Fluvial fan sedimentary characteristics of distributive fluvial system

Abstract

The characteristics of fan sediment in the distributive fluvial system are evaluated based on a thorough analysis of modern silt in the Guertu river distributive fluvial system, as well as data obtained from UAV aerial photography and satellite remote sensing. The Guertu River Distributive Fluvial System (DFS) is classified into three stages, namely "proximal," "middle," and "distal," based on the examination of river morphology, sediment variations, sedimentary attributes, and other relevant aspects throughout different sections of its tributaries, from the source to the mouth. At the upstream section of the downstream fining sequence, the slope is the steepest and the hydrodynamic conditions are intense, resulting in the formation of a predominantly big gravel braided river. The river bed section has a morphology resembling a combination of a "V" and a "U" shape. It is characterized by a narrow and deep configuration, with a relatively short breadth. The sediment primarily consists of medium to large-sized gravel with minimal sand content. The gravel exhibits good roundness and displays a considerable degree of orientation. The primary microfacies present are braided channel and flood plain. The slope of the central area is decreased in comparison to the nearby end, primarily due to the presence of extensive braided rivers. The river bed has a greater width, with minor eolian dunes visible in the river channel. The gravel particles are predominantly fine to medium in size, and there is an increased amount of sand present. The predominant microfacies are braided channels, floodplains, and eolian dunes. At the distal end, the slope is minimal, the landscape is level, the braided river transitions into a meandering river, the sediment consists primarily of sand, and the signs of bioturbation are clearly visible. The primary microfacies consist of braided channels, meandering channels, floodplains, eolian dunes, lakes, and swamps.

Introduction

Sedimentology research traditionally places significant emphasis on the sedimentary properties and models of sedimentary units such as alluvial fans, braided rivers, meandering rivers, and deltas. These studies provide insights into the sediment transport process under various situations^1,2,3,4,5. Nevertheless, the current geological models typically focus on a single sedimentary unit as the primary component and fail to adequately consider the interconnectedness between different stratigraphic units. Consequently, it becomes challenging to comprehend the distribution of comprehensive sedimentary systems across the basin^6,7,8. In 2010, Weissmann et al. and Mikesell et al. introduced the concept of the Distributive Fluvial System (DFS). This term describes the process by which sediment particles transported by a river are deposited in a radial pattern within a basin. Hartley et al. conducted a comprehensive investigation on the characteristics of DFS, including its type, distribution, extension length, tectonic background, and environmental features^9,10. They also identified six channel models and five terminal kinds of DFS development¹¹. The correlation between the spatial arrangement of DFS morphology and the underlying geological setting, as well as the level of tectonic activity, prevailing climatic circumstances, and hydrodynamic parameters¹⁰. The development, distribution, and structure of DFS are influenced by authigenic and exogenous factors, including basin structure, climate, sediment source, topography, river flow, flow velocity, and sediment concentration. External elements that influence the environment include structure, climate, sediment supply, and geography. These aspects are sometimes referred to as exogenous influences. The local sedimentary environment, sedimentary topography, and sedimentary micro-topography in DFS are influenced by river flow, flow velocity, and sediment concentration. These elements are considered internal and autogenic influences. Endogenous factors are influenced by exogenous factors, while exogenous elements exert their influence through endogenous factors. Both entities collaborate to oversee the creation and advancement of DFS^12,13,14. Out of the elements mentioned, the structure plays a crucial role in determining the development position and scale of DFS. The climate directly influences the shape, size, and distribution of depositional features in river systems (DFS) by impacting the water and sediment conditions, vegetation, as well as the structure of river networks and the evolution of river types. The origin of sediments has an impact on the variations in composition and grain size of DFS sediments. The rate of flow, velocity, and yearly fluctuations of the river influence the type and geometry of the DFS river, thereby changing the morphology of its sand deposits and the structure of its sediment. Zhang et al.¹⁵ categorized DFS based on fan size (radius and area) into three types: alluvial fan (small: radius less than 30km, area less than 100 km²), fluvial fan (large: radius between 30 ~ 100km, area between 100 ~ 1000km²), and giant fan (giant: radius greater than 100km, area between 1000 ~ 10,000 km²).

