Interpretation of sedimentary facies based on river particle size analysis (Case study: Haji-Arab River _ Qazvin Province)

Haji-Arab River is a seasonal river in the south of Qazvin province. Quaternary alluvial deposits in Qazvin province spread on wide range. Since many coherent studies in terms of geology, sedimentary environment, mineralogy and climatic conditions have not been done in the mentioned sediments, so this study has been done to study the sediments of this river and determine its sedimentary facies. Therefore, the results of this study can provide a more precise understanding of the sedimentology status and sedimentary environment of this area.

This study was conducted to investigate the sediments of this river and to determine its sedimentary facies. Sampling was done from 4 boreholes and trenches around the ancient Qara Tappeh hill. Sediment granulation has been done by dry sieving, wet sieving and laser shaker sieving. Sedimentological studies show that the occurrence of frequent floods in the region has changed the texture of channel sediments and changed various parameters of particle size. Haji-Arab River, flows from south to north direction. This river originates from the central mountains and extends to Sagz-Abad. Sampling has been done around the ancient hill of Sagz-Abad (Qara-Tappeh) with an altitude of 1274.9m above sea level. This hill is located 52 km south of Qazvin (7 km north of Sagz-abad of environs Buin-Zahra city) and 132 km west of Tehran. Sampling were done in September 2016. The boreholes and trenches were excavated by Department of Archaeology, University of Tehran. From 4 boreholes with a maximum depth of 8.3 meters, 57 samples have been taken from around the ancient hill of Qara-Tappeh. Collected samples were dried due to humidity outdoors. 15% of the samples were granulated in two forms: dry and wet. Since the result difference was more than 10% due to grains adhesion, other samples were sieved by wet method. Particles smaller than 4µm were granulated by laser shaker sieving. Statistical parameters such as kurtosis, sorting, skewness and mean were calculated using Gradistat software. Also In this study, sedimentary facies were studied using field and laboratory methods. Sediment type was named based on Folk (1974) classification and facies classification based on Miall classification (1996, 2006). Finally, by combining all the data, a sedimentary model for Haji-Arab River was offered.

The study of sediments in this river led to the identification of three categories of sedimentary facies, which are: mud facies (Fl, Fm, Fr), sandy facies (Sm, Sp, Sl, Sh) and gravel (Gh, Gcm, Gms, Gci, Gmg, Gmm), which have settled in different parts of the river environment. Based on the identified facies, structural elements Floodplain Fines (FF), Sandy Bedforms (SB), gravelly Bars and Bedforms (GB), Sediment Gravity flow deposits (SG) and channels (CH) have been formed in this sedimentary basin. Sedimentary models of this river include Fine-Grained Meandering River, Sand-Bed Meandering River, Gravel-Bed Braided River and Shallow-Gravel-Bed Braided River.

Generally, coarse-grained, sandy and fine-grained sediments, poorly to very poorly sorting, absence of fossils, light color of sediments, the presence of plant traces indicate that the Quaternary sediments of the study area form in river system which include sub-environments such as, Channel, off-channel and floodplain deposits. Examination of sediment sequence in the study area shows the transition from a humid and low-energy environment to a dry and high-energy environment during the Quaternary. According to field studies and studies of sedimentary facies of Haji-Arab River, silt sediments are the most widespread in Haji-Arab catchment. Gravel with lower percentage are observed in limited parts of the basin and in the upper parts of the studied sequences. Therefore, based on the above evidence and the characteristics of facies in the Miall (1996) classification, fine-grained sediments have been deposited in the meandering River and coarse-grained sediments of this region have been deposited in a braided river. According to the studies, the architectural elements of FF, CH and SB and to a lesser extent GB and SG can be considered for this basin. Based on this, Can be considered 4 river models including fine-grained meandering rivers, Sandy meandering river, gravel wandering river and shallow gravel bed braided can be used for Haji-Arab River in the study area.