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   چینه نگاری، رخساره‌های رسوبی و جایگاه تکتونیکی ماسه‌سنگ‌های بخش میانی سازند قرمز بالایی در بُرش کانسار چهرآباد، شمال باختر زنجان  
   
نویسنده رجب زاده علی ,کوهستانی حسین ,مختاری میر علی اصغر ,زهدی افشین
منبع رخساره هاي رسوبي - 1398 - دوره : 12 - شماره : 1 - صفحه:75 -90
چکیده    منطقه چهرآباد در شمال خاور ماه‌نشان و در فاصله 75 کیلومتری شمال ‌باختر زنجان قرار دارد. واحدهای سنگی رخنمون‌ یافته در این منطقه متعلق به سازندهای قرمز زیرین، قم و قرمز بالایی می‌باشند. سازند قرمز بالایی در این منطقه 980 متر ستبرا داشته و از 3 بخش اصلی تشکیل شده‌ است. این بخش‌ها به‌‌ترتیب شامل واحدهای تبخیری با ستبرای 235 متر در پایین، تناوبی از گل‏سنگ و ماسه‌سنگ‌های خاکستری‌ تا قرمز رنگ با ستبرای 590 متر در بخش میانی و افق‌هایی از گل‏سنگ‌‌های سبز با میان‌لایه‌های ژیپس با 155 ستبرا در بخش بالایی می‌باشد. توالی انتخاب شده جهت مطالعات چینه‌شناسی و رسوب‌شناسی مربوط به بخشی از قسمت دوم سازند قرمز بالایی با 231 متر ستبرا می‌باشد. این توالی شامل هفت لایه‌ ماسه‌سنگ خاکستری و قرمز رنگ است که به‌ صورت متناوب با واحد‌های گل‏سنگی قرمز رنگ قرار گرفته‌اند. بر پایه مطالعات پتروگرافی، این ماسه‌سنگ‌ها به‌ طور میانگین حاوی 21% کوارتز، 10% فلدسپات و 25% خُرده‌سنگ بوده و از نوع فلدسپاتیک‌لیت‌آرنایت و لیت‌آرنایت‌ می‌باشند. بر اساس شواهدی نظیر چینه‌بندی متقاطع، حضور آثار گیاهی، نبود رسوبات گراولی، ریپل‌مارک‌های متقارن و آثار موجودات حفار و شناسایی 7 رخسار‌ه‌ ماسه‌سنگی گل‌سنگی (fl، fm، sm، sh، sr، sp و st) در منطقه‌ چهرآباد می‌توان بیان داشت که محیط رسوبی این واحد‌های سنگی، رودخانه‌های مئاندری ماسه‌ای با پیچش زیاد نزدیک به محیط ساحل می‌باشد. براساس نتایج آنالیز مودال و بررسی نسبت بین کوارتزها، خُرده‌سنگ‌ها و فلدسپات‌ها، موقعیت تکتونیکی این ماسه‌سنگ‌ها حاشیه‌های فعّال‌ قارّه‌ای و برپایه‌ اطلاعات ژئوشیمیایی و نسبت‌های sio2/al2o3 در مقابل k2o/na2o، حاشیه‌های فعّال‌ قارّه‌ای و حوضه‌های فورلندی می‌باشد.
کلیدواژه رخساره های رسوبی، جایگاه تکتونیکی، ماسه سنگ، سازند قرمز بالایی، چهرآباد، زنجان
آدرس دانشگاه زنجان, دانشکده علوم, گروه زمین شناسی, ایران, دانشگاه زنجان, دانشکده علوم, گروه زمین شناسی, ایران, دانشگاه زنجان, دانشکده علوم, گروه زمین شناسی, ایران, دانشگاه زنجان, دانشکده علوم, گروه زمین شناسی, ایران
پست الکترونیکی afshin.zohdi@znu.ac.ir
 
   Stratigraphy, sedimentary facies and tectonic setting of sandstones of the middle part of the Upper Red Formation at Chehrabad deposit section, northwest of Zanjan  
   
