Depositional Architecture of Aggrading Delta Front Distributary Channels and Corresponding Depositional Evolution Process in Ordos Basin: Implications for Deltaic Reservoir Prediction

Abstract

Distributary channels at the delta front of lacustrine basins play a crucial role in transporting terrigenous sediments and redistributing depositional facies along the basin margin. These channels are also significant reservoirs for oil and gas. This study investigates the Triassic Yanchang Formation in the Southeastern Ordos Basin (China), emphasizing the sedimentary characteristics, hydrodynamic processes, and evolutionary patterns of delta front distributary channels. Special focus is given to the response of sedimentary filling to paleotopographic changes along the basin margin to enhance reservoir prediction. Through field profiling and quantification of channel morphological parameters, two distinct topographic types were identified: transitions from gentle to steep slopes and from steep to gentle slopes. The findings reveal that the morphology, evolution, and distribution patterns of distributary channels were primarily influenced by the paleotopographic gradient, with sediment grain size playing a supplementary role. Detailed analysis highlights the topographic control on sediment transport mechanisms: in gentle terrain, friction-driven processes dominate (rolling/suspension), whereas in steep terrain, inertial forces prevail (rolling/saltation). Channel architecture correlates strongly with paleotopography: gentle to steep transitions form isolated, vertically stacked sand bodies with thick mouth bars, while steep to gentle transitions produce sheet-like sands with lateral migration features. This study establishes a predictive framework linking slope thresholds to reservoir morphology, offering prioritized targets for hydrocarbon exploration. The methodology is applicable to the margins of lacustrine basins in intracratonic settings, reducing subsurface uncertainty by integrating paleotopographic reconstructions with channel aspect ratios and migration rates.

1. Introduction

Lacustrine basin delta fronts refer to regions formed by the interaction of river and lake processes, characterized by high sedimentation rates, and a strong influence of fluvial processes and base level fluctuations [1,2]. These regions serve as depositional areas for terrigenous sediments and act as active zones for the transportation of these sediments into lake basins [3,4,5]. Among these, the delta front distributary channels function as the primary conduits for the transport of terrigenous sediments, thereby significantly influencing sediment distribution and facies within the lake basin [6,7,8,9]. During the same period, numerous distributary channels developed within the river-controlled delta front at the lake basin’s edge. These channels are generally shallower and narrower compared to the main distributary channels and display significant directional variations, leading to a lobe or bird’s foot shape of the delta. Consequently, delta front distributary channels are regarded as highly significant topographic features at the basin’s edge over shorter geological timescales, and they have become a central focus of numerous studies [8,10,11,12,13].

The shapes of the delta front distributary channels are puzzling. In lacustrine basins with relatively stable tectonic environments, the evolution of river-controlled delta front distributary channels is controlled by many factors during the depositional period [13,14,15,16,17,18,19,20,21,22,23,24]. To investigate the growth processes of delta front distributary channels and deltas in lacustrine basins, researchers commonly use numerical simulations or flume experiments, adjusting parameters such as slope, initial levels, wave conditions, river flow velocities, and sediment characteristics to model the overall evolution of delta front. They suggested that the patterns of delta front are primarily the result of fluvial activity, characterized by the incomplete development of barriers and levees and indistinct distributary channel boundaries, and the delta formation process, evolutionary characteristics, and sediment body stacking patterns are controlled by internal sediment transport channels [25,26,27,28,29,30,31]. The gently sloping regions of lake basins refer to large-scale areas; however, local irregular topographic can significantly influence the morphology, sediment characteristics, sedimentary evolution, bifurcation, and confluence of delta front distributary channels. Increasingly, research indicates that topographic is a critical controlling factor for the development of delta front in lacustrine basins [28,32,33,34,35].

Topographic, particularly slope (“topographic slope” refers to the inclination of ancient land surfaces, which governed sediment transport and depositional architecture), directly influences the morphology of delta front channels. Over millennial scales (e.g., ~10 kyr), topographic slope exerts first-order control on sedimentation and delta front channel evolution, while fluvial processes, wave action, accommodation space, and climatic changes exhibit subordinate influences due to their higher temporal variability or localized impacts [36,37]. Fluvial sediment supply may fluctuate over sub-millennial scales, and wave energy is often buffered by basin geometry in lacustrine systems [38]. This hierarchy of controls highlights slope-driven gravitational forces as the dominant stabilizer of channel architecture at this timescale [39]. Channels on gentle slopes typically exhibit wider and shallower forms. Normally, a larger scale topographic contributes to the sediments dispersing, leading to a broader delta front being developed. Sediments are distributed more uniformly across this larger area, resulting in increased channel sinuosity and the development of complex distributary systems. Compared to steep slopes, sediment distribution on gentle slopes is more gradual and continuous [5,31,33,40,41,42,43]. Consequently, the response of delta front channels to topographic features provides insights into morphological evolution and aggradation patterns. Notably, there is a paucity of studies linking topographic changes with the evolution of delta front channels [28,30,44]. Therefore, it is essential to strengthen the linkage between topography and sedimentology in the study of delta front channels to elucidate the sedimentary characteristics and evolutionary processes of these channels across different topographic settings [45,46]. Furthermore, this linkage facilitates the differentiation of channel morphology and characteristics under various topographic conditions in ancient outcrops of gently sloping margins of lacustrine basins and supports the development of a comprehensive pattern of delta front channel evolution in response to specific topography changes. This study analyzes the sediments of delta front distributary channels at different locations within a single gently sloping, river-controlled delta system, focusing on describing sedimentary facies and facies associations, and analyzing significant morphological and depositional differences to understand the filling mechanisms of distributary channels and their characteristics across various topographic settings.

The study examines the formation and evolution of distributary channels, offering a novel interpretive framework for ancient delta front deposits influenced by fluvial processes, featuring multiple distributary channels of varying scales. This framework aims to enhance the accuracy of predictions regarding the evolution and morphology of similar topographic, river-controlled deltaic sand bodies. For a long time, the primary reservoir interval in the Ordos Basin has been characterized as a gently sloping delta system within a lacustrine basin. [47,48]. The outcrops of the Late Triassic delta transitional facies in the Southeastern Ordos Basin provide a valuable opportunity to study the sedimentary characteristics of the delta front distributary channels. This paper examines the geometric form, filling pattern, superposition mode, lithofacies characteristics, and sedimentary evolution of delta front distributary channels. A comparison and classification scheme for sand transport channels within delta systems is proposed. The mechanism of topography, which acts as a crucial controlling factor in the deposition of distributary channels at the front of deltas, was clarified, further augmenting our comprehension of the sedimentary evolution and depositional patterns of such deltas.

2. Geological Setting

The study area is located in the Ordos Basin, Central China (Figure 1A), a large, multi-cycle intracratonic basin with a relatively simple overall structure, which has undergone multiple periods of tectonic uplift and depression migration [48]. The Ordos Basin covers an area of approximately 320,000 km², making it the second-largest sedimentary basin in China. According to its structural features and the nature of the basin, it can be divided into six secondary structural units, including the Northern Yimeng Uplift, the Central Northern Shaanxi Slope, the Southern Weibei Uplift, the Eastern Jinxi Flexure Belt, the Tianhuan Depression, and the Western Thrust Belt on the western edge (Figure 1B). The Ordos Basin has been accumulating sediments since the Middle Paleozoic era, with the strata reaching an average thickness of 4 to 5 km by the end of the Neogene period [49]. During the Late Triassic to Cretaceous periods, the Ordos Basin was an inland depression basin [50,51]. The sedimentary system during this period primarily comprised five types of transitional marine–terrestrial deposits: fan delta, braided river delta, meandering river delta, deep water delta, and shallow water delta [48]. The research area extends from Yanchang County to Yichuan County and Yijun County, situated in the southeastern part of the Ordos Basin on the Yishan Slope and the Weinan Uplift (Figure 1B). The strata overall exhibit gentle and stable structures, forming a westward-dipping monocline with a dip angle of less than 1°. They are predominantly made of fluvial, lacustrine facies and their transitional facies [52].

The Upper Triassic Yanchang Formation unconformably overlies the Middle Triassic marine Zhifang Formation and is overlain by Early Jurassic strata. Forming 800–1200 m thick inland fluvial lacustrine strata are dominated by fluvial and lacustrine facies, accompanied by meandering river delta and other sedimentary systems [52]. It was divided into 10 members (Ch1 through Ch10) by the petroleum industry (Figure 2A) [53,54]. The study interval of interest, Ch7 upper members, underwent a short rise in water level followed by a rapid fall (Figure 2), forming a well-developed delta [55,56].

This study examines four well-exposed outcrops of the Ch7 upper members in the Upper Triassic Yanchang Formation within the Ordos Basin, specifically the Yueliangwan, Zhujiawan, Tielongwan, and Fudihu outcrops (Figure 1C). The Zhujiawan outcrop is located in Yanchang County, on the southeastern margin of the Ordos Basin. The Yueliangwan outcrop is in Yichuan County, approximately 47 km south of the Zhujiawan outcrop. The Fudihu outcrop is located in Yijun County, about 162 km southwest of the Yueliangwan outcrop (Figure 1C and Figure 2B). These sediments were deposited in the delta front to semi-deep lacustrine environments [53,57]. The cross-sections of the three outcrops are perpendicular to the paleocurrent direction and are continuously exposed along the highway cut. The outcrops exhibit a gentle dip angle of less than 1°, extending over a considerable distance, which facilitates large-scale tracking and comparative study (Figure 2B).

3. Data and Methods

This study primarily focuses on the detailed analysis of four outcrop sections from the Yueliangwan, Zhujiawan, Tielongwan, and Fudihu exposures in the Southeastern Ordos Basin. The selected observation locations exhibit prominent sedimentary structures, extend significantly in the transverse direction, and have cross-sectional orientations perpendicular to the paleocurrent direction, facilitating comparative studies of the distributary channel structures. Over 1000 high-resolution photographs of typical sedimentary structures were taken. The measured thickness of the geological strata is approximately 80 m, which represents the sum of the four sections. High-precision total station instruments were utilized to measure the thickness and width of the entire distributary channel sand bodies at the outcrops.

