Introduction

Submarine landslides are ubiquitous features on continental slopes worldwide, raising major concerns regarding their underlying controls and geohazards potential1. The spatial correlation between submarine landslides and layer-parallel glide planes suggests that specific stratigraphic layers can strongly influence slope stability2,3. These layers are further related to lower strength to focus the development of the glide plane during landslides and are defined as the ‘weak layers’2. On glaciated continental margins, mainly in middle- to high-latitude oceans, changes of sediment types during glacial-interglacial cycles can result in distinct lithological contrasts and ‘inherited’ weak layers, of which the shear strength is inherently lower than adjacent units4,5,6,7. In parallel, the strength of layers can be reduced by external processes, such as overpressure from rapid sedimentation as glacial sediments arrive in large quantities by ice streams8 or gas hydrate dissociation by warming bottom waters9, leading to the formation of ‘induced’ weak layers10.

Although specific biological productivity and tectonics, such as diatom oozes and constricted ocean gateways, are known to result in distinct lithological contrasts10,11, the influence of climate changes on weak layer formation on non-glaciated margins, mostly located in low-latitude oceans, remains poorly documented. Although the external factors for ‘induced’ weak layers might still exist, the lack of glacigenic sediments input on non-glaciated margins results in the relatively subtle or absent lithological contrasts12. The presence of numerous giant submarine landslides, coupled with millions of people living in proximity, highlights the critical need to improve our understanding of the formation mechanisms of weak layers in low-latitude oceans13,14,15.

The non-glaciated South China Sea (SCS), located at low latitudes (approximately 15° N and 114° E), is one of the largest marginal seas in the western Pacific Ocean (Fig. 1a). On its northern continental slope, the boundaries between Mesozoic crustal segments have provided a strike-slip faults domain during diachronous rifting and opening of the SCS, further acting as conduits of magma and thermogenic gas to present16,17,18. Since the cessation of SCS spreading in the early Miocene, a combination of tectonic events, sediment supply, and global sea-level fluctuations has controlled the migration of the shelf break and coastal onlaps in the northern SCS (Fig. 1b)19,20,21,22,23. The deposition of coarse-grained turbidites (P1-P7 in Fig. 1a) and fine-grained contourites (Site 18, 29, 69, 138 and 145 in Fig. 1a) records the influence of turbidity and contour currents. Coarser-grained deposits reflect periods of sea-level drop and lowstand, whereas finer-grained sediments were deposited during sea-level rise and highstand24,25.

Fig. 1: Regional setting and seismic–well correlation for the Baiyun Slide area, northern South China Sea.
figure 1

a Location of study area in the South China Sea. Black polygons and lines represent the coverage of seismic data and profiles used in this research, respectively. The locations of the shelf break are adapted from Zhuo et al. (2019). Grain-size distribution on the modern seafloor is based on data from Zhong et al. (2017) (marked by blue dots) and Wang et al. (2018, 2020) (marked by red dots). Multibeam data are from CNOOC. b Seismic-well integration at BY7-1-1, with correlation to coastal onlap and global sea-level changes21,31. Porosity, permeability, and grain size were estimated from well-log data by CNOOC. Biostratigraphic correlation was processed by CNOOC20. SWC Surface Water Currents, IWC Intermediate Water Currents, DWC-Deep Water Currents, SCS South China Sea, TW Taiwan, HI Hainan Island, VN-Vietnam, PH Philippines, LCGU Lower coarse-grained unit, UCGU Upper coarse-grained unit, FGU Fine-grained unit, CGL Coarse-grained layer, FGL Fine-grained layer.

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A wide variety of slope instabilities has been identified on the northern South China Sea margin, such as the giant creep zone, exceeding 800 km2 with high hazard potential26, and the Baiyun Slide Complex, affecting an area of ~10,000 km² and displacing ~1000 km³ of sediments27,28,29. Although layer-parallel glide planes suggest the involvement of weak layers below the Baiyun Slide Complex29,30, their physical properties and depositional processes remain inadequately understood. To elucidate the development of these weak layers, we analyzed geophysical and chronological data from an industry well on the intact upper slope (Fig. 1a). By tracing the seismic reflection to the logged sequence, we can identify the distinct geophysical properties of the weak layer and the depositional systems that contain them. Combined with the correlation between sea-level changes and the depositional systems, we aim to develop a widely applicable model for the formation mechanism of weak layer, enhancing our understanding of the influence of sea-level variations, which reflect climatic changes, on slope instability in non-glaciated low-latitude oceans.

Results

Sedimentology and physical properties of sediments on the unfailed slope

The headwall region of the Baiyun Slide Complex was evacuated at ~0.54 Ma (Fig. 2a,f). The bedding-parallel glide plane, H1, is identified as the top of the uppermost seismic reflections that are continuous across headwall, at the bottom of MTDs with chaotic seismic facies (Fig. 2f). The headwall of Baiyun Slide is identified at the boundary between the intact upper slope and disrupted lower slope with continuous and chaotic seismic facies, respectively (Fig. 2f). Blocky structures within the MTDs manifest as triangle shapes on the seismic profile and can still be recognized on the modern seafloor as local positive relief (marked in Fig. 2f). Exploration well BY7-1-1 is located at the intact upper slope 40 km away from the headwall of giant Baiyun Slide Complex, covered by both 2D and 3D seismic surveys (Figs. 1a and 2a). Core-log-seismic integration shows few distinct units, including a lower coarse-grained unit (LCGU) between H4 and H3, a fine-grained unit (FGU) between H3 and H2, and an upper coarse-grained unit (UCGU) above H2.

