The effect of downstream fining in river sediments: case study, Rayen River in southeast Kerman

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.
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).
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).
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.