According to the Udden–Wentworth grain size classification, detrital sediments in the study area were categorized as coarse sandstone (0.5–1.0 mm), medium sandstone (0.25–0.50 mm), fine sandstone (0.125–0.25 mm), very fine sandstone (0.0625–0.1250 mm), siltstone (0.0039–0.0625 mm), and mudstone (<0.0039 mm) [58,59]. The primarily division scheme by A.D. Miall and the interpretation of fluvial, deltaic, and lacustrine deposits by William E. Galloway, John S. Bridge, and Gary Nichols were used to categorize lithofacies. Lithofacies associations are utilized to infer sedimentary environments [60,61,62,63,64,65]. In this study, we employed a comprehensive facies analysis framework tailored to deltaic environments, integrating methodologies from A.D. Miall’s classification system and the depositional models proposed by William E. Galloway, John S. Bridge, and Gary Nichols. This integrated approach facilitated the categorization of lithofacies and the interpretation of sedimentary environments within the deltaic context. We utilized A.D. Miall’s classification scheme, which is widely recognized for its applicability to fluvial deposits [40,41,62]. To adapt this framework to deltaic settings, we incorporated depositional models that account for the unique characteristics of deltaic systems. Specifically, we referred to Galloway’s model, which emphasizes the interplay of fluvial, tidal, and wave processes in deltaic environments, and Bridge and Nichols’ interpretations, which provide insights into the sedimentary structures and facies associations typical of deltaic deposits [61,63,64]. Lithofacies associations were analyzed to infer sedimentary environments, with a particular focus on deltaic processes. We examined vertical and lateral facies stacking patterns to reconstruct depositional settings and understand the dynamics of sediment transport and deposition within the deltaic system.

The categorization of facies was determined through a comprehensive analysis of their depositional characteristics, incorporating both vertical and lateral facies stacking patterns. The identified facies and their associations were then used to infer the depositional processes and environments. Channel units were classified using a three-tiered approach. Sand-on-sand contacts, characterized by abrupt grain size changes, and delineated by thin clay/silt layers at the lower and upper boundaries were recognized. Channel geometries (e.g., ribbon, sheet, pinch-out) based on interpreted photomosaics, with a focus on width/thickness ratios and lateral continuity, were classified. Vertical (aggradational vs. filled) and lateral (compensational vs. clustered) facies arrangements to reconstruct architectural evolution were documented.

4. Results and Analysis

4.1. Lithofacies Analysis

The lithofacies and lithofacies association analyses were conducted on the four outcrops. The lithofacies comprise 16 types, including sandstones, siltstones, and mudstones, with distinct sub-lithofacies based on grain size patterns and sedimentary structures (Figure 3). These lithofacies were interpreted in terms of the sedimentary processes that lead to their deposition (Table 1).

Figure 3. Lithofacies division of the delta front depositional system in the Ch7 member in the Upper Triassic Yanchang Formation in the Ordos Basin.

Figure 3. Lithofacies division of the delta front depositional system in the Ch7 member in the Upper Triassic Yanchang Formation in the Ordos Basin.

Table 1. Types and characteristics of lithofacies in delta front deposits of Ch7 member, Yanchang Formation [62].

Lithofacies Code	Lithofacies Title	Grain Size	Thickness Range (m)	Description	Interpretation
Sth	Heterocentric trough cross-stratified Sandstone	Coarse to medium-grained sand	0.50–3.00	Lithofacies Sth occurs at the base of grayish-white sandstone. Its lower boundary has a curved erosional surface where troughed sets intersect consistent lamina planes at oblique angles. There are varying degrees of depression between the cross-bedded sets, with trough widths ranging from 1.0 to 3.0 m (Figure 3 and Figure 4A,C).	Trough cross-bedding forms at the base of a channel due to intense hydrodynamic erosion linked to channel incision [66]. Lithofacies Sth indicates channel migration [67].
Stc	Concentric trough cross-stratified Sandstone	Coarse to medium-grained sand	0.50–2.00	Lithofacies Stc consists of well-sorted, rounded gray-white sandstone. The base exhibit subtle erosion surface. The troughs exhibit symmetrical shapes, with rapidly varying thicknesses and concave bottoms. The concave laminae and sets are consistent and vertically stacked. The trough widths range from 1.0 to 2.0 m (Figure 3 and Figure 5A,B,D).	Trough cross-bedding forms at the base of a channel as a result of strong hydrodynamic erosion associated with channel incision. Lithofacies Stc reflects rapid channel infilling [68,69].
Sp	Planar cross-stratified Sandstone	Coarse to medium-grained sand	0.50–5.00	The cross-bedding is observed at the base of the sets within gray-white sandstone. The sets exhibit a flat, parallel, and plate-like morphology, with minimal variation in thickness. The laminae intersect the sets at various angles, with the majority of laminae within a set aligned in the same direction. The angle between the laminae and the sets exceeds 10° (Figure 3 and Figure 4B,D,E,G).	Lithofacies Sp reflects the lateral or foreset deposition of sediments transported by strong forces. The cross-bedding indicates the downstream flow direction of bedload migration and typically forms within the microfacies of distributary channels and mouth bars [70,71].
Sc	Convolute bedded Sandstone	Medium to fine-grained sand	0.30–1.00	Convolute bedding is observed in gray sandstone, characterized by continuous open “synclines” and tight “anticlines” (Figure 3). The distorted laminae gradually return to their normal orientation toward the base of the rock layer and are truncated by the overlying strata, with diameters ranging from 0.3 to 0.8 m (Figure 4C).	During flooding, channel overflow generated excessive vertical force at the edges, causing sediment to roll and deposit into the interdistributary bay. This process led to the curling of internal sediment layers, forming convolute bedding [72,73].
Sm	Massive Sandstone	Medium to fine-grained sand	0.20–2.00	The rock stratum consists of well-sorted gray sandstone, positioned above St and Sp (Figure 5D). It is characterized by a uniform appearance and the absence of laminae, with a relatively consistent composition and structure (Figure 3).	Lithofacies Sm forms from the rapid vertical accumulation of suspended particles, indicating a high-energy sedimentary environment with strong hydrodynamic conditions. It is commonly found in the middle and upper sections of distributary channels [74,75].
Sg	Fining-upward Sandstone	Medium to fine-grained sand	1.00–3.00	It consists of gray sandstone (Figure 4B,G). The fining-upward sandstone is a sedimentary unit characterized by a gradual decrease in particle size from medium to fine grains. The bedding planes are parallel, and laminae are absent (Figure 3).	Lithofacies Sg forms as water velocity progressively decreases during deposition. It is typically found in the middle and upper sections of mouth bars. As deposition occurs, reduced water flow leads to smaller sediment grain sizes and suppresses the development of bedding structures [76].
Sr	Current ripple cross-stratified Sandstone	Medium to fine-grained sand	0.10–1.00	The middle and upper portions of the well-rounded, gray-white sandstone stratum display multiple layers of asymmetrical, wavy laminae. The bedding consists of overlapping, wave-shaped textures formed by the forward migration and upward growth of sandy ripple marks during sediment deposition, with an extension ranging from 1.0 to 2.0 m (Figure 3 and Figure 4D).	Lithofacies Sr typically forms in the upper sections of the rock stratum and is associated with overflow deposition, commonly observed at the top of a channel or within the upper portions of a flood fan [77,78].
Sh	Parallel-bedded Sandstone	Fine-grained sand	0.20–0.50	Lithofacies Sh is observed in gray-white sandstone, where the layers are parallel and commonly exhibit fracture lineation. These layers display uniform thickness and consist of well-sorted grains (Figure 3). The laminae range in thickness from 0.5 to 1.0 cm (Figure 4B,G).	The absence of cross-bedding and significant bioturbation suggests stable depositional conditions during formation. Parallel-bedded sandstone typically forms under stable, unidirectional high-flow conditions and is commonly found at distributary channel tops or in central mouth bars [71,79].
Fm	Massive Siltstone	Silt	0.10–2.00	Lithofacies Fm consists of well-sorted, gray siltstone with a thickness under 2.0 m, interbedded with lithologies Mm and Mh (Figure 6). It is characterized by a fine-grained, homogeneous texture with minimal internal structure. This lithology lacks notable laminations or bioturbation and exhibits a tendency to break into large, block-like fragments due to its cohesive properties (Figure 3).	Lithofacies Fm is formed in high-energy environments with moderate rates of sedimentation. Periodic flooding events facilitate the deposition of fine sediments with minimal reworking. The lithofacies develops rapidly from suspended fine sediments and typically occurs in terminal distributary channels and distal bars [80].
Fr	Current ripple cross-stratified Siltstone	Silt	0.10–1.00	The facies is found in dark gray siltstone, characterized by ripple heights of less than 0.5 cm and wavelengths ranging from 2.0 to 4.0 cm (Figure 5G). The laminae exhibit alternating layers of fine-grained silt, distinguished by well-defined ripple features. This is typically observed in (Figure 3). The thickness of individual laminae ranges from 0.1 to 1.0 cm, which contributes to their finely banded appearance.	Lithofacies Fr, which indicates the direction of sediment transport, reflects the movement and accumulation of sandy sediments under unidirectional flow conditions with low hydrodynamic forces. This facies occurs in distal bars and sheet sand deposits [71,80].
Fh	Horizontal bedded Siltstone	Silt	0.10–0.50	Lithofacies Fh is developed in dark gray siltstone and interbedded with Mh (Figure 4F and Figure 5H). It is characterized by parallel layers of fine-grained silt with minimal internal structure (Figure 3). These beds exhibit uniform thickness and consistent grain size. The laminae are poorly defined, with thickness ranging from 1.0 to 2.0 mm.	Under stable hydrodynamic conditions, materials precipitate from suspension or solution, forming horizontal bedding composed mainly of fine-grained silt and mud. This lithofacies is common in floodplain settings, representing periods of calm water, where fine sediments settle from suspension. It is found in interdistributary bays [81].
Fs	Deformation structured Siltstone	Silt	0.40–2.00	The dark gray, siltstone is characterized by deformation, fragmentation, folding, and fracturing (Figure 6G,H). This lithofacies contains a mixture of mudstone, with a breccia-like appearance and a mudstone sliding surface at its base. Common structural elements include folds, faults, and irregular lamination (Figure 3). The lengths of the sliding and slump bodies range from 4.0 to 5.0 m, with thicknesses between 2.0 and 2.5 m.	Sliding and slumping structures are deformations caused by the movement of unconsolidated sediments on slopes due to gravity. (1) Sliding structures form when sediments move downslope under gravity. (2) Slumping structures occur when liquefaction or fluidization causes silty sand to roll or become suspended, triggered by earthquakes. These structures develop in rapidly deposited sediments in steep environments, particularly within the slope break zone of the prodelta [82,83].
Mm	Massive Mudstone	Mud	0.10–2.00	Lithofacies Mm is a dark gray lithology with a homogeneous, fine-grained texture predominantly composed of clay- and silt-sized particles. This rock type is notable for its lack of stratification, internal structure, and bioclasts (Figure 3). It is commonly interbedded with fine sandstone and siltstone (Figure 5A and Figure 6).	This lithofacies indicates a stable depositional environment with low-energy conditions. The absence of distinct bedding planes suggests rapid sediment deposition, often associated with events like flooding or sediment overloading. It is commonly found in interdistributary bays [81].
Mh	Horizontal bedded Mudstone	Mud	0.10–0.50	Lithology Mh consists of dark gray mudstone with high organic matter content. The laminae are parallel and straight, with no discernible bedding planes, and exhibit excellent continuity. The laminae thicknesses range from 1.0 to 2.0 mm (Figure 3 and Figure 4F).	It reflects sediment deposition in stable, low-energy environments, typical of low-flow conditions. This lithofacies is commonly found in floodplains, riverbanks, interdistributary bays, and prodelta regions, where fine sediments accumulate in calm water [74,81].
Mc	Carbonaceous Mudstone	Mud	0.10–1.00	This lithofacies is dark gray to black, characterized by high organic matter content, imparting a distinctive color and often a greasy texture. It consists of fine-grained clay and silt particles, with visible plant remains (Figure 3). It has an organic carbon content ranging from 10.0% to 15.0% (Figure 5B and Figure 6C).	Lithofacies Mc indicates deposition in low-energy environments that facilitate the preservation of organic matter. It is found in interdistributary bay environments, where fine sediments accumulate without significant disturbance [69].
Mp	Phytodetritus Mudstone	Mud	0.10–1.00	Lithofacies Mp is composed of dark gray clay and mud particles mixed with significant amounts of plant debris, resulting in a high organic matter content (Figure 3). Phytodetritus ranges in size from 2.0 to 30.0 cm (Figure 5E and Figure 6D).	This sedimentary environment is characterized by low energy, with plant debris indicating proximity to the lake shore, where mud sediments accumulate. It represents a periodically flooded depositional area within the interdistributary bay of the delta front [84].

