پديد آورندگان :
قلعه‌نويي، سهيلا نويسنده دانشجوي كارشناسي ارشد رسوب‌شناسي و سنگ‌شناسي رسوبي دانشگاه فردوسي مشهد , , خانه‌باد، محمد نويسنده استاديار، گروه زمين‌شناسي دانشگاه فردوسي مشهد , , موسوي حرمي ، سيد رضا نويسنده استاد، گروه زمين‌شناسي دانشگاه فردوسي مشهد , , محبوبي ، اسداله نويسنده استاد گروه زمين شناسي، دانشكده علوم، دانشگاه فردوسي مشهد، مشهد، ايران Mahboubi,
چكيده لاتين :
Introduction
The river is a dynamic system that climate, geologic, physiographic and tectonic activity affect on their rate of flow, depth, sediment transport rate and finally play an important role in the formation of sedimentary facies (Sear and Newson 2003; Friend and Dade 2005; Demoulin 2011). Many studies have done on the changes of grain size toward downstream and all of them confirmed the reduction of grain size toward downstream as a result of sorting and abrasion (Parker 1991a, b; Ferguson et al. 1996; Lewin and Brewer 2002). Sedimentary facies deposited within and outside of the river channel (Kjemperud et al. 2008; Mannai-Tayech and Otero 2005) can be used for the reconstruction of the environment (Kwon et al 2002). The purpose of this study is to evaluate the sedimentary facies and structural elements, changes in grain size, the affective factors in the fining of grain size toward downstream, determining affective factors in the creation of sedimentary discontinuities and their correlation with textural components.
Material & Methods
Using the geological map 1: 250,000 Sabzevar (Sahandi, 1992), the main stream, minor waterways and sub basins were identified and the area was calculated for each sub basin. 54 sediment samples at a distance of 16.5 kilometers in length of the main channel from upstream toward downstream of the channel floor were collected. After preparation and measurement of the samples, weight percentage of gravel, sand and mud were calculated and Folk classification (1974) was used. The grain size parameters, such as mean, median, sorting, skewness and kurtosis were calculated using comprehensive graphical method of Folk (1974). Based on evidence and field studies along the Sadkharv River from upstream toward downstream, sedimentary facies in this river were classified based on Miall (2006) scheme. All of the maps, such as geologic, main channel of Sadkharv River and other waterways, ways of access to this basin, longitudinal and transverse profiles were produced by GIS software. To investigate the role of the lithology surrounding the watershed in Sediment production, 20 thin sections and 10 polished thin sections were prepared.
Discussion, result and conclusion
In most rivers with gravel bed load, grain size decreases exponentially toward downstream. Many studies have been done on this issue and confirmed this principle that fining toward downstream was a result of hydraulic sorting and abrasion (Ferguson et al. 1996). Sedimentology of the main channel in this area indicates that the changes in grain size, except in a few cases that is negligible, followed the exponential decreasing pattern toward downstream. This study shows that the sorting along the Sadkharv River increases toward downstream. Most samples in this basin are poorly sorted (range of 0.89 to 2.02?). Ranges of kurtosis in sediments of this river are between 0.12 to 1.65 and the samples are very platykurtic to leptokurtic. In Sadkharv River, in upstream, fine grain sediments are less therefore sediments have negative skewness, but toward downstream, because of the presence of fine-grained sediments more than upstream the sediments show positive skewness. Ranges of skewness in sediments of this river are between -0.86 to 0.9. Sediment with positive skewness represents more fine grain between coarse grain sediments. This suggests that there was not enough time for washing the fine-grain sediments by flow (Rice 1999). Based on texture, such as grain size from upstream toward downstream along of the main rivers, sedimentary facies were divided into three groups that gravel facies include Gmm and Gcm, sandy facies include St, Sp and Sm, and muddy facies include Fm and Fl. According to the sedimentary facies and structural elements (CH, SG, GB and FF) that have been identified in the study area and also on the basis of Schumm classification (1985, 1981), Sadkharv River is a braided river with gravel-sand bedload and in accordance with the classification of Sensarma et al (2008) this river is component small rivers and temporary. Recent studies revealed that this area has two types of lithology. The first type related to the ophiolites that indicate to collision between Iran plate in the north and Turan plate in the south. The second type includes sedimentary rocks and consists of sandstone, marl, shale and conglomerate of Eocene age. In general, most minerals in sediments of this river are related to the ophiolites that contain a lot of pyroxene, amphibole, olivine, carbonate, plagioclase, opaque minerals and minor amount of minerals related to sedimentary rocks.
Sedimentological studies in Sadkharv River basin indicate this basin consist of main river channel with some waterways. Based on the median and mean of particle diameter, size of grains reduced toward the downstream. The main reason is hydraulic sorting and abrasion. Moving toward the downstream, sorting and kurtosis are increased. In upstream sediments, skewness is negative but toward downstream skewness is often positive. Units of geology in this area consist of ophiolite mélange and limestone that are exposed in the upstream, Conglomerate and sandstone in the middle part and shale and marl are exposed in the downstream. Based on sedimentological studied in this basin, seven sedimentary facies include Gcm, Gmm, Sm, St, Sp, Fm and Fl was identified.
Keywords: sedimentary facies, sedimentary parameters, hydraulic sorting, abrasion, Sadkharv River
