marginal marine siliciclastic environment. To investigate the characteristics of the Hojedk Formation in the subsurface, the electrical facies were defined in an exploration borehole in the eastern part of Parvadeh Mine. In this borehole, the Hojedk Formation is 48 meters thick, and four electro/log facies have been identified by considering the gamma-ray motif and resistivity logs. These electro facies are compatible with the sub-environments of coastal sand flats, tidal channels and coastal plain swamps. Keywords: Tabas Block, Hojedk Formation, Middle Jurassic, Facies, Log facies Introduction Central-East-Iranian-Microcontinent is a part of Iran Plate, which can be structurally divided into Lut, Tabas and Yazd blocks (Takin 1972; Aghanabati 2004). The tectono-sedimentary evolution of east-central Iran during the Late Triassic to the Late Jurassic is governed by the Cimmerian orogenic events (Fürsich et al. 2005; Seyed-Emami et al. 2004, 2020; Wilmsen et al. 2009, 2021). The Hojedk Formation is the last lithostratigraphic unit of the Shemshak Group in east-central Iran, which during the Middle Jurassic in this area was affected by Cimmerian tectonic events resulting in large thickness and facies variations (Wilmsen et al. 2009). Biostratigraphic studies have led to determining the age of this formation (early Bajocian) in eastern central Iran (Seyed-Emami et al. 2020). Macrofloras, palynology and sedimentary environment analysis have been conducted on the Hojedk Formation in Tabas Block, which has led to the reconstruction of the palaeoenvironmental and palaeoclimatic conditions of this formation (Vaez-Javadi and Mirzaei-Ataabadi 2006; Vaez-Javadi 2016; Badihagh et al. 2019; Khalilizadeh et al. 2023; Salehi et al. 2023). Based on preliminary studies conducted in the northern part of Tabas Block, the Hojedk Formation was formed in different sub-environments including lakes, coastal plains, deltas, fluvial and flood plains as well as the marine environment (Seyed-Emami et al. 2006). Due to the importance of the Hojedk Formation in the reconstruction of the tectonic-sedimentary evolution of northern Tabas Block during the Middle Jurassic as well as its coal resources, this succession has been studied in the outcrop and subsurface sections in the Parvadeh Coal Mine. The facies and sedimentary environment of this formation are still less known in many areas of the northern Tabas Block, including the Parvadeh Coal Mine area. Considering the extensive coal reserve of 1.1 billion tons in the northern Tabas Block, this region is important for detailed geological studies for the exploration of workable coal seams. To reconstruct the palaeoenvironmental deposition of the Hojedk Formation, field, subsurface and laboratory studies were carried out and all the data were integrated with the geological information of the area. Furthermore, the identification of log facies of this formation in the exploration boreholes, drilled in eastern Parvadeh Mine, is useful for coal exploration, discovery and production from the coal-bearing strata of the studied area. Material & Methods The outcrop section of the Middle Jurassic Hojedk Formation was logged and measured by Jacob's staff (Sdzuy and Monninger 1985) in the western part of the Parvadeh Coal Mine. Sampling was done systematically and sometimes according to the identical facies changes. In total 25 hand specimens were collected and thin sections were prepared from the sandstone and limestone samples. Clastic facies have been identified and classified based on Miall's classification (Miall 1985, 2014). Carbonate facies have been classified based on Dunham’s scheme for carbonate rocks (Dunham 1962), and microfacies character defined based on criteria described by Flügel (2010). In addition, a subsurface study has been carried out on an exploration borehole (BH#341), which was drilled in the eastern part of the Parvadeh Mine. The lithostratigraphy and sedimentological characteristics of this formation in the borehole were compared with the adjacent studied outcrop section. Graphical and Gamma-ray well logs motifs of the borehole have been used in this study for electrical/log facies identification. Discussion of Results & Conclusions In this research, the Hojedk Formation is studied in the north of Parvadeh-Nayband coal basin, 90 km south of Tabas (Parvadeh Coal Mine). Based on field studies, five lithostratigraphic units have been identified. The succession mainly consists of sandstones, shales, conglomerates, siltstones and thin bioclastic sandy limestone. Clastic facies were defined in the field by considering lithology, sedimentary structure, geometry, stratal surface, palaeocurrent and fossil/trace fossil content. Four groups of facies including coarse-, medium- and fine-grained clastic facies and one carbonate facies have been identified. Two coarse- (Gcm and Gt), four medium- (St, Sp, Sh and Sr), and two fine-grained facies including Fl and Fm were identified in the field. Some medium-grained facies are bioturbated and sometimes show vertical cylindrical burrows (e.g. Skolithos). The carbonate microfacies recognized based on rock texture (echinoid brachiopod grainstone-packstone), is equivalent to the standard microfacies (SMF) 18. The latter facies were formed in carbonate bioclastic sand shoals in a marginal shallow marine mixed environment. In order