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Article

Erosional and Depositional Features along the Axis of a Canyon in the Northern South China Sea and Their Implications: Insights from High-Resolution AUV-Based Geophysical Data

by
Xishuang Li
1,
Lejun Liu
1,*,
Bigui Huang
2,
Qingjie Zhou
1 and
Chengyi Zhang
1
1
First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China
2
China National Offshore Oil Corporation (CNOOC) Research Institute Ltd, Beijing 100027, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2024, 12(4), 599; https://doi.org/10.3390/jmse12040599
Submission received: 28 February 2024 / Revised: 21 March 2024 / Accepted: 25 March 2024 / Published: 30 March 2024
(This article belongs to the Special Issue Morphological Processes and Evolution of Marine Geomorphology: Observations, Modeling and Applications)
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Abstract

:
Autonomous Underwater Vehicle (AUV)-based multibeam bathymetry, sub-bottom profiles, and side-scan sonar images were collected in 2009 and 2010 to map the geomorphic features along the axial zone of a canyon (referred to as C4) within the canyon system developed on the northern slope of the South China Sea. These data significantly improved the spatial resolution of acoustic data, leading to a better understanding of the sedimentary processes within the modern canyon system. The bathymetric data reveal that sections across the canyon axis exhibit either asymmetrical or symmetrical characteristics, which differ from the overall asymmetrical pattern of the entire canyon. This suggests that the overall asymmetrical pattern of the canyon is not primarily due to axial incision. Various morphological elements were identified along the canyon axis, including failure scars, undulating features, knickpoints, flat terraces, furrows, and mass transport deposits (MTDs). Landslides, predominantly located in the upper canyon, were formed after at least 5000 years BP. Between the beginning of the canyon and a water depth of approximately 1300 m, there are alternating flat terraces and knickpoints. The large knickpoints’ low slope gradients are likely formed by the presence of undulating features. The internal configurations of undulating features suggest that they are depositional structures rather than sediment deformation. The formation of small-scale furrows below a depth of 1200 m may be associated with occasional gravity flows down the canyon. It is suggested that the canyon was generally inactive during the Holocene but experienced sporadic processes of sediment erosion, transport, and re-deposition in the axial zone that were triggered by landslide events occasionally in the upper canyon.

1. Introduction

Slope-confined canyons, which are also referred to as blind or headless canyons, are commonly observed features along continental margins globally [1,2,3,4,5]. Similar to shelf-indenting canyons, they function as conduits for sediment transport [6,7]. In contrast to shelf canyons, which are active during both low and high sea level periods [8], slope-confined canyons are typically inactive during high sea level periods. However, there remains a lack of comprehensive understanding regarding the intricate sedimentary processes and dynamics that occur within these canyons, especially along their central axis.
The most active part of a submarine canyon is the axial zone, where processes such as sediment transport, accumulation, erosion, and re-suspension are influenced by bottom currents. The cutting of the canyon axis plays a crucial role in shaping the canyon and affecting its evolution [9,10,11,12]. Various geomorphic features within the canyon, such as turbidity current deposits, scours, and terraces, offer insights into its short- or long-term morphodynamics [10,13,14,15,16,17,18,19]. For instance, the residual bordering terraces at different levels in the submarine canyon system off the Peru–Chile forearc indicate axial meandering during its early development [13], while small-scale terraces like the crescent-shaped bedforms in Monterey Canyon suggest the presence of active turbidity flows [17].
The South China Sea (SCS) has a broad northern margin with numerous submarine canyons connecting the shelf and the deep sea [20,21,22,23,24]. A specific canyon system is situated on the northern slope outside the modern Pearl River mouth, comprising 17 small slope-confined canyons [25] (Figure 1a). Previous research has primarily focused on the morphology [25], landslides [26,27,28], and evolution of these canons [22,29,30,31] using shipborne geophysical data. However, our understanding of the sedimentary processes along the modern canyon axes and the factors influencing thalweg migration remains limited.
Technological advancements, particularly the utilization of Autonomous Underwater Vehicles (AUVs) equipped with ultrahigh-resolution bathymetric echo sounders, have significantly improved our comprehension of submarine canyon systems. These AUVs have enhanced the spatial resolution of acoustic data, enabling a better understanding of the formation processes of submarine canyons and their interactions with external environments [17,32,33,34,35]. In 2009 and 2010, the China Offshore Service Limited (COSL) conducted a study utilizing AUV-based multibeam bathymetric data, side-scan sonar data, and sub-bottom profiles in the central zone of a canyon known as C4 within the canyon system. The collected subbottom profiles, with a high vertical resolution of approximately 0.3 m, and multibeam bathymetry, with a horizontal resolution of approximately 1 m, facilitated the identification of small-scale sedimentary features along the canyon axis. The objectives of this study are: (1) to identify and describe detailed geomorphological elements along the C4 canyon axis; (2) to investigate the origin of erosional and depositional features; and (3) to examine the canyon’s activity during the Holocene and the sedimentary processes along its axis.

