Analysis of the Sedimentary Characteristics of a Modern Distributive Fluvial System: A Case Study of the Great Halten River in the Sugan Lake Basin, Qinghai, China

0. INTRODUCTION

As modern earth information technology continuously develops, the field of observation of fluvial sedimentary geomorphology continues to expand (Śledź et al., 2021; Mahdianpari et al., 2018; Gulliford et al., 2017; Neugirg et al., 2016). Fluvial sedimentologists and geomorphologists through the study of alluvial system have proposed the concept of "distributive fluvial system" (DFS) (Weissmann et al., 2010; Hartley et al., 2010a), which is defined as "the deposit of a fluvial system which in planform displays a radial, distributive channel pattern"(Hartley et al., 2017, 2013, 2010b). It's also deemed that DFS is a common alluvial geomorphologic system which is composed of alluvial fan, fluvial fan, or mega fan and develops in the adjacent mountainous region (Weissmann et al., 2010; Zhang et al., 2020a). The geomorphic features include: (1) a well-defined apex from which the fluvial system (active and abandoned) is gradually distributed into the downstream area; (2) a gradual downstream decrease in slope from the apex to the toe and lateral margins; (3) a downstream decrease in channel size; (4) no tributaries merge into the DFS below the apex; and (5) from the apex, active and abandoned channels spread out in a distributive, radial shape.

As the DFS concept has developed, an increasing number of researchers have recognized and accepted it (Soares et al., 2020; Shi et al., 2019; Bilmes and Veiga, 2018; Zhang et al., 2017a; Feng et al., 2017; Yin et al., 2013; Fielding et al., 2012; Sambrook Smith et al., 2010). In 2010, Hartley et al., for research the distribution and developmental characteristics of the DFS, identified six channel types (Hartley et al., 2010b). Davidson et al. discussed the topography and landforms in the modern distributive fluvial system, as well as the quantitative research methods of the DFS in modern and ancient continental basins (Davidson et al., 2013). In 2015, Quartero et al. proposed the possibility of architectural elements and as part of a DFS in the Cordillera foreland basin, in Alberta, Canada (Quartero et al., 2015). In 2015, Owen et al. analyzed the vertical trend of the Salt Wash DFS in the Southwestern United States (Owen et al., 2017) and analyzed the Morrison Formation of the Salt Wash DFS (Owen et al., 2015). In 2016, they also pointed out that the distribution of uranium can be used as a proxy for the basin-wide fluid flow in a DFS (Owen et al., 2016). An increasing number of scholars have studied DFSs, and the theory has been continuously improved (Zhang X H et al., 2021; Zhang C M et al., 2020b, c, 2017b). However, few studies have focused on the anatomy and facies division of typical DFSs (Davidson et al., 2013).

In this study, satellite images, field investigations and literature were used to research the fluvial system of the Great Halten River in the Sugan Lake Basin (Fig. 1). First, the morphology in plan view was delineated by satellite images and radar digital elevation data. Second, field survey data were used to describe the sedimentary characteristics of each facies belt. Finally, the sedimentary characteristics are explained by element analysis. The DFS of the Great Halten River was described in detail at a range of scales to provide a comprehensive understanding of the sedimentary characteristics at these scales and to provide a template for DFS developed in similar climatic and tectonics regimes. The study will also aid in understanding the distribution and likely connectivity of different facies during the extraction and storage of fluids in the subsurface.

Figure  1.  Geographical location of the study area. (a) The Sugan Lake Basin is located in Northwest China; (b) characteristics of topography types in the East-West direction varying with elevation in the Sugan Lake Basin; (c) major topography in the study area.

