حوضه آبریز رودخانه مولید در جنوب شرقی شهرستان قاین، استان خراسان جنوبی واقع شده است. در این مطالعه، بررسی اختصاصات رسوب شناسی (شامل مطالعات بافتی، رخساره های رسوبی، عناصر ساختاری و سبک کانال رودخانه بر اساس رسوبات رخنمون یافته در دیواره کانال) و هیدرولوژیکی دیرینه (با استفاده از حداکثر اندازه قطعات و ضخامت دسته طبقات مورب در هر رخساره رسوبی گراولی) رودخانه مولید صورت گرفته است. آنالیز اندازه دانه نشان می دهد که روند تغییر اندازه ذرات از الگوی نمایی کاهش به سمت پایین دست بطور کامل پیروی نمی کند. این تغییر الگو به دلیل میزان مقاومت سازندهای اطراف رودخانه در برابر فرسایش و فعالیت کشاورزی در داخل و حواشی کانال رودخانه است. عوامل فوق در بعضی از موارد باعث کاهش اثر جورشدگی انتخابی و سایش در طول رودخانه شده و باعث ایجاد دو ناپیوستگی و سه پیوستگی رسوبی شده است. این حوضه شامل مجموعه ای از نهشته های گراولی (Gmm, Gmg, Gcm, Gci, Gh)، ماسه ای (Sh) و گلی (Fl, Fm) است. رخساره های موجود در پنج عنصر ساختاری رسوبات پر کننده کانال (CH)، سدها و اشکال لایه ای گراولی(GB)، نهشته های حاصل از جریان گراویته ای رسوب (SG)، اشکال لایه ای ماسه ای ((SB و رسوبات دانه ریز دشت سیلابی (FF) شکل گرفته اند. بر اساس رخساره های رسوبی و عناصر ساختاری شناسایی شده و بر مبنای مدلهای رخساره ای رودخانه ای ارائه شده، دو مدل رسوبی برای رودخانه مولید پیشنهاد شده است. تخمین پارامترهای هیدرولوژیکی دیرینه بر اساس اختصاصات رسوبات گراولی انجام شده است.
عنوان مقاله [English]
The investigation of sedimentary facies and stacking pattern in the Mulid River (Southeastern Qayen)
In the most gravel bed rivers, particle size exponentially decreases to the downstream. The study of particle size fining trend to the downstream and determination of the effective processes on it along the recent rivers is accomplished in the different parts of Iran. The river sedimentary facies are deposited in the channel and overbank areas and they are provided important information about sedimentary environment and deposition rate, the extent and development of the river channel and floodplain. These sedimentary facies that are deposited in the different depositional conditions have been achieved from variations of flow regime and/ or variation in the depositional environment in the large scale. The aim of this study is to investigate of the particle size variations and the effective controllers of fining trend to downstream, to determine of the important factors in creating sedimentary discontinuities and to study of the sedimentary facies, architectural elements, determination of depositional model and some paleohydraulic parameters of river. The Mulid River catchment with elongated shape is located in 120 km of southeast Qayen in the Southern Khorasan Province, in the 33̊ 24ʹ 44.3ʺ to 33̊ 35ʹ 11.4ʺ east latitude and 59̊ 56ʹ 42.5ʺ to 59̊ 58ʹ 44ʺ north longitude. According to the geological classification of Iran, this basin is a part of the East Iran flysch and mélange belt that is located in the east of the Lut Block.
Materials and Methods
In order to sedimentological studies, 30 sediment samples unsystematically were collected from upstream to downstream and from about 20 cm depth of the main channel bottom of river (with 30 km long). The granulometry analysis of the studied samples were achieved using the dry sieving method with 0.5 φ intervals and weight percent of gravel, sand and mud size particles were estimated. The sediment naming is done using Folk (1980) classification and the estimation of sorting, skewness and kurtosis parameters, after drawing related graphs, are achieved based on the inclusive graphic method of Folk (1980). The determination of sedimentary facies and architectural elements in the studied river channel wall and using codes for them were based the Miall (1996, 2006) classification. In this study, to reconstruct the paleodischarge rate, the paleochannel dimensions and sedimentary characteristics have been used and the presence of empirical equations have been used to estimate of paleohydraulic parameters.
Discussion of Results and Conclusions
The sedimentological studies along the channel in this river catchment are shown that the trends of particle size variations completely does not follow the exponentially pattern of decreasing to downstream. This pattern variation is related to different lithological characteristics and difference in the geological units sensitivity to erosion in the river path and sediment supply from the river channel walls. The above factors are caused to decrease selective sorting and abrasion effects and to form two sedimentary discontinuities and three sedimentary links along the Mulid River. Also, as respects the most values of sediments are composed of coarse particles, the selective sorting process that are depended to the grain size, is due to increase of sorting in each sub trends.
In the study area three sedimentary facies sets are observed in the channel walls that are included gravely (Gmm, Gmg, Gci, Gcm, Gh, Gp), sandy (Sh) and muddy (Fm, Fl) sedimentary facies. The gravely facies have the most abundance in the study area and probably are formed by debris flows with high viscosity and power and/ or turbulence flows with high strength shear stress. Sandy facies that has horizontal stratification, is deposited by unidirectional traction currents (lower flow regime). Muddy (Fm, Fl) facies usually are deposited by low energy traction flows or by waning high strength flood events. Also, five architectural elements are recognized based on geometry and boundary surfaces and are used for depositional interpretation. These elements are included channel sediment fills (CH), gravel bars and bedforms (GB), sediment gravity flow deposits (SG), sandy bedforms (SB) and overbank fines (FF).
The wide range of sedimentary processes are controlled the fluvial style. Therefore the river channel morphology usually vary from upstream to downstream that these variations are due to change of some factors such as valley slope, sediment supply, climate and tectonic of the studied area. According to recognized sedimentary facies and architectural elements in the Mulid River channel walls and based on provided models of Miall (2006) two sedimentary models are proposed for this river: a) The gravel- bed braided river with sediment gravity flow deposits and b) the shallow gravel- bed braided river. The difference of a and b models is in frequency of various architectural elements and abundance of sedimentary facies. The first sedimentary model is formed in the proximal and high relief area where the slope, the discharge rate and sediment supply are high and the sediment transported currents are gravity flows. In this reason, the abundance of architectural elements and sandy and fine sedimentary facies that are results of traction currents, are low. In the second sedimentary model, away from the source area, the slope of longitudinal profile of river decreases and therefore the discharge rate of flow and sediment supply decrease. The abundance of debris flows are very low and the sediment gravity flow deposits are not observed in the end parts of rivers. The most important flow that is transported the sediments, is the traction current in this model. In due to decreasing sediment supply and increasing the accommodation space, frequency of sandy and muddy facies are increased with respect to previous model.
The paleohydraulic parameters are estimated with facies variations for three gravely terraces with suitable exposures in three position of the studied river. The capacity and power of paleoflow are calculated using the maximum clast size in the various gravely sedimentary units. The paleodischarge rate estimation is achieved based on channel cross section area and flow velocity. The cross bedding are used to estimate the maximum paleodepth of current. The vertical variation of sedimentary facies, the thickness variation of cross bedding sets and difference clast size are suggested that the hydrologic conditions are fluctuated. The maximum current power and annual discharge rate is compatible with sediment gravity flow deposits (SG element) and the minimum current power and annual discharge rate is related to gravel bars and bedforms (GB element).