Authors Zohdi Afshin ,Rajabzadeh Ali ,kouhestani Hossein ,Mokhtari Mir Ali Asghar
Abstract    IntroductionThe Upper Red Formation (URF), with the age of Miocene, is mainly composed of clastic sediment including interbedded marl, sandstones and slightly conglomerate layers as well as some evaporate layers mostly in lower parts of the formation. The lithology, color and thickness of this formation are variable in a different locality (Aghanabati, 2004). This formation is the only cap rock for the Qum Formation in the Central Iran zone especially into the Sarajeh and Alborz gasfields (e.g., Morley et al., 2008). The URF has significant distribution in the northwestern of Iran and hosts significant Cu and PbZn deposits (e.g.; Sadati et al., 2016). The constituents of this formation, particularly in the sandstone layers, provide valuable information in relation to the sedimentary environment and the geodynamic location of this formation (Rieser et al., 2005). In this research, based on the facies analysis (Miall, 1996, 2000), modal analysis and geochemical data, sedimentary environment and tectonic setting of these clastic layers in the Chehrabad deposit section, northwest of Zanjan, are interpreted. Materials and methodsThis research is based on a detailed study of lithology, sedimentology and geochemical data of the URF. During field observations, the thickness of sandstone layers and their colors were clearly defined. In order to interpret the sedimentary environments of this formation, a detailed lithofacies have been analyzed during this study. Lateral and vertical variations in all layers have been considered. About 23 thin sections from collected samples are studied by polarizing microscope at the University of Zanjan. In each thin section, the 250points, based on the GazziDickinson method, were counted. To investigate the tectonic setting of these sandstones, 9 samples with the least amount of weathering and calcium carbonate were selected for geochemical analyses by XRF methods. DiscussionChehrabad area is located in the northeast of Mahneshan, approximately 75 km, northwest of Zanjan. Rock units exposed in this area belong to the Lower Red, Qom and Upper Red formations. The thickness of the URF in this area is about 980 m and consists mainly of three main units. These units, from bottom to top, consist of evaporate layers, alternation of mudstone and grey to red sandstone and finally mudstone with interbedded gypsum layers with a thickness of 235, 590, and 155 m respectively. The studied sequence is a part of the middle portion of the URF, with 231 m thickness and has the highest amounts of sandstone layers. Based on the field observation, the middle parts of the formation including 7 gray to red color sandstone, which is alternate with the red mudstones. The sandstones in the Chehrabad area are grey to red and have poor imbrication. According to the sorting and roundness parameters of the grains and also the low amounts of clay matrix (less than 5%), these sandstones are perhaps to be mature in terms of texture maturity. Based on the types of sandstone grains and the Folk (1980) classification, the URF sandstones in the study area is classified as feldspathic litharenite to litharenite. Facies analyses, the color of layers, presence of crossbedding and plant fragments, lack of gravel grains, all represent an oxidized continental environment, such as a fluvial system with a highly sinuous channel (meandering river). Also, the presence of symmetric ripple marks and marine trace fossils indicate that the sedimentation of some parts of this formation has taken place to a tidal condition and most likely close to the coastal environment. In addition, based on field studies and facies analysis, identified lithofacies in Chehrabad area include Fl, Sm, Sh, Sr, Sp, St and Fm. According to the characteristics of each facies and based on the method of Miall (1996), these sandstones were deposited in fluvial and tidal depositional systems. The results of petrography and geochemical studies have been used to interpret the tectonic setting of sandstones in the middle parts of the URF. Based on triangular diagrams of Dickinson and Suczek (1979) (QtFL) and Ingersoll and Suczek (1979) (QpLvmLsm and LvLmLs), also using binary variables graphs of Bhatia (1983) and Roser and Korsch (1986), the tectonic setting of these sandstones is active continental margins and probably foreland basin. ConclusionThe URF in the Chehrabad area consists of 3 parts and the thickness of the middle part of this formation is about 231 m, with 7 sandstone layers, which alternation with red mudstone beds. Based on microscopic studies, these sandstones are classified as feldspathic litharenite to litharenite. Based on field evidence and the presence of crossbedding, plant fragments, lack of gravel grains, symmetric ripple marks, presence of trace fossils and also the type of facies, seven lithofacies (including Fl, Sm, Sh, Sr, Sp, St, and Fm) are recognized. The data obtained from pointcount and geochemical studies clearly show that the tectonic setting of these sandstones in the Chehrabad area is an active continental margin. AcknowledgmentThe authors are grateful to the University of Zanjan Grant Commission for research funding. We acknowledge their support. Constructive reviews by three Sedimentary Facies reviewers greatly benefited the paper. Keywords; Sedimentary facies; tectonic setting; sandstone; Upper Red Formation; Chehrabad; Zanjan. References Aghanabati, A., 2004. Geology of Iran. Geological Survey of Iran, 606 pp. (in Persian).Bhatia, M.R., 1983. Plate tectonics and geochemical composition of sandstones. Journal of Geology, 91: 611–627.Dickinson, W.R., Suczek, C., 1979. Plate tectonics and sandstone composition. American Association of Petroleum Geologists Bulletin, 63: 2164–2182.Folk, R.L., 1980. Petrology of Sedimentary Rocks. Austin, Texas, Hemphill, 159 p.Ingersoll, R.V., Suczek, C.A., 1979. Petrology and provenance of Neogene sand from Nicobar and Bengal fans. DSDP sites 211 and 218. Journal of Sedimentary Petrology, 49: 1217–1228.Ingersoll, R.V., Bulard, T.F., Ford, R.L., Grimn, J.P., Pickle, J.P., Sares, S.W., 1984. The effect of grain size on detrital modes: a test of GazziDickinson point counting method. Journal of sedimentary Petrology, 54: 103–116.Miall, A.D., 1996. The Geology of Fluvial Deposits: Sedimentary Facies, Basin Analysis, and Petroleum Geology. Berlin, SpringerVerlag, 582 p.Miall, A.D., 2000. Principle of sedimentary basin analysis, 3rd edition. SpringerVerlag, NewYork, 668 p.Morley, C.K., Kongwung, B., Waples, D., Warren, J., Julapour, A.A., Abdolghafourian, M., Hajian, M., Otterdoom, H., Srisuriyon, K., Kazemi, H. Rawanchaikul, M., 2008. Impact of structural history and style on the petroleum system of the Central Basin in the SavehQom area, Iran. 8th Middle East Geosciences Conference, GEO 2008, GeoArabia, 13, 206.Rieser, A.B., Neubauer, F., Liu, Y., Ge, X., 2005. Sandstone provenance of northwestern sectors of the intracontinental Cenozoic Qaidam basin, western China: Tectonic and climate control. Sedimentary Geology, 177: 1–18.Roser, B.P., Korsch, R.J., 1986. Determination of tectonic setting of sandstonemudstone suites using SiO2 content and K2O/Na2O ratio. Journal of Geology, 94: 635–650.Sadati, S.N., Yazdi, M., Mao, J., Behzadi, M., Adabi, M.H., Lingang, X., Zhenyu, C., Moktari, M.A.A., 2016. Sulfide mineral chemistry investigation of sedimenthosted stratiform copper deposits, NahandIvand area, NW Iran. 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