Since the inception of the distributive fluvial system idea, numerous researchers have conducted extensive research on contemporary, exposed, and subterranean distributive fluvial systems. According to Lawton et al.¹⁶, the Cretaceous Drip Tank section of the Straight Cliffs Formation in the Grand Staircase area of Utah, located in the southern part of the Cordilleran foreland basin, consists of sand-rich, coarse-grained, low-curvature fluvial deposits. These deposits are primarily composed of sandy river megafans or DFS. Rittersbacher et al.¹⁷ conducted a study on the fluvial architecture of the Blackhawk Formation in the Wasatch Plateau, Utah, United States. They used geological measurements obtained from a lidar model collected by a helicopter. The researchers determined that the whole Blackhawk Formation represents a significant deposit of debris flow sediment. Davidson et al.¹⁸ conducted a quantitative study on the relationship between the DFS area and the basin area. They discovered that this relationship can be used to reconstruct the scale of ancient basin systems, predict the size of underground sedimentary bodies, and gain insight into sediment flow upon reaching a sedimentary basin. Gulliford et al.¹⁹ discovered that the crevasse splay of the Beaufort Formation in South Africa, specifically in the Permian–Triassic Lower Beaufort Formation, exhibits a geometric structure that closely resembles that of contemporary rivers. Gulliford et al.²⁰ conducted an analysis of the chronological and spatial variations in river types and stacking patterns on the river sediment outcrops of the Triassic Beaufort Formation, covering a distance of several kilometers. The Beaufort Formation is characterized by a progradational distributive fluvial system (DFS). In their study, Chesley et al.²¹ employed unmanned aerial vehicles (UAVs) and structural recovery motion (SfM) photogrammetry to create three-dimensional representations of medium-scale outcrops, allowing for the reconstruction of sedimentary properties. The channel sand bodies in the central and distal regions of Salt Wash are classified into four distinct groups based on their recorded direction, size, and plane shape. The periodic sedimentation of the Salt Wash section demonstrates its conformity to contemporary depositional facies systems. Burnham et al.²² employed a terrestrial laser scanner to precisely analyze and contrast the sedimentary architecture of the DFS in the Pennsylvania Pikeville Formation and the Hyden Formation in the central Appalachian Basin of the United States. Their objectives were to ascertain the primary factors influencing sedimentary patterns and establish criteria for identifying sedimentary architecture. However, the research on fluvial fans is limited due to the constraints of geomorphological conditions. Most studies have primarily focused on the scale, characteristics, and classification of distributive fluvial systems. However, there is a lack of sufficient typical examples that examine the sedimentary characteristics of fluvial fans in modern distributive fluvial systems.

This study aims to analyze the geomorphological characteristics and sedimentary facies development of various places within the big distributive fluvial system, using satellite influence data, modern sedimentary inquiry, and gravel measuring findings obtained from the river channel. This study examines the gravel gradient law and sedimentary model of the present distributive fluvial system of Guertu River by analyzing the variations in sediment particle size, gravel orientation, and channel type characteristics.

Geographical background

The Junggar Basin is situated in the northern region of the Xinjiang Uygur Autonomous Region in northwest China, bordered by the Kelameili Mountain, the Zaire Mountain, and the North Tianshan Mountains. The foreland basin depicted in Fig. 1A has a slightly triangular shape. The southern boundary of the Junggar Basin is situated at the intersection of the Junggar Basin and the Tianshan orogenic belt. The creation and evolution of the area are primarily influenced by the tectonic processes of the North Tianshan and Bogda orogenic belts²³. After the ocean basin closed, it underwent three distinct phases of tectonic development: the Permian foreland basin, the Triassic-Paleogene intracontinental depression, and the Neogene-Quaternary regeneration foreland basin²⁴. In the Cenozoic era, namely during the Neogene-Quaternary period, the thrust fault zone located in the southern boundary of the Tianshan Mountains had significant activity due to the influence of the Himalayan movement. The Guertu River distributive fluvial system is situated in the southern boundary of the Junggar Basin, between N44°10'–44°51' and E83°40'-83°53'. It is classified as a distributive fluvial system within the context of compressional structure²⁵. The altitude of the source is 4100 m. After traversing a southwestern to northeastern course, the river exits Hala and the mountain pass, continuing its flow in a northerly direction into the northwest corner of the 124th regiment, where it merges with the Sikeshu River. The total length of the river is 108 km, and the average yearly amount of water flowing through it is 3.42 × 108 km² (Fig. 1A). The Guertu River DFS has a vertex height of 1067 m and an end height of 313 m. The fan has a radius of 50.9 km. The relative height difference from the vertex to the end is 714 m. The average slope is gentle, measuring 1.1°. The topography of the region is characterized by elevated terrain in the southwest and lower terrain in the northeast. Human activities have an impact on the distributive fluvial system of the Guertu River, and a dam has been constructed near the vertex (Fig. 4A). The predominant water flow occurs within the man-made canal. Currently, the river channel has a reduced water flow, making it easier for us to observe the interior of the channel.

Fig. 1

Geographical location and main geomorphological characteristics of the distributive fluvial system of Guertu River. (A) Map of the area's location; (B) Geographic location of study area; (C) Gravel alluvial plain; (D) Eolian dune; (E) Fine-grained sedimentation. Data source: Google Earrth™ satellite image (Landsat/Copernicus, June 2, 2023).

The Guertu River distributive fluvial system exhibits two distinct geomorphological characteristics: a stone plain zone and a fine soil plain zone (Fig. 1A). The area located 22.3 km below the DFS vertices consists mostly of a gravel plain with an average slope of 1.2°. The predominant geomorphic features in this area are mild Gobi, as shown in Fig. 1B. The transition between the gravel plain zone and the fine soil plain zone is not abrupt, but rather occurs gradually. At the terminus of the gravel plain belt, one can find diminutive sand dunes within the river. The dimensions of the sand dunes are around 0.55 m in height, 2.1 m in length, and 0.73 m in width (Fig. 1C). The northeastern section of the fan body consists of a flat plain with fertile soil. It spans a width of 28.6 km and has an average slope of 0.9°. The topography in this area is very low-lying. Vegetation is dispersed along the river channel, whereas dunes form in the spaces between the rivers (Fig. 1D).

