دانشگاه اصفهان پژوهش های چینه نگاری و رسوب شناسی 2008-7888 42 3 2026 09 23 Miocene deposits of the folded Zagros (Asmari Formation, Lorestan Region) ریزرخساره‎‍ها، محیط رسوبی و توالی‎‍های دیاژنزی نهشته‎‍های الیگوسن- میوسن زاگرس چین‎‍خورده (سازند آسماری، پهنۀ لرستان) 23 52 30376 10.22108/jssr.2026.147601.1323 FA محسن آشنا دانشجوی دکتری گروه زمین‎‍شناسی، واحد خرم‎‍آباد، دانشگاه آزاد اسلامی، خرم‎‍آباد، ایران کبری میربیک سبزواری استادیار گروه زمین‎‍شناسی، واحد خرم‎‍آباد، دانشگاه آزاد اسلامی، خرم‎‍آباد، ایران محمدصادق دهقانیان دانشیار گروه زمین‎‍شناسی، واحد بندرعباس، دانشگاه آزاد اسلامی، بندرعباس، ایران سیدوحید شاهرخی دانشیار گروه زمین‎‍شناسی، واحد خرم‎‍آباد، دانشگاه آزاد اسلامی، خرم‎‍آباد، ایران سیامک بهاروند دانشیار گروه زمین‎‍شناسی، واحد خرم‎‍آباد، دانشگاه آزاد اسلامی، خرم‎‍آباد، ایران Journal Article 2025 12 13 <strong>Abstract</strong><br />In this study, the depositional and post-depositional history of carbonate rocks of the Asmari Formation in the Lorestan region was investigated. Three stratigraphic sections were analyzed: the northern Khorramabad section (92.3 m), the southeastern Khorramabad section (90 m), and the southwestern Khorramabad section (190 m). A total of 380 samples were collected for petrographic examination. The lower boundary of the Asmari Formation with the Shahbazan Formation is a conformable contact, while its upper boundary with the Gachsaran Formation represents a conformable unconformity. Thirteen microfacies were identified, including various dolomudstones, wackestones, packstones, grainstones, and rudstones containing different types of foraminifera, echinoderms, bryozoans, coralline algae, and anhydrite. These microfacies represent deposition on a carbonate ramp ranging from inner to mid and outer ramp settings. Several diagenetic processes were recognized, such as micritization, neomorphism, marine, meteoric, and burial cementation, dissolution, development of both fabric-selective and non-fabric-selective porosity, and replacement processes including pyritization, silicification, and dolomitization. Based on petrographic evidence, the paragenetic sequence of the Asmari Formation carbonates was interpreted to have formed in four diagenetic environments: marine, meteoric, burial, and uplift. These environments correspond to stages of early diagenesis (eogenesis), middle diagenesis (mesogenesis), and late diagenesis (telogenesis).<br /><strong>Keywords:</strong> Asmari Formation, depositional environment, diagenesis, Zagros, Lorestan<br /><strong> </strong><br /><strong> </strong><br /><strong>Introduction     </strong><br />The Asmari Formation, with its dominant carbonate lithology, is rich in benthic foraminifera of Oligocene-Miocene age. Therefore, using this valuable fossil content and other skeletal components in this formation, the type of its depositional environment can be determined according to the models presented by Flügel (2010) and Wilson (1986). The Asmari Formation deposits can be considered as the last widespread marine transgression in the Zagros Basin (Amiri Bakhtiar and Noraiyanejad 2022). In terms of age, it begins in the Oligocene and continues up to the Burdigalian of the Early Miocene (Motiei 2000). Based on its fossils and age, the Asmari Formation can be divided into three parts: Lower, Middle, and Upper Asmari (Motiei 2000). In most outcrops in Lorestan, the Lower Asmari limestones are deposited directly over the carbonate rocks of the Shahbazan Formation, which are of Late Eocene age. In more northern areas, the Lower Asmari is absent, and it appears that the interval from the Late Eocene to the Oligocene was subaerial. Toward the southern regions, the Late Eocene–Oligocene limestones of the Lower Asmari grade into the deeper-water shales of the Pabdeh Formation (Paleocene–Oligocene). From Lorestan toward the Dezful Embayment, the Lower Asmari limestones were deposited along the margin of the deeper Pabdeh basin. The study area is situated within the Zagros Fold-Thrust Belt, specifically in the Lorestan sedimentary province. Three stratigraphic