Geomorphological Characteristics and Evolutionary Process of a Typical Isolated Carbonate Platform Slope in the Xisha Sea: A Case Study of the Northwestern Dongdao Platform

Abstract

The northwestern slope of the Dongdao Platform in the Xisha Sea exhibits a complex geomorphological structure. Utilizing high-resolution multibeam bathymetric data and 2D seismic profiles, this study systematically reconstructs the slope morphology and its evolutionary processes. The study area displays a distinct threefold zonation: the upper slope (160–700 m water depth) has a steep gradient of 15°–25°, characterized by deeply incised V-shaped channels and slump deposits, primarily shaped by gravity-driven erosion; the middle slope (700–1200 m water depth) features a gentler gradient of 10°–15°, where channels stabilize, adopting U-shaped cross-sections with the development of lateral accretion deposits; the lower slope (1200–1500 m water depth) exhibits a milder gradient of 5°–10°, dominated by a mixture of fine-grained carbonate sediments and hemipelagic mud–marine sediments originating partly from the open ocean and partly from the nearby continental margin. The slope extends from 160 m to 1500 m water depth, hosting the C1–C4 channel system. Seismic facies analysis reveals mass-transport deposits, channel-fill facies, and facies modified by bottom currents—currents near the seafloor that redistribute sediments laterally—highlighting the interplay between fluid activity and gravity-driven processes. The slope evolution follows a four-stage model: (1) the pockmark formation stage, where overpressured gas migrates vertically through chimneys, inducing localized sediment instability and forming discrete pockmarks; (2) the initial channel development stage, during which gravity flows exploit the pockmark chains as preferential erosional pathways, establishing nascent incised channels; (3) the channel expansion and maturation stage, marked by intensified erosion from high-density debris flows, resulting in a stepped longitudinal profile, while bottom-current reworking enhances lateral sediment differentiation; (4) the stable transport stage, wherein the channels fully integrate with the Sansha Canyon, forming a well-connected “platform-to-canyon” sediment transport system.

1. Introduction

Isolated carbonate platforms are typically located far from the continent and commonly develop in a typical shallow-water carbonate sedimentary environment within a deep-water depositional setting [1,2,3,4]. The platform margin slope generally represents a transition from a shallow-water carbonate depositional environment to a deep-water setting. Based on the geomorphological characteristics of platform margin slopes, they can be classified into two types. The first type is a gently sloping margin, where the depositional environment gradually transitions into the deep-water basin [5,6]. The second type is a rimmed carbonate platform margin, where the depositional environment exhibits a rapid deepening [7,8,9]. In essence, the fundamental difference between these two slope types lies in the disparity in sedimentation rates between the platform and the platform slope. When the platform exports sediment to the periphery at a high frequency and in large volumes, a gently sloping margin typically develops. Conversely, when the platform exports sediment to the periphery at a low frequency and in small volumes, a rimmed margin slope is more likely to form.

For isolated carbonate platforms, which are typically located far from the continent, the influence of terrigenous clastic input is minimal. As a result, the platform margin slope is generally characterized by mixed deposits of allochthonous carbonates and hemipelagic muds [10,11,12,13]. Allochthonous carbonate deposits refer to carbonates that have been transported from their original location due to external forces before being redeposited elsewhere. These are also known as resedimented carbonates [14]. Mixed sedimentation refers to the coexistence of two different types of sedimentary materials—terrigenous siliciclastic debris and carbonate fragments—either as a mixture or in alternating deposits within the same region. This represents an intermediate transition between purely siliciclastic and purely carbonate depositional environments [15]. The evolution of platform margin slopes is fundamentally governed by the dynamic interplay between erosion and deposition. This process is influenced by a range of factors, including sea-level fluctuations, monsoonal variability, and bottom-current activity along the platform margin, all of which play critical roles in shaping slope morphology and sediment distribution [1,4,16]. Slope collapse is a gravitational instability phenomenon that typically occurs during periods of elevated carbonate production on the platform, which often correspond to high sea-level stands [17,18]. Bottom-current-induced turbidity currents can erode the platform margin slope, leading to the formation of channel-like geomorphic features [19]. Furthermore, persistent bottom-current activity can sculpt the seafloor, generating contourite channels that encircle the platform. This continuous erosional process further destabilizes the slope, potentially triggering successive gravity-driven sediment transport events [20]. Additional key depositional features on the slope include drift deposits and carbonate mounds, both of which predominantly develop under the influence of bottom-current activity [4].