The predominant source of water vapor in the basin is the westerly breeze. The amount of rainfall in the western region exceeds that in the eastern region, the periphery receives more precipitation than the central area, and the windward slope has greater rainfall compared to the leeward slope. During the winter, the basin consistently maintains a layer of snow that remains constant. Additionally, the amount of precipitation received during the winter and spring months makes up around 30% to 45% of the total annual precipitation. Aside from the Irtysh River serving as the main outflow river, the remaining rivers within the basin are classified as inland rivers, while the lower regions of the basin consist of sedimentary lands²⁶. The Guertu distributive fluvial system is situated in the Kuitun River basin, and there are notable climatic distinctions between the northern and southern regions²⁷. The yearly precipitation is minimal, whereas evaporation is substantial. The daily temperature difference exhibits significant seasonal change, resulting in a dry environment with extended periods of sunshine. In terms of yearly precipitation, the combined rainfall in May and August made up 66% of the total, with an average annual precipitation of 193.7 mm. The annual evaporation ranged from 1710.5 to 1930.0 mm, which was tenfold greater than the total annual precipitation²⁸. The annual potential sunshine hours amount to 4444 h, whereas the actual sunshine hours range from 2600 to 2800 h. The period from April to September is the primary duration of abundant sunlight, with the number of sunshine hours ranging from 1680 to 1800 h. The yearly mean temperature is 7.4 °C, with a maximum average temperature of 22.8 °C in July and a minimum monthly average temperature of − 12.0 °C. These temperature conditions are characteristic of a typical continental north temperate arid climate²⁹.

Data and methods

This study utilized Google Earth software to acquire satellite pictures (specifically Landsat / Copernicus images)(2014/4/10, resolution is 30m), DEM (Digital Elevation Model) data from Global Mapper (specifically NASA DEM Elevation Data), and meteorological data from Xing Kun et al.³⁰ covering the period from 1974 to 2014. Data collection containing average temperature and monthly precipitation data. The study primarily compared geomorphological features, alterations in river morphology, variations in vertex-to-end slope, fluctuations in average river width, and changes in the ratio of flood plains to rivers. Google Earth software was utilized to acquire satellite images for analysis, including supplementary information . Additionally, Global Mapper2021 software was employed to obtain statistical data on slope using Digital Elevation Model (DEM, resolution is 30m) data, with a specific resolution. The field data comprises information on the gravel's significant diameter and direction. The camera, drone, and other equipment are utilized for conducting on-site investigations and capturing footage. The Nano Measurer 1.2 software is employed to measure the gravel data in the photograph during the later step. The orientation data is represented as a rose diagram created with Grapher 10. An analysis of sedimentary microfacies characteristics is conducted by integrating contemporary sedimentary profiles and modern geomorphological features.

The current sedimentary profile in the field was examined on-site, guided by the principles of sedimentology. The sediment variations at different positions were characterized in detail by measuring criteria such as particle size, orientation, and roundness of gravel. Huang Yuanguang et al.³¹ suggested a quantitative characterization approach to quantify the size and orientation of gravel particles throughout the measuring procedure. The particle size of the gravel is determined by the length of its longest flat axis. The orientation of the gravel is measured using a rose diagram, which shows the relative apparent dip angle of the long axis of the gravel. This includes the maximum value (a) of the orientation of any three adjacent small sector radii in the rose diagram, as well as the deviation degree (σ) of the small sector radii. The study region consisted of a total of 7 observation points, namely p1, p2, p3, p4, p5, p6, and p7. Among these, p1, p3, and p7 were drone aerial points, while p2, p3, p4, and p5 were gravel analysis places. A grand total of 1079 gravel particle sizes and 1249 gravel orientation data points were meticulously counted. By conducting a comparative analysis of the alterations in gravel diameter, orientation, and roundness across different locations, it is possible to unveil the variations in hydrodynamic conditions inside the river. The hydrodynamic force increases in proportion to the diameter of the pebbles. The orientation of particles can aid in assessing the dynamic characteristics and robustness of the medium in the sedimentary setting of large-grained sediments. Greater roundness results in increased sediment transport distance. The aforementioned data can guarantee the precision and genuineness of pertinent study.

Sedimentary characteristics of Guertu River DFS

The structural properties of the Guertu River distributive fluvial system are investigated using remote sensing satellite pictures and contemporary sedimentary survey data. The Guertu River DFS has a fan radius of 50.9 km, a circumference of 166 km, a fan top angle of 67°, and an area of 1419 km². The Guertu River DFS is classified into three zones based on variations in geomorphological parameters, slope (Fig. 2E), and river morphology: proximal, middle, and distal (Fig. 2A). The study focuses on the analysis of the distribution of sedimentary zones and the distribution model of a current distributive fluvial system based on variations in grain size, sedimentary structure, and sedimentary microfacies within each zone. The distributive fluvial system comprises the river channel and inter-channel area, as well as the river network system and river channel type. Regarding river morphology, the upstream section is characterized by a wide and interconnected network of channels, forming a substantial braided river (Fig. 2B). The central section also exhibits a braided pattern, with multiple channels weaving through the landscape (Fig. 2C). Finally, the downstream section has a sinuous meandering pattern, with a single channel winding its way through the terrain (Fig. 2D).