to identify the architectural elements in the Hojedk Formation, features such as the lithology, stratal surface, geometry, and their associated facies have been considered. In the studied succession, three architectural elements including channel and point bars of a meandering tidal creek and coastal plain swamp have been identified in the Hojedk Formation. This formation also has been investigated in the subsurface section (exploration borehole No. 341) in the eastern part of the Parvadeh Coal Mine. In this borehole, the Hojedk Formation is 48 meters thick and consists of sandstones, shales, limestones and siltstones. In this study, four electrical facies (log facies) were identified in the Hojedk Formation, which is compatible with the sub-environments of coastal sand flats, tidal channels, crevasse splay and coastal plain swamps. According to the identified lithofacies, microfacies, architectural elements and log facies in vertical and lateral distribution, the sedimentary environment of the Hojedk Formation in the studied outcrop and borehole is attributed to a tidal flat of a marginal marine siliciclastic environment. سازند هجدک با سن ژوراسیک میانی در شمال حوضۀ زغال‌دار پروده-نایبند و در فاصلۀ 90 کیلومتری جنوب طبس در محدودۀ معدن زغال‌سنگ پروده‌ و از‌نظر ساختاری بر بلوک طبس در زون شرقی ایران مرکزی مطالعه شده است. این سازند در برش‌ مطالعه‌شده، ضخامت 40 متر دارد. در این توالی رسوبی پنج واحد چینۀ سنگی شناسایی و تفکیک شده است که عمدتاَ شامل ماسه‌سنگ، شیل، کنگلومرا و سیلتستون است. بر‌اساس مطالعات صحرایی و مطالعات پتروگرافی در سازند هجدک چهار دسته رخسارۀ سنگی دانه‌درشت (Gcm و Gt)، متوسط (St، Sp، Sh و Sr)، ریز (Fl و Fm) و کربناته (براکیوپود اکینوئید گرینستون-پکستون) شناسایی شده است. به‌طور کلی عناصر ساختاری کانال و پرکنندۀ کانال مربوط به کانال‌های جزر و مدی و باتلاق دشت ساحلی در رسوبات سازند هجدک شناسایی شده است و شرایط رسوب‌گذاری آن‌ به محیط پهنۀ ساحلی از یک محیط حاشیه دریایی آواری نسبت داده می‌شود. به‌منظور بررسی ویژگی‌های الکتریکی رخساره‌های سازند هجدک در زیر سطح این سازند در یک گمانۀ اکتشافی در محدودۀ معدن پروده نیز‌ بررسی شد. در این گمانه سازند هجدک 48 متر ضخامت دارد و با در نظر گرفتن تغییرات شکل لاگ گاما و مقاومت چهار رخسارۀ الکتریکی تشخیص داده شد که با زیرمحیط‎‍های پهنه‌های ماسه‎‍ای ساحلی، کانال‎‍های جزرومدی کوچک و باتلاق دشت ساحلی انطباق‌دادنی‌اند.

https://jssr.ui.ac.ir/article_28549_ac2625e5fa3602d177afffa4e91dec96.pdf

دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 40 2 2024 06 21 Investigating the role of basement structures in controlling facies characteristics, depositional models, and diagenetic alterations of the Sarvak Formation in the eastern and western regions of the Persian Gulf بررسی نقش ساختارهای پی‎‍سنگی در کنترل خصوصیات رخساره‌ای، مدل رسوب‌گذاری و تحولات دیاژنزی سازند سروک در نواحی شرقی و غربی خلیج‌فارس 27 58 28445 10.22108/jssr.2024.141034.1281 FA مرضیه محمودی دانشجوی کارشناسی ارشد دانشکدۀ زمین‌شناسی، دانشکدگان علوم، دانشگاه تهران، تهران، ایران حمزه مهرابی استادیار دانشکدۀ زمین‌شناسی، دانشکدگان علوم، دانشگاه تهران، تهران، ایران 0000-0002-2211-4899 رضا نوزعیم استادیار دانشکدۀ زمین‌شناسی، دانشکدگان علوم، دانشگاه تهران، تهران، ایران Journal Article 2024 04 02 Abstract The Sarvak Formation, as one of Iran's most important hydrocarbon reservoirs, has been deposited and evolved under active tectonic conditions in various regions of the Zagros, including the Persian Gulf. The present study aims to investigate the effects of tectonic activities on the facies characteristics and diagenetic history of the Sarvak Formation in the eastern and western regions of the Persian Gulf. Facies analysis results indicate the deposition of the Sarvak Formation on a carbonate platform of ramp type. In the eastern part of the Persian Gulf, sedimentary facies of the Mishrif Member (Upper Sarvak) are predominantly deposited in inner platform areas (lagoon, shoal, reef deposits), while facies of the Khatiyah Member (Lower Sarvak) are mainly found in deep and outer platform areas (outer ramp). In the western part of the Persian Gulf, the Sarvak Formation encompasses a spectrum of shallow to deep-water facies, with open marine facies (middle and outer ramp) being notably more abundant. Two lower-order sequences (fourth-order) and one higher-order sequence (third-order) have been identified in this formation, indicating a high correlation with other areas of the Zagros and the Arabian Plate. Diagenetic effects related to a palaeoexposure surface have been observed in the uppermost part of the Sarvak Formation in both the eastern and western regions of the Persian Gulf. Dissolution porosity, brecciation, and iron oxide staining are among the most important diagenetic processes associated with these exposure surfaces. The thickness of the Sarvak Formation shows considerable variations in different parts of the Persian Gulf. Additionally, sedimentary facies of this formation exhibit significant lateral variations. The changes in thickness and facies variability of the Sarvak Formation were directly related to basement structures, indicating their activity during the deposition of this formation in the Persian Gulf. These tectonic events have had a significant impact on the burial history of the Sarvak Formation. Ultimately, the culmination of these facies and diagenetic characteristics has influenced the reservoir quality of the Sarvak Formation in the study areas. Keywords: Sarvak Formation, Persian