بررسی رخساره‌ها، محیط رسوبی، چینه‌نگاری سکانسی و فرایندهای دیاژنزی موثر بر نهشته‌های سازند دورود در برش ارم (جنوب شهرستان نکا) در دامنه شمالی البرز مرکزی

نهشته‌های پرمین (سازند دورود) در گستره وسیعی از ناحیه البرز مرکزی رخنمون دارند. در این پژوهش یک برش چینه‌شناسی از نهشته‌های پرمین در برش ارم (جنوب شهرستان نکا) برای بررسی رخساره‌ها، محیط رسوبی، چینه‌نگاری‌ سکانسی و فرایندهای دیاژنزی موثر بر آنها انتخاب شده است. ضخامت سازند دورود در این برش، 320 متربوده و به‌طور عمده از تناوب سنگ آهک و ماسه‌سنگ تشکیل شده است. بررسی‌های صحرایی و پتروگرافی مقاطع نازک میکروسکوپی، امکان تفکیک 4 رخساره آواری و 3 رخساره کربناته را میسر کرده است. این رخساره‌ها در چهار کمربند رخساره‌ای رودخانه‌ای، ساحلی، لاگون و تپه‌های ماسه‌ای‌ زیرآبی نهشته شده‌اند. تنوع کم رخساره‌ها و تدریجی‌بودن روند تبدیل کمربندهای رخساره‌ای به یکدیگر، نبود نهشته‌های توربیدایتی و نیز نبود آثار ریف‌های بزرگ سدی نشان می‌دهد بخش‌های آواری نهشته‌های پرمین در ناحیه بررسی‌شده در محیط ساحلی و رودخانه‌ای و بخش‌های کربناته در یک پلتفرم کربناته از نوع رمپ با شیب یکنواخت ته‌نهشت یافته‌اند. در توالی مطالعه‌شده از سازند دورود، دو سکانس رسوبی درجه 3 شناسایی شده است. در ناحیه بررسی‌شده، فرایندهای گوناگون دیاژنز بر توالی مطالعه‌شده این سازند موثر بوده که مهم‌ترین آنها، دولومیتی‌شدن، سیمانی‌شدن، فشردگی مکانیکی و شیمیایی و پیریتی‌شدن، شکستگی و نوشکلی است.

Microfacies, depositional environment, sequence stratigraphy and diagenetic process affected the Doroud Formation in the Eram Section (south Neka), north flank of central Alborz

Abstract The Permian deposits (Doroud Formation) are exposed in a wide area of central Alborz. A stratigraphic section of Permian deposits in the Eram section (South Neka) has been selected for the study of facies, depositional environments, sequence stratigraphy, and diagenetic processes. The 320 m thick Doroud Formation in the study area mainly consists of intercalation of limestones and sandstones. Field and petrographic studies of thin sections led to identifying four siliciclastic petrofacies and three carbonate microfacies. These facies are deposited in four facies belts, including fluvial, coastal, lagoon, and shoal. The low diversity of facies and the gradual change of facies belts, the absence of turbidites, and the absence of large barrier reef indicate that the Permian deposits in the study area were deposited on a carbonate ramp. Also, based on the absence of turbidite deposits of a deep marine environment, it can be deduced that the depositional environment of the Doroud Formation is homoclinal. Two thirdorder depositional sequences have been identified in the studied interval of the Doroud Formation. Different diagenetic processes have been affected on the studied interval in the study area, most notably dolomitization, cementation, mechanical and chemical compaction, pyritization, fracturing, and neomorphism. Keywords: Doroud Formation, Facies, Sedimentary environment, Diagenesis, Permian, Alborz   Introduction The Alborz zone is one of the important geological zones of Iran, which is spread eastwest in the northern part of Iran and extends from Azerbaijan to Khorasan (Aghanabati 2004). The study of the rock units of the Permian system in Iran has been done by Assereto (Assereto 1963) and Glaus (Glaus 1964), so that the type section of Doroud and Ruteh formations was first introduced by Assereto (Assereto 1963). During the Permian, carbonate sediments belonging to shallow marine environments were mainly composed of small and large benthic foraminifera along with green and red algae (Vachard et al. 2001; 2002; 2011; BoudagherFadel 2008; Parvizi et al. 2013). The studied stratigraphic section is located in the south of Neka city and the west of Eram village and is located in the geographical position of 36◦ 25’ 28 north latitude and 53◦ 36’ 42 east longitude in the northern flank of Alborz. The lower and upper boundaries of Doroud Formation in the Eram section with Mobarak and Ruteh formations are in the form of erosive discontinuities. Sofar, numerous paleontological, sedimentological, and stratigraphic studies have been studied on the Doroud, Ruteh and Nesen formations, especially in the western and central Alborz and to a lesser extent, the eastern Alborz. In this research, the determination of the palaeoenvironmental model, sequence stratigraphy, and diagenesis processes of the Doroud Formation in the Eram section are the main targets.   Materials & Methods This research is conducted in two stages including field and laboratory. Overall the number of 26 hand specimens collected from 320 meters thickness of the Doroud Formation in the Eram stratigraphic section. The petrographic analysis was performed by a polarizing microscope. Pettijohn classification (1987) and Dunham classification (Dunham, 1962) have been used to name clastic and carbonate rocks, respectively. The investigation of the sedimentary environment and facies types are based on the study of their lateral and vertical changes, and comparison with modern and ancient environments (Flugel 2010). The combination of the results obtained from field and laboratory studies in the study section has led to the identification and description of the main carbonate and clastic facies. Sequence stratigraphic studies have been done by considering methods described by Hunt and Tucker (Hunt and Tucker 1992).   Discussion of Results & Conclusions The Doroud Formation in Eram section with a thickness of 320 meters is one of the remarkable outcrops of Lower Permian deposits in the northern flank of Central Alborz. Field observations and detailed petrographic examination of samples of the Doroud Formation in the Eram section led to the identification of seven siliciclastic and carbonate facies types (subarkose, chertarnaite, mixed calcareous sandstone, imperforate foraminifera packstone, oncoids packstonegrainstone, bioclast grainstone, and ooid grainstone) belonging to the four facies belts of the fluvial, coastal, lagoon, and sand shoal (Wilson 1975, Lasemi; 1990, Flugel 2010). The low diversity of facies and the gradual change of facies belts, the absence of turbiditic deposits, and the absence of large barrier reef indicate that the Permian deposits in the study area were deposited on a carbonate ramp. In this study, two thirdorder depositional sequences have been identified in the studied interval of the Doroud Formation. The most important diagnostic diagenetic processes affecting the siliciclastic and carbonate deposits of Doroud Formation can be micritization, boring, cementation, neomorphism, mechanical and chemical compaction, dolomitization and pyritization, silicification and fracturing. According to the field and petrographic studies on Permian deposits in the study area, it can be concluded that the deposits have gone through three stages of eugenics, mesogenesis, and telogenesis diagenetic environments.