Fig. 2

Slope change and channel morphological characteristics of each zone of Guertu River DFS. (A) DEM data map of the Guertu River DFS, featuring the regional locations of three phase zones and 1–7 observation stations. where the measurement line from the vertex to the end is denoted by Z-Z’; (B) The closest braided channel; (C) The central channel's eolian dunes; (D) The large-scale dunes and distal meander channels; (E) Variations in the average slope in various zone. Data source: National Aeronautics and Space Administration, NASA published global 30 m resolution ASTER GDEM data on its data website (earthdata.nasa.gov).

Proximal sedimentary characteristics

The distributive fluvial system of the Guertu River is characterized by gravel sedimentary plains in its proximal geomorphological features (Fig. 1B). The dominant river morphology consists of wide braided rivers, with a river channel width of around 146.28 m, with a slope of 1.2° (Fig. 2E). During the current sedimentary study in this area, one site was designated for gravel analysis (refer to p1 in Table 1) and two spots were designated for sedimentary observation. Observation point 1 is the most elevated location accessible by walking during the investigation. The location of the site is in close proximity to the mouth of the river, where the geological features and gravel data obtained are very indicative of the nearby area. Observation point 2 (p2) is a contemporary sedimentary formation. The profile has excellent exposure and remarkable preservation, with minimal accumulation of slope deposits. The Guertu River's distributive fluvial system begins 22.3 km from the river discharge and covers 13.3% of the entire area of the fan body. The topographical characteristics of this region consist of a gravel sedimentary plain (Fig. 1b) with a gradient of 1.2%. The river has a predominantly braided shape, with a width of around 146.28 m. The Guertu River distributive fluvial system primarily consists of debris flow deposits, braided river channels, and flood plain microfacies. The ratio of flood plain to braided river channel is 7.36. Within this region, there is primarily one location for analyzing gravel (refer to point p1 in Table 1) and two places for observing sedimentary formations. A gravel point (p1) near observation point 1 was used to measure a total of 354 particle size data and 171 directional data. The gravel particle size ranges from a minimum of 22.02 mm to a maximum of 705.82 mm, with an average size of 80.56 mm. The orientation of the gravel's greatest flat surface ranges from 10° to 40°, as seen in Table 1 and Fig. 3. The analysis indicates that the sediments in close proximity are composed of medium to large conglomerates that exhibit favorable orientation and roundness.

Table 1 Gravel analysis data of Guertu River DFS.

Fig. 3

Particle size characteristics of proximal p1 gravel analysis point. The gravel particle size ranges from a minimum of 22.02 mm to a maximum of 705.82 mm, with an average size of 80.56 mm. The orientation of the gravel's greatest flat surface ranges from 10° to 40°.

An artificial dam was constructed at observation point 1, causing the water to flow downstream from the adjacent man-made channel (Fig. 4A). The Guertu River is impacted by human intervention, resulting in a reduced water flow in the river channel. This allows us to easily access the river channel and study the sedimentary properties in close proximity. The majority of the riverbed around the end of Guertu River DFS is characterized by the accumulation of gravel. Following the application of artificial treatment, the water flow transports an increased amount of fine-grained sediments. Within the channel, one can observe deposits of gravel arranged in a cone shape. The barrier gravel has a diameter of approximately 37 cm, and the tailing length measures around 95 cm (Fig. 4B). Of the various components, the tailing material is predominantly sandy, whereas the outermost part of the tailing contains minor deposits of gravel. The braided channel contains predominantly sandy sediments with low hydrodynamic force. At the bottom of the channel, there is a visible sand body with a wavy pattern (Fig. 4C).

Fig. 4

Sedimentary characteristics of proximal channel of distributive fluvial system. (A) Proximal cut channel; (B) Gravelly barrier deposits with sand paths; (C) Channel bottom type that undulates.

The proximal observation point 2 (p2), situated at coordinates N44° 21′ 30.14", E 83° 51′ 5.60", is positioned to the south (downstream) of the vertex at an elevation of 716 m. The distance from the vertex line is 10.6 km, and the position is indicated in Fig. 2. The entire segment is predominantly composed of gravel deposits, with a lower proportion of sandy components. The vertical variation in the river channel period is clearly evident, and it can be categorized based on the arrangement of large gravels at the bottom and the formation of bedding (Fig. 5A). The development of braided channels is evident in the profile, with locally formed high-angle tabular cross-bedding, and debris flow deposits may be observed at the bottom (Fig. 5B). The gravels are densely packed and organized in a stratified or overlapping fashion (Fig. 5C).

Fig. 5

Modern sedimentary section of the proximal channel of the distributive fluvial system. (A) The modern sedimentary profile of a braided channel; (B) Cross-bedding of plates; (C) Clearly visible gravel orientation.

Sedimentary characteristics in the middle

The intermediate section of the Guertu River distributive fluvial system is situated between 22.3 and 45.6 km downstream from the proximal end, making up 35.63% of the entire fan area. The predominant geomorphological features of this region consist of desert, swamp, and sandy vegetation growth, with a slope measuring 0.9 degrees (Fig. 2E). The river's breadth is rather wide, ranging from 90 to 157 m, due to the modest branching and swing amplitude of the upstream river. The river morphology is primarily characterized by the presence of extensive braided rivers. The middle section of the area mostly consists of braided channels, flood plains, and eolian sand dunes, with the eolian sand dunes being relatively minor in size. The position of the transition from the gravel plain zone to the fine soil plain zone is best indicated by observation point 3. The sediment shift from gravelly to sandy dominated features may be seen in the sediment data and sedimentary characteristics at this location, which is close to the proximal and middle limits. The sedimentary profile at observation point 4 (p4) is contemporary. There are very few slope deposits and the profile is well-preserved and visible. A hand-dug hole at Observation Point 5, which is situated on the floodplain, allows visitors to see the floodplain's sedimentary profile. In the area, there are three primary locations for observing sedimentary features: p3, p4, and p5. Among these, p3 and p4 are specifically designated for analyzing gravel.

