Distributive Fluvial System (DFS) refers to “a sedimentary system of river entering a sedimentary basin and dispersing from an apex”. This definition is firstly proposed by Weissmann and Hartley et al. in 2010, based on a statistical study of fluvial sedimentary systems in more than 700 modern sedimentary basins all over the world by using Google Earth. It is deemed that DFS is a common alluvial geomorphologic system which is composed of alluvial fan, fluvial fan or giant fan and develops in the adjacent mountainous region^[1,2]. In the same year, Hartley et al.^[3] further summarized the DFS recognition criterion and highlighted the channels in a DFS are branched, but may not be active simultaneously^[4,5]. The definition of DFS is questioned by some scholars^[6,7], but the continuous development of modern geo-information technologies expands vision for observing the geomorphologic features on the earth surface, and deepens the understanding on alluvial system of fluvial sedimentologists and geomorphologists, so the concept is accepted and utilized by more and more scholars^{[8,9,10,11,12]}. At present, the studies on DFS focus on its scale, characteristics and classification^[13,14,15]. Weissmann et al. put forward the progradation model of DFS based on the variation in hydrodynamic intensity, sediment transport velocity and sediment grain size along the orientation of source in 2013, and classified the DFS into 4 types based on their further understandings on continental basins in 2015^[5]. Davidson et al.^[16] divided the DFS into 3 or 4 facies belts, after comprehensively analyzing modern DFS sedimentary environments. By studying the sedimentary characteristics of channel sand bodies in different formations, Trendell et al.^[17] recognized a total of 11 lithofacies in the channel sand bodies developed in DFS. Based on the DFS viewpoint, Owen^[18] showed the areal characteristics, vertical sequence and sedimentary architecture of the Morrison Formation-Salt Wash Member sedimentary system. The studies mostly focus on the scale, characteristic and classification of DFS with no attempt to analyze typical cases of modern DFS. There are rarely reports in China on the differences in the areal and vertical characteristics of DFS.

Modern DFSs are widely distributed in arid areas of northwestern China^{[19,20,21,22]}. In order to determine the differences in sedimentary characteristics in different zones of arid-type DFS, this paper combines the modern information technologies (satellite remote sensing and UAV) with the classic sedimentological theory to analyze the differences in geomorphologic characteristics in different areas of the Shule River DFS. Then, the differences in sediment grain size, gravel directionality and channel type developed in different areas are described, and the distribution characteristics of sedimentary facies belts of the Shule River DFS are studied. Finally, the sedimentary model is established.

The Shule River DFS is located in the northwestern Gansu Province^[23], the west of the Hexi Corridor^[24], 96°6°-97°24°E and 39°56°-40°36°N (Fig. 1). It originates from the Shule South Mountain of the Qilian Mountains, with the largest catchment area across the western segment of the Qilian Mountains^[25]. The river is perennial with total length of 620 km. After passing through the Zhaobi Mountain Valley, the Shule River disperses into the Hexi Corridor plain to the north of the Altyn Tagh fault (Fig. 1). Geomorphologies related to the Shule River DFS include the gravel plain belt and the oasis fine-soil plain belt (Fig. 1a). The gravel plain belt is located within 35 km to 45 km from the apex of DFS downward, with a slope of 1.05%, and geomorphological characteristics of gentle Gobi (Fig. 1b). In the north of the fan body is an oasis fine-soil plain with a width of 12 km to 20 km and a slope of 0.51%, where the topography is low and flat and swamps and troughs are widespread. And an N-S trending gully with the incision depth of 10 m is formed at the toe of the fine-soil plain belt due to the overflow of underground water (Fig. 1c).

The Shule River DFS is situated inland, where climate is dry, with low rainfall, high evaporation, large difference in temperature between day and night, and long sunshine duration^[26]. Annual precipitation occurs mainly from June to September and averages 39.9 mm, annual evaporation is 2486 mm, and annual sunshine hours is 3246.7^[27]. Annual average temperature is 9.4 °C, monthly average maximum temperature is 23.1 °C, monthly average minimum temperature is -9.4 °C, extreme maximum temperature is 43.6 °C, extreme minimum temperature is -28.5 °C, and annual average frost-free period is 142 d, indicating a typical temperate arid climate^[26,27].

