Sedimentology, facies and depositional model of Qazvin alluvial fan

Abstract

Abstract Qazvin alluvial fan is the largest alluvial fan in the northern margin of the Qazvin Plain. This research focuses on describing and interpreting the origin of the sediments and facies model of this alluvial fan using 24 sediment samples from 11 sections and two surface samples. The results of facies granulometry show that the size of the sediments of the alluvial fan shows wide variations from gravel to clay, and the texture of the sediments of this alluvial fan is mainly gravel, gravelly sand and sandy gravel with very poorly sorting and fine skewness. The sedimentary rock fragments are mostly igneous and limestone. Facies study led to the determination of eight facies including coarse to medium-grained lithofacies (Gmp, Gcp, Gcp-Gmp/Sm, Gmg, Glns, Spl, and Sm), and calcrete facies (P) which grouped into two facies association. The results of this study show that the Qazvin alluvial fan can be considered a waterlaid-dominated alluvial fan due to the superiority of fluvial facies and the absence of the episodic matrix to clast-supported gravel (interbedded with a subordinate) and red matrix-supported gravel. Keywords: Alluvial fan, Facies, Sheet flood, Antidune, Qazvin Plain. Introduction Alluvial deposits are often formed in arid and semi-arid regions and along mountain fronts. Their morphology and sedimentary characteristics provide valuable information about the paleoclimate, tectonics, lithology and drainage basin (Quigley et al. 2007; Hedrick et al. 2013; Ozpolate et al. 2022). Facies and structure of alluvial fan deposits are affected by: 1) diversity of sedimentary environments; 2) interactions of the alluvial fan with the surrounding environment; 3) allocyclic factors; and 4) small-scale and more local factors including climate, topography, base level and sediment supply (Gough 2015). Despite the scientific community's interest in alluvial processes and the relevant geological hazards, very few studies have been conducted on the description and interpretation of sedimentary features, stratigraphy and sedimentary processes of alluvial fans (Blair 1998; Moscariello 2017; Deynoux et al. 2005). This is due to the difficulty of directly observing and describing sedimentary processes, which are often associated with instantaneous, hazardous and unpredictable events; therefore, to fully investigate the evolution of the alluvial fan, predict the dominant sedimentary processes and evaluate its potential risks, a correct understanding of the facies and sedimentary structure of the alluvial deposits is required (Maraga et al. 1998). In the northern margin of the Qazvin Plain, the tectonic activity of the North Qazvin Fault has led to the formation of numerous alluvial fans. However, climatic changes and lithology of the drainage basin play a significant role in the occurrence of their facies. The objectives of this study are 1) to investigate the sedimentological characteristics and sedimentary facies of Qazvin alluvial fan, which can represent the most common sedimentary processes in the northern margin of Qazvin Plain, and 2) to discuss the model of the Qazvin alluvial fan with emphasis on the grain size distribution and lithological features of the drainage basin and sedimentary processes. Material & Methods In order to study the Qazvin alluvial fan, 26 samples of different facies were collected in the form of 11 sediment profiles and two surface samples. The characteristics of representative profiles including sedimentary structures, texture, geometry, and lithology were used to describe fan depositional facies. In addition, palaeocurrent directions were depicted using azimuth measurements of imbricated pebbles. Facies were described following Miall’s (2006) lithofacies code system. Also, in order to study thin sections, nine samples from the sand fractions were prepared to determine the origin of sediments and microscopic characteristics after impregnation with resin (Carver 1971). Folk’s (1980) classification was used to name gravelly and sandy sediments in terms of composition. Modal analysis for sandstone samples was done by counting more than 250 points in the thin section based on the Gazzi-Dickinson method (Gazzi 1966; Dickinson 1970). Also, four powder samples and two oriented clay samples were analyzed to determine the mineralogy of clay fraction. The nineralogical composition of representative bulk and oriented samples was investigated by X-ray diffraction (XRD) at Malayer University, Iran (Italstructures, 40 Kv, Cukα 30mA). Finally, by combining field, laboratory data, the sedimentary facies, the origin of sediments, the model of the Qazvin alluvial fan, the diagenetic features and the pedogenesis of its sediments were identified and analyzed. Discussion of Results & Conclusions According to the analysis and the results obtained from granulometry, the Qazvin alluvial fan is mainly composed of a wide range of semi-coarse to fine-grained sediments. The percentage of boulders in this alluvial fan is very low and coarse pebbles are rarely present. In other words, among the gravels larger than 6 cm, the superiority is with gravels with a diameter of 6 to 12 cm (50 to 80 percent). This shows that coarse pebbles are rarely formed in the catchment area. The main reason for the formation of such relatively fine-grained deposits are the rocks that make up the catchment area, which was mostly derived from fine-grained volcanic rocks, limestone (especially marl), sandstone, siltstone, and shale, which are easily decomposed and crushed. The insignificant abundance of pebbles can also occur where strong tectonic cuts have pulverized the rocks of the catchment basin (Blair 2003). However, according to the type of parent rocks in the catchment area, the lithological composition of the parent rock is the main reason for the formation of such deposits. In addition, the presence of calcrete in the Qazvin alluvial fan clearly shows the contribution of calcareous parent rock in the lithology composition of the catchment basin. Also, the granulometry results show that the amount of mud in the analyzed samples from the study alluvial fan is very low (usually less than 5%). Since the silty and clay parts are the product of feldspar hydrolysis and secondary minerals or they were formed through strong tectonic fracturing (Blair 1999); Therefore, such reactions are very slow in hot and dry climates and lead to a negligible amount of clay fraction (Blair and McPherson 2009). The presence of different types of calcrete in alluvial fans, as well as the clay mineral palygorskite, which is a clay mineral specific to calcrete (AlShuaibi and Khalaf 2011; Zucca et al. 2017), are other clear indicators to confirm the hot and dry climate. The mentioned peculiarities and the abundance of facies caused by water processes indicate the formation of the Qazvin alluvial fan due to fluvial processes in a hot and dry climate. On the other hand, the analysis of the palaeoflow direction based on the clast imbrications shows that the sediments of this alluvial fan were transported mostly from the north to the south; therefore, the tectonic uplift caused by the southern Alborz Fault has a significant contribution to the formation of alluvial fans on the northern margin of the Qazvin Plain. Qazvin alluvial fan, the largest alluvial fan of Qazvin plain, has an active and suitable river system that provides a large volume of debris sediments up to a radius of 22 km. According to the mentioned features and also the abundance of sand sediments compared to gravel, insignificant mud (less than 6.5%), the slope is much lower than other alluvial fans of the Qazvin plain, the occurrence of typical river facies such as Gmp, Gcp, Gmg, Glns and Sm, facies caused by sheet flood (Gmp/Smm-Gcp), antidunes, numerous scour-and-fill structures, and mostly semi-rounded to rounded pebbles with moderate to good imbrication in most facies, Qazvin alluvial fan can be introduced as a waterlaid dominated alluvial fan.