Table 1. Types and characteristics of lithofacies in delta front deposits of Ch7 member, Yanchang Formation [62].

Lithofacies Code	Lithofacies Title	Grain Size	Thickness Range (m)	Description	Interpretation
Sth	Heterocentric trough cross-stratified Sandstone	Coarse to medium-grained sand	0.50–3.00	Lithofacies Sth occurs at the base of grayish-white sandstone. Its lower boundary has a curved erosional surface where troughed sets intersect consistent lamina planes at oblique angles. There are varying degrees of depression between the cross-bedded sets, with trough widths ranging from 1.0 to 3.0 m (Figure 3 and Figure 4A,C).	Trough cross-bedding forms at the base of a channel due to intense hydrodynamic erosion linked to channel incision [66]. Lithofacies Sth indicates channel migration [67].
Stc	Concentric trough cross-stratified Sandstone	Coarse to medium-grained sand	0.50–2.00	Lithofacies Stc consists of well-sorted, rounded gray-white sandstone. The base exhibit subtle erosion surface. The troughs exhibit symmetrical shapes, with rapidly varying thicknesses and concave bottoms. The concave laminae and sets are consistent and vertically stacked. The trough widths range from 1.0 to 2.0 m (Figure 3 and Figure 5A,B,D).	Trough cross-bedding forms at the base of a channel as a result of strong hydrodynamic erosion associated with channel incision. Lithofacies Stc reflects rapid channel infilling [68,69].
Sp	Planar cross-stratified Sandstone	Coarse to medium-grained sand	0.50–5.00	The cross-bedding is observed at the base of the sets within gray-white sandstone. The sets exhibit a flat, parallel, and plate-like morphology, with minimal variation in thickness. The laminae intersect the sets at various angles, with the majority of laminae within a set aligned in the same direction. The angle between the laminae and the sets exceeds 10° (Figure 3 and Figure 4B,D,E,G).	Lithofacies Sp reflects the lateral or foreset deposition of sediments transported by strong forces. The cross-bedding indicates the downstream flow direction of bedload migration and typically forms within the microfacies of distributary channels and mouth bars [70,71].
Sc	Convolute bedded Sandstone	Medium to fine-grained sand	0.30–1.00	Convolute bedding is observed in gray sandstone, characterized by continuous open “synclines” and tight “anticlines” (Figure 3). The distorted laminae gradually return to their normal orientation toward the base of the rock layer and are truncated by the overlying strata, with diameters ranging from 0.3 to 0.8 m (Figure 4C).	During flooding, channel overflow generated excessive vertical force at the edges, causing sediment to roll and deposit into the interdistributary bay. This process led to the curling of internal sediment layers, forming convolute bedding [72,73].
Sm	Massive Sandstone	Medium to fine-grained sand	0.20–2.00	The rock stratum consists of well-sorted gray sandstone, positioned above St and Sp (Figure 5D). It is characterized by a uniform appearance and the absence of laminae, with a relatively consistent composition and structure (Figure 3).	Lithofacies Sm forms from the rapid vertical accumulation of suspended particles, indicating a high-energy sedimentary environment with strong hydrodynamic conditions. It is commonly found in the middle and upper sections of distributary channels [74,75].
Sg	Fining-upward Sandstone	Medium to fine-grained sand	1.00–3.00	It consists of gray sandstone (Figure 4B,G). The fining-upward sandstone is a sedimentary unit characterized by a gradual decrease in particle size from medium to fine grains. The bedding planes are parallel, and laminae are absent (Figure 3).	Lithofacies Sg forms as water velocity progressively decreases during deposition. It is typically found in the middle and upper sections of mouth bars. As deposition occurs, reduced water flow leads to smaller sediment grain sizes and suppresses the development of bedding structures [76].
Sr	Current ripple cross-stratified Sandstone	Medium to fine-grained sand	0.10–1.00	The middle and upper portions of the well-rounded, gray-white sandstone stratum display multiple layers of asymmetrical, wavy laminae. The bedding consists of overlapping, wave-shaped textures formed by the forward migration and upward growth of sandy ripple marks during sediment deposition, with an extension ranging from 1.0 to 2.0 m (Figure 3 and Figure 4D).	Lithofacies Sr typically forms in the upper sections of the rock stratum and is associated with overflow deposition, commonly observed at the top of a channel or within the upper portions of a flood fan [77,78].
Sh	Parallel-bedded Sandstone	Fine-grained sand	0.20–0.50	Lithofacies Sh is observed in gray-white sandstone, where the layers are parallel and commonly exhibit fracture lineation. These layers display uniform thickness and consist of well-sorted grains (Figure 3). The laminae range in thickness from 0.5 to 1.0 cm (Figure 4B,G).	The absence of cross-bedding and significant bioturbation suggests stable depositional conditions during formation. Parallel-bedded sandstone typically forms under stable, unidirectional high-flow conditions and is commonly found at distributary channel tops or in central mouth bars [71,79].
Fm	Massive Siltstone	Silt	0.10–2.00	Lithofacies Fm consists of well-sorted, gray siltstone with a thickness under 2.0 m, interbedded with lithologies Mm and Mh (Figure 6). It is characterized by a fine-grained, homogeneous texture with minimal internal structure. This lithology lacks notable laminations or bioturbation and exhibits a tendency to break into large, block-like fragments due to its cohesive properties (Figure 3).	Lithofacies Fm is formed in high-energy environments with moderate rates of sedimentation. Periodic flooding events facilitate the deposition of fine sediments with minimal reworking. The lithofacies develops rapidly from suspended fine sediments and typically occurs in terminal distributary channels and distal bars [80].
Fr	Current ripple cross-stratified Siltstone	Silt	0.10–1.00	The facies is found in dark gray siltstone, characterized by ripple heights of less than 0.5 cm and wavelengths ranging from 2.0 to 4.0 cm (Figure 5G). The laminae exhibit alternating layers of fine-grained silt, distinguished by well-defined ripple features. This is typically observed in (Figure 3). The thickness of individual laminae ranges from 0.1 to 1.0 cm, which contributes to their finely banded appearance.	Lithofacies Fr, which indicates the direction of sediment transport, reflects the movement and accumulation of sandy sediments under unidirectional flow conditions with low hydrodynamic forces. This facies occurs in distal bars and sheet sand deposits [71,80].
Fh	Horizontal bedded Siltstone	Silt	0.10–0.50	Lithofacies Fh is developed in dark gray siltstone and interbedded with Mh (Figure 4F and Figure 5H). It is characterized by parallel layers of fine-grained silt with minimal internal structure (Figure 3). These beds exhibit uniform thickness and consistent grain size. The laminae are poorly defined, with thickness ranging from 1.0 to 2.0 mm.	Under stable hydrodynamic conditions, materials precipitate from suspension or solution, forming horizontal bedding composed mainly of fine-grained silt and mud. This lithofacies is common in floodplain settings, representing periods of calm water, where fine sediments settle from suspension. It is found in interdistributary bays [81].
Fs	Deformation structured Siltstone	Silt	0.40–2.00	The dark gray, siltstone is characterized by deformation, fragmentation, folding, and fracturing (Figure 6G,H). This lithofacies contains a mixture of mudstone, with a breccia-like appearance and a mudstone sliding surface at its base. Common structural elements include folds, faults, and irregular lamination (Figure 3). The lengths of the sliding and slump bodies range from 4.0 to 5.0 m, with thicknesses between 2.0 and 2.5 m.	Sliding and slumping structures are deformations caused by the movement of unconsolidated sediments on slopes due to gravity. (1) Sliding structures form when sediments move downslope under gravity. (2) Slumping structures occur when liquefaction or fluidization causes silty sand to roll or become suspended, triggered by earthquakes. These structures develop in rapidly deposited sediments in steep environments, particularly within the slope break zone of the prodelta [82,83].
Mm	Massive Mudstone	Mud	0.10–2.00	Lithofacies Mm is a dark gray lithology with a homogeneous, fine-grained texture predominantly composed of clay- and silt-sized particles. This rock type is notable for its lack of stratification, internal structure, and bioclasts (Figure 3). It is commonly interbedded with fine sandstone and siltstone (Figure 5A and Figure 6).	This lithofacies indicates a stable depositional environment with low-energy conditions. The absence of distinct bedding planes suggests rapid sediment deposition, often associated with events like flooding or sediment overloading. It is commonly found in interdistributary bays [81].
Mh	Horizontal bedded Mudstone	Mud	0.10–0.50	Lithology Mh consists of dark gray mudstone with high organic matter content. The laminae are parallel and straight, with no discernible bedding planes, and exhibit excellent continuity. The laminae thicknesses range from 1.0 to 2.0 mm (Figure 3 and Figure 4F).	It reflects sediment deposition in stable, low-energy environments, typical of low-flow conditions. This lithofacies is commonly found in floodplains, riverbanks, interdistributary bays, and prodelta regions, where fine sediments accumulate in calm water [74,81].
Mc	Carbonaceous Mudstone	Mud	0.10–1.00	This lithofacies is dark gray to black, characterized by high organic matter content, imparting a distinctive color and often a greasy texture. It consists of fine-grained clay and silt particles, with visible plant remains (Figure 3). It has an organic carbon content ranging from 10.0% to 15.0% (Figure 5B and Figure 6C).	Lithofacies Mc indicates deposition in low-energy environments that facilitate the preservation of organic matter. It is found in interdistributary bay environments, where fine sediments accumulate without significant disturbance [69].
Mp	Phytodetritus Mudstone	Mud	0.10–1.00	Lithofacies Mp is composed of dark gray clay and mud particles mixed with significant amounts of plant debris, resulting in a high organic matter content (Figure 3). Phytodetritus ranges in size from 2.0 to 30.0 cm (Figure 5E and Figure 6D).	This sedimentary environment is characterized by low energy, with plant debris indicating proximity to the lake shore, where mud sediments accumulate. It represents a periodically flooded depositional area within the interdistributary bay of the delta front [84].

