تحلیل رسوبات رودخانه ای در دیرینه ی حوضه ی میقان (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
حوضه های بسته، به دلیل حساسیت بالا به بالانس تبخیر و بارش، مناسب ترین سیستم برای مطالعات پالئو و شرایط دیرینه هستند. ازآنجاکه رودخانه ها از مهم ترین اجزای حوضه های رسوبی هستند، پژوهش حاضر با هدف بازسازی شرایط محیطی دیرینه در حوضه ی میقان با کمک گرفتن از شواهد رسوبی پادگانه رودخانه ی فصلی گچلان (بخش شمال شرقی حوضه )، مدل رقومی ارتفاع و تصاویر ماهواره ای منطقه انجام پذیرفته است. در این مطالعه سعی شد با استفاده از مشاهده ی میدانی، نمونه برداری و انجام مطالعات دانه سنجی و مورفوسکوپی و هم چنین آزمایش های XRD و XRF بر روی 12 نمونه ی رسوبی، فرآیندهای حاکم بر حوضه ی میقان در گذشته بازسازی گردد. جورشدگی ضعیف، گردشدگی نسبتاً مناسب، چند نمایی بودن، تمایل به ذرات ریزدانه، بلوغ متوسط و هوازدگی پایین و میانه از مهم ترین ویژگی های اغلب نمونه های رسوبی برداشتی بودند. هم چنین وجود فسیل تکستولاریا از نوع فرامینی فر که قادر به زندگی در محیط های دریاچه ای تا دریای عمیق بوده اند، می تواند نشانه ای از بالا بودن تراز آب دریاچه ی میقان و پیشروی آن به داخل رودخانه های حاشیه ی پلایا در دوره های یخچالی کواترنری باشد. وجود کانی های هماتیت و مگنتیت نیز در بخشی از مقطع پادگانه بیانگر وقوع هوازدگی بوده و نشان می دهد رسوبات مقاطع فوق در طول دوره ی کواترنر، به طور متناوب خارج از آب به سر برده اند.Analysis of river sediments in the paleo- Meyghan basin
Introduction
The territory of central Iran is now affected by the hot and dry climate. But, several evidences, such as the extreme erosion of running waters, huge alluvial fans, wide alluvial terraces, and flood deposits indicate colder and wetter climate in the territory (Ghahroodi Tali, 2011: 3).
The rivers belong to the most important parts of the sedimentary basin. In fact, the rivers are one of the existing elements of the continental perspective. Based on their sizes, the riverine sediments are deposited in different parts of the river (Opreanu et al, 2007: 23). Alluvial landscapes are very complicated because of the discontinuity in vertical sequence that are controlled by various factors (climate, geology, geomorphology, and human), and it may be difficult to interpret sequence sequences. Even, the interpretation of the sequences may become difficult. By studying the particle size distribution of the deposited sediments along the rivers, such as the alluvial basin, we can gain important information about the particle granulation, the shape, size, slope, and elongation of the sediments (Randel et al., 2006:58).
The particle size of the sediments, showing the external and inherent conditions of the environment, which are a reaction to the geomorphological and climatic changes, show the morphodynamical balance of the sedimentary environment (Geupta and et al, 2002: 239). By selective sorting of the particles, due to their different transfers, we can identify the sediment phases (Solis-Castillo, 2013:10). This is especially evident in the mineral characteristics, such as their concentration, and shapes, and the particle sizes. Hence, sorting these characteristics can indicate the paleo-sedimentary environments, and the origin of the sediments (Komar, 2007: 10).
This study aims to rebuild the original environment and the paleo-sedimentary conditions in Meyghan plans by the help from sedimentary basin evidences of Gachalan seasonal River (Northeastern part of the basin). In this study, it has been tried to reconstruct the processes of Meyghan Basin, by using field observations, sampling, and doing granulametrical Morphoscopy, XRD, and XRF tests to analyze the sedimentary samples of alluvial basin Gachalan, in the Southeast of Meyghan plans.
Materials and methods
To analyze the Paleo conditions of Meyghan basin, firstly, Geochalan River, in the Northeastern part of the basin was selected. Then, by the help from the digital elevation model of the district, the exact pathway of the river was extracted. Next, on field observations, 12 sedimentary samples were gathered from the riverine level. Granola-metric studies and Morphoscopy and XRD, and XRF tests were performed on the samples.
Discussion and Results
According to Folck’s classification, the studied sedimentary samples were put in the groups of slightly gravelly sand to sand gravel. The interpretation of the statistical parameters from the granulation of the sediments showed that: except for samples 7, and 12, which had average sorting (index of coast sediments), all other sediments had poor sorting, which is an attribute of riverine sediments. None of the samples showed their sorting is less than 0.8. This indicates that, the only dominant process of transferring the sediments in the studied layers was an aquatic process (Khanbabaee et al, 2016: 141). Poor sorting, also, indicates that the sediments have different origins. Tilting curves of layers 2, 1 and 5 were toward coarse particles, and other layers inclined to fine particles.
XRD and XRF tests showed that; in A and E sections, quartz mineral had the highest frequency. Also, in 2 and 3 sections, calcite mineral had the most frequency. In 2, 3, 4, and E sections, the amounts of Hematite mineral were observed to be increasing. The presence of these minerals confirms the occurrence of weathering and indicates that the above-mentioned sections of the sediments have been periodically out-of-water. The presence of Magnetite and Hematite in the layers shows the presence of weathering and oxidation more or less in all the sections (except section 1). The presence of some amounts of the heavy mineral of Magnetic in section 2 shows the sedimentary origin of quartz type, especially meta-Quartzite, Biotite, and Mesquite type.
The ratio of Al2o3/SIo2 more than 5 or 6 in the sediment stones is indicative of high sediment maturity (Roster et al, 1996: 14). The ratio in the samples of the studied area is averagely equal to 4.37. It is indicative of medium sedimentary maturity of the studied area. Among them, the sediments of sample A with 3.62, and the sediments of 3 samples with 5.01 have the minimum and maximum maturity, respectively.
By calculating the ratio of aluminum to tantalum, the amounts of the studied samples equaled averagely to 15.2, which we put than in the class of intermediate plutonic stones. To calculate chemical weathering, the Culers Index was used (Cullers, 2000: 150). The closer the index is to 50 and closer to 100, indicating more weathering in the region of origin. The value of this index for the studied region varied from at least 19.6% in sample c to a maximum of 27.54% in sample 1, indicating medium and low weathering in the source region in all periods.
Conclusions
formation of volcanic facies in the era of Chromin (the end of the Quaternary end) in the Gachalan river basin and the basaltic andesitic masses caused by the occurrence of the main Pasadena's phase the Sahand-Bazman belt caused the andesitic-basaltic rocks to provide the majority of sediments transported by processes Blue to Peacock Play. On the other hand, in the glacial periods, currents have always been the main process of sedimentation and basin formation in the northwestern part. As in these courses, with the rise of the lake's water level and salinity reduction, the water level was raised in the Gachalan River and the possibility of transfer of the Foraminifera fos sils to the river's environment was provided. Although in all periods, the chemical decomposition of rocks has been low to moderate, but physical degradation has always dominated the formation of stones in the source region as well as the texture of the deposited sediments.
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