Gulf, Basement structure, Facies, Diagenesis, Palaeoclimate Introduction Facies characteristics and sedimentary environment, diagenetic processes, and fractures are the most important factors affecting the quality of hydrocarbon reservoirs (Ahr 2008). Therefore, considering tectonic features as one of the most important influencing factors on all the mentioned characteristics is a necessity in any integrated reservoir study (Nelson 2001; Hollis 2011; Lai et al. 2021). During the Late Cretaceous, especially in the Cenomanian-Turonian interval, tectonics was one of the main factors controlling the sedimentary basin of the Zagros region (Alavi 2007; Farahpour and Hessami 2012). These changes in the tectonic regime caused the reactivation of faults and salt domes in the region, creating a period of highly dynamic tectonic activity (van Buchem et al. 1996, 2002, 2011; Immenhauser et al. 2000, 2001; Sharp et al. 2010; Hollis 2011; Vincent et al. 2015). The Sarvak Formation, the second most important oil reservoir in Iran, has been significantly influenced by the aforementioned tectonic events, affecting its deposition and diagenetic history in various parts of the Zagros region. This is evidenced by the considerable variation in thickness and facies changes of the Sarvak Formation across different areas of the Zagros (Farahpour and Hessami 2012; Mehrabi et al. 2015; Bagherpour et al. 2021; Sadeghi et al. 2023). This study aims to integrate sedimentological data, including facies studies, diagenetic processes, and sequence stratigraphy, with existing information about the tectonic setting of the Persian Gulf during the Late Cretaceous, to examine the impacts of tectonic factors on the facies characteristics and diagenetic evolution of the Sarvak Formation in the eastern and western regions of the Persian Gulf. Material & Methods In this research, core data, thin-section microscopy, and X-ray diffraction (XRD) analysis results from two wells in two different fields in the eastern and western parts of the Persian Gulf, referred to as wells A and B, are utilized. The studied core length in Field-B is 23 meters, and in Field-A, it is 61 meters. The number of thin sections is 67 for Field-B and 265 for Field-A. The number of XRD analyses for well A is 28, and for well B, it is 12. The Embry and Klovan (1971) classification is used for the textural nomenclature of carbonate rocks. For facies analysis and reconstruction of the depositional environment, the standard model by Flügel (2013) is applied. The sequence stratigraphic division is based on the identification of certain significant surfaces (i.e., sequence boundaries and maximum flooding surfaces) that act as timelines in creating the chronostratigraphic framework of the target formation (Vail et al. 1977). In this study, the Transgressive-Regressive (T-R) sequence stratigraphy method is adopted for the depositional sequences of the Sarvak Formation. This method was mainly introduced by Curray (1964) and later developed by Embry and Johannessen (1993) and Embry (1993; 1995). Discussion of Results & Conclusions The Sarvak Formation was deposited in a carbonate ramp setting, which includes facies belts of the inner ramp, middle ramp, and outer ramp. Transitional facies between the inner and middle ramp areas are separated by the fair-weather wave base (FWWB). High-energy shoal complexes in the inner ramp are recorded in the Sarvak Formation (MF-5 and MF-6). These complexes are represented by grainstones and packstones containing benthic foraminifera, peloids, and skeletal fragments. The allochems of the shoal facies are mainly rudist debris and other bivalves, with less frequent fragments containing echinoderms, foraminifera, and peloids. The middle ramp facies assemblage encompasses a wide range of shallow to relatively deep marine facies, including mud to grain-supported (mudstone/wackestone to packstone) facies with planktic and benthic fossil assemblages (MF-1 and MF-7). Large benthic foraminifera and skeletal fragments (mainly bivalves and echinoderms) characterize packstones to wackestones attributed to the proximal middle ramp, while foraminifera (benthic and planktic) and skeletal fragments (mainly echinoderms, bryozoans, and red algae) characterize mudstones to wackestones in the distal middle ramp. Peloids and intraclasts are significant non-skeletal grains in these facies. The deep marine facies assemblage, including the outer ramp and basinal facies, forms a substantial part of the Upper Cretaceous carbonate sequences in the studied wells. They are recorded as low-energy, mud-dominated facies (mudstone to wackestone) where planktic organisms (such as planktic foraminifera and oligosteginids) are the dominant components (MF-2, MF-3, and MF-8). The outer ramp facies of the Sarvak Formation are primarily recorded as oligosteginid-rich and planktic foraminifera-rich (such as Hedbergella sp., Whitinella sp., Globigerinelloides sp., and Rotalipora sp.) mudstones to wackestones associated with middle ramp facies (distal parts). Diagenetic studies have shown that the Sarvak Formation has undergone marine diagenetic environments, meteoric diagenesis, followed by shallow and deep burial diagenesis. Based on the textural relationships of the diagenetic features and with the assistance of previous studies on the Sarvak Formation, the paragenetic