Quaternary Journal of Iran

Abstract The cause of fining grain size toward downstream is a subject for decades, because grain size variation can be effected by sediment transport, bed roughness and hydraulic sorting (Frings et al., 2010: 831). Two processes have been considered to explain downstream grain size decreases in gravel-bed rivers: (1) abrasion of single grains that reduce their size and (2) hydraulic sorting or selective transport due to differential transport (Moussavi-Harami et al., 2004: 474). The size of gravel bed material typically decreases with increasing distance downstream, can be attributed to a combination of abrasion during transport, abrasion of gravel in place, weathering, selective entrainment and/or enhanced transport of finer sediment sizes and tributary effect (Pizzuto, 1995: 753). The textural characteristic of sediments is widely affected by several factors including the source area, climate, distance and the sediment transport energy in sedimentary environments (Kumar Maity and Maiti, 2016: 1128). This paper examines the downstream fining and factors that control the rate of fining in the Rayen River in southeast Kerman, which can be used as a model for interpretation of continental silici clastic rocks in the geological record. There are three main aims in this study including textural variation toward downstream, to identify a model for downstream fining and recognize sedimentary facies in the Rayen River.
Materials and methods
During the field study, 42 samples were collected from the main channel of the river and the location of the samples were recorded by GPS. The collected sample transferred to laboratory and after drying, dry sieves with 0.5Ф intervals (from -5.5to 4Ф) used for analysis and then the weight of sediment on each sieve was weighted by a scale and then classified in to different categories by the size and using Excel program for calculation of different parameters. Then 10 thin sections from sediments on the sieve with the size 1 phi (0.5mm) prepared and examined for the composition of sediments and abrasion. The textural parameters of sediments, including mean, median, sorting, skweness and kurtosis, were measured using graphic comprehensive method of Folk (1980). The Rayen River is located in Kerman Province and about 14 km southwest of Rayen city. This study area is a part of Hezar volcanic rocks of the Urumieh- Dokhtar magmatic assemblage in south Kerman that show the latest volcano phase of the Eocene in this area and is composed of lava, pyroclastic and siliciclastic sediments. The lava consists of basaltic- andesite, andesitic- basalt and basalt. The pyroclastic deposits are volcanic breccia, volcanic sandstone and the sedimentary rocks including limestone, siltstone and sandstone. These rocks probably resulted from old compound volcanos with successive eruptions (Eocene) that had periodically activity and the eruptions were very explosive (Ahmadipor and Maleki, 2009; 47).
Results and discussion
After analysis of sediments and calculation of the percentage of mud, sand and gravel, sediments are classified according to Folk (1980) classification. Measurement of average particle size show that most of this river sediments are sandy gravel and gravelly sand. According to triangular diagrams of Sneed and Folk (1958), most of particles are in the range of bladed and compact bladed. Due to composition of sediments and initial form, based on the andesitic origin of sediments, bladed and compact bladed are observed even in the downstream of the basin. Based on thin sections study, the river sediments formed mainly from volcanic rock fragments, containing minerals such as pyroxene, amphibole, olivine, plagioclase, opaque and quartz. Sedimentological studies along the main channel of this catchment area show that the process of changing the grain size toward downstream follows the exponential pattern decrease. The causes of fining toward downstream in the Rayen River include:

Geomorphological factors such as: decreases in elevation and gradient toward downstream.
Sedimentological factors including: abrasion and hydraulic sorting.