The geomorphological features, channel style and distribution range of DFS were surveyed and statistically analyzed using the data of satellite image, radar digital elevation and UAV aerial photography (Table 1). The variations in geomorphological features and river morphology were compared by means of the Google Earth satellite image. The changes in the slope from the apex to the toe, the average channel width and the flood plain to channel ratio were statistically analyzed mainly by using the radar digital elevation data. Channel width was the distance between the outermost banks, which was measured along the direction perpendicular to the river axis, including sand bar and channel bar.

Guided by sedimentological theory, the field modern sedimentary sections were surveyed on site. The variation characteristics of sediments at different positions were described precisely by measuring gravel parameters, e.g., grain size, directionality and roundness (Table 2). The river morphological variations in different areas were delineated precisely on the basis of satellite image by using the UAV aerial photography technology. The characterization of gravel grain size and directionality in the process of measurement was conducted by using the quantitative gravel directionality characterization method proposed by Huang et al.^[28]. The grain size was determined from the long-axis and short-axis size^[29]. The directionality of gravels was determined using the rose plot for long-axis relative apparent dip, including the maximum value (a) of the directionality of radii of 3 adjacent small fans and the degree of deviation (σ) for radius of small fan. Roundness was classified according to the criterion proposed by Tao et al.^[30] in 2018 that involves 5 types, i.e., angular, subangular, subangular-subrounded, subrounded and rounded. The study area was surveyed at 8 points (p₁ to p₈), of which 2 were UAV aerial photography points (p₂ and p₈) and 5 were gravel analysis points (p₁ to p₅), with 1932 grain size data and 1322 directionality data (Table 2).

The morphological features of the Shule River DFS were analyzed based on remote sensing image data. On plane, the Shule River DFS appears as a nearly symmetric fan with a radius of 59.6 km, arc length of 128 km, chord length of 113 km and fan apex angle of 96°, covering an area of 3660 km². According to the differences in geomorphological features, slope and river morphology, the Shule River DFS is divided into the proximal, middle and distal belts (Fig. 2). Geomorphologically, it is characterized by gravel Gobi in the proximal belt, psammophytic vegetation in the middle belt, and oasis plain in the distal belt. Speaking of slope, there is a trend of decreasing from the proximal belt through the middle belt to the distal belt (Fig. 2b). The river is morphologically large- scale braided river in the proximal belt, bifurcating braided river in the middle belt and meandering river in the distal belt. The differences in sedimentary microfacies types and sedimentary characteristics allow us to determine the distribution law and distribution model of sedimentary facies in modern DFSs.

The proximal belt of the Shule River DFS is located within 10 km from the site where river runs out of the mountain and accounts for 5.87% of the total fan body area. It is geomorphologically characterized by the coverage of gravel Gobi (Figs. 1b and 3b), with a slope of 1.25% (Fig. 2b). The river morphology is dominated by large-scale braided river (Fig. 3) with channel width of 400 m to 500 m. Major subfacies developed in the proximal belt of the Shule River DFS include flood plain and braided channel, whose ratio is 6.64. There is one sedimentation survey point (p₁ in Fig. 2) and two gravel analysis points (p_1-1 and p_1-2 in Table 2) in this belt.

The location of p₁ is shown in Fig. 2. The whole section is dominated by gravel deposits with less sand composition. Strong stage variation of channel occurs laterally. The development stage of channel can be divided based on the directional arrangement of larger gravels at the bottom and the directional growth of vegetation near the top (Fig. 4a). Gravels are closely contacted and show bedded or imbricate arrangement. Braided river channel is visible, with locally developed high-angle incised tabular cross-bedding and lag deposits at the bottom (Fig. 4b). Gravels are sub-rounded owing to the long-distance transport through the source area of the Shule River (Fig. 4c). Vertically, channels of different stages fine upward (Fig. 4d), and the gravel bed at the bottom (Fig. 4c, 4d) shows distinctive directionality.