sequence of the diagenetic characteristics of this formation is constructed. Due to local and regional sea level fluctuations during the Cenomanian-Turonian, the Sarvak Formation has experienced a complex diagenetic history in various parts of the Zagros region (Mehrabi and Rahimpour-Bonab 2014). Palaeoexposure surfaces had a major control on the diagenetic evolution of the Sarvak Formation (Rahimpour-Bonab et al. 2013; Hajikazemi et al. 2017; Mehrabi et al. 2022a, b). Depending on the presence or absence of palaeohighs (such as salt domes and horst structures), the number of these palaeoexposure surfaces and the intensity of the associated diagenetic alterations vary across different parts of the Zagros region. The four main stages of diagenetic evolution of the Khatiyah and Mishrif members of the Sarvak Formation are as follows: Stage 1: Marine transgression, sedimentation, and marine diagenesis of the Khatiyah Member in the early to mid-Cenomanian. Stage 2: Highstand of sea level, sedimentation, and marine diagenesis of the Mishrif Member in the mid- to late-Cenomanian. Stage 3: Relative sea level fall, emergence of the platform, exposure and meteoric diagenesis of the Mishrif Member at the Cenomanian-Turonian boundary. Stage 4: Deposition of shale facies in a lacustrine environment (Laffan Shale) during this stage. The diagenetic processes of the Sarvak Formation can be divided into two categories: 1) Diagenetic processes related to discontinuities, and 2) Diagenetic processes unrelated to discontinuities. Processes related to discontinuities represent a meteoric diagenetic environment and indicate exposure surfaces, such as extensive dissolution termed karstification, meteoric cementation, formation of paleosols, formation of pisoid horizons, dissolution-related brecciation and collapse, iron oxide staining, silicification, and neomorphism. Diagenetic processes unrelated to discontinuities include micritization, bioturbation, burial dolomitization, mechanical and chemical compaction, pyritization, and burial or marine cementation. Significant variations in the thickness of the Sarvak Formation from different parts of the Zagros sedimentary basin have previously been measured and reported by researchers (Mehrabi et al. 2015b). Additionally, remarkable changes in the nature of the facies and depositional sub-environments of the Sarvak Formation have also been reported from various regions of the Zagros (Esrafili-Dizaji et al. 2015). Similarly, such changes are clearly observable in terms of the thickness and facies of the Sarvak Formation in the eastern and western parts of the Persian Gulf. In the eastern Persian Gulf, salt domes have created the most significant tectonic structures, often associated with the formation of hydrocarbon fields in this area. In contrast, in the western part of the Persian Gulf, the pre-existing basement faults and uplifted structures (such as the Kharg and Mish anticlines, and the Hendijan–Bahregansar structures) are the most significant structural features and have formed the main traps for the accumulation of oil and gas in fields like Hendijan, Bahregansar, etc. (Kazem Shiroodi et al. 2015; Mohammadrezaei et al. 2020). On these pre-existing highs, the thinnest thickness of the Sarvak Formation has been recorded, and shallow-water facies dominate over them, indicating a shallower depositional environment on these highs. The changes of sedimentary facies in relation to these structures reflect their activity during the deposition of the Sarvak Formation in the eastern and western parts of the Persian Gulf. سازند سروک‌ یکی از مهم‎‍ترین مخازن هیدروکربوری ایران، تحت شرایط فعال زمین‎‍ساختی در نواحی مختلف زاگرس، از‌جمله خلیج‌فارس نهشته شده و تکامل یافته است. مطالعۀ حاضر به‌دنبال بررسی آثار ناشی از فعالیت‎‍های زمین‎‍ساختی بر خصوصیات رخساره‎‍ای و تاریخچۀ تحولات دیاژنزی سازند سروک در نواحی شرقی و غربی خلیج‌فارس بوده است. نتایج مطالعات رخساره‎‍ای حاکی از نهشته‌شدن سازند سروک در یک پلاتفرم کربناتۀ نوع رمپ است. در بخش شرقی خلیج‌فارس، رخساره‎‍های رسوبی عضو میشریف (سروک بالایی) اساساً در بخش‎‍های درونی پلاتفرم (لاگون، شول، واریزه‎‍های ریفی) و رخساره‎‍های عضو خاتیا (سروک پایینی) عمدتاً در بخش‎‍های عمیق و بیرونی پلاتفرم (رمپ بیرونی) نهشته شده‎‍اند. در بخش غربی خلیج‌فارس، سازند سروک طیفی از رخساره‎‍های کم‌عمق تا عمیق را در بر می‌گیرد که فراوانی رخساره‎‍های دریای باز (رمپ میانی و رمپ بیرونی) بسیار چشمگیرترند. دو سکانس رسوبی رده‌پایین (رده‌چهارم) و یک سکانس رسوبی رده‌بالا (رده‌سوم) در این سازند شناسایی شده‎‍اند که انطباق بالایی را با دیگر نواحی زاگرس و صفحۀ عربی نشان می‎‍دهند. آثار دیاژنزی مرتبط با یک سطح رخنمون‌یافتگی قدیمه در بالاترین بخش سازند سروک، در هر دو ناحیۀ شرقی و غربی خلیج‌فارس مشاهده شده‎‍اند. انحلال جوی، گسترش افق‎‍های پیزوئیدی و آغشتگی به اکسیدهای آهن، از‌جمله مهم‎‍ترین فرآیندهای دیاژنزی مرتبط با این سطوح رخنمون‌یافتگی دیرینه‌اند. ضخامت سازند سروک تغییرات درخور توجهی را در بخش‎‍های مختلف خلیج‌فارس نشان می‎‍دهد. همچنین رخساره‎‍های رسوبی این سازند نیز‌ تغییرات جانبی درخور توجهی در ناحیۀ‌ مطالعه‌شده دارند. تأثیرپذیری ضخامت و تغییرات ماهیت رخساره‎‍های رسوبی سازند سروک در ارتباط با ساختارهای پی‎‍سنگی، حاکی از فعالیت آنها در زمان نهشت‌ این سازند در خلیج‌فارس‌اند و این وقایع بالاآمدگی زمین‌ساختی، اثر زیادی بر روند تاریخچۀ تدفین سازند سروک گذاشته‎‍اند. برآیند این خصوصیات رخساره‎‍ای و دیاژنزی در‌نهایت، کیفیت مخزنی سازند سروک را در نواحی‌ مطالعه‌شده تحت تأثیر قرار داده است.