Deep-water channels serve as fundamental conduits within the sedimentary “source-to-sink” system, facilitating the transport of terrigenous clastic material to deep-sea environments through turbidity and gravity flows. The study of their sedimentary dynamics has emerged as a cutting-edge research focus in sedimentology [21,22,23]. Recent advancements in high-resolution 3D seismic imaging, seabed observation technologies, and numerical modeling have led to significant breakthroughs in understanding channel formation and evolution, fluid–bed interactions, and reservoir prediction [20,21,24]. For instance, research on the Niger Delta has demonstrated that channel meandering patterns are governed by the interplay between tectonic deformation rates and sediment supply, giving rise to five distinct morphological configurations, including avulsed bends and confined bends [25]. Similarly, an evolutionary model of the Taixinan Canyon in the South China Sea has elucidated a three-stage channel development process driven by bottom-current modification of pockmark chains (initial gully–immature channel–mature channel), providing new insights into the coupled dynamics of hydrodynamic forces and geomorphic feedback mechanisms [24].

This study systematically investigates the slope sedimentary system in the northwestern Dongdao Platform using high-resolution multibeam bathymetric data and 2D seismic profiles. By integrating geomorphological and seismic sedimentological analysis, the research provides a detailed characterization of the topographic and geomorphic features, as well as the sedimentary processes of four slope channels. Furthermore, it examines the evolutionary mechanisms governing the development of this complex geological system.

2. Geological Background

As the largest marginal sea in the western Pacific, the South China Sea has undergone three major evolutionary stages from the Late Cretaceous to the Early Miocene: continental lithospheric extension and rupture, Cenozoic seafloor spreading, and arc–continent subduction/collision [26,27,28]. These processes have shaped a distinct tectono-sedimentary system. The continental slope is structurally controlled by a complex interplay of NW-trending neotectonic faults and NEE-trending basement faults, forming a chessboard-like fault network. Among these, NE-trending faults primarily dictate the slope morphology [29,30,31]. Since the Quaternary, sedimentation rates have increased by a factor of 2 to 10, accelerating the migration of overpressured fluids and triggering frequent geological hazards, including mud diapirs and gas chimneys [32]. These processes interact dynamically with submarine landslides, establishing a state of equilibrium. Since the Late Miocene, the continuous subduction of the Pacific Plate has further reshaped the stress field along the continental margin, driving the development of complex channel–turbidite fan systems in regions such as the Qiongdongnan Basin [33].

The Xisha Uplift, a key structural unit of the northwestern continental slope of the South China Sea, is underlain by a metamorphic basement [34] and has experienced multiple tectono-sedimentary cycles [35,36]. The uplift is dissected by NE-trending and E-W-trending faults, forming a fault-block uplift zone (Figure 1a). During the Miocene marine transgression, biogenic reef limestones were deposited, reaching thicknesses of up to one kilometer [37]. Quaternary glacial–interglacial cycles induced frequent sea-level fluctuations, shaping a geomorphic landscape characterized by an intricate interplay of coral reef platforms, seamount chains, and deep-sea troughs [2,38,39]. Of particular significance is the Xisha North Trough, an arcuate deep-water channel that reaches depths exceeding 3000 m and connects to the deep-sea plain of the South China Sea. This trough serves as the primary conduit for turbidity current transport toward the ocean basin. Recent observations indicate that bottom-current velocities in this region can reach up to 0.3 m/s, continuously modifying the seafloor pockmark chains and facilitating the formation of unidirectionally migrating channels [40].

The Dongdao Platform, located in the eastern part of the Xisha Uplift, features the initiation segment of the Sansha Canyon in its northern region, as revealed by multibeam surveys. The canyon’s cross-sectional profile exhibits a transition from a V-shape to a U-shape, with a maximum slope of 27°, functioning as a sediment transport corridor linking the shallow reef areas with the northwestern sub-basin (Figure 1). Recent sedimentary dynamics studies suggest that the combined effects of bottom-current erosion along the platform margin and seasonal storm surges drive the transport of coral debris into the deep sea, with an estimated annual sediment discharge reaching millions of tons [17].

3. Materials and Methods

This study utilized the advanced multibeam bathymetric and multichannel seismic surveying technologies of the Guangzhou Marine Geological Survey Bureau to conduct a systematic geophysical survey in the target area, with the multibeam survey covering an area of approximately 300 square kilometers.