2. Study Area

The seafloor in the canyon area exhibits undulations with wavelengths of 1–2 km and heights of dozens of meters. These undulations consist of numerous mounded or elongated mounded terrain units on the canyon sidewalls and the interfluves between adjacent canyons (Figure 1c). The origin of these undulating features is a topic of ongoing debate, with proposed explanations including sediment creeping [26,27,36], sediment waves generated by down-slope turbidity currents [27,36]. Recent studies focusing on slope stability suggest that these undulations on this scale are stable [37].
The canyon system began forming approximately 13.8 million years ago, and its development was influenced by sea-level fluctuations, sediment supply, and bottom currents [22,31,38]. All the canyons in the system underwent similar evolution and periodic infilling [22,31]. Seismic data indicate that the canyon system extended from the outer shelf to the continental slope between 10.5 and 5.5 million years ago but has since been confined to the slope [31]. Basal lags with high-amplitude reflections suggest significant eastward migration during the evolution of the canyon system, especially between 10.5 and 5.5 million years ago (Figure 2). Lateral inclined packages with moderate to good continuity on the canyon sidewalls and interfluves are interpreted as depositional features associated with ancient ocean bottom currents [22,23,31]. The trajectories of the canyon axes show that the lateral migration distance of each canyon decreases from approximately 10 km at the proximal to 1–2 km at the distal end [31], although the exact reasons for this phenomenon remain unclear.

3. Data and Methods

The AUV-based geophysical surveys were carried out in canyon C4, situated in the eastern section of the canyon system (see Figure 1a), to assess the geological conditions of the canyon prior to the installation of the pipeline. The survey encompassed the canyon floor and extended laterally by 1000 m on both sides (Figure 3a), ensuring complete coverage with a width of approximately 1400 m. The HUGIN 1000 AUV, produced by Kongsberg Maritime, Norway, was utilized to gather ultra-high-resolution geophysical data during two missions in 2009 and 2010. The AUV was equipped with an EM2000 multibeam sounder, an Edgetech full-spectrum side-scan sonar, and an Edgetech chirp profiler. The AUV maintained a constant depth of 35 m above the seabed during the survey. The primary survey lines along the canyon were spaced 150 m apart, with an overlap of approximately 20% between adjacent swaths. The AUV achieved a navigation accuracy of 0.05% utilizing inertial navigation with the assistance of Doppler Velocity Log (DVL) and gyrocompass. Positioning accuracy was estimated to be around 5 m, utilizing data from the ultrashort baseline (USBL) system and the Differential Global Position System (DGPS) on the ship. The EM2000 multibeam sonar operated at a frequency of 200 kHz, emitting beams at equal intervals with a mean frequency of 2–3 Hz. The AUV’s positioning data were processed using NavLab, a tool for navigation analysis and data post-processing, and merged with the acquired geophysical data. The bathymetric data were processed using Caris Hips and Sips version 11.4, resulting in a digital elevation model (DEM) with a grid of 1 m × 1 m. It should be noted that while the 1 m × 1 m DEM provides detailed underwater terrain information, its positional accuracy may be limited by the accuracy of the AUV’s position. The Edgetech full-spectrum side-scan sonar system acquired data simultaneously at two frequencies of 105 kHz and 410 kHz. The low-frequency (105 kHz) mode provided data with a coverage width of 220 m and a horizontal resolution of 0.5 m. The Edgetech sub-bottom profiler operated in the frequency range of 2–16 kHz, with the pulse wave limited to 2–10 kHz, and pulses emitted at a frequency of 3 Hz with a length of 20 ms. The obtained sub-bottom profiles had a penetration depth of approximately 40 ms and a vertical resolution of around 0.3 m. The side-scan sonar images, multibeam bathymetric data, and sub-bottom profiles were imported into SB-Interpreter version 2018 for comprehensive interpretation.