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1. STUDY AREA

The Sugan Lake Basin is located between 92.5°–96.5° E and 38°–39.4° N in northwestern Qinghai Province, China (Zhang et al, 2019; Wang, 1988) (Fig. 1a). The northern part of the basin is bounded by the Altun Mountains and the Qilian Mountains, and the eastern part is bounded by the Saishiteng Mountains, which separate the Sugan Lake Basin from the Qaidam Basin (Xue and Yang, 2002). The elevation of the basin range varies within 3 500–4 600 m, with the highest peak at 5 620 m in the Altun Mountains. The overall topography of the basin is high in the east and low in the west (Fig. 1b), with an altitude range of 2 800–3 200 m. The lowest point is the Sugan Lake area, with an altitude of 2 792.6 m. The Great and Small Halten rivers drain between the Qilian Mountains and the Tuergendaban Mountains and form the main water source for the Sugan Lake (Chen et al., 2006) (Fig. 1a). The Great Halten River flows 89 km down from the source area into Sugan Lake. As it flows through the central part of the basin, surface flow declines due to infiltration and evaporation several times and then reappears in the form of spring water downstream. The annual average temperature is 2.75 ℃, the annual precipitation is only 18.7 mm, and the evaporation is more than 2 900 mm. The area has an extremely arid climate. Due to the special natural conditions of the basin, vegetation is rare, and the original ecology is basically maintained. Except for lakeside wetlands and upstream river valley wetlands, which are summer and autumn pastures in Aksai County, the rest of the area consists of desert landscapes such as gobi gravel, gravelly land and sandy lan d (Liu et al., 2017; Hou et al., 2010).

The Sugan Lake Basin is an intermountain basin that has two very different geomorphic units (Li, 2018; Zhao et al., 2013; Shi et al., 2005) (Fig. 1c). The geomorphology in the basin can be divided into five elements: (1) piedmont moraine platforms distributed in the southern Altun Mountains, the central Qilian Mountains and the northern Tuergendaban Mountains, with altitudes of 4 500–5 000 m (Fig. 2a). (1) piedmont plains, which are mainly sloped alluvial belts, distributed in the southern Qilian Mountains, Saishiten Mountains and Tuergendaban Mountains; these features are composed of modern alluvial gravel slopes (Fig. 2b). (2) Alluvial plains distributed in the Great and Small Halten River valleys; their average widths are 1–3 km, and the widest is 6 km. In the Great Halten River, steep ridges formed by incision (Fig. 2c). (3) Limnetic plains, which are located in the eastern Sugan Lake area and are composed of modern alluvial clay. The surfaces are flat, and abundant water, edges and fine soil belt boundaries often have spring surface flow, forming groundwater overflow belts. Under intense evaporation, patches of salt land and small salt marshes are formed (Fig. 2a). (4) The eolian plains, which are mainly distributed in the terminal area of the Great and Small Halten rivers, are composed of modern eolian sand (Zhou, 2007). The surfaces are composed of crescent sand dunes and wavy sand dunes, which are 5–20 m higher than the surface, and the end of the Great Halten River can reach 120 m (Fig. 2d).

Figure  2.  Topographic characteristics of the study area, mainly includes five types of geomorphic. (a) Piedmont moraine platform and limnetic plain; (b) piedmont plain; (c) alluvial plain; (d) eolian plain.

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2. METHODS

Using Google Earth satellite image data (Gracchi et al., 2021; Balzter et al., 2015), with the DFS apex as the center of the circle and 5 km as the radius interval, the DFS of the Great Halten River, extending 89 km in a downstream direction. Channel numbers, width, and sinuosity were measured at 5 km intervals downstream (Table 1). The elevation data provided by SRTM90 were used to measure the slope of each interval (Boothroyd et al., 2021) (Table 1). In the measurements, the channel number represents the sum of the channels intersecting within the radius arc length range corresponding to the DFS of the Great Halten River. The channel width is the distance between the outermost banks, from which the axis of the river is 90 degrees, including sand bars. By measuring the length of the fluvial talweg line in ArcGIS, the channel sinuosity was calculated at 5 km intervals (Li et al., 2021; Fisher et al., 2013) (Fig. 3).

Table  1.  Statistical results based on Google satellite images, a group of data is counted every five kilometers down the vertex, mainly including the number of channels, total width of channels, average width of channels, channel sinuosity and width of floodplain