 The most important factor on the rate of abrasion is, composition, size and initial form of particle (Bertoldi et al., 2010; 348). Accordingly, on the upstream parts of the Rayen River, the size of particle is larger and is more angular, and moving downward, the particle is finer and is more rounded. Plotted data of mean and median, show that particle size from upstream toward downstream of the Rayen River and varies from 0.357 to 14.928 and 0.267 to 22.627 mm respectively. Decrease median and mean from upstream toward downstream of river (R²=0.806, 0.858), indicate that the rate of coarse grain in the range of gravel decreased while the finer grain in the range of sand size increased. Calculations of statistical parameters show that the sediments of the catchment area are poorly and very poorly sorted (from 1.64 to 2.63phi) with coarse skewned to strongly fine skewned (from -0.31 to 0.52) and mesokurtic to platykurtic (from 0.007 to 1.318).
Study of sedimentary facies in this river indicate that 3 different type of lithofacies including gravel (Gmm, Gmc, Gh, Gp, Gmg), sandy (Sm) and Mud facies (Fl) are present. These lithofacies have formed in four architectural elements including Channel (CH), Gravity flow deposits (SG), Gravel bars and Bed forms (GB) and Fine grains clastic deposits (FF).
Conclusion
Two process have been considered to describe the decrease of grain size toward downstream in the Rayen River with gravel bed. Hydraulic sorting, where the smaller grains transported faster than coarse grains, and abrasion, where individual grains based on their composition decreased toward downstream, are responsible for grain size decreases. Based on the result, Rayen catchment area is braided river with sandy gravel bed.

Abstract Barchan dunes and nebkhas are great geomorphologic phenomena of desert areas in south of Damghan. The purpose of this study is to analyze the sedimentology of sand dunes and nebkha deposits in the Haj Aligholi Desert.
More than two-thirds of the Iran's area is arid and semi arid land. The diversity of flora is limited in these areas because of low humidity. This causes the wind to easily affect on the surface of these lands and carries the sand size and finer grains and deposited in another place. Barchan sand dunes are the most abundant type of sand dunes in the Haj Aligholi Desert. Dispersed vegetation plays an important role in sediment transport and nebkhas formation in this area. Nebkhas or sand dunes are formed usually in hot and humid and semi-arid regions and widespread where the sand can be trapped by vegetation. The study area is located about 35km south of Damghan in 54°, 10' to 54°, 30'E longitude and 35°, 45' to 36°, 00'N latitude.
2-Materials and methods:
27 and 15 samples were collected respectively from the sand dunes and nebkhas to determine the sedimentary parameters. Grain size analysis were carried out using dry sieve with a set of 0.5 phi screens for measurement of statistical parameters (mean, median, sorting, skewness and kurtosis). For morphometric investigation the parameters of sand dunes (lee side, stoss side and crest) and nebkhas (long axis, short axis and height length) were measured.
3-Results and discussion:
The mean diameter of the grain size, the average of sorting, skewness and kurtosis in the sand dunes are about 2.435 phi (fine sand), 0.530 phi (moderately well sorted), -0.268 (coarse skewed) and 1.054 (mesokurtic), respectively. The mean diameter of the grain size, sorting, skewness and kurtosis in the nebkhas are about 2.800 phi (fine sand), 0.540 phi (moderately well sorted), -0.070 (near symmetrical) and 1.640 (very leptokurtic), respectively. The comparison between the index of kurtosis and sorting show that with increasing of the grain's kurtosis, the grain's sorting increased.
After doing regression for each of the independent variables versus the dependent variable to height was calculated. The results show that the attribution of the stoss side and the crest show the best correlation. Therefore, with stoss side and crest components, we can calculate the height of sand dunes from the high-resolution images satellite. To compare the relationship between the long axis's length and short axis's length, as well as the long axis's length and height of the nebkhas obtained the regression equation in the study area. The results show that this both variable are suitable for measured of long axis in the nebkhas. 
In this study, both types of nebkhas (active and passive) were identified that cause of the deactivation of them is dropping in the level of groundwater, which its witness is embayment and cracking on the surface of the ground. Due to the prevailing of northwest wind seems to be the accumulation of sediments at this area is closely related to the groundwater table and soil moisture of this region.
4-Conclusion:
The average of sorting calculated for sediments of sand dunes and nebkhas, were moderately well sorted can be show two source for sediments (bimodal) and it cause to decrease the amount of sorting. The average of skewness of sand grains that skewed to the coarse grains which we can found type movement of sediments that it is saltation. Northwest wind that blowing in study area controlled sand dunes type and position and cause of this claim is morphology and situation of sand dunes. The results of regression revealed that the attribution of the stoss side and the Crest in sand dunes shown the best correlation and also in nebkha dunes the attribution of the short axis and height show the best correlation. In the study area both nebkha (active and passive) found which phenomena annihilator of nebkha is falling groundwater level that it's increasing the desertification.