A total of 465 particle size measurements and 758 directional measurements were taken from two gravel locations (p3, p4) located near the observation point 3,4. The observation point 3 has a minimum gravel particle size of 12.42 mm, a maximum particle size of 130.9 mm, and an average particle size of 42.35 cm. The orientation analysis reveals that the predominant alignment of the largest flat surface of the gravel is between 150 and 180 degrees (Table 1, Fig. 6A). Observation point 4 has a reduced overall particle size compared to observation point 3, and the orientation of the gravel is more uniform. The gravel has a minimum particle size of 4.51 mm, a maximum particle size of 45.77 mm, and an average particle size of 13.84 cm. The orientation of the greatest flat surface of gravel is between 100 and 130 degrees (Table 1 and Fig. 6B). The middle section of the gravel consists primarily of fine to medium-sized gravel, which the degree of roundness secondary and orientation consistented. This suggests that the hydrodynamic conditions in the middle part of the distributive fluvial body of the Guertu River are robust, primarily influenced by the forceful movement of sediment through traction flow. This is evident in the presence of a large braided river channel.

Fig. 6

Particle size characteristics of P3 and P4 gravel analysis points in the middle. (A) The minimum gravel particle size of the observation point 3 is 12.42 mm, the maximum particle size is 130.9 mm, and the average particle size is 42.35 cm. The orientation shows that the tendency of the maximum flat surface of the gravel is 150–180°; (B) The minimum gravel particle size of the observation point 3 is 12.42 mm, the maximum particle size is 130.9 mm, and the average particle size is 42.35 cm. The orientation shows that the tendency of the maximum flat surface of the gravel is 150 ~ 180°

The middle observation point 4 ( p4 ), coordinates N 44° 27 ′ 40.99 ', E 83° 53 ′ 10.67 ', the current altitude is 469 m, the distance from the vertex is 25.8 km, the position is shown in Fig. 2. The whole section here is dominated by gravel deposition, with thin layer of sandy deposition. Horizontally, the content of gravel sediments in the upper reaches of the river is significantly smaller than that in the lower reaches, and the sand content is increased (Fig. 7A). By analogy with the morphological characteristics of modern rivers at this point, the changes of sediment grain size and structural characteristics may be the transformation of river morphology. The gravel sedimentary braided channel is the main channel, which can be seen from the gravel bar to the sandy bar, and the gravel to the sandy transition area. Vertically, the gravel has obvious directionality, and the coarse-grained sediments in the bottom layer are relatively developed with a large thickness of about 0.8 m. The sediments in the middle are mainly fine-grained thin-bedded sandstone, developing massive bedding, trough cross bedding and parallel bedding, showing obvious channel deposits. The thickness of fine-grained sandy sediments on the top is small, about 0.2 m (Fig. 7B).

Fig. 7

Modern sediment profile of the middle channel. (A) The modern sedimentary profile; (B) The section's column diagram displays abundant bedding; (C) The gravel has a discernible vertical variation; (D) There is discernible cross-bedding that resembles a trough; (E) The gravel's orientation is clear.

Based on the directional distribution of the larger gravels at the bottom and the change law of particle size (Fig. 7C), the modern portion of the distributive fluvial system's middle channel can be separated into stages. The spaces are partially filled with sand, and the gravels are in point contact or linear contact. The multi-stage particle supporting gravel layer and the supporting gravel layer with support alternate in the braided channel. The supporting gravel typically has a diameter of 4–7 cm. Fine-grained sediments are absent from the spaces between the gravels, and high-angle, plate-shaped cross-beddings have occurred locally. At the bottom, retention deposits are visible (Fig. 7D). There are clear directed arrangement and imbricate structure characteristics in the braided channel deposits close to the upstream (Fig. 7E).

Observation point 5 is situated in the lower section of the distributive fluvial system, specifically in the middle half. The flood plain in this location is further advanced, as shown in Fig. 8A. The channel deposits are primarily composed of fine gravel. Based on the similarities in the physical features of present-day rivers, the sandy lens in question could potentially represent the primary sandbar located within the channel, as depicted in Fig. 8B. The riverbed in the active river channel is primarily composed of gravel, with a thin layer of sand covering it in areas where the hydrodynamic force is weak. At the edge of the riverbed, remnants of bird's footprints can be observed (Figs. 6A, 8C). In comparison to the nearby area, the flood plain in the center region is more advanced in terms of development, and the vegetation is primarily composed of low bushes (Fig. 8D). The fine-grained deposits are retained and the floodplain deposits are thickened in the floodplain deposition area due to the worsening of hydrodynamic conditions near the middle (Fig. 8E).