Distance from apex (km)	River density	Total channel width (m)	Average channel width (m)	Channel sinuosity	Floodplain width/channel width
0	1	998.73	998.73	1.02	0.00
5	2	961.15	480.58	1.02	3.84
10	2	1 426.52	713.26	1.02	3.93
15	3	2 591.32	863.77	1.05	2.58
20	5	1 345.66	269.13	1.02	7.10
25	6	1 225.28	204.21	1.04	8.95
30	6	2 010.89	335.15	1.06	7.06
35	6	1 857.91	309.65	1.01	8.65
40	8	1 603.59	200.45	1.02	11.25
45	12	1 667.87	138.99	1.02	13.22
50	11	2 808.76	255.34	1.02	7.93
55	12	3 337.07	278.09	1.05	6.98
60	18	532.08	29.56	1.05	54.18
65	19	1 190.72	62.67	1.18	27.76
70	17	473.30	27.84	2.18	64.44
75	25	634.22	25.37	2.02	42.35
80	18	337.93	18.77	1.87	51.40
85	4	134.43	44.81	1.98	83.38

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Figure  3.  UAV Matrice 600 equipped with a Zenmuse X5 lens and the river parameters for the statistical method. The measured parameters mainly include channel length, valley length and channel width.

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Geological sections were measured in the field by means of digging and probing and included 5 deposition observation points and 10 sampling points. The Skyray instrument explore 9000 was used for measurement of elements at each sampling point (Figs. 4a, 4b; Table 2).

Figure  4.  Element measurement. (a) measuring equipment Skyray instrument explore 9000 and standard sample gbw03116; (b) X-ray infrared spectrum; (c) X-ray test range micrograph.

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Table  2.  Measurement results of surface sediment elements at each sampling point of the Great Halten River DFS

Number	Element content (%)
	MgO	Al2O3	SiO2	P	S	K	Ca	Ti	Mn	Fe
1	1.683 9	13.667 0	56.858 8	0.008 8	0.000 0	0.075 7	7.939 7	0.022 5	0.005 1	1.185 4
2	0.872 1	15.461 9	56.461 8	0.014 1	0.000 0	0.625 8	5.557 6	0.045 8	0.009 4	2.124 5
3	2.425 2	17.584 0	72.547 8	0.000 0	0.013 7	0.217 7	3.177 8	0.022 3	0.006 4	1.441 5
4	1.189 8	12.376 7	52.024 1	0.000 0	0.000 0	0.090 6	0.539 2	0.014 8	0.002 0	0.819 9
5	0.660 3	12.258 3	51.542 2	0.007 4	0.000 0	0.733 2	3.622 2	0.048 5	0.009 8	2.184 5
6	1.578 1	15.014 7	50.776 2	0.004 9	0.263 6	1.618 4	4.137 0	0.034 3	0.009 8	1.789 2
7	4.296 0	17.199 2	47.285 1	0.056 6	6.563 6	0.192 5	4.061 0	0.031 2	0.007 1	1.318 6
8	1.119 2	17.637 3	47.237 7	0.000 0	0.361 4	3.550 1	4.530 3	0.050 6	0.020 6	3.742 7
9	0.000 0	9.958 3	48.826 8	0.017 4	0.000 0	0.100 0	2.110 9	0.022 8	0.002 7	0.884 2
10	1.436 9	13.817 9	47.944 0	0.000 0	8.786 7	0.000 0	6.047 6	0.036 6	0.007 1	1.105 3

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In the process of element measurement, the sample is dried and weighed first, and then screened with the standard of 2.00 mm aperture (Fig. 4c). In the process of analysis, a standard sample (GBW03116) was added to each batch of samples for data monitoring (Fig. 4a), and 30% of the samples were measured at random to control the error within < 0.3% (Table 2).

In complex topographic areas, unmanned aerial vehicle (UAV) photography is not affected by the topography (Fig. 3). Compared with Google Earth satellite imagery, it is more accurate and yields more comprehensive data. Therefore, aerial photography was used to observe the Great Halten River from the apex to the toe of the DFS.

3. RESULTS

The Great Halten River DFS is mainly formed by the Great Halten River, and is oriented parallel to the basin bounding faults in an axial setting and terminates at Sugan Lake (Fig. 1a). Upstream of the DFS apex, the channel tortuosity is mainly controlled by the scale of the sedimentary system on both sides (Fig. 5a). The channel traverses between valleys, and the sediments are mainly formed by overlapping point bars (Fig. 5b). Downstream of the DFS apex, the channel is not constrained by the sedimentary system on either side, and there is no obvious truncation. The channel position is changed mainly by meandering on the flood plain through crevasses (Fig. 6).