4. Results

4.1. Bathymetric Data

The AUV collected detailed bathymetry data of the canyon’s axial floor, with depths ranging from 550 m to 1500 m. The axial zone of the canyon is approximately 500 m wide and has a slope gradient of about 2° (Figure 3). The thalweg, or the lowest point of the canyon, exhibits a slightly concave shape and can be divided into two segments. The upper segment has an average slope of 2.6° and depths ranging from 650 m to 1200 m, while the lower segment has an average slope of 1.4° with depths exceeding 1200 m (Figure 4). The transverse topographic profiles within the axial zone show both asymmetrical and symmetrical patterns (Figure 4).
In the upper region of the canyon, there are undulating terraces that align roughly parallel to the depth contours above the canyon head (Figure 5a). A rough surface is found in the axial zone and sidewalls of the canyon above a depth of 1000 m (Figure 5a,b). Between depths of 700 m and 1300 m, the seafloor in the axial zone of the canyon alternates between steep slopes and flat terraces (Figure 3c,d and Figure 4). Below a depth of 1300 m, the seafloor is characterized by furrows that run parallel to the canyon axis (Figure 6a,b).

4.2. Sub-Bottom Profiles

AUV-based sub-bottom profiles allow for the identification of two distinct internal reflection facies: stratified reflections and chaotic or translucent reflections. The stratified reflections consist of parallel reflections and wavy reflections with good continuity (Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11). Deposits showing chaotic or translucent reflections are commonly present in the canyon axis and sidewalls above 1000 m depth. Within the depth range of 700 m and 1300 m depth along the canyon axis, thick chaotic units often display a lenticular shape and are frequently observed at the flat terraces (Figure 7 and Figure 10). Thin chaotic layers can be found in some local steep slopes (Figure 7 and Figure 8). Chaotic or translucent reflections below 1300 m depth are primarily restricted to the modern central axis zone (Figure 11) and are also observed between two old, stratified layers (Figure 11).
Deposits displaying parallel or wavy reflections are predominantly located on the canyon sidewalls and are typically eroded at the canyon axis (Figure 8 and Figure 9). The wave crests of wavy reflections often show an upslope migration (Figure 7, Figure 9, and Figure 10), with an orientation roughly parallel to the regional contours in the canyon head area and to the axis within the canyon. At the canyon head, stratified reflections are often cut off by a steep slope, and there are no regressive failures (Figure 7). Along the canyon axis within the depth range of 700 m and 1300 m, layered deposit units with stratified reflections are commonly overlain by chaotic units on flat terraces and are truncated at steep slopes (Figure 7 and Figure 10), sometimes showing failure scars. Below 1300 m depth, the older layered units with stratified reflections are frequently eroded or covered by a younger, thin chaotic layer (Figure 11).

4.3. Side-Scan Sonar Data

The side-scan sonar images display two distinct backscatters including homogeneous backscatters and backscatters with variable amplitudes (Figure 12). Homogeneous backscatters, which indicate a smooth seafloor surface, are primarily observed in the sidewalls of the lower canyon and at the canyon head. Within the canyon axial zone, relatively smooth surfaces showing light gray echo features are typically found on flat terraces, while heterogeneous backscatters with variable amplitudes are predominantly found on local steep slopes. Below a depth of 1200 m, furrows and scours in the central canyon exhibit linear striped backscatter patterns, which sharply contrast with smooth surfaces with homogeneous backscatters on either side.

4.4. Geomorphological Elements in the Axial Zone

Various geomorphic elements were identified based on geophysical data, and they include depositional features, erosion features, and features associated with submarine landslides (Figure 13).

4.4.1. Depositional Features

Depositional features in the axial zone consist of undulating features and flat terraces. The undulating features are identified by wavy reflections with varying amplitudes and are frequently truncated at steep slopes (Figure 7, Figure 9, Figure 10 and Figure 11). These features are typically situated at water depths ranging from approximately 650 m to 1300 m and are also observed beneath mass transport deposits (MTDs) (Figure 7, Figure 9, Figure 10 and Figure 11). Sub-bottom profiles indicate that these undulating features develop in alignment with the regional contours of the continental slope or with the canyon axis, with wavelengths estimated to be several hundreds of meters to more than one kilometer.
The flat terraces exhibit inclinations of less than 1° and are commonly positioned at the base of knickpoints. The deposits within the flat terraces typically display chaotic or translucent reflections (Figure 7 and Figure 9).