A total of 328 grain size data and 249 directionality data were measured at two gravel points (p_1-1 and p_1-2) near the p₁ survey point (Fig. 5). The minimum, maximum and average grain sizes are 1.47 cm, 23.94 cm and 5.18 cm, respectively. Directionality is reflected as a dip of 30° to 60° along the maximum flat horizontal surface of gravel (Table 2 and Fig. 5). Good directionality and roundness of gravels suggest that the hydrodynamic condition in the proximal belt of Shule River DFS is quite strong and dominated by traction flow, as evidenced by the emergence of braided channels with high-energy environment.

Inter-channel flood plains near the survey point p₁ are relatively developed (Fig. 3), where abandoned channels are visible (Fig. 6a). The areal and vertical analysis on the position corresponding to the flood plain shows that proximal flood plain is characterized by thin mud layers (Fig. 6b) on vertical section of modern deposits (Fig. 6b) and white mud skin on the plane (Fig. 6c). In addition, the scattered gravels that are present in inter-channel flood plain in the site near the mountain pass (Fig. 6c) are products of the sheet flow from channel during the flood period. Based on the comparison of the p₁ section with the modern flood plain deposits (Fig. 6a), these thin mud layers are poorly preserved on regional section due to the effect of strong hydrodynamic conditions at the proximal portion and the scouring action of the next-phase flooding.

The middle belt of the Shule River DFS is located within 30 km below the proximal belt and accounts for 46.25% of total fan area. It is geomorphologically characterized by Gobi desert and psammophytic vegetation (Fig. 7b), presenting a slope of 0.94% (Fig. 2b). The fluvial channels are relatively narrow, ranging between 100 m and 200 m, due to the bifurcating and infiltration of the upstream river. The river morphology is dominated by highly branched braided river (Fig. 7a). Major sedimentary microfacies developed in the middle belt include braided channel, flood plain and aeolian dune. The flood plain to braided channel ratio is 15.29. Aeolian dunes are limited in scale and developed near the distal belt (Fig. 7c). There are three sedimentation survey points (p₂, p₃ and p₄ in Fig. 2) and corresponding three gravel analysis points (p_2-1, p_3-1 and p_4-1 in Table 2) in this belt.

At the survey point p_2, channel bifurcating seriously (Fig. 2a) and flood plain developed widely (Fig. 7a). Due to the influence of bifurcating and infiltration, hydrodynamic force weakens, branch channels dry up and water flow occurs only in the main channel (Fig. 7b). The channel bar transits from gravel in the proximal belt to sand (Fig. 7c). Gravel analysis show that the gravels at this survey point are mainly subrounded with the minimum grain size of 1.04 cm, the maximum grain size of 14.45 cm and average grain size of 3.85 cm, and their directionality is reflected by a dip of 120° to 150° along the long-axis (Table 2).

The survey point p₃ is near the margin of the DFS (Fig. 2a), where flood plain is relatively developed. Both p₂ and p₃ lie in the upper part of the middle facies belt, but p₂ is located within the main runoff of the Shule River with stronger hydrodynamic force, so its grain size of sediments is coarser than that at p₃. A total of 347 grain size data and 276 directionality data are recorded at p₃. The minimum, maximum and average grains size of gravel are 0.60 cm, 6.87 cm and 2.04 cm, respectively (Table 2). The dip along the maximum flat horizontal surface of gravels ranges from 90° to 150°. The flood plain between the channels near p₃ is more developed than that in the proximal belt (Table 1 and Fig. 8a), and the vegetation coverage is higher (Fig. 8b), with plant roots on the vertical section of modern sedimentation (Fig. 8c). The middle belt where the hydrodynamic force weakens allows preservation of fine-grained deposits in flood plain area and thus has thicker flood plain deposits than the proximal (Figs. 6b and 8c).

Deposits at the p₄ are dominated by gravel (Figs. 2a and 9a), with lenticular sand body developed (Fig. 9a, 9b). Gravels show clear vertical directionality and imbricate arrangement. Deposits grade from about 0.8 m thick coarse grain at the bottom to about 0.2 m thick fine grain at the top, showing typical feature of channel deposits. The analogy with modern river morphology at this survey point indicates that the lenticular sand body may be a small channel bar within the channel (Fig. 9b, 9c). Gravel analysis shows that the gravels at this survey point are mainly subrounded, with the minimum grain size of 0.97 cm, maximum grain size of 5.72 cm and average grain size of 2.43 cm. The long-axis dip is 90°-150° (p_4-1 in Fig. 10).