Multibeam bathymetric data with a horizontal resolution of 50 m were acquired using the SeaBeam 2112 multibeam echo sounder system by L3Harris (Melbourne, FL, USA), equipped with 151 beams and a maximum swath angle of 150°. The system maintains a measurement accuracy of better than 0.3% at water depths of up to 4000 m, ensuring high-resolution bathymetric mapping. During data acquisition, GNSS positioning data, POS MV inertial navigation system attitude parameters, and AML Minos X sound velocity profiler (AML Oceanographic Ltd., Victoria, BC, Canada) data were simultaneously recorded to ensure data integrity and accuracy. Multibeam data processing was conducted using Caris HIPS v11.4 software, following a comprehensive workflow:

(1). Sound Velocity Field Correction: A dynamic sound velocity profile interpolation technique was applied to mitigate the effects of diurnal temperature variations (ΔT ≈ 5 °C) and tidal salinity fluctuations (ΔS ≈ 3‰) in shallow waters, effectively reducing swath alignment errors to within 0.25 m.

(2). Motion Compensation: Real-time corrections for roll (±10°), pitch (±7°), and heave (±0.5 m) were implemented using inertial navigation data to mitigate wave-induced beam distortions, ensuring high-precision bathymetric measurements.

(3). Topographic Modeling: A high-resolution 50 m × 50 m gridded digital elevation model (DEM) was generated. Three-dimensional terrain visualization and automated contour generation were performed using Global Mapper v17, achieving accuracy standards that comply with the International Hydrographic Organization (IHO) Special Order classification.

Multichannel seismic data were acquired in 2019 using the Guangzhou Marine Geological Survey Bureau’s offshore survey vessel. The seismic source consisted of a GI-gun array with a total volume of 8849 cm³, deployed at a depth of 5 m, with a shot interval of 25 m and a minimum offset of 150 m. The signal reception system utilized a 360-channel Seal-type 24-bit digital streamer, with a channel spacing of 6.25 m and a deployment depth of 6 m. The seismic data processing workflow included preprocessing, pre-stack noise attenuation, surface-related multiple elimination (SRME) for suppressing water-column multiples, high-resolution Radon transform for multiple suppression, diffraction multiple attenuation, amplitude balancing, deconvolution, velocity analysis, and pre-stack migration correction. These processing techniques ensured optimal data quality for subsequent structural and stratigraphic interpretation. However, the 2D seismic data are inherently limited by their sparse linear acquisition geometry. Vertical profiles struggle to fully resolve 3D fluvial channel geometries (e.g., bends, bifurcations) and lateral facies variations, as inter-line interpretations rely on extrapolation and may introduce artifacts. The low lateral resolution impedes the identification of narrow channel boundaries or thin sandbody pinch-outs. Additionally, when stratigraphic dips are oblique to the survey lines, seismic reflections may exhibit geometric distortions.

4. Results

4.1. Interpretation of Multibeam Bathymetry Data

4.1.1. Geomorphological Characteristics of the Northwestern Slope of the Dongdao Platform

The northwestern slope of the Dongdao Platform displays a complex seafloor morphology shaped by gravity-driven processes and sediment redistribution along multiple submarine channels (C1–C4; C1 is the longest, approximately 19 km, while C4 is the shortest, approximately 9 km). The slope extends from approximately 160 m to 1450 m water depth (Figure 2) and can be divided into three zones based on variations in slope gradient and morphological features (Figure 3, Table 1).

Upper Slope (160–700 m water depth; 0.5–2 km long): Adjacent to the platform margin, this zone features steep gradients of 15°–25° (Figure 4). It is dominated by deeply incised V-shaped channels and erosional features such as grooves and scours. These reflect active gravity-driven erosion and frequent sediment remobilization, resulting in irregular basal topography and localized slump deposits.

Middle Slope (700–1200 m water depth; 6–8 km long): With a gentler gradient of 10°–15°, this transition zone exhibits widening and deepening channels, reduced incision intensity, and increased sediment accumulation. It marks a balance between downslope transport and deposition.

Lower Slope (1200–1500 m water depth; 0.2–1 km long): Characterized by a gentle gradient of 5°–10°, this zone contains subdued channel features, some partially infilled by fine-grained carbonate and hemipelagic mud. It represents a shift from active transport to a low-energy depositional regime.