4.2. Lithofacies Associations

Six lithofacies associations (LA-1–6) from delta front were identified (Figure 7 and

Table 2. Lithofacies associations in delta successions of Yanchang Formation, Ordos Basin.

Lithofacies Associations	Brief Description	Thickness Range (m)	Component Lithofacies	Environmental Interpretation
LA-1	Channelized medium to fine-grained concentric cross-stratified sandstone	2.0–3.0	Stc-Sr (dominant); Sp, Sm (subordinated)	River mouth to proximal delta front, major distributary channel, mouth bar setting
LA-2	Multilayered medium to fine-grained heterocentric cross-stratified sandstone with subordinated massive mudstone	5.0–6.0	Sth-Sp-Sr (dominant); Sp, Mm (subordinated)	Medial delta front, swinging distributary channel, mouth bar setting
LA-3	Multilayered heterocentric trough cross-stratified sandstone	5.0–6.0	Sth-Sp (dominant); Sh, Fh (subordinated)	Medial delta front, lateral accretion distributary channel, mouth bar setting
LA-4	Horizontal bedded mudstone and siltstone with subordinated channelized sandstone	4.0–5.0	Fh, Mh, Mm (dominant); Sth-Sc-Sh (subordinated)	Medial delta front, interdistributary bay, overflow deposits setting
LA-5	Current ripple cross-stratified siltstone with subordinated channelized massive siltstone	2.5–3.0	Fm Fr (dominant); Mm (subordinated)	Distal delta front, terminal distributary channel, distal bar setting
LA-6	Channelized massive siltstone with subordinated current ripple cross-stratified siltstone	1.0–1.5	Fm Fr (dominant); Mm (subordinated)	Distal delta front to slope break edge, terminal distributary channel, distal bar setting

). These lithofacies associations correspond to the contact relationships and sedimentary processes between channelized and non-channelized fluvial deposits, providing insights into different types of channel patterns.

4.2.1. LA-1: Major Distributary Channel-Fill Deposits

Description: LA-1 is characterized by vertically stacked, normal graded, thick-bedded, coarse to fine-grained sandstones with subtle erosion surfaces. It is mainly composed of lithofacies Stc and Sr, with a thickness of 2.0 to 3.0 m (Figure 7 and Table 2). LA-1 comprises planar cross-stratified sandstones (Sp), concentric trough cross-stratified sandstones (Stc), current ripple cross-stratified sandstones (Sr), and massive sandstone (Sm) from base to top (Figure 7). The lenticular gray sandstone, characterized by a convex bottom and pointed sides, represents the channel sand body. The channel’s superposition pattern is isolated, with no lateral migration. There is a subtle erosion surface between Sp at the base and Stc, which has a thickness ranging from 0.5 to 1.5 m, while Sm at the top has a thickness of 0.5 to 0.6 m.

Interpretation: LA-1 is interpreted as the superposition of major distributary channel-fill and mouth bar deposits. This subtle erosion surface between Sp and Stc, characterized by a concave-up morphology and serving as a distinct channel boundary, imply that the mouth bar is gradually in contact with the channel deposit, with an erosion surface relationship between the channel and mouth bar sand bodies, suggesting that the mouth bar forms prior to the channel [85,86]. When the jet at the river mouth is finger-like and the flow rate is high, the flow reverses, causing sediment to be transported and deposited on both sides, resulting in the formation of thicker sand bars laterally [87]. As the base level declines, the channel remains stabilized in its original position, leading to the development of concentric trough cross-bedding and current ripple cross-bedding that gradually thin upward. The vertical arrangement of lithofacies from Sr to Sm indicates that the hydrodynamics of the channel are diminishing [79,88].

4.2.2. LA-2: Amalgamated Swinging Distributary Channel-Fill Complexes Deposits

Description: LA-2 is characterized by vertically stacked, normal graded, thick-bedded, medium to fine-graded sandstone and mudstone with basal erosion surfaces. It is mainly composed of lithofacies Sth, Sp, and Sr, with a thickness of 5.0–6.0 m (Figure 7 and Table 2). LA-2 comprises planar cross-stratified sandstones (Sp), heterocentric trough cross-stratified sandstones (Sth), planar cross-stratified sandstones (Sp), current ripple cross-stratified sandstones (Sr), and massive mudstones (Mm) from base to top (Figure 7). The bed 5.0 m thickness heterocentric trough cross-stratified sandstones (Sth) overlie planar cross-stratified sandstones (Sp). Multiple intersecting erosion surfaces and coarse-grained sand lag deposits are common in lithofacies Sth. The lithofacies at the top of this associations include lithofacies Sp, Sr, and Mm, with bed thickness varies from 1.0 to 1.5 m.

Interpretation: LA-2 is interpreted as superposition of amalgamated swinging distributary channel center-fill complexes and mouth bar deposits, exhibiting normal grading from coarse-grained lithofacies Sth to fine-grained Mm [89]. The absence of a subtle erosion surface between Sp at the base and Sth indicates that the mouth bar formed prior to the channel, with the normal grading likely recording waning hyperpycnal flows during channel abandonment [24,90]. The medial delta front, situated above a normal wave base in a dynamic environment distal from the river mouth, is sensitive to topographic changes or base level fluctuations that redirect flow pathways [91]. Each erosion surface marks sediment reworking during channel reconfiguration events. The vertical lithofacies transitioning from Sth to Mm reflects systematic energy decay: initial high-energy hyperpycnal flows (coarse Sth) transition to suspended-load sedimentation (fine Mm), as is consistent with channel-fill processes in flood-dominated systems [92].

4.2.3. LA-3: Amalgamated Lateral Accretion Distributary Channel-Fill Complex Deposits

Description: LA-3 is characterized by laterally stacked, normal graded, medium to fine-graded sandstone and mudstone with three-phase parallel erosion surfaces. It is mainly composed of lithofacies Sth and Sp, with a thickness of 5.0–6.0 m (Figure 7 and Table 2). LA-3 comprises parallel-bedded sandstones (Sh), heterocentric trough cross-stratified sandstones (Sth), planar cross-stratified sandstones (Sp), and horizontal bedded siltstone (Fh) from base to top (Figure 7). The bed 5.0 m thickness heterocentric trough cross-stratified sandstones (Sth) and planar cross-stratified sandstones (Sp) are stacked at an angle upon parallel-bedded sandstones (Sh). Three sets of erosion surfaces and lithofacies Sth and Sp, arranged from base to top, are laterally superimposed. The erosion surfaces are relatively parallel to one another, which is the channel boundary. The lithofacies at the top of this association is lithofacies Fh, with bed thicknesses varying from 0.5 to 1.0 m.

Interpretation: LA-3 is interpreted as a superposition of amalgamated lateral accretion distributary channel center-fill complex deposits [93,94]. The frequent reappearance of banded associations between lithofacies Sth and Sp—characterized by the unidirectional lateral displacement of channel sand bodies—reflects paleocurrent direction and macroscopic channel migration patterns, demonstrating lateral accretion of distributary channels through successive sedimentary cycles [95,96]. Changes in flow direction due to factors like shifts in sediment supply or water level can cause the channel to migrate laterally [69]. Unlike LA-2, the lithofacies Sth and Sp are followed by lithofacies Sh, which represents a mouth bar formed by unidirectional high-speed flow. Above the lithofacies Sp lies the lithofacies Fh, a natural levee formed by unidirectional low-velocity flow.

4.2.4. LA-4: Interdistributary Bay and Overflow Deposits

Description: LA-4 is characterized by laterally stacked, reverse graded, medium to fine-graded sandstone, siltstone, and mudstone with erosion surface. It is mainly composed of lithofacies Fh, Mh, and Mm, with a thickness of 4.0–5.0 m (Figure 7 and Table 2). LA-4 comprises horizontal bedded siltstone (Fh), horizontal bedded mudstone (Mh), heterocentric trough cross-stratified sandstones (Sth), convolute bedded sandstone (Sc), parallel-bedded sandstones (Sh), and massive mudstone (Mm) from base to top (Figure 7). The bed 2.0–3.0 m thickness horizontal bedded siltstone (Fh) and horizontal bedded mudstone (Mh) exhibit characteristics of multilayer interbeds of silt and mud, with grain size typically thinning outwards. The bed 1.5 m thickness heterocentric trough cross-stratified sandstone (Sth), convolute bedded sandstone (Sc), and parallel-bedded sandstone (Sh) is stacked at an angle upon horizontal bedded siltstone (Fh) and horizontal bedded mudstone (Mh). The channelized surface is evident at the base, with lithofacies Sth, Sc, and Sh developing upward. In the channel sand body, three convolute bedded deposits are arranged laterally and gradually shrink in the direction of sand body thinning. The lithofacies at the top of this association is lithofacies Mm, with bed thicknesses varying from 0.2 to 0.3 m.