Figure  5.  Upstream morphology of the DFS apex of the Great Halten River. (a) Upstream of the apex, the river is confined by valleys and (b) may have a wandering planform.

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Figure  6.  Characteristics of the DFS shape in plan view and the division of each zone in the Great Halten River DFS.

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Based on the remote sensing image data, the horizontal morphological characteristics of the Great Halten River DFS were analyzed. In plan view, it forms long strip shape, with a fan radius of 92.07 km, a fan crest angle of 34.10°, and an area of 1 690 km² (Fig. 1a, Fig. 6). Based on the field survey and UAV aerial photography, according to fluvial morphological characteristics and sedimentary characteristics of different areas of the DFS, three zones were identified in the DFS of the Great Halten River (Fig. 6).

The proximal fluvial morphologies are mainly large braided channels. The medial fluvial morphologies are bifurcated braided channels. The distal fluvial morphologies transition from the braided channel to the meandering channel. In terms of topographic change, the proximal slope is greater than the medial slope, and the distal slope is the smallest (Fig. 6). In addition to fluvial morphology and geomorphic morphology, there are great differences in the sedimentary characteristics and element content among different regions. This study mainly analyzes Al₂O₃, SiO₂, MgO, Fe₂O₃, CaO, TiO₂, MnO, CaCO₃ and other indicators in sediments (Table 2). The results show that the major elements in surface sediments are Al₂O₃ and SiO₂, with an average content of 14.50% and 53.15% respectively, accounting for 67.65% of the total elements (Fig. 7).

Figure  7.  Histogram of average element content in surface sediments from the apex to the toe of the Great Halten River DFS.

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3.1 Proximal

The proximal zone of the Great Halten River is located within 36–40 km from the apex of the DFS, accounting for 30.76% of the total DFS area. The altitude at the apex of the DFS is 3 280 m, and the horizontal distance changes with altitude. The apex is steeper than other areas, with a slope of 0.68% (Fig. 6). In the four element analysis points at the proximal, the content of Al₂O₃ and SiO₂ is 14.77% and 59.47%, respectively (Fig. 7). The position near the apex is affected by the sedimentary system at the end of the Small Halten River the continuous sand dunes are present. Near the medial part is mainly gravel gobi. The Great Halten River crevasses at the incision and is divided into two main rivers downstream (Fig. 6).

In plan view, the width of the channel is large and ranges from 300 to 600 m. The flood plain to braided channel width ratio is 6.06 (Table 1). In the large braided channel, sand bars are developed (Figs. 8a, 8b, 8d). In the source area, the source range is wide, and some source areas are covered with snow year-round (Fig. 8c). There is clear erosion on both sides of the river because of strong hydrodynamic (Fig. 8b). The river is mainly perennial. The vegetation in this area is relatively lush because of the sufficient water source, which is the summer and autumn pasture in Aksai County (Fig. 8c).

Figure  8.  Proximal fluvial morphology in the Great Halten River DFS. (a), (b) The proximal fluvial channel is mainly a braided channel, with obvious erosion on both sides; (c) the proximal topography is mainly grassland, and glaciers are present at the source; (d) part of the channels were dry, only the main river has water flowing.

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Vertically, the modern sedimentary section in the proximal region is mainly gravelly, with minor sandy content (Fig. 9). Horizontally, the hierarchical changes in the channel are obvious, which can be divided according to the directional arrangement of large gravels at the bottom (Fig. 9a). The gravels are in close contact and arranged in bedding or are imbricated (Fig. 9b). In addition, braided channels are developed (Fig. 8d). Correspondingly, the large tabular cross-bedding is partially developed, and lag deposits are visible at the bottom (Fig. 9b). The gravel is sub-rounded (Fig. 9c). Vertically, each stage of the channel is from bottom to top, showing obvious changes from coarse to fine (Fig. 9c). In particular, the gravel layer at the bottom (Figs. 9b, 9c) has an obvious orientation.

Figure  9.  Characteristics of proximal modern sedimentary sections in the Great Halten River DFS. (a) Characteristics from the proximal source direction; (b) obvious orientation of bottom gravel; (c) obvious cyclicity in the vertical direction.