This three-part zonation reflects a dynamic interplay between erosion and deposition, modulated by slope gradient, sediment supply, and hydrodynamic forces.

4.1.2. Microtopographic Features of the Slope

Beyond the large-scale zonation, the slope hosts a variety of microtopographic features—steep slope zones, erosional grooves, slump deposits, sediment wedges, and variable channel margins—which reflect ongoing sedimentary and erosional processes.

Steep Slope Zones (160–700 m): Found mainly on the upper slope, these areas exhibit strong incision and slope gradients of 15°–25° (Figure 4), where V-shaped channels indicate significant gravity-driven erosion. Localized slumps suggest active sediment instability, likely triggered by slope oversteepening or seismic activity.

Erosional Grooves: Prominent in channels C1–C3 (Figure 3), these features are 500–1500 m wide and 200–400 m deep, with V- or U-shaped cross-sections. Their limited infill suggests sustained high-energy turbidity or gravity flows. Slope margin collapse and asymmetric erosion point to long-term hydrodynamic shaping.

Slump Deposits: Concentrated near steep zones in C1 and C2, these small-scale features (100–300 m diameter) are indicative of episodic slope failure events driven by overloading or liquefaction.

Sediment Wedges: Found on the lower slope (1200–1500 m), these features formed by gravity-driven deposition—likely turbidity currents—show stratification that suggests episodic transport and accumulation.

Channel Boundaries: Channel margins vary in sharpness and slope, reflecting differing erosional and depositional histories (Figure 4 and Figure 5). C1 features steep, sharp margins from intense erosion; C2 shows gradual, stable edges from sustained sediment accumulation; and C3–C4 exhibit diffuse boundaries, likely reworked by bottom currents.

4.2. Interpretation of 2D Seismic Data

The seismic profiles of the study area exhibit a diverse range of seismic facies, reflecting complex depositional environments and geological processes. Based on reflection continuity, amplitude, internal architecture, and external morphology, the seismic facies within the study area can be classified into distinct units, including mass transport deposits (MTDs), channel-fill facies, drift facies, bright reflection facies, volcanic facies, and carbonate debris facies (Figure 6 and Figure 7).

Mass transport deposits (MTDs) are predominantly distributed in the upper slope region and exhibit characteristic chaotic seismic reflections. These reflections are marked by low amplitude, low continuity, and disordered patterns, with localized occurrences of lens-shaped or blocky geometries, indicative of sediment remobilization driven by gravity flows or slump events. Furthermore, MTDs are frequently associated with basal unconformities, suggesting extensive sediment transport and redeposition (Figure 6). Their genesis is likely controlled by tectonic activity or gravitational instability on steep slopes.

The channel-fill facies are predominantly observed within the C1, C2, and C3 channel systems in the seismic profiles, exhibiting U-shaped or V-shaped depressions. These channels are infilled with medium- to high-amplitude, relatively continuous reflectors, suggesting that they underwent significant erosional processes followed by later sediment infill. From a sedimentary dynamics perspective, these valleys are likely the result of early turbidity current erosion, with subsequent infill deposits associated with suspended sediment accumulation or density flow processes in low-energy environments (Figure 6).

The drift facies are characterized by arcuate reflections in the seismic profiles, predominantly found in the slope regions. These deposits exhibit medium-amplitude, moderately continuous stratification, with localized onlapping observed in some areas, reflecting the influence of bottom currents or oceanic circulation. The presence of drift deposits indicates that the study area was influenced by ocean currents over a significant period, potentially associated with regional variations in the marine dynamic environment (Figure 7).

Distinct high-amplitude bright reflections are observed in multiple seismic profiles, typically distributed along sedimentary laminations, and are potentially associated with the presence of gas or fluid accumulations. The occurrence of bright reflection facies may be linked to the presence of gas hydrates, gas migration, or variations in pore fluids within the sediments. The identification of this seismic facies provides valuable insights for further investigations into the occurrence and distribution of hydrocarbons or gas hydrates (Figure 6).

A typical volcanic seismic facies is identified in the northeastern part of the study area, characterized by high-amplitude, low-continuity chaotic reflections, with localized lens-shaped structures (Figure 6). These characteristics suggest the presence of volcanic ejecta or volcanically influenced sediments in the region. This feature indicates that the study area may have been influenced by volcanic activity during a specific period, and the development of volcanic rocks may have exerted significant control over the sedimentary processes.

