سنگ رخساره ها و شرايط ته نشيني سازند كهريزك (پليستوسن مياني - پسين) در خاور تهران

Lithofacies and depositional environment of Kahrizak Formation in the east of Tehran

دوره كواترنري، كوتاه ترين دوره زمين شناسي است كه مشخصه هاي متمايز زيادي دارد. از جمله اين مشخصه ها، گستردگي زياد نهشته هاي آن در سطح زمين و ماهيت آواري بسياري از اين نهشته ها است. سازند كهريزك از جمله نهشته هاي كواترنري در دامنه جنوبي البرز مركزي و در محدوده تهران است كه با توجه به موقعيت چينه شناسي خود، داراي محدوده سني پلئستوسن مياني- پسين مي باشد. در اين تحقيق 4 توالي از اين سازند در خاور شهر تهران، در مسير تهران- رودهن مورد بررسي قرار گرفت. اجزاي متشكله اين سازند از فرسايش نهشته هاي قديمي تر بويژه سازند كرج (توف، سنگ آهك، شيل و بازالت) و سازند هزار دره ناشي شده اند. براساس مطالعات صحرايي و آزمايشگاهي 4 ليتوفاسيس گراولي (Gmm، Gcm، Gh و Gci)، 2 ليتوفاسيس ماسه اي (Sh و Sm) و 2 ليتوفاسيس دانه ريز (Fl و Fm) مورد شناسايي قرار گرفت كه 3 عنصر ساختاري CH، SG و FF را تشكيل مي دهند. وجود اين عناصر ساختاري در كنار ساير مشخصه هاي رسوبي نشان مي دهد كه نهشته هاي آواري سازند كهريزك در يك سيستم رودخانه اي بريده بريده با بستر گراولي و ته نشست هاي جريان ثقلي- رسوبي بر جاي گذاشته شده است.

The Quaternary system contains many events despite of the time constraints، which characterized from all systems of geologic. These distinctions include structural conditions، landform، climate conditions، glaciers، melting ice، development of life especially in relation to human life. In Iran، two-thirds of outcrops belong to the Quaternary deposits. Iran has noticeable geologically dynamic during the Quaternary. Neotectonic motions have been observed in the forms: new activity faulting، folding، volcanic، high extreme weathering and erosion of the oldest deposits. These activities are led to form a wide range of water and wind deposits in the Quaternary of Iran. After deposition Hezar- Dare Formation and its folding with Pasadenian phase، the youngest of Late Alpine orogenic phase، Kahrizak Formation has been settled on Hezar- Dare Formation by unconformity and under the alluvial deposits of Tehran. After Formation of thousands of valleys and folds it with Pasadenian phase، the youngest Late Alpine orogenic phase Formation unconformity on thousands of valley just Kahrizak and Tehran has been under alluvial deposits. Conglomerate of Formation Kahrizak exposed in northern Tehran unconsolidated conglomerate and in southern Tehran showed fine grain with clay composition. Methodology: Study area based on the UTM system is located between 35°44′ to 35°45′ north latitudes and 51°42′ to 58°48′ east longitudes in the South foot of central Alborz and a distance of about 10 km from the city Tehran. In the study area geological structures such as folding and fractures are E- W. In this study، in order to identify lithofacies and depositional environment of Kahrizak Formation in the east of Tehran were studied four sequences. Stratigraphic column of these sequences were drawn based on vertical distribution facies. In order to identify the nature of the components of Kahrizak Formation، sedimentary deposits were studied from the point of view macroscopic and microscopic and were classified based on folk (1974). For study of fine-grain sediments and clay minerals 4 samples (one example from each section) were sent to the laboratory of the Geological Survey and Mineral Explorations for XRD analysis. Lithofacies has been named by Miall (2006) and according to the architectural element obtained depositional model were interpreted for Kahrizak Formation. Results and Discussion: According to the Folk classification (1974) identify three major deposits of gravel، sand and clay and according to the architectural element of Miall (2006) identify 4 gravelly lithofacies (Gmg، Gcm، Gh and Gci)، 2 sandy lithofacies (Sh، Sm) and 2 fine- grained lithofacies (Fl and Fm) that makes 3 architectural elements CH، SG and FF. XRD results showed that clay minerals، including montmorillonite، illite and chlorite are the main constituent and calcite، quartz and small percentages of sanidine، orthoclase and albite are non-clay components. Petrographical studies also showed that pyroclastic rocks، volcanoes، limestone and shale in Kahrizak Formation have originated directly Karaj Formation (Middle Eocene) or indirectly (erosion of Hezar- Dare Formation). Lithofacies are field data includes texture and structure characteristic which one of the most important tools in the interpretation of clastic sedimentary environments. According to field study and structures for example normal graded bedding، inverse graded bedding، horizontally bedded and massive، clastic deposits of massive structure and finning grain upward، Kahrizak Formation may have been deposited in gravel bed braided river with sediment- gravity flow deposits located on an alluvial fan. Conclusion: The Quaternary system is the most important systems in Earth's history. Kahrizak Formation is deposits of this system. Due to the position of Kahrizak Formation among Hezar- Dare Formation and alluvial، this Formation can be age the middle- late Pleistocene. Identification 8 lithofacies of gravelly، sandy and fine-grained and 3 architectural elements CH، SG and FF in clastic deposits of Kahrizak Formation indicated these sediments have been settled in gravel bed braided river with sediment- gravity flow deposits located on an alluvial fan، under the effects of low rainfall and high rainfall. Finning grain upward and studied structures supported this interpretation. Petrographical studies show sediments derived from erosion of Karaj Formation and include tuff، limestone، basalt، sandstone and shale.