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3.2 Medial

The middle zone of the Great Halten River DFS is located within 20–25 km from the proximal zone, accounting for 24.96% of the total DFS area. The average altitude in the middle is 2 980 m. The change in the horizontal distance with altitude is slower than that in the proximal zone, with a slope of 0.62% (Fig. 6). In the four element analysis points at the medial, the content of Al₂O₃ and SiO₂ is 15.53% and 49.21%, respectively (Fig. 7). The geomorphic characteristics of this area are mainly gravel gobi and sandy vegetation. The morphological characteristics of the channel are mainly bifurcation braiding (Fig. 10).

Figure  10.  Fluvial morphology in the medial zone of the Great Halten River DFS. (a) The medial fluvial channel is mainly a bifurcated braided channel; (b) some tributary channels are dried up, and only the trunk channel has water flow.

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In plan view, the channel width is smaller than the proximal channel width due to the branching and infiltration of the fluvial channel. The average channel width varies within 100–200 m, and the flood plain to channel width ratio is 20.58 (Table 1). The scale of the bar is smaller than that in the proximal scale (Figs. 9a, 10b). The hydrodynamics is weakened because of the influence of branching and infiltration, from UAV aerial photography also can observed the following phenomena. Some tributary channels have dried up, and only the trunk channel is flowing (Fig. 10b). The flood plain is widely developed, and the bar transitions from gravel in the proximal area to sandy (Fig. 10b).

Vertically, the modern sedimentary section in the medial zone is mainly sandy gravel deposits, with finer gravel and a particle size of approximately 2–3 cm. The sandy composition increases compared with that of the proximal zone (Fig. 11).

Figure  11.  Characteristics of modern sedimentary sections in the medial zone of the Great Halten River DFS. (a), (b) Characteristics from the source direction in the medial zone; (c) in the axial direction of the channel, with obvious cyclicity in the vertical direction, cross-bedding develops.

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According to measurements of the modern DFS section in the source and axial directions, the channel stratification is more obvious in each phase in the source direction (Fig. 11a), and the gravel has obvious cyclicity (Figs. 11b, 11c). The gravels have good roundness. The channel contains mainly gravel and has a thin layer of sand bodies (Fig. 11b). Additionally, in the axial direction of the channel (Fig. 11c), large cross-bedding is visible because of the intercutting of channels (Fig. 11c). Vertically, the hierarchy of the channel is obvious (Fig. 11c). Compared with the proximal, the medial floodplain is wide. Due to the influence of branching and infiltration, some rivers dried up in the dry season and showed temporary channel deposition. Therefore, abandoned lobes often formed in the central zone. These abandoned lobes do not indicate no river development, but they are not in the same period as the current river.

3.3 Distal

The distal zone of the Great Halten River DFS is located below the spring line (Fig. 6), and the regional boundary is obvious. The upper boundary is 58–60 km away from the apex of the DFS, and the lower boundary is the lake shore (Fig. 1a). The distal zone accounts for 44.28% of the total DFS area. The average altitude in the distal region is 2 850 m. The horizontal distance changes most slowly with altitude, with a slope of 0.27% (Fig. 6). In the two element analysis points at the distal, the content of Al₂O₃ and SiO₂ is 11.89% and 48.38%, respectively (Fig. 7). The geomorphic characteristics mainly include swamp, with a large amount of vegetation (Fig. 12).

Figure  12.  Fluvial morphology in the distal zone of the Great Halten River DFS. (a) The distal zone of the river mainly includes a meandering channel; (b) salt separates near the shoreline, and the sediments are finer gravel; (c) salt marsh, pond and meadow; (d) the groundwater layer is approximately 3.8 m away from the surface.

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In plan view, the fluvial morphology is transformed from the braided channel to the meandering channel (Fig. 12a). In the area near the lake, it is the meandering channel (Fig. 12a). The average channel width varies from 20 to 50 m. The ratio of the flood plain width to the channel width is 53.86 (Table 1). There is groundwater seepage because of the low altitude and relatively gentle slope. The channel near the spring line is pinched out due to branching and infiltration upstream. Moreover, as it extends to the lake area, the channel gushes out of the surface in the form of groundwater to form small and flat channels (Figs. 12a, 12c). Swamps and small lakes are relatively developed distally, and the sediments contain finer gravels (Fig. 12b). The surface mineralization near the shore of the lake is high, which has salt frost precipitates (Fig. 12b).