In certain seismic profiles, clastic sedimentary facies associated with carbonates are observed (Figure 7). These facies are characterized by high-amplitude, low-continuity reflection units, often accompanied by collapse or sliding features. The presence of such deposits indicates that the study area may have experienced carbonate degradation due to sea-level fluctuations or tectonic activity, resulting in the formation of gravity-driven carbonate clastic sediments.

5. Discussion

5.1. Formation and Evolution of Channels

Channel systems typically evolve through successive episodes of erosion and sediment deposition, forming a composite structure over multiple time periods [41]. Their sedimentary architecture can be classified into lateral stacking and vertical stacking models. In deep-water confined channel systems, vertical stacking tends to develop in steeper slope regions, while lateral stacking dominates in gentler slope areas [42]. The marginal region of the Xisha Uplift contains significant gas reservoirs, with gas expulsion leading to the widespread development of pockmarks on the seafloor [43,44]; larger pockmark strings have been identified around the study area (Figure 1). The bright reflections observed in the seismic profiles are interpreted as gas accumulations, and distinct gas chimneys are evident in the underlying strata beneath the channel (Figure 8). The gas chimney provides a pathway for vertical gas migration, leading to the development of pockmarks on the slope.

In the study area, the initial development of channels is closely associated with the release of subsurface fluids or other escape mechanisms, which led to the formation of large-scale pockmarks. These pockmarks, acting as localized depressions, create conditions conducive to sediment instability. Over time, gravity-driven sediment flows gradually connected multiple pockmarks, ultimately forming a continuous channel system. Once established, the ongoing release of fluids or gases further intensified gravity flow activity, resulting in the gradual enlargement of the channels. This process caused significant deep and lateral erosion of the slope, leading to substantial modifications in the slope topography. As the channel system evolved, sediment bypass increased, facilitating the transport of carbonate platform sediments into deeper waters. Additionally, this process enhanced the connectivity of sediment transport pathways, promoting the efficient redistribution of slope sediments.

5.2. The Modification of Slope Topography by Gravity Flows

Gravity flows, including turbidity currents and debris flows, play a pivotal role in sediment transport within the channel systems [45]. These gravity flows primarily originate from the Dong Island carbonate platform, where unstable carbonate clastic deposits are periodically reactivated and mobilized under specific conditions. Seismic facies analysis reveals that high-density, sediment-laden gravity flows deposit within the channels, transitioning from chaotic to layered reflections, which reflect the influence of high-energy fluid depositional processes. The presence of MTDs and channel incisions suggests potential slope failure hazards and sediment bypass.

During periods of enhanced sediment input, the intensity of gravity flow activity increases, driving the further deepening and widening of the channels. As these flows move downslope, they continuously transport additional sediment, thereby amplifying the system’s erosional capacity. This mechanism accounts for the continued expansion of the channels and the eventual transport of sediments into deeper waters, particularly within the Sansha Canyon. When compared to other gravity flow-dominated sedimentary systems, it is clear that the interaction between high-energy fluids and subsurface fluid release plays a significant role in shaping slope morphology. The periodic activity of gravity flows ensures the sustained openness of the channel system, preventing rapid infilling and maintaining its long-term activity.

Early gravity flows were controlled by the pockmark topography, with flow paths being discrete in nature. Seismic profiles indicate that the deposits from this stage are primarily composed of thin layers of medium- to fine-grained sands, exhibiting parallel bedding structures that reflect the characteristics of low-density turbidity currents. Geological weaknesses along the edges of the pockmarks became preferential sites for erosion by gravity flows, leading to the lateral expansion of individual pockmarks. Once the channel system was established, the energy of the gravity flows became concentrated along the channel axis, forming high-density debris flows. Multibeam data reveal that, in the middle reaches of the channel (at depths of 700–1200 m), the incision depth reaches up to 400 m, with sidewall slopes exceeding 25° and the development of large-scale bedforms (wavelengths of 50–100 m and wave heights of 2–3 m). The coarser sediment grains during this stage indicate a significant increase in transport capacity. As the channel extends downslope, fine-grained material is laterally sorted, forming asymmetric levees, with the eastern side being 15–20 m higher than the western side. Coarse-grained deposits along the channel axis and fine-grained silt at the levees form a typical “binary structure” (depicted in the interaction zone between Channels and Current in Figure 9d). Ultimately, the gravity flows transport more than 80% of the coarse material to the head of the Sansha Canyon, forming depositional lobes with an aggradation angle of up to 12° at the canyon’s front (shown as the front edge of the Sansha Canyon in Figure 9d). While our study primarily focuses on gravity-driven processes, the secondary influences of tectonic activity, changes in sediment supply, and sea-level fluctuations may have also contributed to the slope evolution.