4.4.2. Erosional Features

Erosional features found in the axial zone include knickpoints, furrows, and scours. Knickpoints are defined as steep slope sections or steps in the longitudinal profile of a fluvial system, submarine canyon, or channel [39,40]. Based on bathymetric data and sub-bottom profiles, eight knickpoints were identified at depths ranging from 700 m to 1300 m along the canyon axis. Knickpoints correspond to steep slopes where older depositional units with parallel or wavy reflections are often exposed (Figure 7 and Figure 10). These knickpoints have a height of tens of meters and a mean slope gradient of approximately 5°. The lips of the knickpoints, where the slope begins to become steeper, are typically located at the lower sides of the undulations (Figure 7 and Figure 10). A suspicious jump pool, which is related to hydraulic jumps [41], was found at a water depth of 850 m (Figure 7).
Furrows and scours are common features in the axial zone of the canyon below a depth of 1200 m. Furrows are usually 1 to 2 m deep, around 10 m wide, and several hundred to roughly 1000 m long (Figure 6a,b). They exhibit seismic diffractions at their edges, resulting in a saw-tooth shape of the seafloor in sub-bottom profiles (Figure 6c,d). Scours, on the other hand, are observed at the edge of the furrow field (Figure 6a,b) and are typically several tens of meters in length, much shorter than the furrows. Both furrows and scours can be identified in side-sonar images by their linear backscatters with varying amplitude (Figure 12d). The density of furrows across the canyon can reach up to 30 furrows per 1 km in some areas. Downstream, the density of furrows generally increases slightly. Furrow merge can be observed in the local area, indicating different flood events or the confluence of flows (Figure 6a).

4.4.3. Features Related to Submarine Landslides

Bathymetric data and side-sonar images show that failure scars have elliptical, tongue-shaped, or irregular shapes (Figure 5a,b and Figure 12b). Most failure scars are smaller than 2 km2, with the slope gradients at their trailing edge typically steeper than 10°. These scars are mainly situated in the canyon head area and canyon sidewalls at depths less than 1000 m.
MTDs typically commonly exhibit chaotic or translucent facies with irregular external forms in sub-bottom profiles [42,43,44]. These diagnostic criteria are also applicable in our study area [37]. MDTs are predominantly found in the canyon axis zone (Figure 7 and Figure 9). Their thickness near the failure scars is more than 10 m, decreasing to less than 5 m down the canyon (Figure 11).

5. Discussion

5.1. Origin of Geomorphological Elements

5.1.1. Undulating Features

Extensive research has been conducted on the undulating features within the studied canyon system [26,27,36,37,45]. However, there is still ongoing debate regarding the formation of these features. The undulating features located along the canyon axis generally align with the canyon axis. Their wavelength is slightly shorter than that of the undulating features found in the canyon head area [27,36]. High-resolution sub-bottom profiles revealed continuous internal reflections between waves, indicating no faults, and an apparent upslope migration of the crests (Figure 7, Figure 8, Figure 9 and Figure 10). These characteristics suggest that the undulating features are not a result of sediment deformation, such as sediment creeping, but depositional structures associated with specific hydrodynamic conditions, such as down-slope turbidities or along-slope bottom currents [14,46]. The internal configurations and external forms of these undulating features resemble lateral inclined packages observed in exploration seismic profiles [22,29,31]. These features play a crucial role in understanding canyon evolution as they are widely located both in the canyon walls and along the canyon axis. Further investigation into ocean hydrodynamic in this area can offer more insights into the formation of these features.

5.1.2. Knickpoints

In contrast to the small knickpoints typically found in many canyons or channels, such as the Monterey and Soquel submarine canyons [17], the Capbreton submarine canyon [47] and the submarine channel in Bute Inlet [48],which are several meters to a dozen meters high, several meters to tens of meters wide, and have a large slope gradient (>10°), the knickpoints in the study area are larger in size and have lower slope gradients (Figure 4). Small-scale knickpoints in canyons or channels are typically caused by slope instability due to gravity flows and sediments properties [15]. Conversely, larger-scale knickpoints are often influenced by structural factors [49]. In the study canyon, the lips of the knickpoints, where the slope begins to become steep, are often located at the lower sides of the undulations (Figure 7 and Figure 10). This is similar to those knickpoints along the thalweg of a sinuous submarine channel–levee system on the slope of the western Niger Delta, which are controlled by folds [49]. Therefore, we hypothesize that these knickpoints originated from slope failures at the crests of the undulating features.