Interpretation: LA-4 is interpreted as a superposition of interdistributary bay and overflow deposits. The lithofacies Fh and Mh represent interdistributary bay deposits, with the horizontal bedding of sediments reflecting changes in flow dynamics and the gradual weakening of flow near the channel [69]. The erosional surface between lithofacies Fh, Mh, and Sth serves as the channel boundary, while the convolute bedding within the channel forms due to changes in water velocity when the distributary channel overflows its banks during flooding, influencing the depositional pattern. The sharp contact between Fh/Mh and overlying Sth exhibits concave-up scour geometries, as is consistent with channel margin collapse during bankfull discharge. Decimeter-scale soft-sediment deformation within the Sth layers dips consistently toward the paleochannel axis, recording shear stress during unidirectional overflow events [97]. Interdistributary bays also form as a result of overflow from diversion channels [98,99].

4.2.5. LA-5: Distal Bar with Terminal Distributary Channel Erosion-Fill Deposits

Description: LA-5 is characterized by vertically stacked, normal graded, thick-bedded siltstones and mudstones with erosion surfaces. It is mainly composed of lithofacies Fm and Fr with a thickness of 2.5–3.0 m (Figure 7 and Table 2). LA-5 comprises massive siltstone (Fm), current ripple cross-stratified siltstone (Fr), massive mudstone (Mm), and massive siltstone (Fm) from base to top (Figure 7). The bed 2.0 m thickness massive siltstone (Fm) and current ripple cross-stratified siltstone (Fr) includes muddy current ripple cross-stratifications in its middle section and exhibits an isolated channel type erosion surface. The lithologic grain size at the top of the channel-filling deposits gradually becomes fine and becomes mudstone. The upper section consists of Fm, with a thickness of 0.5 to 0.7 m.

Interpretation: LA-5 is interpreted as a superposition of distal bar with terminal distributary channel erosion-fill deposits [74]. The lithofacies Fm and Fr represent distal bar deposits. When a river leaves its main channel and flows into a wider distributary area, the water’s direction and speed change. This leads to the deposition and movement of sand grains, forming laterally extending sand bars. The isolated channel exhibits gradually weakened hydrodynamics, resulting in the formation of a muddy deposit. Lithofacies Fm at the top of the lithofacies association is a thin layer of distal bar deposits [8,100].

4.2.6. LA-6: Terminal Distributary Channel Erosion-Fill Deposits

Description: LA-6 is characterized by vertically stacked, normal graded, thin-bedded siltstones and mudstones with erosion surfaces. It is mainly composed of lithofacies Fm and Fr with a thickness of 1.0–1.5 m (Figure 7 and Table 2). LA-6 comprises current ripple cross-stratified siltstone (Fr), massive mudstone (Mm), massive siltstone (Fm), and massive mudstone (Mm) from base to top (Figure 7). The bed 0.5–0.6 m thickness current ripple cross-stratified siltstone (Fr) includes muddy current ripple cross-stratifications. The 0.5–0.6 m thickness massive sandstone (Fm) is an isolated channel type, exhibiting an erosion surface at its base that has eroded the underlying massive mudstone (Mm). The lithofacies Mm is at the top of this association, with a bed thickness of 0.2 m.

Interpretation: LA-6 is interpreted as a superposition of terminal distributary channel erosion-fill deposits [8]. The lithofacies Fm in the upper part of the sequence differs from LA-5 by exhibiting a distinct erosion surface, resulting from cutting and filling in the outer delta front channel. The lower part of the sequence consists of lithofacies Fr, representing a distal bar deposit formed under low hydrodynamics. This lithofacies association illustrates the downcutting and rapid filling processes of distributary channels in the steep slope zone of the outer delta front during a relative sea level drop [101,102].

4.3. Morphology, Fill Patterns, and Facies Zone of Distributary Channels

Four distinct channel cross-sectional morphologies were identified: aggradation channel with erosion base, swinging channel with erosion base, lateral accretion channel with erosion base, and filled channel with erosion base (Figure 8). Types 1 and 4 are isolated channels, while Types 2 and 3 are amalgamated channel complexes.

4.3.1. Type 1: Aggradation Channel with Erosion Base

This channel type is observed at the Yueliangwan outcrop (Figure 9A). The channel sand body is lenticular and isolated within the outcrop, with a flat surface at the top and subtle erosion surface at the bottom. Channel dimensions range from 9.06 to 27.01 m in width and 0.9 to 2.45 m in depth, with aspect ratios of depth to width varying from 9.52 to 11.39.

Description: The outcrop features a rising cycle with over 90% sandy content (Figure 9A). The lenticular gray sandstone, characterized by a convex bottom and pointed sides, represents the channel sand body (Figure 9E). The lithofacies associations of the 1a, 1b, and 1c channels in this outcrop are classified as LA-1. The strata underlying the channel consist of lithofacies Sp. This channel type was filled with lithofacies Stc, which thickens upward over multiple periods (LA-1). These channels share the same morphology and filling structure (Type 1), and their superposition pattern is isolated, with no lateral migration (Figure 9A,E). Isolated distributary channels with well-defined boundaries are developed on the profile and superimposed on pre-channelization deposits (Figure 9G). The upper part of the channel was filled with lithofacies Sr. The entire channel is embedded within the underlying fine sandstone, with the fine sandstone on both sides displaying planar cross-bedding (lithofacies Sp), serving as the channel levee (Figure 8A and Figure 9E). At the base of the outcrop is a dark gray mudstone (lithofacies Mp) rich in carbon and phytodetritus, with particle sizes ranging from 1.0 to 3.5 cm (Figure 9C). At the base of the lithofacies Fm, a stem fossil of Neocalamites, measuring 20.0 cm in length and 8 cm in diameter, stands upright in the rock stratum. The stem fossil is segmented into four sections, each with distinct nodes and internodes, the latter featuring longitudinal ridges and grooves (Figure 9B). Lithofacies Sp, Sr, and Sh with a thickness of 3–4 m can be seen in the middle outcrop (Figure 9D).

Interpretation: This area, situated above the slope break zone, provides increased accommodation space, indicating river mouth to proximal delta front setting (shallow water). The surface of the sandy infillings does not exceed the level of the channel margins, indicating that the channels are under-filled or filled to balance. The hydrodynamic force gradually decreased, resulting in an accelerated rate of channel filling. The lithofacies Mp, rich in carbon and phytodetritus, indicates a short transport distance, low hydrodynamic conditions, and abundant vegetation [103]. The fluctuating water level at the lake margin suggests sediment deposition in the interdistributary bay of the delta front [7,104]. Neocalamites is a genus commonly found in China from the Upper Triassic to Middle Jurassic, typically growing in shallow water settings such as coastal lakes, with water depths generally not exceeding 5.0 m [105,106]. Neocalamites grew at the edge of the lake basin and were rapidly buried, leading to the formation of fossilized stems. The gray and white massive sandstone exhibits large cross-bedding (lithofacies Stc and Sp), indicative of a strong hydrodynamic environment during sedimentation, primarily formed by lateral accretion or progradation of sediments driven by strong hydrodynamic forces. The overall morphological characteristics of the sandstone, combined with other factors, suggest a delta front mouth bar setting. The development of lithofacies Stc with upward thickening reflects accelerated channel infilling and a gradual connection with the underlying mouth bar, indicating a closely symbiotic relationship (Figure 9E,F).

4.3.2. Type 2: Swinging Channel with Erosion Base

This channel type is observed at the Zhujiawan outcrop (Figure 10A). The channel complex forms a long, linear strip with a flat top surface and subtle erosion surface at the bottom. The complex is 151.46 m wide and 5.85 m deep, yielding an aspect ratio (depth to width) of 25.85.

Description: The outcrop features a rising cycle with over 85% sandy content (Figure 10D). On the southeast side of the outcrop, two phases of gray and white mouth bar sand bodies are present, with the upper phase overlying a banded channel complex (Figure 10D,E). The lithofacies associations of the 2a, 2b, and 2c channels in this outcrop are classified into LA-2 (Figure 10). The strata underlying the channel consist of lithofacies Sp. This channel type is filled from the bottom upward with lithofacies Sth, Sp, and Sr (LA-2). Channels 2a, 2b, and 2c exhibit similar morphology and filling structures (Type 2). The flanks of the complex are composed of lithofacies Sr, forming levees (Figure 8B and Figure 10D). Ferric concretions are present in lithofacies Mm, with a size of 0.2 to 0.3 cm (Figure 4E). At the transition from the mouth bar to the distributary channel, a series of long-axis-oriented shaly and silty gravels are observed (Figure 4G). Evidence of lateral deposition is observed in the form of down-cut planar cross-bedding within the gray-white fine sandstone (lithofacies Sp) (Figure 4B). The well-preserved plant leaf fossils in the gray-white fine sandstone have a size of 0.1 to 0.2 cm (Figure 4H).

Interpretation: This area exhibits significant accommodation space, indicating a medial delta front setting in shallow water. Multiple intersecting erosion surfaces within the sand body of the channel complex indicate channel swinging. The channel type exhibits a mixed superimposition pattern, where multi-stage distributary channel sand bodies intersect and overlap both vertically and laterally. Due to frequent intercutting, it is difficult to preserve distributary channels with a single origin and integrity. Ferric concretions, with a substantial number forming in environments with fluctuating lake levels, such as tidal flats and lake facies, also occur in briefly exposed areas [107]. At the transition from the mouth bar to the distributary channel, a series of long-axis-oriented shaly and silty gravels are observed (Figure 4G). This structure typically results from the erosion of silty natural levees by water flow during floods, leading to collapse and accumulation in place or short-distance transport [108]. The well-preserved plant leaf fossils in the lithofacies Sm (Figure 4H) were transported over a short distance due to rapid burial in a nearshore shallow water setting.

4.3.3. Type 3: Lateral Accretion Channel with Erosion Base

This channel type is observed at the Zhujiawan outcrop (Figure 10A). The morphological characteristics are similar to those of channel Type 2. The channel dimensions range from 28.89 to 116.28 m in width and from 1.08 to 4.73 m in depth, with aspect ratios (depth-to-width) varying from 14.10 to 27.38.