https://jssr.ui.ac.ir/article_28445_fdb76424138aaf89458e597252dd419a.pdf

دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 40 2 2024 06 21 Biostratigraphy of the Gurpi Formation in Chenareh section based on calcareous nannofossils زیست‌ چینه‌نگاری سازند گورپی در برش تاقدیس چناره بر‌اساس نانوفسیل‌های آهکی 59 86 28603 10.22108/jssr.2024.141481.1289 FA زینب بیات دانشجوی دکتری چینه‌نگاری و دیرینه‌شناسی، گروه حوضه‌های رسوبی و نفت، دانشکدۀ علوم زمین، دانشگاه شهید بهشتی، تهران، ایران 0009-0002-7245-7918 انوشیروان کنی دانشیار، گروه حوضه‌های رسوبی و نفت، دانشکدۀ علوم زمین، دانشگاه شهید بهشتی، تهران، ایران جهانبخش دانشیان استاد، دانشکدۀ علوم زمین، دانشگاه خوارزمی، تهران، ایران Journal Article 2024 05 20 Abstract The Gurpi Formation was measured and sampled in a stratigraphic section located on the northern limb of the Chenareh anticline to the north of Andimeshk. The thickness of the Gurpi Formation in this section is 288.5 meters and its lower boundary with the Ilam Formation is abrupt while its upper boundary with purple shales of the Pabdeh Formation is gradual and continuous. Paleontological studies led to the identification of 67 species belonging to 45 genera from 20 families of calcareous nannofossils, which made it possible to recognize CC18 to CC26 biozones from Sissingh standard classification scheme and NP1 to NP5 biozones from Martini standard classification scheme equivalent to biozones CNP1 to CNP7 from Agnini et al.. Based on the identified biozones, the duration of deposition of the Gurpi Formation was determined to be early Campanian–Selandian. In this section, the Emam Hassan Limestone Member was recognized with a thickness of 112.8 meters having been deposited during the early Maastrichtian. Both the lower and the upper boundaries of the Emam Hassan Limestone Member within the Gurpi Formation are continuous and gradual. The position of the Campanian–Maastrichtian, Maastrichtian–Danian (Cretaceous–Paleogene) and Danian–Selandian transitions in the Gurpi Formation were also determined in the studied section. Keywords: Gurpi Formation, Chenareh anticline, Calcareous nannofossils, Cretaceous–Paleogene transition Introduction The Gurpi Formation was deposited in a deep sedimentary basin during the Late Cretaceous–Paleocene in the geological zone of Zagros. The type section of the Gurpi Formation in Mount Gurpi located in Khuzestan province, consists of 320 meters of marls and bluish-gray shales and subtly includes thin layers of clayey limestones (Stöcklin and Setudehnia 1970). The Gurpi Formation has two formal members, Emam Hassan Limestone (light gray and weathered white) and Seymareh Limestone (brown in appearance), and an informal member called Mansuri limestone (Motiei 1993). The Type section of the Emam Hassan Limestone Member in Mount Emam Hassan, is located in Lorestan province and includes 114 meters of clayey limestones with interlayers of marls. The Emam Hassan Member is spread only in Lorestan and part of Khuzestan (Stöcklin and Setudehnia 1970). Calcareous nannofossils are one of the most powerful tools used in the biostratigraphy of the Mesozoic (especially Cretaceous) and Cenozoic rocks, and they have always been emphasized in the Geologic Time Scale (GTS) along with foraminifera and ammonites. In many cases, in sediments with soft lithology such as the Gurpi Formation, using calcareous nannofossils, it leads to achieving a very clear biostratigraphic classification. The age of the Gurpi Formation in Lorestan and part of Khuzestan is from Campanian to Paleocene and in Fars and another part of Khuzestan from Santonian to Maastrichtian. The Emam Hassan Limestone Member was deposited during the Maastrichtian period (Stöcklin and Setudehnia 1970). In the Lorestan subzone, recent paleontological studies in the Gurpi Formation often confirm the Campanian–Paleocene age range. But some studies also mentioned Santonian (Hadavi and Rasaizadi 2009; Solgi 2015; Maghfouri Moghaddam 2015; Taherzadeh 2018; Mahmoodian 2019; Mohammadian 2019). The purpose of this study is to investigate the lithostratigraphy and biostratigraphy (based on calcareous nannofossils) and finally to determine the age of the Gurpi Formation in the Chenareh section. Material & Methods Chenareh geological section is located in the Chenareh anticline, within theLorestan subzone of the Zagros Basin (Fig. 1). The geographic location of this section is between two provinces of Lorestan and Khuzestan. Due to the lack of a complete succession outcrop, this section was taken as a composite section: The coordinates of the base and the top of the first part of the Chenareh section are respectively 32° 54ʹ 03ʹʹ N and 48° 06ʹ 47ʹʹ E and 32° 54ʹ 32.45ʹʹ N and 48° 05ʹ 57ʹʹ E (from base of the section to 279 meters). The coordinates of the base and the top of the second part of the section are respectively 32° 53ʹ 86.22ʹʹ N and 48°08ʹ 52.17ʹʹ E and 32° 53ʹ 89.22ʹʹ N and 48° 08ʹ 08.18ʹʹ E (top final 21 meters). In total 