5.1.3. Furrows and Scours

The furrows identified in the study area are significantly smaller in size compared with those observed in open abyssal plain environments [50]. Furrows are typically formed in regions characterized by recurring, directionally stable, and episodically strong currents. They can be created by coarse-grained sediments or cohesive sediment blocks by carving longitudinal troughs into the seafloor [51]. In order to determine whether the furrows and scours in the canyon were scoured by the alternating currents, we calculated the critical flow velocity (Vc) necessary for cohesive sediment particles to initiate movement using a formula proposed by Xu et al. [52] (Formula (1)):
V c = C 1 ( ρ m − ρ ) C 2 κ 2 ρ l n ⁡ 9 h ν C 1 ( ρ m − ρ ) C 2 ρ − l n ⁡ ( h ) − 1
where C1 = 1.59 × 10−8 [Nm/kg], C2 = 3.06, κ is the Karman constant (approximated as 0.4), h the thickness of the flow [m], ρm is the wet density of sediment [kg/m3], ρ is the density of seawater [kg/m3], and ν is the kinematic viscosity of seawater [m2/s].
When h = 50 m, ρm = 1400 kg/m3, ρ = 1030 kg/m3, and ν = 0.00083 m2/s, we estimated the value of Vc to be approximately 40 cm/s. This velocity exceeds the recorded flow velocities of less than 30 cm/s observed in a long-term mooring [53]. Hence, it is probable that the furrows and scours are a consequence of episodic gravity flows rather than oceanic currents within the canyon. Sand waves and submerged coral reefs have been documented at the shelf margin above the canyon head [54,55], and they could potentially be the origin of coarse particles or debris transported by the gravity flows, resulting in the formation of longitudinal furrows along the canyon’s axial zone.

5.1.4. Landslides

Landslides can be indicated by failure scars, which are remains of previous landslide events [56]. In our study area, detailed bathymetric data and side-sonar images show that landslides predominantly occurred in water depths shallower than 1000 m, where the slopes are steeper compared with other regions. The causes of landslides within the canyon system have been extensively investigated [26,28,57], and geotechnical numerical simulations suggest that sediment overload is the primary triggering factor [28].

5.2. Sedimentary Processes along the Canyon Axis

5.2.1. Sediment Erosion, Transport, and Re-Deposition

The erosional and depositional features along the canyon axis offer valuable insights into geological processes related to sediment erosion, transport, and re-deposition [10,14,15,16,18]. Geophysical data in the study area indicate that landslides primarily occurred in the upper canyon (water depth < 1000 m), as evidenced by the rugged seabed and the absence of younger sediments above these scars (Figure 5, Figure 7, Figure 10, and Figure 12). Sediment accumulation rates within the canyon system during the Holocene range from 10 to 26 cm per millennium [58,59]. The detection of a 0.5 m thick sedimentary layer in the sub-bottom profiles allows us to estimate that these small landslides were formed after at least 5000 years BP. Longitudinal sub-bottom profiles reveal that the thick chaotic packages are primarily situated near the failure scars (Figure 7, Figure 10, and Figure 11), indicating that mass wasting resulting from landslides traveled a limited distance. The average thickness of the MTDs decreases downstream, from over 10 m at depths between 700 m and 1100 m to less than 5 m at a depth of 1500 m. The presence of undulating features is crucial in retaining the mass wasting within the axial zone of the canyon.

5.2.2. Axial Incision

The morphology of the canyon is influenced by axial incision, as evidenced by the concave-up shape of its longitudinal profile [8,9,10,12]. In the study area, the most pronounced concavity is situated at a water depth of about 1200 m (Figure 4). Knickpoints are present above this point, while shallow erosional furrows are predominantly observed below it (Figure 3 and Figure 12). There is no indication of retreating slope failures in the canyon head or along its axis, consistent with observations from other canyons within the system [60]. Knickpoints along the canyon axis play a crucial role in changing its geomorphic forms [15]. Sediment erosion typically occurs at the faces of knickpoints, where old strata are exposed and evidence of failure scars is present (Figure 7 and Figure 10). The upstream migration of knickpoints (i.e., leaving behind cut terraces [39,40]) is not observed, suggesting that axial incision is not intense in the present-day canyon. Furrows identified through sub-bottom profiles and bathymetric data are not buried by younger sediments and exhibit a width-to-spacing ratio larger than 1/2 (Figure 6), indicating an erosional or depositional equilibrium [51].
Previous shipborne multibeam bathymetric surveys have shown that the canyon system displays an asymmetrical cross-sectional configuration, with steeper canyon walls on the eastern side compared with the west side [25]. However, AUV-based bathymetric data reveal that the terrain profiles across the axial zone of the canyon can exhibit asymmetrical or symmetrical characteristics, contrasting with the overall asymmetry of the canyon. This indicates that axial erosion is not the primary factor driving the asymmetrical pattern and thalweg migration.

5.2.3. Sediment Gravity Flows along the Canyon

Sediment gravity flows are essential for both the erosion and deposition of sediment within canyons [12,61,62]. Geomorphic features provide valuable information about the occurrence of gravity flows within canyons [10,13,15,16,17]. In this research, we identified erosional features such as failure scars, knickpoints, furrows, and scours in the axial zone (Figure 13), as well as thin-layered MTDs in the lower part of the canyon (Figure 11), indicating the presence of gravity flows. The presence of slope failures suggests that the primary sediment source of gravity flows is mass wasting in the upper canyon. However, these gravity flows may be relatively weak for the following reasons: (1) the absence of a distinct watercourse in the axial zone (Figure 3); (2) knickpoints not exhibiting significant upward migration from axial erosion (Figure 7 and Figure 10); (3) the lack of obvious hydrodynamics jumps, as indicated by jump pools (Figure 7, Figure 10, and Figure 11); and (4) small landslides observed in the upper canyon, with most mass wasting being confined near failure scars (Figure 9).