5.3. Evolutionary Process of the Slope

The sedimentary evolution of the northwestern slope of the Dongdao Platform is shaped by the interplay between channel development and gravity-driven sediment transport. This evolution can be divided into four distinct stages: pockmark formation, initial channel development, channel expansion and maturation, and stable transport (Figure 9).

In the pockmark formation stage, high-porosity strata and active upward migration of subsurface fluids lead to the buildup of overpressured gas. This gas pierces through the sedimentary layers, creating vertical chimneys that vent at the seafloor, triggering localized sediment instability and forming numerous discrete pockmarks. These early-stage pockmarks are typically small, scattered, and remain unconnected. Sedimentation occurs primarily during quiescent periods, while limited gravity-driven transport redistributes a minor portion of the material downslope. At this point, no well-defined transport channels have developed.

During the initial channel development stage, ongoing fluid expulsion gradually alters the slope gradient, steering gravity flows to follow paths along the pockmark chains. This results in the partial linkage of individual pockmarks and the formation of early incised channel structures. Gravity flows at this stage are mainly low-concentration turbidity currents, transporting coral debris from the platform margin. Sediment flux remains low, producing thin, laterally extensive silt layers. Although bottom currents are not yet a dominant factor, localized liquefaction features can be observed around pockmark edges. Erosion intensifies in low-lying areas where gravity flows begin to converge.

In the channel expansion and maturation stage, the slope profile develops a stepped morphology, with each step aligned with the location of a former pockmark—highlighting the combined influence of fluid escape and gravity-driven erosion. Flow energy increases substantially, evidenced by frequent high-density debris flows capable of mobilizing coral reef blocks several meters in diameter. Erosional fill complexes become common in the middle reaches of the channels, and sidewall slopes can exceed 25°. Internally, large-scale bedforms indicate sustained high-energy conditions. The bottom-current activity becomes more prominent during this phase, as shown by asymmetrical channel flanks, with the eastern banks slightly elevated. These bottom currents contribute to the lateral sorting of fine-grained material, initiating sediment differentiation. By the end of this stage, the channel network has become largely continuous, forming a well-defined pathway for sediment delivery to deeper water.

In the stable transport stage, the channel system fully connects with the Sansha Canyon, establishing a mature “platform-to-canyon” sediment routing system. Coarse carbonate debris is funneled downslope via gravity flows and accumulates near the canyon head, forming progradational sediment lobes with slope angles ranging from 10° to 15°. Fine-grained sediments are primarily transported and deposited within the channels, although a portion is further redistributed into the canyon by tidal currents.

6. Conclusions

This study, based on multibeam bathymetric data and 2D seismic profile analysis, elucidates the geomorphological structure and evolution of the northwestern slope of the Dong Platform in the Xisha Sea. It primarily investigates the influence of gravity flows and fluid migration on the slope sedimentary environment. The key findings are as follows:

(1). The study area is divided into three zones: the upper slope (160–700 m water depth), the middle slope (700–1200 m water depth), and the lower slope (1200–1500 m water depth). The upper slope is characterized by a steep gradient (15°–25°) and deeply incised channels (C1, C2, C3, C4), which are influenced by gravitational processes, exhibiting erosional gullies and slump deposits. The middle slope has a gentler gradient (10°–15°), where the channels stabilize and sediment accumulation begins. The lower slope has a shallower gradient (5°–10°), with near-complete channel infilling, transitioning from an erosion-dominated to a deposition-dominated environment, mainly composed of fine-grained carbonates and hemipelagic mud. Microtopographic features reveal intense erosion in the upper parts of the channels, with slump deposits along the sidewalls, underscoring the shaping role of gravity flows in the slope morphology.

(2). The formation of the channel system progresses through four stages: in the pockmark formation stage, fluid expulsion induces localized sediment instability, resulting in pockmarks; in the initial channel development stage, enhanced fluid release leads to increased sediment erosion, connecting depressions and forming early-stage channels; in the channel expansion and maturation stage, gravity flows deepen and widen the channels, while sediment levees develop along the channel margins; in the stable transport stage, the channels function as pathways for sediment transport, delivering carbonate debris to deeper water environments.

