ریزرخساره‌ها، شرایط رسوب‌گذاری و چینه نگاری سکانسی سازند بهرام (دونین میانی و بالایی) در برش کوه بندعبدالحسین در جنوب خاوری انارک

به منظور بررسی ریزرخساره‌ها و محیط رسوبی سازند بهرام در جنوب خاوری انارک، برش چینه شناسی کوه بندعبدالحسین انتخاب و مورد مطالعه قرار گرفت. سازند بهرام در این برش 366 متر ضخامت دارد و سنگ شناسی آن شامل سنگ آهک‌ های نازک تا ضخیم لایه، تناوب سنگ آهک‌ ها و شیل‌های نازک لایه، سنگ آهک ‌های ماسه ‌ای و سنگ آهک ‌های مارنی است. براساس تغییرات رخساره ای و ویژگی‌های سنگ شناختی 16 ریزرخساره در این برش تشخیص داده شده است. بررسی رخساره‌های شناسایی شده حاکی از ته‌نشینی این ریزرخساره‌ها در یک رمپ کربناته در زمان دونین میانی و پسین است. بررسی ‌های چینه نگاری سکانسی سازند بهرام بر مبنای تغییرات عمودی ریزرخساره‌ها و مقایسه آن‌ها با منحنی تغییرات سطح آب دریا به شناسایی 7 سکانس درجه 4 در برش مورد مطالعه منجر شده است. با توجه به اطلاعات به دست آمده، سکانس رسوبی اول و هفتم قسمتی از سکانس ‌های رسوبی می‌ باشند و سکانس‌ های رسوبی دوم تا ششم سکانس‌ های رسوبی کامل و دارای دسته رخساره‌های پیش‌رونده، پس‌رونده، بیشینه سطح غرقابی و مرز های سکانسی هستند.

Microfacies, depositional condition and sequence stratigraphy of the Bahram Formation (Middle–Upper Devonian) in Kuh-e-Bande-Abdol-Hossein section, SE Anarak

IntroductionDuring the Silurian and Devonian time, some parts of Iran, including Central Iran, Alborz and SanandajSirjan basins, along with the Turkey and Afghanistan plates had been connected to the Arabian and African plates; located on the northwestern margin of the Gondwana and southern Paleotethys (Berberian and King, 1981; Husseini, 1991; Sharland et al., 2001; Ruban et al., 2007; AlJuboury and ALHadidy, 2009). The Devonian deposits represent a considerable distribution in Tabas Block and Central Iran. During the Devonian time, the Iranian platform was located in the southern hemisphere, near 30° latitudes. Outcrops of the Upper Paleozoic around Isfahan region mostly limited to the northern Isfahan (Soh and Natanz districts, Najhaf, Neqeleh, Varcamar, North Tar, east Kesheh sections; Bahrami et al., 2015), Northeast of Isfahan (Zefreh, Chahriseh, and Dizlu sections; Königshof et al., 2016) and south of Isfahan (Shahreza–Ramsheh area; Bahram et al., 2014). In order to investigate the microfacies and sedimentary environment of Bahram Formation in southeastern Anarak, KuheBande AbdolHossein section was selected and studied. KuheBandeAbdolHossein section is located about 32 km southeastern Anarak and northeastern Isfahan in E: 53° 52’ 55& and N: 33° 10’ 90 & coordinates. The Anarak route toward Khur is the way to the section; after two kilometers, through a 25 km long unpaved road the study section could be accessed. Materials and MethodsIn order to study the sedimentary environment and identify the sedimentation process during the Middle and Upper Devonian KuhbandeAbdolhossein section in Anarak region, 230 thin sections and a few polished slabs were prepared and process with laboratory techniques to study and identify the skeletal and nonskeletal components, texture and other microscopic characteristics. Microfacies textures were identified according to Dunham (1962), Embry and Klovan, 1971 schemes, and also the further standard models of Wilson (1975) and Flugel (2010) have been used. In order to distinguish calcite mineral from dolomite, all samples were stained by Alizarin red S solution (Dickson, 1965). DiscussionThe Bahram Formation in this section is 366 m thick and lithologically is composed of thin to thickbedded limestone, an alternation of limestone and thinbedded shale, sandy limestone and marly limestone. Based on facies changes and petrological properties, 16 microfacies were identified in this section. Investigation of the identified facies indicates the deposition of these microfacies has taken place in a carbonate ramp during the Middle and Late Devonian time in the study section in Anarak region. On the basis of vertical and lateral microfacies variations and their comparison with the sea level changes’ curve, sequence stratigraphic studies of the Bahram Formation led to the identification of seven fourthorder sequences in the study section. According to the obtained data, the first and the seventh sedimentary sequences are as parts of complete sedimentary sequences, while second to sixth sedimentary sequences are complete sequences including transgressive system tracts, high stand system tracts and maximum flooding surface as well as sequence boundaries. ConclusionThe Bahram Formation in the KuheBandeAbdolHossein section, SE Anarak with a thickness of 366 m is composed of thin to thickbedded limestone, an alternation of limestone and thinbedded shale, sandy limestone, and marl limestone. Microfacies analysis has led to the recognition of 16 microfacies and their lateral and vertical distribution show that carbonates were deposited on a ramp type carbonate platform. Storm deposit or very slight steepened slope resulted in the deposition of very rare redeposited finegrained bioclasts in the deeper part of the platform. According to the sequence, stratigraphy analysis seventh fourthorder depositional sequences are recognized in the Bahram Formation. Microfacies analysis indicates that this section shows rather deeper paleoenvironments in comparison to the previously studied sections of the Bahram Formation in Central Iran. Keywords: Bahram Formation; microfacies; sedimentary environment; sequence stratigraphy; Upper GivetianUpper Famennian; KuheBandeAbdolHossein. ReferenceAlJuboury, A.I., ALHadidy, A.H., 2009, Petrology and depositional evolution of the Paleozoic rocks of Iraq. Marine and Petroleum Geology, 26: 208231.Bahrami, A., Boncheva, I., Königshof, P., Yazdi, M., Ebrahimi KhanAbadi, A., 2014. Conodonts of the Mississippian/Pennsylvanian boundary interval in Central Iran. 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