Vertically, the gravel layer gradually thins from bottom to top, and the strip-shaped lenticular sand layer gradually becomes thicker (Fig. 13a). The thickness of fine-grained sandy sediment in the top layer is similar to that of gravelly sediment in the bottom layer. The fluvial shape begins to change from the braided channel to the meandering channel. In addition to retaining the typical binary structure of the channel, thick mud layers are visible vertically and are approximately 35 cm (Fig. 13b). Compared with the proximal and middle sediment particle size, the distal sediment particle size is significantly smaller. Some small ripple cross-bedding is also developed distally as the hydrodynamic weaken (Fig. 13c). Compared with the proximal and medial zones (Fig. 5, Fig.8), the distal mainly develops landform features such as swamps and small ponds. In the open and flat area of the distal, groundwater seepage is very obvious and can converge to form a Spring River in some areas.

Figure  13.  Characteristics of modern sedimentary sections in the distal zone of the Great Halten River DFS. (a) Floodplains were widely developed in the distal zone; (b) a thick mud layer is developed near the lake area; (c) ripple cross-lamination is developed.

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4. DISCUSSION

4.1 Sedimentary Model of the Distributive Fluvial System

4.1.1   Element characteristic analysis

Due to the different physical and chemical properties of sedimentary rocks, there are obvious differences in the migration and enrichment of geochemical elements in the weathering, transportation and deposition. The differentiation of elements is controlled by the sedimentary environment. The characteristics of sedimentary environment in each zone of DFS can be distinguished by using element content and ratio (Deng and Qian., 1993). In the Great Halten River DFS from the top to toe, with the grain size of surface sediment becoming finer, the content of SiO₂ gradually decreases and the content of Al₂O₃ gradually increases (Tables 2, 3; Fig. 7). Under drought conditions, Mg, Ca, Mn and other elements in the sedimentary medium precipitate in large quantities and deposit at the bottom of the water, while Fe, Al and other elements precipitate rapidly in the humid environment. The Mg/Ca value is mainly distributed between 0.17 and 0.35 (Table 3), showing a decreasing trend from the proximal to the medial and to the distal. It reflects the climate from drought to slightly humid, which is consistent with the geomorphic transition from desert Gobi to near Lake area. Similarly, Mg/Al, Fe/Mn and Fe + Al/Ca + Mg also show regular (Table 3).

Table  3.  Proportional relationship of elements in each zone of the Great Halten River DFS

Zone	Element ratios					Element change rate
	Mg/Ca	Mg/Al	Fe/Mn	Fe + Al/Ca + Mg		Al	Si	Ti
proximal	0.358 5	0.104 4	243.063 0	2.765 0		14.772 4	59.473 1	0.026 3
medial	0.468 1	0.123 2	190.924 0	2.963 9		15.527 4	49.210 3	0.041 2
distal	0.176 1	0.060 4	204.012 8	2.685 2		11.888 1	48.385 4	0.029 7

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By analyzing the changes of element content in different positions from the apex to toe of the Great Halten River DFS, this study discussed the sedimentary dynamic environment and sediment transport mode of surface sediments. It has a certain reference value for the corresponding relationship between element characteristics and sedimentary environment.

4.1.2   Sedimentary characteristics analysis

The sedimentary characteristics of channels at different locations were described in detail by micro-observation of the DFS (Fig. 7).

The proximal channel is the gravel braided channel, which has mainly traction currents. The water mainly originates from ice and snow water in the source area (Fig. 9). The particle size is coarse, mainly medium to large boulders (Table 4). The gravels have obvious orientation, less sand content and poor sorting, showing a high-energy water environment (Fig. 9).

