Analysis of sedimentary facies and sediment fining in Sarghayeh-Sarnish watershed (South of Mashhad)

The Sarghaye-Sarnish watershed, with elongated shape and about 70.654 Km2 area, is located in south of Mashhad. Sarghaye and Sarnish rivers are the main ones in this watershed. The purpose of this study is to investigate grain size varietions based in sedimentary links and its relation to textural parameters such as sorting, skewness and kurtosis, as well as identification of sedimentary lithofacies. Six lithofaceis including matrix supported gravel (Gmm), grain supported gravel (Gcm), grain supported gravel with horizontal bedding (Gh(, massive Sand (Sm), laminated mud (Fl) and massive mud (Fm) are recognized along the wall of studied rivers. These lithofaceis have formed in 4 architectural elements including Channel (CH), Gravity flow deposits (SG), Gravel bars and Bed forms (GB) and Fine grain clastic deposits (FF). Accordingly, Sarghayeh and Sarnish are proposed as gravelly braided river systems. To understand the grain size variation and effective factors in downstream fining, 60 samples from the main channels sediments have been analyzed. The results indicate that grain size changes toward downstream is not exponentional and has three discontinuities and four isolated sedimentary links in Sarghayeh river, and one discontinuities and two sedimentary links in Sarnish river. These discontinuities resulted from laterally input of coarse grain sediments from tributaries, increase of bed slope, emergence bed rock, change in lithology and tectonic activities such as faulting. The isolated sedimentary links have fining trend toward downstream due to hydraulic sorting and abrasion. However, based on sedimentological studies, we represent that the sediments are mostly poor sorted, positively skewed and platykurtic in studied rivers.