Description: The outcrop features a rising cycle with over 85% sandy content (Figure 10B). In the upper section of the outcrop, the banded gray fine sandstone represents a complex of distributary channels that overlies the mouth bar. Distinct lateral deposits are visible within the channel (Figure 10B,C). The 3a, 3b, 3c, and 3d channels are categorized as LA-3 and LA-4 lithofacies associations (Figure 10). These Type 3 channels display uniform morphologies and infill architectures, marked by amalgamated superposition of multiphase distributary channel sand bodies through lateral intersection and overlap. The strata underlying the channel consist of lithofacies Sp and lithofacies Mm. The channel is filled with lithofacies Sth, Sp, and Sc from the bottom upward (LA-3). On one side of the channel complex, lithofacies Fm and Fh (LA-4) are present, while on the opposite side, lithofacies Sp forms the channel’s levee (Figure 8C and Figure 10B). In the lower section, gray-black lithofacies Mh with lithofacies Fh (Figure 4F) transitions laterally into gray lithofacies Stc and Sh (Figure 10C), with lithofacies Sc (Figure 4C). Moreover, the shale interlayers between the distributary channels are poorly developed.

Interpretation: This area exhibits significant accommodation space, indicating a medial delta front setting in shallow water. The presence of multiple subparallel erosional surfaces within the channel complex sand bodies indicates unidirectional channel migration. This channel type exhibits a depositional pattern characterized by the unidirectional stacking of multiphase distributary channel sand bodies both vertically and laterally. Frequent unidirectional channel migration promotes the development of laterally extensive, directionally aligned sand bodies through the amalgamation of distributary channels, rather than isolated single-channel deposits. The lithofacies association of Mh, Fh, Sth, and Sc occurs when a distributary channel overflows its banks during a flood, forming an in-terdistributary bay.

4.3.4. Type 4: Filled Channel with Erosion Base

This channel type is observed at the Tielongwan outcrop (Figure 11A) and the Fudihu outcrop (Figure 12A). The channel sand body is lenticular and isolated, with a flat surface at the top and an erosion surface at the bottom. The channel dimensions range from 7.20 to 18.22 m in width and from 0.22 to 0.79 m in depth, with aspect ratios (depth-to-width) varying from 14.40 to 24.62.

Description: The Tielongwan outcrop features a rising cycle with under 70% sandy content, while the Fudihu outcrop features a rising cycle with under 50% sandy content. The strata underlying the channel consist of mudstone (LA-5) (Figure 12F) or siltstone (LA-6) (Figure 11F). The channel was filled with lithofacies Fm (LA-5,6). The siltstone on both sides forms the levees of the channel (Figure 12E). The channel sand body is overlain by mudstone or siltstone.

In the Tielongwan outcrop, phytoclasts are visible in the thick-bedded dark lithofacies Mp, ranging in size from 1.00 to 2.00 cm (Figure 11C). Lithofacies Fr is on the outcrop top (Figure 11D). A channel (4a), measuring 3.70 m in width and 0.32 m in depth, is observed at the top of the distal bar (Figure 11E). The channel bottom consists of massive siltstone, while the upper part comprises mudstone with a high silt content in the fill. In the upper section of the outcrop, multi-stage thin-layer sand bodies (lithofacies Fm and Fr) are present, with mudstones (lithofacies Mm, Mp, and Mc) developed between them (Figure 11F).

In the Fudihu outcrop, a series of highly deformed sandy (lithofacies Fs) lenses have formed (Figure 12B,C). The sand and mud are mixed, with a distinct mudstone sliding surface, sandstone coiled stratum (Figure 12B), and sandy deformation structures (Figure 12C). Sand lenses are surrounded by peripheral mudstone. The convex lenticular silty sand body at the top of the flat bottom represents the terminal distributary channel, filled with massive siltstone (lithofacies Fm), and eroding mudstone (lithofacies Mm) at the base (Figure 12D,E). The increasing sand content in the upper profile, with the deposition of multi-stage, long-strip intermediate silty sand bodies (lithofacies Fm and Fr). Sand bodies in vertically oriented, single-layer distributary channels are isolated and do not migrate laterally. Isolated distributary channels with well-defined channel boundaries develop along the profile and exhibit characteristics of “mud-covered sand”.

Interpretation: The Tielongwan and Fudihu outcrops, located above the slope break zone, exhibit a distal delta front to slope break edge setting.

The Tielongwan outcrop features banded dark-gray and gray lithofacies Fm and Fr with a flat base and convex top (Figure 11B,D), indicating reduced hydrodynamic energy and the presence of a distal bar. Phytoclasts range in size from 1.00 to 2.00 cm (Figure 11C), indicating a long transport distance [109]. This outcrop reflects the depositional environment of the distal delta front, situated far from the lake shoreline, with multiple intervals of interbedding between the distal bar and prodelta mud [70]. The terminal distributary channel is the primary source channel for the distal bar. In this area, the water is deep, the supply is insufficient, the hydrodynamic energy is weak, and the channel is prone to abandonment and infilling with prodelta mudstone [33,79]. In the upper section of the outcrop, multi-stage thin-layer distal bar sand bodies are present, with prodelta mudstones developed between them (Figure 11F).

In the Fudihu outcrop, the formation of standard dune structures reveals characteristics of slip/slump sediments that eroded earlier sediments and transported them [110,111]. This deformation structure results from the instability of the sedimentary sand body at the slope break of the outer delta front due to external forces, leading to sliding. When stress exceeds a certain threshold, deformation and liquefaction occur within the sliding sand body, leading to the formation of slumped sediments under the influence of gravity [112,113]. This phenomenon may be attributed to volcanic eruptions, seismic events, and tectonic activities during the Late Triassic [114,115]. The increasing sand content in the upper profile indicates a gradual shallowing of the water body, with the deposition of multi-stage, long-strip intermediate silty sand bodies, representing the distal bar deposit of the outer delta front. The convex lenticular silty sand body at the top of the flat bottom represents the terminal distributary channel, filled with massive siltstone, and eroding the outer delta front mudstone or distal bar siltstone at the base (Figure 12D,E).

4.4. The Paleoslope–Channel Morphology Relationship

Five channel morphology parameters of seventeen channel units (1a, 1b, 1c, 2a, 2b, 2c, 3a, 3b, 3c, 3d, 4a, 4b, 4c, 4d, 4e, 4f, 4g) were measured in the study area, showing a strong correlation with paleoslope (paleoslope refers to the inclination of ancient land surfaces, controlling paleosediment transport pathways and depositional architecture through gravity-driven processes), including thickness, width, width-to-thickness ratio, degree of symmetry, and migration frequency [116,117,118,119]. Channels in

Table 3. The distributary channel morphology parameters of the delta front in the study area include the thickness, width, width-to-thickness ratio, degree of symmetry, and migration frequency.

Channel Units	1a	1b	1c	2a	2b	2c	3a	3b	3c	3d	4a	4b	4c	4d	4e	4f	4g
Thickness (m)	2.45	0.90	0.96	5.85	3.50	1.07	1.79	1.43	1.56	4.73	0.32	0.55	0.50	0.70	0.50	0.79	0.80
Width (m)	27.91	9.06	9.14	151.46	49.35	29.30	35.02	30.78	28.89	116.28	3.70	7.60	7.20	10.60	8.54	11.53	14.10
W/T	11.39	10.07	9.52	25.89	14.10	27.38	19.56	21.52	18.52	24.58	11.56	13.82	14.40	15.14	17.08	14.59	17.63
Degree of symmetry	0.83	0.97	0.81	0.71	0.54	0.58	0.66	0.42	0.63	0.72	0.87	0.89	0.72	0.69	0.76	0.88	0.99
Migration frequency	1	1	1	3	3	3	4	4	4	2	1	1	1	1	1	1	1
Relative paleoslope	0.73	0.97	0.85	0.09	0.13	0.07	0.08	0.05	0.09	0.15	0.75	0.64	0.50	0.47	0.44	0.60	0.56

are categorized by channel type (Table 3): channels 1a, 1b, and 1c are aggradation channels with erosion base (Type 1); 2a, 2b, and 2c are swinging channels with erosion base (Type 2); 3a, 3b, 3c, and 3d are lateral accretion channels with erosion base (Type 3); 4a, 4b, 4c, 4d, 4e, 4f, and 4g are filled channels with erosion base (Type 4). Based on this categorization, the geometric relationship between channel morphological parameters and paleoslope is analyzed.

4.4.1. Width-to-Thickness Ratio

In order to distinguish the paleoslope of different types of channels, data from each channel unit were analyzed using W/T (Figure 13A).

Paleoslope is inversely proportional to the channel width-to-thickness ratio, with high ratios indicating channels formed in gentle slopes, resulting in wide, shallow profiles that exhibit slow water flow and limited erosion capacity. In contrast, low ratios correspond to steep slopes, where channels are narrow and deep, characterized by fast-flowing water and significant erosional power [28,33,120]. Steeper channels support higher gravitational forces, leading to faster, more erosive flows, while flatter terrains facilitate wider, shallower channels due to broader water dispersion. Sediment transport capacity is also influenced by slope, with higher slopes allowing for the transport of coarser sediments through narrow channels [121]. Models of channel morphology suggest that under equilibrium conditions, steeper slopes necessitate smaller width-to-thickness ratios to effectively convey sediment and water, maintaining hydraulic efficiency.

For the 1a, 1b, and 1c channels, the width-to-thickness ratio ranges from 9.52 to 11.39, with an average of 10.33. The fitted curve is represented by y = 0.0808x + 0.1945 (R² = 0.9991). For the 2a, 2b, and 2c channels, the width-to-thickness ratio ranges from 14.10 to 27.38, with an average of 22.46. The fitted curve is represented by y = 0.0339x + 0.8742 (R² = 0.8627). For the 3a, 3b, 3c, and 3d channels, the width-to-thickness ratio ranges from 18.52 to 24.58, with an average of 21.05. The fitted curve is represented by y = 0.0371x + 0.4231 (R² = 0.9964). For the 4a, 4b, 4c, 4d, 4e, 4f, and 4g channels, the width-to-thickness ratio ranges from 11.56 to 17.63, with an average of 14.89. The fitted curve is represented by y = 0.0339x + 0.8742 (R² = 0.8627) (Table 3 and Figure 13B). The fitted curve demonstrates that the paleoslope of Type 1 channels is the steepest. In contrast, Type 4 channels exhibit a moderate paleoslope, while the fitted curves for Type 2 and Type 3 channels are comparable, both characterized by relatively gentle paleoslopes.