5.3. Activity of the Canyon during the Holocene

When gravity flows transport sediments and alter the shape of a submarine canyon through erosion and deposition, the canyon is classified as active [62,63,64,65]. On the other hand, when gravity flows occur infrequently or not at all, with only pelagic deposits present, the canyon is considered inactive [18,65,66]. In our study area, detailed geophysical data indicated the absence of hemi-pelagic or pelagic suspended sediment layers along the canyon axis. We also observed small-scale landslides, erosional furrows, and MTDs, suggesting sporadic low-energy gravity flows. Despite these findings, there are no significant downcutting or widening features along the canyon axis. Therefore, we concluded that the canyon was generally inactive during the Holocene, but experienced intermittent processes of sediment erosion, transport, and re-deposition in the axial zone.

6. Conclusions

This study utilized ultra-high-resolution AUV-based geophysical data to investigate the morphologic elements of a canyon (C4) in the slope-confined canyon system in the northern SCS and discuss their origin. We examined geological processes associated with sediment erosion, transport, and re-deposition along the canyon axis. The main findings are as follows:
(1)
The longitudinal profile of the canyon axis exhibits a slightly upward concave shape, while the transacted pattern can be asymmetrical or symmetrical, contrasting with the asymmetrical pattern of the entire canyon.
(2)
Some morphological elements were identified in the axial zone, including failure scars, knickpoints, flat terraces, undulating features, small-scare furrows, and MTDs. Landslides predominantly found in the upper canyon were formed after at least 5000 years BP, with associated MTDs often observed at the base of the canyon walls or on the flat terraces. Flat terraces and knickpoints alternate from the canyon head to a water depth of approximately 1300 m, while small-scale erosional furrows are located below a water depth of 1200 m.
(3)
The internal configurations of undulating features indicate that they are depositional structures, but further investigations into their origin are needed. Knickpoints exhibit large forms and lower slope gradients, with their formation possibly linked to slope failures at the crests of undulating features. The small-scale erosional furrows may result from gravity flows along the canyon.
(4)
The canyon was generally considered inactive during the Holocene, although sporadic processes of sediment erosion, transport, and re-deposition occurred in the axial zone, triggered by occasional landslide events in the upper canyon.

Author Contributions

Data processing, Q.Z. and B.H.; data analysis, X.L. and B.H.; writing—original draft preparation, X.L. and C.Z.; writing—review and editing, L.L.; project administration, L.L. All authors have read and agreed to the published version of the manuscript.

Funding

We appreciate the National Natural Science Foundation of China (NSFC) for supporting Project 41876061, including this study.

Data Availability Statement

The data are available from the corresponding author upon reasonable request.