The grain size of sediments is finer and the flood plain is more widespread in the middle belt than in the proximal belt. In open areas with flat topography, the reworking action of wind on deposits is obvious, allowing the formation of aeolian dune locally (Fig. 7c). Due to the influence of bifurcating and infiltration, some rivers dry up during the dry season and appear as the temporary channel sedimentation. Also, abandoned lobes are often formed in the middle belt of DFS, which does not deny the development of channels but suggest they are different from the current river in the formation period.

The distal belt of the Shule River DNS is located within 15 km below the middle belt and accounts for 47.88% of total fan body area. This area is geomorphologically characterized by oasis plain with high coverage of vegetation, showing a slope of 0.51% (Fig. 2b). Channels become narrower with a width ranging from 80 m to 100 m. The river morphology transforms from braided to meandering in this belt (Fig. 11a, 11b) owing to the gentle water flow as a result of decreased slope. Sedimentary microfacies developed below the spring line include braided channel, meandering channel, flood plain, dune, swamp and ponds (Fig. 11a). There are four survey points (p₅, p₆, p₇ and p₈ in Fig. 2) and one gravel analysis point (p_5-1 in Table 2) in the distal belt.

The section at the survey point p₅ shows the upward thinning gravel bed and thickening lenticular sand bed in band shape (Figs. 2a and 12a). The fine-grained sandy deposits at the top have similar thickness with the gravel deposits at the bottom, marking the transition of channel morphology from braided to meandering. Gravel analysis indicates that the gravels at p₅ are subrounded with the maximum grain size of 4.06 cm, the minimum grain size of 0.30 cm and average grain size of 0.90 cm (Table 2). The directionality is ambiguous, as reflected by a dip of 70° to 160° along the long-axis (p_5-1 in Fig. 13).

Thick sand bed topped with a set of clay bed is deposited at the survey point p₆ (Figs. 2a and 12b). Wormtrail, plant root trace and other pores are visible on the section with calcareous bed developed, indicating the development of levee deposits (Fig. 12b). Barrier markers formed as a result of screening by small gravels are discovered on the riverbed near this point (p₆ in Fig. 13).

The section at the survey point p₇ consists of a sand body in the upper section with trough cross-bedding and a gravel bed at the bottom, showing a positive rhythm overall from the bottom to the top (Figs. 2a and 12c). Some backwater depressions and small lakes are formed in low-lying areas near this point (p₇ in Fig. 13).

The survey point p₈ lies in the distal belt of DFS. Deposits transiting from river bed lag facies with distinct scouring interface at the bottom (Fig. 12e) to massive fine- to silt-level at the top, which occurs nearly 10 m above the river bed (Figs. 2b and 12d). The cutoff of river is revealed by UAV aerial photo (p₈ in Fig. 13).

The distal deposits are much finer than the proximal and middle deposits. Major sedimentary microfacies developed in the distal belt include swamp, meandering river and small lake, which caused by weakened hydrodynamic condition. In the distal belt of the Shule River DFS, the deposits are dominated by argillaceous and sandy deposits, with less gravel deposits, representing the sedimentary feature of meandering river in the channel abandonment stage.

As the studies on continental petroliferous basins are deepened continuously, studying the DFS provides a new idea for building the depositional model of continental petroliferous basin. The depositional system is analyzed comprehensively by comparing the large-scale variations in the Shule River DFS from the apex to the toe, including slope variation along the source trend, river width variation, ratio of flood plain to channel variation and river morphologic variation. In the proximal belt, the piedmont highland transiting to the piedmont plain and the slope is the largest, which enables the river to break from the topographic control at both sides of the apex and appear as large-scale braided river with larger channel width and lower ratio of flood plain to channel. Major sedimentary microfacies developed in the proximal belt include braided channel and flood plain (Table 3). In the middle belt, the slope is smaller than that in the proximal belt. Due to the influence of the upstream bifurcating and infiltration, river gets narrower and occurs as highly bifurcating braided river, and flood plain is developed widely. Major sedimentary microfacies developed in the middle belt include braided channel, flood plain and aeolian dune (Table 3). In the distal belt, which is close to the depocenter of the basin, the slope is the lowest, so water flow is hindered and backwater depressions and small lakes are developed extensively. The river morphology transiting from braided to meandering. Major sedimentary microfacies developed in this belt include braided channel, meandering channel, flood plain, aeolian dune, swamp and lake (Table 3). The depositional characteristics in a broader scale than the single channel facies model can be presented in the depositional model of DFS by studying the continuous gradation of river morphology from large-scale braded river to meandering river through bifurcating braided river.