4.4.2. Degree of Symmetry

In order to differentiate the paleoslopes of the various cross-sections of the channel, the degree of symmetry for each channel unit’s cross-section is analyzed. The degree of symmetry of the channel is calculated as follows (Figure 13A):

Degree of symmetry = W_s/W_l

(1)

Ws and Wl are determined by drawing a vertical line from the thickest part of the channel sand body to the top of the sand body. The linear distances from the intersection point to the top two endpoints of the channel sand body and are defined as Ws (the smaller value) and Wl (the larger value). In this study, the degree of symmetry for an axisymmetric channel sand body is defined as 1 (i.e., Ws = Wl). A lower degree of symmetry indicates poorer symmetry of the channel, while a value closer to 1 signifies better symmetry (Figure 13A).

The paleoslope is directly proportional to the degree of symmetry in the channel cross-section. In meandering river systems, gentle slopes promote greater curvature due to reduced flow velocities, which allow for increased lateral erosion and meander formation, resulting in more tortuous channels with lower symmetry [33]. Conversely, steeper slopes tend to produce straighter channels, as higher flow velocities reduce lateral movement of water, thus maintaining more direct flow paths and enhancing channel symmetry. This relationship is further supported by empirical studies and hydrodynamic simulations, which demonstrate that lower slopes encourage meandering and greater curvature, thereby reducing symmetry, while steeper slopes foster straighter channels with higher symmetry [122,123]. Therefore, a higher degree of symmetry (closer to 1) corresponds to a steeper paleoslope, while a lower degree of symmetry (closer to 0) indicates a gentler paleoslope [9,31,34].

The degree of symmetry for the 1a, 1b, and 1c channels ranges from 0.81 to 0.97. The degree of symmetry for the 2a, 2b, and 2c channels ranges from 0.54 to 0.71. The degree of symmetry for the 3a, 3b, 3c, and 3d channels ranges from 0.42 to 0.72. The degree of symmetry for the 4a, 4b, 4c, 4d, 4e 4f, and 4g channels ranges from 0.71 to 0.99 (Table 3). The overall degree of symmetry of the 1a, 1b, and 1c, and the 4a, 4b, 4c, 4d, 4e 4f, and 4g channels exceeds 0.71, while the overall degree of symmetry of the 2a, 2b, and 2c, and the 3a, 3b, 3c, and 3d channels remains below 0.72. These differences are significant (Figure 13C). Type 1 channels exhibit the steepest paleoslope. Type 4 channels have a lower paleoslope compared to Type 1. Type 2 and Type 3 channels show the gentlest paleoslope.

4.4.3. Migration Frequency

In order to differentiate the paleoslopes of the various channels, the migration frequency for each channel unit’s cross-section is analyzed. A river develops a set of genetically related channel complexes through multiple periods of migration and swinging [8,9,28,33]. In this study, the migration frequency of channels is defined as the number of distinct channels identified within a set of genetically related channel complexes (Figure 13A).

The formation of delta systems is influenced by the topographic slope of the land. As the slope decreases, delta channels become more prone to splitting and shunting, resulting in increased channel curvature. This, in turn, leads to higher channel migration, meandering, and overflow deposits. Studies show that on gentler slopes, channels have higher migration frequencies due to the reduced slope, which allows for more lateral movement and sediment deposition [33]. In contrast, steeper slopes reduce channel migration and make the channels more stable [124]. Channels 1a, 1b, and 1c, and 4a, 4b, 4c, 4d, 4e, 4f, and 4g have a migration frequency of 1, indicating minimal to no migration, and they typically develop in environments with steep paleoslopes. In contrast, the 2a, 2b, and 2c channels exhibit migration frequencies of 3, while the 3a, 3b, 3c, and 3d channels display migration frequencies of 2 or 4, forming in areas with gentler paleoslopes (Table 3 and Figure 13D).

4.4.4. Channel Lithologic Character

As the channel type progresses from Type 1 to Type 4, the sandstone filling becomes progressively finer, transitioning from fine sandstone to siltstone. Concurrently, the mud content in the channel filling increases, while the sand content shows a gradual decrease (

Table 4. Classification and sedimentary characteristics of distributary channels in study area.

Channel Units	1a	1b	1c	2a	2b	2c	3a	3b	3c	3d	4a	4b	4c	4d	4e	4f	4g
Geometry	Complete	Complete	Complete	Partial	Partial	Partial	Partial	Partial	Partial	Partial	Complete	Complete	Complete	Complete	Complete	Complete	Complete
Sediment	Rich sand	Rich sand	Rich sand	Sand, Mud	Sand, Mud	Sand, Mud	Sand, Mud	Sand, Mud	Sand, Mud	Sand, Mud	Silt, Mud	Silt, Mud	Silt, Mud	Silt, Mud	Silt, Mud	Silt, Mud	Silt, Mud
Sand content rate	95%	96%	95%	80%	90%	85%	90%	85%	86%	90%	55%	88%	80%	83%	80%	75%	77%
Filling structure	Aggradation			Swinging			Lateral accretion				Filled
Superimposed mode
Lithofacies associations	LA-1				LA-2				LA-3, LA-4				LA-5, LA-6
Transport mechanism	Saltation dominated				Suspended-load dominated								Rolling dominated
Facies zone	River mouth to proximal delta front				Medial delta front								Distal delta front to slope break edge
Channel morphology and Paleotopographic
Relative paleoslope	0.73	0.97	0.85	0.09	0.13	0.07	0.08	0.05	0.09	0.15	0.75	0.64	0.50	0.47	0.44	0.60	0.56
Paleoslope type	SS I			GS II							SS III
Topographic	I: Gentle to steep slopes			II: Steep to gentle slopes							III: Gentle to steep slopes

and Figure 13E).

The sedimentary setting of Type 1 to Type 4 channels range from proximal delta front to distal delta front. Changes in topographic slope led to variations in transport energy and water depth, which in turn influence the selective deposition of clastic sediments. In a delta front environment, a gentle slope reduces hydrodynamic force and results in finer sediment granularity. Therefore, paleoslope is positively correlated with the granularity of clastic sediment [31,33,125,126].

4.4.5. Paleoslope Classification

This study introduces the concept of “Relative paleoslope” to characterize the terrain of the four types of delta front distributary channels. The term “Relative paleoslope” does not denote the actual slope of the channel development environment but is used to describe the relative position and evolutionary process of the four types of delta front distributary channels. The ratio of width-to-thickness of a channel exhibits a negative correlation with the paleoslope. The degree of symmetry of a channel shows a positive correlation with the paleoslope. The migration frequency of the channel demonstrates a negative correlation with the paleoslope. The relative paleoslope is calculated as follows:

Relative paleoslope = (1/Width-to-thickness ratio) × Degree of symmetry × (1/Migration frequency) × A

(2)

Note: The relative paleoslope has no specific unit; it is represented by a single value. A is a constant used to scale the “relative paleoslope” value for easier reading, where A = 10.

The relative paleoslope for the 1a, 1b, and 1c channels ranges from 0.73 to 0.97. The relative paleoslope for the 2a, 2b, and 2c channels ranges from 0.07 to 0.13. The relative paleoslope for the 3a, 3b, 3c, and 3d channels ranges from 0.05 to 0.15. The relative paleoslope for the 4a, 4b, 4c, 4d, 4e, 4f, and 4g channels ranges from 0.44 to 0.75 (Table 3). The paleoslopes of these four types of channels can be distinctly categorized into three groups.

The paleoslopes of these 17 of channels can be classified into 3 categories: Steep Slope I (SS I), where the 1a, 1b, and 1c (Type 1) channels are located; Gentle Slope II (GS II), where the 2a, 2b, 2c, 3a, 3b, 3c, and3d (Type 2 and Type 3) channels are located; and Steep Slope III (SS III), where the 4a, 4b, 4c, 4d, 4e, 4f, and 4g (Type 4) channels are found (Figure 13F).

5. Discussion

5.1. Facies Zone–Topography Control on Channel

The facies zone–topography control on channels refers to the concept that the spatial arrangement of sedimentary environments (facies zones) and landform characteristics (topography) collectively influence the development and morphology of river channels [127,128]. This integrated perspective is essential for understanding channel evolution and the processes of sediment transport and deposition, which are crucial for geological studies as well as oil and gas exploration. Understanding the facies zone–topography control is also key to reconstructing past environments and predicting the distribution of channel sand bodies in the subsurface of delta systems [94].

5.1.1. River Mouth to Proximal Delta Front-Topography I Control on Type 1 Channel

In conjunction with the analysis of depositional facies zone and paleoslope, the formation of Type 1 channels is governed by the transition from the gentle slope of the delta plain to the steep slope of the delta front (SS I) topography, through which it passes Slope Break-1 (Topography I) (Table 4).

In the sedimentary context of the river mouth to the proximal delta front zone of a lacustrine basin, abundant sediments are supplied from distributary channels at the delta front into the basin [129]. However, due to the insufficient accommodation space at the river mouth, thick deposits cannot form, and sediments are primarily transported by rolling and saltation (Table 4). After passing through the slope break, the transition from gentle to steep slopes increases the accommodation space [130]. The jets of distributary channels experience gravity-driven flow with intense turbulence, which enhances material mixing and transport, with sediment transport primarily governed by inertial forces. The stronger gravity-driven jets rapidly increase hydrodynamic energy, continuing to transport sediments primarily by rolling and saltation, forming thicker mouth bar deposits [129,131]. Notably, the flow closer to the river mouth has higher energy, and the distributary channels radiate outward, often associated with mouth bars [71]. Due to the strong hydrodynamic influences, the channels do not bifurcate and resist lateral migration, resulting in good symmetry. Under a regressive lake setting, the channels gradually infill, forming concentric trough cross-bedding indicative of aggradation, with well-developed mouth bars on both sides and isolated channel sand bodies within the stratum (Table 4).

5.1.2. Medial Delta Front-Topography II Control on Type 2 and 3 Channels

In conjunction with the analysis of depositional facies zone and paleoslope, the formation of Type 2 and Type 3 channels is controlled by the transition from the steep slope of the proximal delta front (SS I) to the gentle slope of the medial delta front (GS II), passing through the Slope Toe-2 (Topography II) (Table 4).