sections were selected in this region for detailed analysisThe study of the Asmari Formation in the Lorestan sedimentary basin is highly important, primarily due to its role as one of the most significant carbonate hydrocarbon reservoirs in the Zagros Basin. The primary objective of investigating this formation in this part of the basin is to identify facies variations, reconstruct depositional environments, and understand its diagenetic evolution, thereby enabling the prediction of reservoir quality and porosity distribution across different segments of the basin. A precise analysis of the petrophysical properties and sequence stratigraphy of the Asmari Formation in Lorestan provides a framework to improve reservoir modelling, evaluate the processes controlling hydrocarbon migration and accumulation in the area, and reduce exploration risks during the drilling of development and appraisal wells.<br /> <br /> <br /><strong>Material & Methods    </strong><br />In this research, following multiple field visits and the study of the 1:250,000 scale geological maps of Khorramabad and Pol-e Dokhtar, three suitable outcrop sections of the Asmari Formation were selected within the Zagros Fold-Thrust Belt (Lorestan sedimentary basin). <strong>Section 1:</strong> Located on the northern limb of the Khorramabad َnticline, near Sarab-e Chenar village (Bastam area). The geographical coordinates are 48° 8’ 58" E longitude and 33° 41’ 36" N latitude. <strong>Section 2:</strong> Located southeast of Khorramabad, along the road to Sepiddasht. The geographical coordinates are 48° 13’ 10" E longitude and 33° 36’ 40" N latitude<strong>. Section 3: </strong>Located approximately 85 km south of Khorramabad (Varah-Zard village) and 11 km north of Pol-e Dokhtar. The geographical coordinates are 47° 43’ 2" E longitude and 33° 13’ 26" N latitude. These sections were chosen to ensure they exhibit maximum thickness, significant lithological variations, easy accessibility, and minimal cover. A total of 380 rock samples were collected from these sections: 90 samples from the northern Khorramabad section, 90 samples from the southeastern Khorramabad section, and 200 samples from the southern Khorramabad section. Thin sections were prepared from all samples for petrographic analysis. These thin sections were prepared at Lorestan University and studied and photographed using an Olympus BH2 polarizing microscope equipped with a Nikon D70 camera. Carbonate rock nomenclature followed the method proposed by Dunham (1962) and the modified scheme from Embry and Klovan (1971). The interpretation of microfacies and depositional environments was conducted based on the works of Flügel (2010) and Wilson (1975).<br /> <br /><strong>Discussion of Results & Conclusions </strong><br />In the three studied sections, 13 carbonate microfacies have been identified. These include: Anhydrite microfacies, fenestral dolomudstone, nodular dolomudstone, wackestone/packstone with extraclasts, mudstone to wackestone with bioturbation, wackestone/packstone with imperforate foraminifera, wackestone/packstone with imperforate and perforate foraminifera, wackestone/packstone with perforate foraminifera, bioclastic grainstone containing foraminifera, packstone to grainstone with echinoderms, bryozoans, and coralline algae, rudstone/floatstone containing foraminifera and coralline algae, floatstone/rudstone with coralline red algae, mudstone to wackestone with planktonic foraminifera. These microfacies were deposited within three distinct facies belts corresponding to a carbonate ramp: the tidal flat, lagoon, and open marine environments. Based on field evidence and the identified microfacies in the studied stratigraphic sections, the depositional environment of the Asmari Formation exhibits a continuous and gradual transition from the mudstone, wackestone, and packstone microfacies of the low-energy lagoonal environment associated with the inner ramp, to the grainstone facies of the high-energy lagoon at the beginning of the mid-ramp. This is followed by a