194 samples were collected from 300 meters of thickness in this section, the first three samples (0– 1 meter level of the section) belong to the Ilam Formation and the last eight samples (in the last 10.5 meters) belong to the Pabdeh Formation (Purple Shale Member). A total of 187 samples have been prepared for the study of nannofossils by the gravity-settling method of Bown and Young (1998). The prepared slides were studied by a Nikon Optiphot II Pol optical microscope equipped with a Nikon D-3300 digital camera at 1000x magnification. In order to identify calcareous nannofossils, a combination of references such as Perch-Nielsen (1985a and b), Young and Bown (1997), Young et al. (1997), Burnett (1998) and the Nannotax website were used. Biostratigraphic classification for the Late Cretaceous is based on the standard scheme of Sissingh (1977) revised by Perch-Nielsen (1985a) (CC) and for the Paleocene is based on the two schemes of Martini (1971) revised by Perch-Nielsen (1985b) (NP) and Agnini et al. (2014) (CNP). Discussion of Results & Conclusions The Gurpi Formation in the Chenareh section (288.5 meters thick) can be divided into seven lithological units and is predominantly composed of shales (including units 2, 3, 4, 6 and 7) interbedded with some argillaceous limestone layers and subordinate marls (units 1, 2) (Fig. 2). Unit 5 or the Emam Hassan Limestone Member is 112.8 meters thick. According to the 1:100,000 geological map of Balarud (Sahabi and Macleod, 1969), the Gurpi Formation is covered by the Amiran Formation on the northern limb of the Chenareh anticline, and the Pabdeh Formation on its southern limb. Despite the presence of the Chenareh section on the northern limb, the top of the Gurpi Formation is definitely overlain by the Purple Shale Member of the Pabdeh Formation (Figs. 1 and 3). The time of deposition of the Gurpi Formation was determined early Campanian up to the earliest part of the Selandian (the earliest part of NP5 biozone). The Emam Hassan Limestone Member was determined to have the earliest Maastrichtian age range (CC23b to CC25a). The last part of the Ilam Formation (last 1 m) was assigned to the middle of the early Campanian (biozone CC18a) and the beginning part of the Pabdeh Formation was assigned to the Selandian (NP5). According to the field observations, the lower boundary of the Gurpi Formation with Ilam Formation is sharp while the upper boundary with the Purple Shales of the Pabdeh Formation is continuous and gradual. Based on the laboratory studies, the transition of the Ilam to Gurpi formations is ambiguous, but the boundary of Gurpi to Pabdeh formations is chronologically continuous. The lower and upper boundaries of the Emam Hassan Limestone Member within the Gurpi Formation based on both field observations and laboratory-obtained data are continuous and gradual. Three stratigraphic transitions of Campanian–Maastrichtian, Maastrichtian–Danian (Mesozoic–Cenozoic) and Danian–Selandian were identified in the Chenareh section. The Campanian–Maastrichtian transition (Maastrichtian base) is placed within the immediate limestone-shale sequence of the base of the Emam Hassan Member and between the two events of the last appearance of A. parcus and the last appearance of Q. trifidum (closer to the last appearance of A. parcus). The Maastrichtian–Danian transition (Danian base) was identified by the first abundant appearance of the genus Thoracosphaera at the boundary between the shale-limestone sequence (unit 6) and the dark grey shales (unit 7). Thus, the first sample of the dark shales of unit 7 is attributed to the Danian age (NP1). The Danian–Selandian transitions (base of Selandian) was determined based on the first appearance of Fasciculithus ulii and F. billii, F. janii, F. pileatus within the dark grey shales of unit 7 (the last sample in unit 7) and immediately before the beginning of the base of the Purple Shales (Fig. 6). نهشته‌های سازند گورپی در برشی واقع در یال شمالی تاقدیس چناره در شمال اندیمشک، نمونه‌برداری و مطالعه شد. ضخامت سازند گورپی در این برش 5/288 متر است. در این برش مرز زیرین سازند گورپی با سازند ایلام به‌صورت ناگهانی و مرز بالایی آن با شیل‌های ارغوانی سازند پابده به‌صورت پیوسته و تدریجی مشاهده شده است. مطالعات فسیل‌شناسی به شناسایی 67 گونه متعلق به 45 جنس از 20 خانواده از نانوفسیل‌های آهکی منجر شد که امکان تبیین بیوزون‌های CC18تا CC26 از طبقه‌بندی استاندارد Sissingh 1977 و بیوزون‌های استاندارد NP1 تا NP5 از طبقه‌بندی Martini 1971 معادل با بیوزون‌های CNP1 تا CNP7 از Agnini et al. 2014 را فراهم آورده است. بر پایۀ بیوزون‌های شناسایی‌شده، بازۀ زمانی ته‌نشست سازند گورپی در برش چناره کامپانین پیشین- سلاندین تشخیص داده شد. در این برش عضو آهک امام حسن با ضخامت 8/112 متر و دارای محدودۀ سنی مائستریشتین پیشین تعیین شد. مرز زیرین و بالایی عضو آهک امام حسن درون سازند گورپی هر دو، پیوسته و تدریجی است. موقعیت گذر کامپانین- مائستریشتین، مائستریشتین- دانین (کرتاسه- پالئوژن) و دانین- سلاندین در سازند گورپی در برش مطالعه‌شده،‌ قابل رهگیری است.