Table  4.  Lithofacies association developed in the Great Halten River DFS and their sedimentary characteristics

Position	Grain size	Sorting	Roundness	Directionality	Hydrodynamic force	Lithofacies association
Proximal	Medium to coarse gravel	Poor	Subrounded	Clear	Traction flow	Braided channel, flood plain
Medial	Fine to medium gravel, sandy lamina	Moderate	Subrounded	Certain	Traction flow	Braided channel, flood plain, aeolian dune
Distal	Fine gravel, thick sand	Good	Subrounded to rounded	Ambiguous	Traction flow	Braided channel, meandering channel, flood plain, aeolian dune, marsh, lake

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The medial channel is also mainly a gravelly riverbed. Small lenticular sand bodies are visible. The gravel particle size is mainly fine to medium gravel (Table 4), with increased sand content and medium sorting. The gravel has a certain orientation, showing a transition from a high-energy water environment to a low-energy water environment (Fig. 15). The hydrodynamics in the medial zone are weak by branching and infiltration. Some tributary channels dry up in the dry season, which is manifested as temporary channels (Fig. 10). The temporary channel of the DFS is mainly affected by rainfall. From June to September, precipitation is abundant, runoff from the Great Halten River is large, and the mud content in the runoff is high. The temporary channel has water flow, but with the impending dry season, these temporary channels dry up rapidly and form a layer of mud skin on the surface of the river. With the arrival of the next flood period, the fine mud skin is washed downstream. The mud skin that is developed in the temporary river should be difficult to preserve by comparing underground study (Fig. 11).

Figure  14.  Correlation analysis of river morphological parameters in the Great Halten River DFS.

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Figure  15.  Sedimentary characteristics and depositional model of the Great Halten River DFS. From macro to micro, large-scale to small-scale, systematically reveal the sedimentary characteristics of different locations.

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In the distal zone, the river begins to change from the braided channel to the meandering channel, and the sediments are mainly sandy (Table 4). Small lakes are relatively developed in this area due to the influence of slope. Near the edge of the active lobe, bar deposits are developed, and the sand is trough-shaped (Fig. 13). At the extreme end of the facies belt, the fluvial channel is meandering. Lag deposits at the bottom are developed, indicating a low-energy water environment (Fig. 15).

The change in the coarse gravel braided channel to the fine-grained meandering channel occurs from the apex to the toe. It is part of the progressive decrease in the DFS internal sedimentary system rather than the particle size difference caused by the development of the braided fluvial and meandering channels (Fig. 15).

4.1.3   Geomorphic characteristics analysis

Comparing the large-scale changes and characteristics of the distributive fluvial system of the Great Halten River from the apex to the toe, can help to understand the overall sedimentary system (Fig. 15). This comparison includes in the slope along the source direction, the change in the channel width, in the flood plain to channel ratio and in the fluvial morphology. According to the statistical river morphology parameters, we can observe that the channel width is negatively correlated with sinuosity, number of channel and flood plain width (Fig. 14). There is a strong correlation between the width of channel and flood plain, related coefficient R² = 0.925 9 (Fig. 14).

The proximal area is mainly the transition stage from the piedmont highland to the alluvial plain and has the maximum slope. The fluvial channel is free from the topographic control on both sides of the apex and is characterized by a large braided channel. The width of the channel is large. The ratio of the floodplain to the channel is small. Lithofacies associations such as braided channels and floodplains are mainly developed (Fig. 8). Influenced by the sedimentary system in the distal zone of the Small Halten River, eolian dunes occurred at the proximal of the Great Halten River, occur (Table 4).

The slope in the medial region is smaller than that in the proximal region. The channel width becomes narrower, and the floodplain is widely developed and affected by branching and infiltration upstream (Fig. 10). The fluvial channel is characterized by a highly bifurcated braided channel, mainly developing lithofacies associations such as braided channels, floodplains and eolian dunes (Table 4).

The distal zone is close to the subsidence center of the Sugan Lake Basin. The slope is relatively slow, the water flow is blocked, and geomorphic units such as wetlands and small ponds are widely developed (Fig. 12). The fluvial morphology is transformed from a braided channel to a meandering channel. The lithofacies association includes braided channels, meandering channels, floodplains, eolian dunes, swamps and lakes (Table 4).

As described the sedimentary system characteristics of the DFS show a wider range than the single channel facies model (Fig. 15).

