Sedimentology, facies and depositional model of the alluvial fan of Abyek, Qazvin

Abstract

Abstract This study was done to describe and interpret the facies, the provenance of sediments, pedogenesis processes, and the model of the Abyek alluvial fan in the northern margin of Qazvin Plain, using 45 sediment samples in the form of nine profiles and nine surface samples. The facies study led to the determination of six facies grouped into two facies associations including coarse-grained lithofacies (Gms, Gcs, Gci, Gcp/Gmp, Gmg, and Glns), and calcrete facies (Plc). This alluvial fan is dominated by the episodic matrix to clast-supported gravel (interbedded with a subordinate) and red, matrix-supported gravel which were deposited by non-cohesive debris flow. The results of granulometry analysis showed 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 and sandy gravel with very poorly sorting and fine skewness. The study of macromorphology and micromorphology of calcrete showed that their occurrence was controlled by pedogenic processes. Micromorphological studies also revealed alpha and beta features such as coated grains, pisoid, laminar crust and strong brecciation. Keywords: Alluvial fan, facies, Calcrete, Qazvin Plain, Abyek. Introduction Alluvial fans are, more often, coarse-grained and serve as an excellent proxy for unrevealing past changes in climate, hinterland tectonics and sea/lake level. Besides tectonics, climate and hinterland lithology exert significant influence on the volume and grain size of sediments received in such systems (Chakraborty and Paul 2013). In this regard, the classification of alluvial fans by Blair and McPherson (1994) based on sedimentary processes strengthens the old hypothesis of dry and wet alluvial fans. Although the validity of this climate-response hypothesis has been questioned. The purposes of this study are 1) to investigate of sedimentary features and description and interpretation of sedimentary facies to identify sedimentary facies and the facies model of the Abyek alluvial fan and 2) to discuss the role of climate, tectonics, and lithology of the source area on the sedimentary facies. Material & Methods In order to study Abyek alluvial fan, 45 samples of different facies were collected in the form of nine sediment profiles and nine surface samples. The characteristics of representative profiles including sedimentary structures, texture, bed geometries, and lithology were used to describe fan facies. In addition, paleocurrent directions were depicted using azimuth measurements of imbricated pebbles. Facies were described following Miall’s (2006) facies classification. Also, in order to study thin sections, 11 samples from the sand-size sediments and 12 calcrete samples were prepared, respectively, in order to determine the provenance of sediments and microscopic characteristics (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 each section based on the Gazzi-Dickinson method (Gazzi 1966; Dickinson 1970). In this study, in order to analyze calcrete samples, six thin section samples and two blocks with dimensions of 1 x 1 cm were prepared and imaged by FESEM electron microscope. Scanning electron microscopy (SEM) was performed on the representative samples at the Beamgostar Laboratory, Iran (Mira3-TESKAN Scanning Electron Microscope, 20KV). Also, five powder samples and three oriented clay samples were analyzed to determine the mineralogy of clay fraction. The mineralogical composition of representative bulk and oriented samples was investigated by X-ray diffraction (XRD) in Malayer University, Iran (Italstructures, 40 Kv, Cukα 30mA). Finally, by combining field, laboratory data, the sedimentary facies, the origin of sediments and the model of the Abyek alluvial fan and the features of diagenesis and pedogenesis of its sediments were identified and analyzed. Discussion of Results & Conclusions The study of facies of Abyek alluvial fan led to the identification of six facies in two groups: the first group includes coarse-grained lithofacies, which includes matrix-supported gravel (Gms), clast-supported gravel (Gcs), inversely graded clast-supported gravel (Gci), rhythmic gravelly and sandy planar couplets (Gcm and Gmm/Sm), graded clast- to matrix-supported gravel (Gmg) and Grain- to ground-supported lenticular gravel (Glns) and the second group includes calcrete (Plc). The debris-flow deposits, red-coloration, interbedded mudflow, polygonal mud cracks, and calcretes of the studied areas are indicative of a generally warm and arid climate (Gile et al. 1965; Hayward 1983; Kraus 1999; Clyde et al. 2010), which also exists in other areas of the Qazvin Plain. Subaerial debris flows require abundant clastic debris, a steep slope, and a high discharge for their initiation. Abundant clastic detritus resulting from mechanical weathering during long dry periods are transported by flash floods, with little vegetation to inhibit run-off (Miall 1977). Also, non-cohesive debris flows are caused by watersheds with a small amount of mud (especially clay). In this study, the granulometry results show that the amount of mud in the analyzed samples is very small (less than 5%). Since the silty and clay fractions are the product of hydrolysis of feldspar and secondary minerals or they were formed through severe tectonic cuts (Blair 1999); therefore, such reactions are very slow in warm and dry climates and lead to an insignificant amount of mud fraction (Blair and McPherson 2009). On the other hand, the presence of ancient soil (red horizons) indicates periodic sedimentation and warm and dry climate conditions (Yan et al. 2007). The presence of calcrete in the alluvium as well as the palygorskite clay mineral, which is a clay mineral specific to calcrete (Zucca et al. 2017), are other clear signs to confirm the warm and dry climate. On the other hand, the composition and analysis of the palaeoflow direction based on the imbrication of pebbles shows that the alluvial sediments were mostly transported from north to south; Therefore, the tectonic uplift caused by the southern Alborz fault has a significant contribution to the formation of alluvial fans on the northern edge of the Qazvin Plain. Stable tectonic conditions and warm and dry climates (Reeves 1983; Wright and Tucker 1991) are the main factors controlling the formation of calcrete in this alluvial fan. According to granulometry, the sediments of this alluvial fan are mainly composed of a wide range of semi-coarse to fine-grain sediments. The percentage of boulders is very low and coarse pebbles are rarely found in the studied samples. In other words, among the gravel clasts larger than 6 cm, the superiority is with gravels with a diameter of 6 to 12 cm (40 to 90 percent). This shows that coarse rock fragments rarely form in the catchment area. The main reason for the formation of relatively fine-grained deposits in the alluvial fan is the watershed lithology, which is mostly limestone (especially marl), fine-grained volcanic rocks, sandstone, siltstone and shale derived that easily have been decomposed and crushed. A small abundance of coarse rock fragments can also occur where strong tectonic shears have pulverized the rocks of the catchment area (Blair 2003). However, according to the watershed lithology in the catchment basin, the lithological composition of the parent rock is the main reason for the formation of such deposits. In addition, the most mature type of calcrete has been found in the Abyek alluvial fan with a calcareous lithological composition, which clearly shows the dominant contribution of calcareous parent rock to the formation of calcrete. Abyek alluvial fan due to specific features such as the frequency (80–95%) of Gms facies, the presence of red horizon, the amount of mud less than 5%, and the small radius (approximately 3.5 km) can be considered as a fan caused by the accumulation of non-cohesive debris flow (Blair and McPherson 2009). This alluvial fan is a special type of fan resulting from non-cohesive deposits (with a very low percentage of clay) that were formed during sudden discharges. The very small abundance (10–15%) of gravelly facies caused by runoff shows that the contribution of floods to the initial accumulation of alluvial fan sediments was very small and only leads to the transportation and redeposition of sediments locally.

Keywords

• alluvial fan
• facies
• calcrete
• qazvin plain
• abyek