https://jssr.ui.ac.ir/article_28603_25c8e85adb90215436e3e61c5cb82336.pdf

دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 40 2 2024 06 21 Biostratigraphy and palaeoecology of Ostracodes of the Members c1 to c4 and e of the Qom Formation in the South of Shurab, Southeast of Qom بیوستراتیگرافی و پالئواکولوژی استراکدهای بخش‌هایc1 تاc4 و eسازند قم در جنوب شوراب، جنوب شرق قم 87 112 28577 10.22108/jssr.2024.141330.1286 FA مریم گودرزی دانشجوی دکتری چینه و فسیل‌شناسی، گروه حوضه‏ های رسوبی و نفت، دانشکدة علوم زمین‌، دانشگاه شهید بهشتی، تهران، ایران عباس صادقی استاد، گروه حوضه ‏های رسوبی و نفت، دانشکدۀ علوم زمین‌، دانشگاه شهید بهشتی‌، تهران، ایران 0000-0002-5515-0781 محمود جلالی دکتری چینه و فسیل‌شناسی، مدیریت اکتشاف، شرکت ملی نفت ایران‌، تهران نسرین هداوندخانی دکتری چینه و فسیل‌شناسی، دانشکدۀ علوم زمین، دانشگاه شهید بهشتی، تهران، ایران مریم اختری کارشناس ارشد، مدیریت اکتشاف، شرکت ملی نفت ایران‌، تهران Journal Article 2024 05 05 Abstract: In order to study the Qom Formation, the ostracodes of members c1 to c4 and e of this formation were studied in the south Shurab section. The studied strata in this section have a significant abundance and diversity of ostracods. Changes in the abundance and diversity of species in the ostracodes of the studied members show that Member c2 was deposited in a shallower environment compared to the middle and upper part and the base of Member c3. The high abundance of psychrophilic species at the base of Member c4 and the presence of thermophilic forms at the top of Member c4 indicate that the basin was deeper at the time of deposition of the base Member c4 sediments than at its top. Also, the visible changes in the ostracods collection in marly Member e indicate frequent fluctuations in the depth of the environment during the deposition of this member in the studied area. In general, it can be inferred that the environment of members c1 to c4 and e of the Qom Formation was a warm with moderate salinity sea water. In the biostratigraphic studies, 42 species belonging to 36 genera of ostracoda and four biozones have been identified. Based on the biostratigraphic studies, the Chattian age was determined for the sediments of the top of Member b and c1, the Aquitanian for the sediments of members c2 to c4, and the Burdigalian for the sediments of Member e of the Qom Formation in the Shurab section was determined. Keywords: Qom Formation, Ostracoda, South of Shurab, Biostratigraphy, Palaeoecology Introduction The Qom Formation is a significant geological unit due to its hydrocarbon resources, making it a focus of geological studies. This formation in its extensive area in central Iran, within the Sanandaj-Sirjan and Urumieh-Dokhtar zones exhibits remarkable changes both laterally and vertically, making it a subject of geological interest both in terms of surface outcrops and subsurface sections. Diverse fossil contents and significant facies changes have prompted stratigraphic researchers to investigate the Qom Formation based on its available fossil contents, in order to understand more about this formation, and also solve existing problems and ambiguities , and complete its study chain. In this regard, one of the fossil groups with significant diversity and abundance in the Qom Formation is the ostracods, which has received less attention and few studies have been conducted on it. The significance of studying the Qom Formation from one side and the importance of ostracodes as a useful tool in paleontological studies, particularly for determining the age of different parts of the Qom Formation and identifying its palaeoenvironment, on the other hand, has led this research to deal with biostratigraphic and palaeoecological studies of members c1 to c4 and e of the Qom Formation in the south Shurab section, located southeast of Qom based on ostracod findings in the area. Material & Methods In this study, research was conducted in two phases: fieldwork and laboratory work (sample preparation and ostracod identification using a stereomicroscope). A total of 255 samples were collected and examined from members c1 to c4 and e of the Shurab section. Sample preparation by washing method included the use of 35, 70, 80, and 120 mesh sieves. Separation of ostracods from the sediments was performed using a stereomicroscope, which led to the identification of 2685 ostracodes, including 42 species belonging to 36 genera from 12 families and two suborders. After identifying the ostracodes, their range chart was drawn, and then, - the ages of members c1 to c4 and e of the Qom Formation in the Shurab section were determined along with their biostratigraphic zones.. In the palaeoecology study, counting and statistical analysis of ostracod species was done so that the number of species present in each sample was identified separately and the percentage of abundance of each species in relation to the total identified ostracodes was calculated. Finally, the palaeoenvironment of the studied members of the Qom Formation in the south of the Shurab section was predicted based on ostracodes. Discussion