The river depositional characteristics at various positions are described in detail based on large-scale DFS survey (Fig. 14). In the proximal belt, the river occurs as the gravelly braided river bed, the hydrodynamic force is dominated by the traction flow, and the ice and snow melting water feeds the river. The grain size is relatively coarse and dominated by medium size to boulder. Gravels are characterized by distinct directionality, low sand content and poor sorting, indicating a high-energy environment^[31]. In the middle belt, the river also occurs as the gravelly river bed, with small-scale lenticular sand bodies. The grain size is dominated by fine to medium with higher sand content. Gravels show certain directionality and moderate sorting, indicating the transition from high-energy to low-energy water environment (Fig. 14). Due to the influence of bifurcating and infiltration in middle belt, hydrodynamic force weakens, and some branch rivers dry up in dry season and occur as ephemeral channels. The ephemeral channels within the Shule River DFS are influenced mainly by rainfall. Due to abundant rainoff from June to Septermber, the runoff flow in the Shule River is large, the argillaceous content in sediments carried by the river is high and water flow occurs in ephemeral channels. When the dry season arrives, however, ephemeral channels dry up rapidly, leaving a layer of mud skin on the channel surface (Fig. 7b). As the next-stage flooding comes, fine mud skin will be transported to downstream, so those deposited in ehemeral channales are poorly preserved. In the distal belt, river morphology transiting from braided to meandering, and it is mainly sandy deposite. As a result of slope variation, smal lakes are developed, and point bar deposits are present near the margin of active lobes to form sand body with trough cross-bedding. At the end of the distal belt, the river morphology transiting to meandering, with lag deposits at the bottom, indicating a low-energy water environment (Fig. 14). The gradation from coarse braided river in the proximal belt into fine meandering river in the distal belt is essentially a result of degradation inside the DFS depositional system, instead of the grain size difference caused by development of two depositonal systems of braided river and menadering river (Fig. 14).

Within the Shule River DFS, the slope declines from the proximal belt to the middle belt, and then to distal belt. River disperses from the apex of DFS. The channel width decreases gradually and the flood plain area increases due to the influence of bifurcating and infiltration. It is geomorphologically characterized by the coverage of gravel Gobi in the proximal belt, the presence of psammophytic vegetation in the middle belt, and oasis plain in the distal belt. The river in the Shule River DFS is morphologically a large-scale braided river without the topographic control at both sides of the source area in the proximal belt, a highly bifurcating braided river in the middle belt, a transition from braided river to meandering river in the middle-distal transfer zone, and a meandering river in the distal belt. As for the sedimentary characteristics in the Shule River DFS, gravels are predominately subrounded medium gravel to boulder with good directionality in the proximal belt, fine to medium gravel with good roundness and directionality in the middle belt, and rounded sandy grains with low gravel content and ambiguous directionality in the distal belt. On a scale of general depositional model variation, the Shule River DFS appears to be sand-rich in the distal belt and less sand in the proximal belt. Braided river facies are deposited mainly in the proximal and middle belts, while the meandering river facies is dominant in the distal belt, enabling the development of abundant facies belts, such as abandoned channel, oxbow lake, lake and swamp.

a—the maximum value of the directionality of any three adjacent small sector radii, %;

bin—the angular distance of the fan graph, (°);

g_max—the maximum grain size, mm;

g_min—the minimum particle size, mm;

n—the number of samples; α

α—the slope of the field profile, %; α

α_avg—oriented average gradient of gravel, (°);

σ—small fan radius deviation degree, dimensionless.