Fig. 8

Interchannel sedimentary characteristics. (A) Satellite images of floodplains and central rivers; (B) modern interchannel sedimentary profile; (C) Bird foot remains in transient river channels; (D) interchannel geomorphology and vegetation features; E. contemporary interchannel sedimentary profiles of floodplain.

Distal characteristic sedimentary characteristics

The distal end of the Guertu River DFS is situated at a distance of 45.6–51.3km downstream from the midpoint, with a slope of 0.5° (Fig. 2E). The geomorphic characteristics of this region primarily consist of the growth of vegetation along river channels, the presence of stagnant lakes within these channels, and the existence of deserts in places that are distant from the river channels (Fig. 9A). Based on the UAV aerial photographs, it is evident that the river has a rather limited width, ranging from 80–100 m, and its shape is predominantly meandering (Fig. 9B). The primary microfacies at the distal end include meandering river, flood plain, eolian dune, and temporary lake. In this area, there are two locations where sedimentary observations can be made, as well as one point (p6) specifically designated for gravel analysis.

Fig. 9

Distal sedimentary characteristics of Guertu River DFS. (A) Remote oases and dunes captured on satellite imagery; (B) a meandering river; (C) transient lakes; (D) sunk grooves displaying dark organic materials in floodplains; (E) enormous animal traces seen in deserted river channels.

The total of 260 particle size measurements and 320 directional measurements were taken at gravel point p6, which is located near observation point 6. The gravel particle size ranges from a minimum of 0.47 mm to a maximum of 4.78 mm, with an average size of 1.64 mm. The orientation is determined by the inclination of the biggest level area of gravel, which ranges from 80° to 110° (Table 1, Fig. 10). The orientation is indistinct, nearly round, and exhibits satisfactory roundness, suggesting that the hydrodynamic conditions at the far end of the Guertu River's distributive fluvial system are feeble.

Fig. 10

Particle size characteristics of p6 gravel analysis points in the distal. The minimum gravel particle size is 0.47 mm, the maximum particle size is 4.78 mm, and the average particle size is 1.64 mm. The orientation is characterized by the tendency of the largest flat surface of gravel at 80°–110°

Observation site 7 (p7) is situated at the terminus of the distributive fluvial system. The predominant sediments consist of fine sand and mud. Additionally, there are stagnant depressions and tiny lakes present in the adjacent low-lying areas (Fig. 9C). Humus is seen in winding channels (Fig. 9D), while curving channels show the presence of big animal tracks (Fig. 9E).

Discussion

In his study, Hartley highlighted the concept of a distributive fluvial system to describe the channel system where rivers enter a basin from a specific point and radiate outwards. He also emphasised the importance of understanding the channel distribution patterns in the sedimentary system, which can be categorised into six modes: braided branch type, single branch type, braided meander type, single meander type, multi-meander type, and single meander type¹⁰. By comparing satellite photos of the Guertu River DFS before and after artificial treatment during the dry and wet periods, it has been observed that there are certain issues with this classification model. In the dry season, the Guertu River distributive fluvial system has a unidirectional meandering channel distribution pattern. During the rainy season, it assumes a unibraid configuration. However, it is important to mention that the size of the fan remains relatively constant throughout time. The reason for this is that the terminal type of the Guertu River DFS merges with the axial Sikeshu river. During the rainy season, the fan body is unable to traverse the axial river. In their study, Zhang Changmin et al.¹⁵ employed a classification scheme that takes into account the scale, and identified the Guertu River DFS as a significant distributive fluvial system. The key elements to consider in the analysis of distributive fluvial systems are the tectonic background, climatic backdrop, fan size, slope, geomorphic features, channel type, and facies association. The primary focus of this study will be on the analysis and classification of geomorphic units, channel types, and sedimentary model.

Geomorphological units and channel types

This study conducted a comprehensive analysis of the sedimentary system by comparing the sedimentary features of the Guertu River's distributive fluvial system from its source to its mouth. The analysis included parameters such as slope, river width, ratio of flood plain to river channel, and river morphology. The contemporary sedimentary data derived from satellite pictures provide information on the slope, channel width, channel to flood plain ratio, and geomorphological properties of the distributive fluvial system of Guertu River(Table 2). Weissmann et al.¹¹ observed that the distributive fluvial system is arranged in a radial pattern extending from the apex. They also noted that the size of the channels generally reduces as you move downstream, and the proportion of flood plain to channel increases. In their 2021 study, Charlottel et al.³² examined the extent and size of sediment deposits in the Lower Jurassic Kanyenta Formation in the southwestern United States. They found that the size of sediment grains and the width of channels reduced in more distant areas, but the proportion of flood plain to channel area increased. The findings of these studies exhibit a similar pattern to the results of this study, however the precise ratios are not in agreement. The inconsistency in the climate backdrop and parent rock qualities of various distributive fluvial systems, along with the specific ratio of flood plain to river channel area, is the cause of this result.

Table 2 Statistics of related parameters in different zones of the Guertu River DFS.