Acknowledgments

Great thanks are extended to the technical staff and crew who participated in the fieldwork. The authors are also grateful for the reviewers and their constructive comments that improved the quality of this manuscript.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Bathymetric map (a) showing the canyon system and undulating seafloor outside of the Pearl River mouth, the northern slope of the South China Sea. The map (b) in the upper left corner indicates the location of the canyon system. The zoomed-in bathymetric map (c) in the lower right corner provides a closer view of the elongated mounded terrain units (EMTUs), which are undulating features within the canyon system. The yellow dotted line represents a long escarpment beneath the canyon system. PRM refers to the Pearl River mouth. Seventeen canyons are labeled C1 to C17 from east to west. The canyon system is located on the upper continental slope, with water depths less than 2000 m. The canyons are nearly straight and extend downslope along a bearing of ~165°, with sinuosity values less than 1.05. The water depths at the heads of the canyons range from 350 to 900 m, gradually increasing from west to east.
Figure 1. Bathymetric map (a) showing the canyon system and undulating seafloor outside of the Pearl River mouth, the northern slope of the South China Sea. The map (b) in the upper left corner indicates the location of the canyon system. The zoomed-in bathymetric map (c) in the lower right corner provides a closer view of the elongated mounded terrain units (EMTUs), which are undulating features within the canyon system. The yellow dotted line represents a long escarpment beneath the canyon system. PRM refers to the Pearl River mouth. Seventeen canyons are labeled C1 to C17 from east to west. The canyon system is located on the upper continental slope, with water depths less than 2000 m. The canyons are nearly straight and extend downslope along a bearing of ~165°, with sinuosity values less than 1.05. The water depths at the heads of the canyons range from 350 to 900 m, gradually increasing from west to east.
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Figure 2. Seismic architectures and thalweg migration in the canyon system (Modified after Zhu et al. [22]. Profile MCS-1 (a) cuts across the canyon heads and MCS-2 (b) cuts across the middle of the canyons. See Figure 1b for the location of profiles. In the late Miocene stage (10.5–5.5 million years ago), the canyon system carved deeply into the slope. The migration of the thalweg is indicated by red dotted lines with arrows. Lateral Inclined Packages (LIPs) and Chaotic Packages with high amplitudes (HCPs) are labeled. Blue dashed lines on seismic profiles denote the primary reflective interfaces.
Figure 2. Seismic architectures and thalweg migration in the canyon system (Modified after Zhu et al. [22]. Profile MCS-1 (a) cuts across the canyon heads and MCS-2 (b) cuts across the middle of the canyons. See Figure 1b for the location of profiles. In the late Miocene stage (10.5–5.5 million years ago), the canyon system carved deeply into the slope. The migration of the thalweg is indicated by red dotted lines with arrows. Lateral Inclined Packages (LIPs) and Chaotic Packages with high amplitudes (HCPs) are labeled. Blue dashed lines on seismic profiles denote the primary reflective interfaces.
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Figure 3. Bathymetric map of the study canyon (a), AUV-based bathymetric map (b), gradient map (c), and contour map (d). The location of the study canyon is indicated in Figure 1a. The black dotted line represents the axial zone, characterized by alternating flat terraces and steep slopes. Black thick line AB in contour map (d) is the thalweg of the canyon.
Figure 3. Bathymetric map of the study canyon (a), AUV-based bathymetric map (b), gradient map (c), and contour map (d). The location of the study canyon is indicated in Figure 1a. The black dotted line represents the axial zone, characterized by alternating flat terraces and steep slopes. Black thick line AB in contour map (d) is the thalweg of the canyon.
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Figure 4. Longitudinal profile showing an overall concave shape with alternating knickpoints and flat terraces. Please refer to Figure 3d for the locations.
Figure 4. Longitudinal profile showing an overall concave shape with alternating knickpoints and flat terraces. Please refer to Figure 3d for the locations.
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Figure 5. Pseudo-3D view showing failure scars (a,b), a knickpoint and flat terrace (c), and furrows (d). Please refer to Figure 3b for the locations of area I to IV. The black dashed lines indicate the crests of undulating features, while the yellow dotted lines represent the rims of failure scars. The thin white lines correspond to furrows.
Figure 5. Pseudo-3D view showing failure scars (a,b), a knickpoint and flat terrace (c), and furrows (d). Please refer to Figure 3b for the locations of area I to IV. The black dashed lines indicate the crests of undulating features, while the yellow dotted lines represent the rims of failure scars. The thin white lines correspond to furrows.
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Figure 6. Furrows and scours in the lower canyon’s axial zone shown on a bathymetric shaded map (a) upstream and (b) downstream, as well as in the sub-bottom profiles (c,d). Please refer to Figure 3b for the location of area V and area VI. The locations of sub-bottom profiles SBP1 and SBP2 are shown in Figure 6a and Figure 6b with red thick lines, respectively. White dashed lines in Figure 6a denote furrows.
Figure 6. Furrows and scours in the lower canyon’s axial zone shown on a bathymetric shaded map (a) upstream and (b) downstream, as well as in the sub-bottom profiles (c,d). Please refer to Figure 3b for the location of area V and area VI. The locations of sub-bottom profiles SBP1 and SBP2 are shown in Figure 6a and Figure 6b with red thick lines, respectively. White dashed lines in Figure 6a denote furrows.