gradual transition to packstone facies with elongated foraminifera and hyaline tests, and floatstone and rudstone microfacies of the distal mid-ramp, eventually leading to mudstone facies containing planktonic foraminifera characteristic of the open marine environment at the beginning of the outer ramp. The presence of evidence such as the abundance of non-porous benthic foraminifera with porcelains shells in a texture ranging from wackestone to packstone, red algae, weak to moderate sorting of allochems and supporting mud texture, the depositional environment of the Asmari Formation deposits in the stratigraphic sections is considered as a homoclinal carbonate ramp. Among the most important diagenetic processes in the studied sections are compaction, cementation, dissolution, porosity and replacement. Based on petrography evidence, the diagenetic sequence of the Asmari Formation occurred during three stages (eogenesis, mesogenesis, and telogenesis) and in four diagenetic environments (marine, burial, freshwater, and uplift).<br />Early Diagenesis (Eogenesis): This stage includes some diagenetic processes, such as micritization and syntaxial rim cement, which are characteristic of early diagenesis in marine environments, have been identified in the studied sections, confirming the initial diagenesis stage.<br />Freshwater Diagenesis: In the freshwater phreatic environment, intergranular pores are continuously filled with water, which can lead to the dissolution of metastable minerals like aragonite and high-magnesium calcite (Heydari and Wade 2014).<br />Intermediate Diagenesis (Mesogenesis): During this stage, sediments are subjected to pressure and temperature resulting from burial at various depths, continuing until the threshold of diagenesis is reached. Processes occurring in this stage include physical and chemical compaction, blocky and drusy cements, dolomitization, and pyritization, all of which have been identified in the studied samples. Chemical compaction leads to the formation of features resulting from pressure dissolution, such as stylolites.<br />Late Diagenesis (Telogenesis): During uplift, iron ions are introduced into the sediments by meteoric waters through fractures, forming hydrated iron oxides (oxidizing conditions), which gradually convert to hematite. Fractures and joints identified in the studied sections formed during this stage and are filled with freshwater cements. در این پژوهش، تاریخچۀ رسوب‎‍گذاری و پس از رسوب‎‍گذاری سنگ‎‍های کربناتۀ سازند آسماری در پهنۀ لرستان مطالعه شده است. سه برش چینه‎‍شناسی (برش شمال خرم‎‍آباد 3/92 متر، برش جنوب شرق خرم‎‍آباد 90 متر و برش جنوب غرب خرم‎‍آباد 190 متر) در راستای این هدف انتخاب و در‎‍مجموع 380 نمونه از این برش‎‍ها برای مطالعات سنگ‎‍شناسی برداشت شد. در این برش‎‍ها مرز زیرین سازند آسماری با سازند شهبازان به‎‍صورت ناپیوستگی هم‌شیب و مرز بالایی آن با سازند گچساران همساز است. در این توالی‎‍ها 13 ریزرخساره، شامل ریزرخساره‎‍های انیدریت، دولومادستون فنسترال، دولومادستون ندولار، وکستون/پکستون داری اکستراکلست، مادستون تا وکستون دارای آشفتگی زیستی، وکستون/پکستون دارای روزن‌داران بدون منفذ، وکستون/پکستون دارای روزن‌داران بدون منفذ و منفذدار، وکستون/پکستون دارای روزن‌داران منفذدار، گرینستون بایوکلستی حاوی روزن‌دار، پکستون تا گرینستون دارای روزن‌داران، اکینودرم، بریوزوئر و کورالین، رودستون/فلوتستون حاوی روزن‌دار و جلبک کورالیناسه‎‍آ، فلوتستون/رودستون بایوکلستی حاوی جلبک قرمز کورالین و ریزرخسارۀ مادستون تا وکستون داری روزن‌داران پلانکتونیک شناسایی شد که بیانگر محیط رسوبی از نوع رمپ کربناته‎‍اند. در این برش‎‍ها چندین فرآیند دیاژنزی ازجمله میکریتی‎‍شدن، نوریختی، سیمانی‎‍شدن (نوع دریایی، آب شیرین و تدفینی)، انحلال و تخلخل (وابسته به فابریک سنگ و غیر وابسته فابریک)‎‍ و جانشینی (پیریتی‎‍شدن، سیلیسی‎‍شدن و دولومیتی‎‍شدن) رخ داده است. براساس شواهد پتروگرافی، توالی پاراژنتیکی نهشته‎‍های سازند آسماری در این برش‎‍ها در چهار محیط دریایی، آب شیرین، تدفینی و بالاآمدگی تفسیر شده است. این فرآیندها مراحل دیاژنزی اولیه (ائوژنز)‎‍، دیاژنز میانی (مزوژنز) و دیاژنز نهایی (تلوژنز) را پشت سر گذاشته‎‍اند. https://jssr.ui.ac.ir/article_30376_ed41fb34491c2698d275684f26ec6494.pdf