Sediments enter the basin from distributary channels at the steep delta front, retaining substantial kinetic energy in the steep slope section. Distributary channels, in the absence of gravity-driven flow, are influenced by water body expansion and wave action, which reduce flow velocities, making jet-driven sediment transport the primary mechanism [132,133]. With sufficient sediment supply, as the transport distance increases, hydrodynamic forces gradually weaken, and sediments are primarily transported by suspension and rolling. After passing the slope toe, the gradient changes from steep to gentle, and the resistance to sediment transport increases rapidly. Under weaker hydrodynamic conditions, the channels rapidly bifurcate and migrate, breaching and oscillating during intermittent flood periods. At this stage, sediments are primarily transported by rolling and suspension [107,134]. The distributary channels have a wider distribution range, poorer symmetry, and greater sediment maturity. The channels are flanked by natural levees or mouth bars. Under a regressive lake background, the channels slowly infill, and the channel sand bodies are present as sheet formations within the stratum.

5.1.3. Distal Delta Front to Slope Break-Topography III Control on Type 4 Channel

In conjunction with the analysis of depositional facies zone and paleoslope, the formation of Type 4 channels is controlled by the transition from the gentle slope of the medial delta front (GS II) to the steep slope of the distal delta front-slope break edge (SS III), passing through Slope Break-3 (Topography III) (Table 4).

In the sedimentary context of the distal delta front to slope break edge zone of a lacustrine basin, similar sedimentary processes occur, but at deeper water locations where the jet action of the river ceases [71,135]. After passing through the slope break, gravity-driven forces primarily influence hydrodynamics [136]. Compared to the gentle slope setting, hydrodynamics here remains strong, causing slight downcutting and erosion of early sediments. Consequently, the channels do not bifurcate or migrate laterally, resulting in good symmetry. At this stage, sediments are primarily transported by rolling and suspension. As sediments travel over longer distances, grain size gradually decreases, and sand content diminishes [137,138]. Rapid infill occurs under a regressive lake background. Due to weaker hydrodynamics compared to shallow water settings, sedimentary structures are underdeveloped, sediment supply is insufficient, and the channels are flanked by low distal bars and natural levees, with channel sand bodies isolated within the stratum.

5.2. Evolution Patterns of Distributary Channels

Research on distributary channels at delta fronts in lacustrine basins indicates that sediment is transported from confined channels to unconfined lacustrine environments, forming mouth bars and distributary channel complexes at the delta front. Due to significant local terrain control, distributary channels at the delta front are highly unstable, frequently branching, migrating (bending), merging, and undergoing minor downcutting due to changes in local steep slope.

The relationship between the four types of delta front distributary channels and the gradient in the study area is as follows: Distributary channels near the river mouth exhibit good symmetry with erosion and occur in areas with steep terrain. Distributary channels in the medial part of the delta front exhibit poor symmetry with erosion and are situated in areas with gentle terrain. Terminal distributary channels exhibit good symmetry with erosion and occur in areas of steep terrain (Table 4). Based on facies zone and topography, the delta front in the study area is divided into three sections to account for the evolution of the four types of distributary channels (Figure 14).

The distributary channel further divides after entering the lake through the river mouth, forming a distributary channel. The hydraulic force is strong, the channel is straight with low W/T, and it is associated with the mouth bar, often situated on the mouth bar. As the delta gradually receded, the channel was filled with sandy sediment, forming an aggradation channel (Type 1).

Distributary channels further from the river mouth enter areas with a gentle slope. Lateral river accretion and channel swinging lead to the formation of a channel complex that typically results in a broad, elongated band formed through successive stages of lateral accretion and channel swinging (Types 2 and 3). The channel is superimposed on the mouth bar and is in transverse contact with both the mouth bar and the interdistributary bay. As the delta gradually receded, the channel was filled with sandy sediment.

Distributary channels that enter the distal delta front evolve into terminal distributary channels. Hydrodynamic forces strengthen, causing the channel to straighten again; it superimposes on or erodes the distal bar and serves as the main supply channel to the distal bar (Type 4). Influenced by volcanic activity, earthquakes, tectonic movements, and other external forces, the sediments on the slope collapsed, forming a sliding and slumping sedimentary body.

5.3. Aggradation Pattern of Channels in Response to Topography

The aggradation patterns of distributary channels in delta front are strongly influenced by the underlying topographic characteristics [34,139]. The topography of a delta system directly governs channel formation, sediment transport, and deposition processes, resulting in distinct aggradation patterns [140,141]. Topographic settings can be broadly categorized into two types: gentle to steep slopes and steep to gentle slopes. Each of these settings contributes to unique channel responses and aggradation behaviors. Discussing these two topographic types provides valuable insights into the global significance of channel aggradation in delta systems, particularly in the context of reconstructing past environments and predicting subsurface hydrocarbon reservoirs.

(1) Gentle to Steep Slopes: In the transition from gentle to steep slopes, the increase in accommodation space (the concept of accommodation space is determined through the analysis of the sedimentary environment, sand content within the section, and topographic changes) due to the steepening slope and the enhanced gravity-driven hydrodynamic forces after the slope break governs channel sedimentation. During this transition, the sedimentary environment typically shifts from low-energy, low hydrodynamic conditions to high-energy, high-hydrodynamic conditions. Such transitions are commonly observed in the slope break zones of delta fronts. As the slope increases, gravity-driven flow intensifies, and sediment transport becomes more efficient, primarily driven by inertial forces such as rolling and jumping. These conditions often result in the formation of blocky or cross-stratified sand bodies, exhibiting a gradual progression of accretion patterns in response to hydrodynamic changes (such as variations in bedding thickness or grain size). Channels tend to stabilize, form isolated paths, and develop symmetrical, well-defined boundaries. Sedimentation occurs predominantly through vertical deposition, rather than lateral migration. This sedimentary structure is widespread in deltaic and alluvial plain environments globally, from the Mississippi River Delta in North America to the Ganges-Brahmaputra Delta in Asia [142,143,144].

(2) Steep to Gentle Slopes: In the transition from steep to gentle slopes, gravity-driven flow weakens, hydraulic forces diminish, and flow velocity decreases. Sediment migration occurs mainly through rolling and suspension, causing the river channel to branch and become more dispersed. The reduction in flow velocity promotes the deposition of larger, more mature sediments, forming multilayered channel sand bodies with intercalated argillaceous layers. At this stage, the channel undergoes significant lateral migration, forming larger sand bodies with greater lateral extent. The transition from high- to low-energy facilitates the lateral diffusion and superposition of sediments, particularly in environments such as piedmont basins, lakes, or marine settings. When fast-flowing water exits a steep slope into a gentler or flat area, sediment is rapidly deposited, forming distributary channels and floodplain deposits. This environment promotes multi-stage superposition and lateral expansion of sand bodies. The transition from steep to gentle slopes is crucial for understanding the depositional patterns of mature delta systems. As flow energy decreases, channels gradually become choked with sediment, and lateral accretion becomes dominant, leading to more complex sedimentary configurations. The sedimentation process is gradual, with large channel sand bodies deposited over extensive areas, while finer sediment fills the interstitial spaces. These sedimentary patterns are found worldwide, from the Amazon Delta to the Niger Delta [145,146].

5.4. Implications for Reservoir Prediction

Based on the characterization of delta front distributary channels and the sand thickness distribution along the basin margin (Table 3 and Table 4, and Figure 14), the most favorable and thick deltaic skeleton sandstone reservoirs can be effectively predicted. The sandstone thickness in the proximal and medial delta front zones (0.90–5.85 m) is significantly greater than that in the distal delta front (0.32–0.80 m). This variation in thickness across facies zones within the delta front is primarily governed by the combined influence of topography and hydrodynamic conditions. Therefore, in delta front facies zones characterized by complex topography, steep and gentle-sloped topographic features are the most promising targets for oil and gas exploration.

The sedimentary processes governing delta front distributary channels and levees are universally relevant to analogous deltaic and lacustrine systems. Variations in gradient play a pivotal role in controlling sediment transport, channel morphology, and deposition patterns, offering insights into channel evolution and sedimentary architecture on a global scale [5,140,147,148]. To effectively predict channel evolution, sediment distribution, and reservoir prediction in both ancient and modern delta systems, it is essential to understand the topographic control mechanisms and analysis methods of channel morphology parameters.

6. Conclusions

Based on the study of distributary channels at the delta front of the YanChang Formation in the Ordos Basin, it is suggested that topographic differences exert a significant influence on the variations in sedimentary characteristics, morphology, and planar distribution features of distributary channels at the delta front of a lacustrine basin.

(1) Based on the cross-sectional geometry of the channel, the channel unit could be divided into four types, namely aggradation channel with erosion base, with isolated superposition style, swinging channel with erosion base, with lateral splicing and vertical cut stack superposition style; lateral accretion channel with erosion base, with lateral splicing and vertical cut stack superposition style; and filled channel with erosion base, with isolated superposition style.

(2) Based on channel morphological, two distinct terrain types within the delta front were identified: transition from gentle to steep slopes and from steep to gentle slopes. Sedimentary characteristics indicate that the transition from gentle to steep slopes can occur in both shallow and deep-water settings. This led to the development of a pattern for the evolution of distributary channels at the delta front.

(3) The aggradation patterns of delta front distributary channels are governed by two distinct topographic transitions: (1) Gentle-to-steep slopes drive vertical accretion through high-energy gravity flows, forming isolated channels with blocky/cross-stratified sands. Enhanced accommodation and bedload transport (rolling/saltation) promote stable vertical stacking. (2) Steep-to-gentle slopes induce lateral migration under waning energy, producing multilayered sand sheets with mud interbeds. Flow deceleration facilitates channel branching and floodplain deposition, creating interconnected reservoirs. This dichotomy between vertical accretion and lateral migration underscores paleoslope gradients as the primary control on deltaic stratigraphy. By correlating geomorphic thresholds with reservoir characteristics (connectivity, heterogeneity), our model significantly improves hydrocarbon exploration strategies in both modern and ancient deltaic systems.

(4) Delta front distributary channel’s sandstone reservoirs are controlled by paleotopography and hydrodynamics, with proximal–medial zones (0.90–5.85 m) hosting thicker sand bodies than distal zones (0.32–0.80 m). Steep to gentle slope transitions are optimal exploration targets. Global relevance of gradient-driven sediment transport and channel–levee architecture highlights the need for integrated paleotopographic and depositional architecture analyses in reservoir prediction.