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Figure 7. Longitudinal sub-bottom profile and its interpretation illustrating erosional and depositional elements along the upper canyon’s axis. The location of sub-bottom profile SBP3 is indicated in Figure 3a. The black dotted lines represent erosional surfaces, while the thick red dashed line with an arrow indicates the migration of the crests of the undulations. Abbreviations: Wavy Rfs.: wavy reflections; mass transport deposits: MTDs.
Figure 7. Longitudinal sub-bottom profile and its interpretation illustrating erosional and depositional elements along the upper canyon’s axis. The location of sub-bottom profile SBP3 is indicated in Figure 3a. The black dotted lines represent erosional surfaces, while the thick red dashed line with an arrow indicates the migration of the crests of the undulations. Abbreviations: Wavy Rfs.: wavy reflections; mass transport deposits: MTDs.
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Figure 8. Cross-sectional sub-bottom profile displaying acoustic architectural elements. The location of sub-bottom profile SBP4 is indicated in Figure 3a. The thick red dashed line with an arrow indicates the migration of the crests of the undulations. A thin layer of MTDs was discovered in the axial zone of the canyon.
Figure 8. Cross-sectional sub-bottom profile displaying acoustic architectural elements. The location of sub-bottom profile SBP4 is indicated in Figure 3a. The thick red dashed line with an arrow indicates the migration of the crests of the undulations. A thin layer of MTDs was discovered in the axial zone of the canyon.
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Figure 9. Cross-sectional sub-bottom profile showing failure scars and a thick layer of MTDs. The location of sub-bottom profile SBP5 is indicated in Figure 3a. The thick red dashed lines with arrows indicate the migration of the crests of the undulating features.
Figure 9. Cross-sectional sub-bottom profile showing failure scars and a thick layer of MTDs. The location of sub-bottom profile SBP5 is indicated in Figure 3a. The thick red dashed lines with arrows indicate the migration of the crests of the undulating features.
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Figure 10. Longitudinal sub-bottom profile and its interpretation revealing various morphological elements in the eastern wall of the upper canyon. The location of sub-bottom profile SBP6 is indicated in Figure 3a. Erosional surfaces are represented by black dotted lines, while the migration of the crests of the undulating features is indicated by thick red dash lines with arrows.
Figure 10. Longitudinal sub-bottom profile and its interpretation revealing various morphological elements in the eastern wall of the upper canyon. The location of sub-bottom profile SBP6 is indicated in Figure 3a. Erosional surfaces are represented by black dotted lines, while the migration of the crests of the undulating features is indicated by thick red dash lines with arrows.
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Figure 11. Longitudinal sub-bottom profile and its interpretation showing various morphological elements along the lower canyon’s axis. The location of sub-bottom profile SBP7 is indicated in Figure 3a. The black dotted lines denote erosional surfaces.
Figure 11. Longitudinal sub-bottom profile and its interpretation showing various morphological elements along the lower canyon’s axis. The location of sub-bottom profile SBP7 is indicated in Figure 3a. The black dotted lines denote erosional surfaces.
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Figure 12. Side-scan sonar images and interpretations. (a) is the mosaic image of the entire area and (b–d) are zoomed-in images for local areas. The black dotted lines indicate the axial zone of the canyon. Red dash half-marks denote the edges of failure scars.
Figure 12. Side-scan sonar images and interpretations. (a) is the mosaic image of the entire area and (b–d) are zoomed-in images for local areas. The black dotted lines indicate the axial zone of the canyon. Red dash half-marks denote the edges of failure scars.
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Figure 13. Distribution of erosional and depositional elements along the canyon axis. The identified morphological features include failure scars in the upper canyon, MDTs along the canyon axis, furrows, and scars mainly in the axial zone below 1200 m depth.
Figure 13. Distribution of erosional and depositional elements along the canyon axis. The identified morphological features include failure scars in the upper canyon, MDTs along the canyon axis, furrows, and scars mainly in the axial zone below 1200 m depth.
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Li, X.; Liu, L.; Huang, B.; Zhou, Q.; Zhang, C. Erosional and Depositional Features along the Axis of a Canyon in the Northern South China Sea and Their Implications: Insights from High-Resolution AUV-Based Geophysical Data. J. Mar. Sci. Eng. 2024, 12, 599. https://doi.org/10.3390/jmse12040599

AMA Style

Li X, Liu L, Huang B, Zhou Q, Zhang C. Erosional and Depositional Features along the Axis of a Canyon in the Northern South China Sea and Their Implications: Insights from High-Resolution AUV-Based Geophysical Data. Journal of Marine Science and Engineering. 2024; 12(4):599. https://doi.org/10.3390/jmse12040599

Chicago/Turabian Style

Li, Xishuang, Lejun Liu, Bigui Huang, Qingjie Zhou, and Chengyi Zhang. 2024. "Erosional and Depositional Features along the Axis of a Canyon in the Northern South China Sea and Their Implications: Insights from High-Resolution AUV-Based Geophysical Data" Journal of Marine Science and Engineering 12, no. 4: 599. https://doi.org/10.3390/jmse12040599

APA Style

Li, X., Liu, L., Huang, B., Zhou, Q., & Zhang, C. (2024). Erosional and Depositional Features along the Axis of a Canyon in the Northern South China Sea and Their Implications: Insights from High-Resolution AUV-Based Geophysical Data. Journal of Marine Science and Engineering, 12(4), 599. https://doi.org/10.3390/jmse12040599

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