of Results & Conclusions The Qom Formation with a thickness of 544 meters is outcropped in the Shurab stratigraphic section located in the southeast of Qom. In this section, members c1, c2, c3, c4, and e respectively with thicknesses of 30, 113.5, 13 and 19 meters, and Member e with a thickness of 49.8 meters were subjected to biostratigraphic and palaeoecological studies. Based on the identification of 2685 ostracodes obtained from 255 samples from the mentioned members, while recognizing 42 species belonging to 36 genera from two suborders in the Podocopida order, the following 4 biozones are identified and introduced. assemblage zone Cyamocytheridea reversa -4- Cytherella vulgate assemblage zone Cytheridea Josephina-3- Neonecidea gerda 2- Quadrocytheris symmetric- Actinocythereis rosefieldensis assemblage zone 1-Cytheretta (Flexus) trifurcate - Cytherella obesa assemblage zone Among the mentioned biozones, biozone 1 belongs to the top of Member b, biozones 2 and 3 belong to members c2 to c4, and biozone 4 belongs to Member e. In Member c1, due to the presence of dense limestone facies and the impossibility of separating ostracodes, the biozones were not introduced. On the basis of the identified biozones and the ostracodes available there, the Aquitanian age for members c2 to c4, and the Burdigalian for Member e is suggested. For sediments at the top of Member b, based on its existing ostracodes, and for Member c1 based on its stratigraphic position and regional studies, the age of the Chattian was determined. Palaeoecological studies of the existing ostracodes in the upper sediments of the members b to e, significant changes in abundance and diversity are seen under the influence of changes in depth, temperature and salinity. These changes show that the environment of the formation of the above-mentioned members had significant fluctuations in depth, so that the presence of thermophilic genera belonging to shallow areas, such as Actinocythereis, at the top of Member b and the base of Member c2. The genera belonging to deeper areas, such as Cytheropteron, in the middle to the end of Member c2 and the marl base of Member c3, indicate the changes in depth from the shallow depth at the top of Member b and the base of Member c2 to a greater depth in the middle part of c2 to the base of c3. This deepening is also evident at the base of Member c4, so that psychrophilic genera such as Cytherella, Krithe and Paracypris have a significant abundance in this part, while towards the top of Member c4, the abundance of psychrophilic genera decreases and thermophilic genera belonging to shallower environments, such as Pontocypris, Actinocythereis, Propontocypris, increase and generally show a shallowing towards the top of this member. In Member e, the changes in the abundance and diversity of ostracodes indicate fluctuations in the water depth at the time of the formation of this member, so that at the beginning of Member e, the abundance of thermophilic species indicates the establishment of a shallow environment in the continuation of the evaporative environment of Member d. But this gradually upwards with the increase of the abundance of psychrophilic genera such as Krithe, Cytherella, and Paracypris, and the environment has became deeper. Similarly, in the second half of Member e, as the the depth decreased, thermophilic species initially increased, and finally as the depth increased, psychrophilic species became more abundant. The ostracodes identified in the studied members show that the most abundant during the deposition of the mentioned succession belonged to thermophilic species. Also, the salinity studies show that the most abundant of species in the aforementioned members belong to the species related to the marine environment with normal salinity. به‌منظور بررسی بیوستراتیگرافی و پالئواکولوژی سازند قم‌، استراکدهای بخش‌هایc1 تاc4 و eسازند قم در برش جنوب شوراب،‌ مطالعه شده است. نهشته‌های مطالعه‌شده در این برش، از فراوانی و تنوع درخور توجهی در استراکدها برخوردارند‌.‌ تغییرات فراوانی و تنوع گونه‌ای در استراکدهای بخش‌های‌ مطالعه‌شده نشان می‌دهد که قاعدۀ بخش c2 در محیط کم‌عمق‌تری نسبت‌به قسمت میانی و بالایی آن و قاعدۀ بخش c3نهشته شده است. فراوانی بالای جنس‌های سرمادوست در قاعدۀ بخش c4 و حضور فرم‌های گرما‌دوست در رأس بخش c4، حاکی از عمق بیشتر حوضه در زمان نهشته‌شدن رسوبات قاعدۀ c4 نسبت‌به رأس آن بوده است. همچنین تغییرات‌ مشاهده‌شده در مجموعۀ استراکدهای موجود در بخش مارنی e، حاکی از نوسانات مکرر عمق محیط در زمان شکل‌گیری نهشته‌های بخش e در این ناحیه است. در‌مجموع با شواهد به دست آمده در این تحقیق، چنین استنباط می‌شود که با وجود نوسانات موجود در شاخص‌هایی چون عمق، درجه‌حرارت و شوری‌، محیط تشکیل بخش‌های c1 تاc4 وe سازند قم در محدودۀ‌ مطالعه‌شده، محیطی گرم با شوری متعادل بوده است. در مطالعات بیوستراتیگرافی، ضمن تشخیص 42 گونه متعلق به36 جنس از استراکدها، چهار زون زیستی شناسایی شده است که براساس آنها سن شاتین برای رسوبات رأس بخش b و بخش c1، اکیتانین برای نهشته‌های بخش‌های c2 تاc4 و بوردیگالین برای نهشته‌های بخش e سازند قم در برش شوراب تعیین شد.

https://jssr.ui.ac.ir/article_28577_08b426e840a1359bc860eb02b2c8c77f.pdf