From a geomorphic standpoint, the proximal section primarily consists of a gravel gobi beach that forms during the transition from the piedmont highland to the piedmont plain. This section has a maximum slope of 1.2°. The river's course is determined by the terrain of the surrounding land and is also affected by human activities, resulting in the formation of an incised braided river. The river has a substantial width, with an average measurement of 146.28 m. The proportion of the flood plain to the river is quite minimal. The primary topographical feature is a region that transitions from a gravelly surface to a sandy surface, with a slope that is 0.9 degrees less steep than the end closest to it. The river channel gradually narrows due to branching and infiltration, while the floodplain gradually widens. The river is a wide meandering watercourse with an average width of 113.32 m. Because it is close to the center of subsidence in the basin, the landform at the further end consists primarily of oasis and sand dunes. The most gradual incline measures 0.5°, resulting in the obstruction of water flow and the extensive formation of geomorphic features such as stagnant depressions and small lakes. The river's form ranges from a braided river to a meandering river, with an average width of 93.6 m. In line with the findings of this research, Goswami Pradeep and Deopa Tanuja³³ examined the lithofacies and sequence of the Lower Siwalik in the southeastern Uttarakhand state of India. They concluded that the meandering river and braided river are present in the central and distal sections of the large fan. In their study, Marcus et al. ³⁴discovered three distributive fluvial systems located on the northeastern edge of the Bauru Basin in southeastern Brazil. They conducted a thorough examination of the river channels and paleosoil profiles, and determined that the deposits and internal architecture of these channels can provide insights into the past climate conditions. This study also demonstrates that in arid climates, the distributive fluvial system forms extensive flat dunes, while in humid climates, the flood plain is more developed. The climate of the Guertu River in this study is characterized as a continental north temperate arid environment. At the distal end, there are large-scale dunes, which aligns with the findings of previous research. In their study, Renske C. et al.³⁵ examined the correlation between the width of the channel, river branching, sediment grain size, and the facies belt of the distributive fluvial system. They utilized satellite imagery and conducted flume simulation experiments to assess these relationships. The distributive fluvial system is characterized by a transition from coarse-grained sediments in proximal areas to fine-grained sediments in distal areas. This transition is accompanied by a change from wide channels to tight channels, and is often accompanied by branching. Luigi Bruno and his colleagues³⁶ conducted a quantitative study on the spatial distribution and downstream trends of the Lower Jurassic Kayenta Formation in the southwestern United States. Their findings revealed several important observations. Firstly, the study showed that the DFS model lacks constraints on the river system. Secondly, there was a decrease in the merging of downstream river channels and sheet-like building elements. Thirdly, although small, there was a decrease in the downstream particle size. Fourthly, there was a decrease in the width-to-thickness ratio of downstream preserved river channels and sheets. Lastly, there was an increase in the percentage of elements found on the downstream riverbanks. The law governing the morphological transformation of river channels in the Guertu river DFS involves a continuous progression from an incised braided river to a branched braided river and finally to a meandering river. The river channel gradually narrows from its source to its mouth, while the floodplain widens downstream. Additionally, the ratio of the floodplain's width to the river channel increases. The findings of these research indicate that the geomorphological features of the distributive fluvial system are influenced by both the tectonic conditions and the climate. The characteristics include a positive correlation between slope and fan scale, the formation of large-scale flat sand dunes in the distributive fluvial system in arid climates, and the enhanced development of flood plains in wet climates.

The river bed in the proximal channel is braided with gravel, and traction flow is the primary hydrodynamic condition. The primary source of water flow is derived from the meltwater of ice and snow in the source region. The particle size is characterized by a gritty texture, predominantly consisting of medium to large-sized gravel. The gravel has clear orientation, contains a lower proportion of sand, and is poorly sorted, indicating a high-energy aquatic habitat. The central channel consists predominantly of a gravel riverbed, with the presence of tiny lenticular sand formations. The gravel in question is predominantly composed of fine to medium-sized particles. The sand content is elevated, and the sorting is moderate. Additionally, it exhibits a certain degree of directionality, which is evident in the transition from a high-energy water environment to a low-energy water environment. The hydrodynamic conditions in the middle section of the river are deteriorated as a result of branching and infiltration. This leads to temporary drying up of some tributaries during the dry season. The downstream river transitioned from a braided pattern to a meandering one, with predominantly sandy deposits. The presence of steep slopes in this location, particularly around the edges of active lobes, contributed to the formation of tiny lakes. along the termination of the facies band, the river exhibits meandering characteristics, and it is evident that sediment accumulation occurs along the riverbed, indicating a low-energy water environment (Fig. 11B). The sedimentary system of DFS exhibits decreasing hydrodynamic conditions from the proximal to the distal end, transitioning from a coarse-grained braided river to a fine-grained meandering river. This change is attributed to the internal dynamics of the sedimentary system, rather than solely the particle size difference resulting from the development of braided and meandering rivers^10,11,37. The sediments in the distributive fluvial system exhibit a steady decrease in particle size from the upper reaches to the lower reaches, with an increase in the proportion of fine-grained sediments towards the distal end. Furthermore, the cruise picture reveals that the sediments near the braided river are predominantly composed of coarse gravel. The sediments in the latter portion of the meandering river are predominantly composed of fine sand. An exhaustive examination conducted throughout the Guertu River, starting from the nearest point to the furthest point, revealed that the sediment grain size of the conventional braided river is greater than that of the meandering river. Furthermore, it demonstrates a consistent decrease within the same river system. Jonathan W. Primm³⁸ also observed this alteration in the examination of The Turonian-Coniacian Smoky Hollow Member of the Straight Cliffs Formation in the Kaiparowits basin of southern Utah. These trends are caused by the reduction in energy and the decreased ability of the river to convey water downstream due to the widening of the river system, the splitting of the river channel, and the high rates of water loss through evapotranspiration and infiltration into the dry ground or avulsion of the channel belt^39,40.