4.2 Particularity of the Distributive Fluvial System

DFS mainly emphasizes the integrity of the continental sedimentary basin system and the relationship with each part. The study of DFS divides the sedimentary system of the continental basin into three types: DFS, Axial System, and Terminal System (Zhang et al., 2020c; Davidson et al., 2013; Weissmann et al., 2010). The axial system includes the axial fluvial system and the intersection part between DFS. The terminal system includes aeolian, lake, and salt lake sedimentary areas. The theory of DFS holds that from alluvial fan to the braided river, the meandering river, and flood plain is a continuous change of the same system, rather than the simple connection between multiple sedimentary systems (Zhang et al., 2020a). For instance, the Huagang Formation in the Xihu Sag of the East China Sea shelf basin is generally represented by a river-delta system. Among them, the braided river-braided river delta system is mainly developed in the Lower Huagang Formation, which is represented by a small braided river delta system on the west slope, a small alluvial fan system on the east slope, and a large braided river-braided river delta supplied by the east-west slope and the northern Hupi reef uplift in the long axis direction. The Upper Huagang Formation mainly develops meandering river-shallow water delta system, which is manifested as meandering river-shallow water delta in the long axis direction and small-scale shallow water delta on the west slope. In general, in the whole Huagang Formation, the large scale of the long axis sedimentary system is the main factor controlling the sedimentation process of the depression, while the short axis provenance system in the slope area on both sides is small, which only controls the sedimentary system with limited scale in the slope area (He et al., 2021). From the perspective of DFS, it can be considered that the Huagang Formation is a set of large-scale branch river system in the east-west direction, and finally terminates on the axial river, which continues to move southward until it enters the lake. It can form a wider range of sedimentary system in the southern region, which has a certain exploration potential (Zhang et al., 2020a). Similarly, the Mahu sag, Junggar Basin, Northwest China, was guided by the traditional alluvial fan model in early oil and gas exploration. As exploration deepened, the alluvial fan model dominated by gravity flow encountered difficulties in explaining the sedimentary system. When the drilling penetrated deep into the center of the sag, the entire sag containing gravel showed that the center of the depression was not the sedimentary center of the basin (Du et al., 2019; Tang et al., 2018). From the viewpoint of the DFS, the Mahu Sag may be an open lake basin. Coarse-grained sediments from the mountains on the northwest edge of the basin flow into the Mahu sag from both sides and are transported to the southern part of the sag through axial fluvial flow. The grain size of sediments generally becomes thinner to the south. It can be inferred that the sandy sedimentary system may have developed downstream of the conglomerate sedimentary system of the Triassic Baikouquan Formation. To summarize, comparing the grading regularity of the sand content in different facies zones of the DFS has certain guiding significance for the direction of oil and gas exploration (Fig. 15).

5. CONCLUSIONS

(1) The slope in the proximal zone of the Great Halten River DFS is greater than that in the medial zone, and the slope in the distal zone is the smallest. The channel is distributed radially downward along the apex of the DFS. The channel width gradually decreases, and the floodplain area increases by branching and infiltration. The geomorphic characteristics in the proximal zone are mainly gravel gobi, the development of sandy vegetation is shown in the medial zone, and the distal zone is mainly the oasis zone.

(2) The fluvial morphology of the Great Halten River DFS varies. The proximal river is not controlled by the topography on both sides of the source area and is shown as a large braided channel. The medial zone is mainly a highly bifurcated braided fluvial channel. The fluvial morphology transitions from the braided to the meandering channel in the transition area from medial to distal. The distal fluvial is mainly shown as a meandering channel.

(3) The sedimentary structure of the Great Halten River DFS varies. In the proximal zone, large tabular cross-bedding is mainly developed with good gravel orientation and mainly medium boulders. In the medial zone, trough cross-bedding and graded bedding are mainly developed, and the gravel is mainly fine to medium. In the distal zone, small trough cross-bedding and ripple cross-lamination are mainly observed, and they are mainly sandy deposits with low gravel content and indistinct orientation.

(4) In the change of element characteristics of the Great Halten River DFS, the major elements of surface sediments are mainly SiO₂ and Al₂O₃. With the grain size of sediments from the top becoming smaller, the SiO₂ content gradually decreases and the Al₂O₃ content gradually increases. The relative proportion change of element content from the top to the end is relatively consistent with the geomorphic characteristics.

(5) The overall sedimentary model of the Great Halten River DFS varies. The sand body distribution shows that the distal sand body content is the largest and that the proximal sand body content is the smallest. Braided fluvial deposits are mainly in the proximal and medial zones. Meandering fluvial deposits are mainly in the distal zone and occur with large numbers of abandoned channels, oxbow lakes, ponds and swamps.