(3). Gravity flows, including high-density turbidity currents and debris flows, exert a dominant influence on slope morphology in the study area. Early gravity flows, primarily driven by fluid escape, led to initial channel erosion and the deposition of fine- to medium-grained sand. As gravity flow energy became concentrated along the channel axes, high-density debris flows were generated, resulting in the development of asymmetric sediment levees (with the eastern levee 15–20 m higher than the western levee). In the downstream regions, coarse-grained sediments accumulated primarily in the channel axes, while fine-grained sediments were laterally redistributed. Ultimately, over 80% of the transported sediments were delivered to the frontal area of the Sansha Canyon, forming depositional lobes.

Compared to other deep-water sedimentary systems, the evolution of gravity flows in this area is significantly influenced by fluid expulsion, particularly the release and migration of gas. This process not only influenced the initial formation of the channels but also played a crucial role in their subsequent expansion. Additionally, the carbonate clast deposits identified in the seismic profiles suggest that the area may have undergone sea-level fluctuations or tectonic activity, which facilitated the gravity-driven transport and redeposition of carbonate sediments.

Author Contributions

Conceptualization, Investigation, Data Curation, and Methodology, X.G. and X.L.; Writing—Original Draft, D.L. and X.F.; Writing—Review and Editing, F.T. and C.X.; Investigation, L.H. and M.C.; Software, L.W. and Z.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All data have been provided in this paper. Further inquiries can be directed to the corresponding authors.

Acknowledgments

We are grateful to the Guangzhou Marine Geological Survey for providing the foundational data for this study.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1 (a) represents the tectonic background map of the northwestern South China Sea, (b) represents the geomorphological map of the study area and its surrounding region, and (c) represents the topographic and geomorphological map of the study area. In map (a), the red solid lines indicate faults, and the red box marks the location of map (b). In map (b), the box marks the location of map (c). In map (c), the black solid lines represent the positions of seismic profiles, and C1–C4 represent four channels.

Figure 2 The upper large image is a 3D geomorphological map of the study area, while the four smaller images below show the depth variations along the four profile lines. a, a′ and b, b′ are two profiles parallel to the platform, while c, c′ and d, d′ are two profiles perpendicular to the platform.

Figure 3 The (upper) image is the slope direction map of the study area. The (below-left) section shows the depth variations at the heads of the four channels, while the (below-right) section shows the depth variations at the tails of the four channels. e, e′ are the profile paths at the tail of the channel; f, f′ are the profile paths at the head of the channel.

Figure 4 The (upper) image is the slope gradient map of the study area. The (below-left) section shows the slope gradient variations at the heads of the four channels, while the (below-right) section shows the slope gradient at the tails of the four channels. g, g′ are the profile paths at the tail of the channel; h, h′ are the profile paths at the head of the channel.

Figure 5 The (top-left) image is the 3D geomorphological map of the study area. The (bottom-left) and (right) sections together comprise four images, each representing the depth variations in the four channels.

Figure 6 Seismic profile of Line 01 (upper) and interpretation (lower).

Figure 7 Seismic profile of Line 02 (upper) and interpretation (lower). The yellow triangle indicates onlap.

Figure 8 Gas migration in profile line 01. The pink solid line outlines the shape of the chimney, and the yellow arrows indicate gas migration.

Figure 9 The evolutionary process of the study area, (a) represents the pockmark formation stage, (b) represents the initial channel development stage, (c) represents the channel expansion and maturation stage, (d) represents the stable transport stage.

Table 1

Summary of geomorphological and sedimentary characteristics of the northwestern slope of the Dongdao Platform.

Slope Segment	Water Depth/m	Gradient/°	Dominant Features	Main Processes	Channel Characteristics
Upper Slope	160–700	15–25	Steep slopes, incised channels, V-shaped grooves, localized slump deposits	Intense gravity-driven erosion, sediment remobilization	Narrow, deeply incised, stepped margins
Middle Slope	700–1200	10–15	Broadened channels, reduced incision, sediment accumulation	Transition from erosion to sedimentation, redistribution	Widened and deepened channels, relatively stable
Lower Slope	1200–1500	5–10	Subdued channel expression, sediment wedges, fine-grained deposits	Low-energy deposition, sediment infill	Channels partially infilled, boundaries diffuse

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