Fig. 11

Sedimentary model of Guertu River DFS. The Guertu River DFS sedimentary model; (B) Modifications to each zone's channel shape, phase combination, and hydrodynamic conditions.

Changes in animal footprints observed in the field during contemporary sedimentary investigations were also noted by this study. At the closest point of the distributive fluvial system, no animal footprints were discovered. The explanation is that footprints cannot be retained in the proximal river channel due to the prevalence of gravel deposition. A bird's foot fragment of approximately 2 cm in diameter was discovered in the river channel in the center of the distributive fluvial system, while a wide variety of large mammal activity remnants were preserved in the curved section of the distal river channel. This outcome is comparable to Richards, Paul J⁴¹ findings. Based on field observations from the south-east Australian hillslope, this study demonstrates that bioturbation in the river sedimentary system is hard to maintain in a gravel-developed river channel and that it increases as hydrodynamic force decreases. The analysis is being done for two reasons. First, the bioturbation can be preserved due to the fine sediments in the middle and distal portions of the distributive fluvial system. The incised braided river's proximal development is the second. The influence of topographic characteristics makes it difficult for creatures to get near rivers.

Sedimentary model

The Guertu River's DFS is created by both the river and flood transport. The alluvial plain and alluvial plain are formed by the accumulated alluvial and alluvial deposits. The wind has altered some of the alluvial deposits, causing physical weathering or aeolian deposits to blanket the surface. The proximal tabular cross-bedding, trough cross-bedding, and massive bedding in the vertical portion of current deposition primarily create microfacies like channel filling deposition and gravel flood plain (Fig. 11A ). The majority of the land is somewhat level, although eolian dunes can emerge in certain places where the wind has clearly altered the sediments. The sediment's particle size is often finer, and the flood plain is more extensively dispersed. There are lakes, rivers, dunes, and other microfacies that meander. The less developed distal gravel sediments in the Guertu River distributive fluvial system are primarily argillaceous and sandy sediments. These sediments reflect the meandering river's sedimentary characteristics in the section of the river channel that has been abandoned, primarily developing meandering rivers, flood plains, aeolian sand dunes, swamps, and lakes. After conducting a phase analysis of the Upper Jurassic sedimentary rocks of the Guará Formation in the arid region of Gondwana, Adriano Domingos dos Reis et al.⁴² came to the conclusion that the arid climate had an impact on the Pirambóia Formation, causing it to develop eolian dunes, eolian sand layers / interdune, and temporary river channels. A quantitative study of the Guara Formation's Upper Jurassic of Gondwana stratigraphic section was done by Adrian. D et al.⁴³, who came to the conclusion that the formation formed an arid distributive fluvial system sedimentary model. Aeolian deposits, channel infill deposits, temporary channels, and flood plains are the four combinations of microfacies that were found. The Guara Formation produced a large DFS that is split into four zones, ranging from proximal to distal, much like this study. Zones 1 and 2 are composed mostly of ephemeral and perennial fluvial channels, respectively. Zones 3 and 4 are deposits outside of the fluvial channel belts, with aeolian and floodplain deposits predominating in each zone. Zone 1 and 2 represent the proximal and center halves of this study, respectively, and Zone 3 and 4 represent the distal portion, in terms of phase combination. Based on the categorisation by Hartley et al.¹⁰, Zone 4 in enormous DFS can be considered as an expansion of the distant area¹⁰, and instances can be observed in strike-slip basins located in desert regions of China and Mongolia⁴⁴.

Conclusions

The the Guertu River DFS is generally thought to be a sizable distributive fluvial system experiencing drought as a result of research into the climatic background of the Junggar Basin. Based on geomorphological features, channel morphological features, and facies combination, the Guertu River DFS may be separated into three facies belts: proximal, middle, and distal. The Guertu River's distal slope is the smallest and its DFS proximal slope is larger than its middle slope. Gravel sedimentary plains make up the majority of the near-end geomorphological characteristics. In the center are aeolian dunes and sparse vegetation, while at the far end are extensive aeolian dunes and flora formed along the river. The topography on either side of the source area controls the proximal river's shape as an incised braided river during the shift in river morphology. The middle river's morphology primarily consists of a braided river with many branches. The braided river transitions to the meandering river in the distal river morphology, primarily as a meandering river.

The proximal gravel exhibits good orientation in the Guertu River DFS sedimentary features shift, with the majority of the gravel being medium-giant and sub-angular in nature. The predominant type of gravel in the center is fine-medium, with good roundness and orientation. Sand deposition predominates in the distal region, with a lesser amount of spherical, directionally fuzzy gravel. The sand body distribution law indicates that the sand body concentration is highest at the far end and lowest at the near end of the Guertu River DFS's general variation in sedimentary pattern. There are several developed lakes, swaths, oxbow lakes, and abandoned river channels. The near and middle ends are dominated by braided river deposits, while the far end is dominated by meandering river deposits.