Fig. 2: Seismic expression of the Baiyun Slide headwall with fluid and bottom-current indicators.
figure 2

Seismic profiles across the headwall region (a, c, f) show key seismic reflections and features of fluid accumulation (b, g, h) and bottom currents (d, e). HAA High-Amplitude Anomalies, SWC Surface Water Currents, IWC Intermediate Water Currents, DWC, Deep Water Currents.

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The LCGU was deposited during 6.3 to 5.0 Ma with interbedded fine- and relatively coarse-grained layers and variable porosity and permeability (Fig. 1b). Global sea-level was relatively stable at the same time, whereas coastal onlaps have shifted landward and oceanward on a scale of ~50 km (Fig. 1b).

Since 5.0 Ma, global sea-level has been rising, reaching a peak of ~20 m higher than the present sea level by 3.8 Ma (Fig. 1b). Coastal onlap records a landward shift of more than 100 km starting at 5.0 Ma, followed by a relatively constant position afterwards (Fig. 1b)31. The FGU was deposited in this period with constant porosity and low permeability.

From 3.8 Ma to 2.4 Ma, a drop in global sea level was accompanied by multiple shifts in coastal onlap in the northern SCS (Fig. 1b). The UCGU was deposited during this period, consisting of interbedded fine- and coarse-grained layers, such as a permeable coarse-grained layer (CGL in Fig. 1b) and the overlying low-permeability fine-grained layer (FGL in Fig. 1b).

Records of turbidity and contour currents

Cut-and-fill structures, including large-scale canyons and relatively small-scale gullies, are recorded in seismic data, illustrating the development and activity of turbidity currents since the Late Miocene (Fig. 2). The bottom of LCGU (H4) separates extensive canyons below and subparallel seismic reflections above, where isolated canyons were developed, indicating the existence of turbidity currents since 6.3 Ma (Fig. 2a). Below the top of LCGU (H3), a group of slightly curved linear features oriented toward lower slope (Fig. 3a) corresponds to small-scale gullies reflecting the turbidity current activity before 5.0 Ma. Cut-and-fill structures were not observed within the FGU but reappeared in the UCGU as gullies and canyons above H2 (Fig. 2c), revealing the suspension of turbidity current activity from 5.0 to 3.8 Ma and a rejuvenation of canyons and gullies afterwards.

Fig. 3: Attribute maps of key seismic reflections showing structures of fluid-related features and relationship with glide plane.
figure 3

a–c Variance maps at 20 ms below H3, H3 and H2; d, e relief maps of seafloor and H3; f thickness of Fine-grained unit (FGU) between H2 and H3.

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Seismic profiles and seafloor relief reveal the moats of contour currents on the modern seafloor (Figs. 1a and 2). In the study area, moats are recognized as erosional depressions within both the LCGU and UCGU (Fig. 2c). Mounded and wavy drifts are observed adjacent to the moats (Fig. 2d,e). These drifts exhibit asymmetric concave structures with internal reflections that are truncated by the moats on their upper slope (see the truncated dotted black lines within the wavy drifts in Fig. 2d,e).

Indicators of focused fluid flow

Seismic data show High-Amplitude Anomalies (HAAs) with polarity reversal indicating the presence of fluid and, in particular, free gas32,33. In our study area, HAAs are widely distributed across multiple stratigraphic levels (Fig. 2b). Strike-slip faults with flower structures and en-echelon patterns, acting as the conduits of magma and thermogenic gas in the northern SCS18, lie adjacent to these HAAs, forming gas migration features such as ‘gas flag’ structures (Fig. 2b). In parallel, pipes, as conduits for focused fluid flow, are identified within the FGU, characterized by sub-circular discontinuities on the structure maps and vertical structures with absence of reflections on the seismic profiles (Figs. 2d,f, 3b,c).

The roots of these pipes, where they terminate downwards, are observed just below the FGU at a depth deeper than ~1500 ms (Fig. 3b,e). The heights of these pipes are a few tens of meters, with a vertically consistent diameter of a few hundred of meters (Fig. 2g,h). Most of the pipes become absent at the top of FGU, where their tops are preserved as sub-circular structures and overlying wavy seismic reflections (Figs. 2d and 3c). These pipes thus record the accumulation of fluids below the FGU and their upward migration through this unit.

Discussion and Conclusion

Mixed contourite-turbidite system formed during sea-level changes

Due to the slow post-rift subsidence since the Late Miocene, relative sea level in the northern SCS follows the global eustatic variations by astronomical forcing, producing repeated shifts of coastal onlaps on continental slopes21,34,35. Migration of the paleo-shelf break indicates that our study area has resided on the continental slope at the same time (Fig. 1a)22,23. The modern water circulation patterns in SCS were established at the same time, with the closure of Indonesian Gateway and the narrowing and shallowing of Luzon Strait36,37,38. Due to the oceanward progradation of the shelf break since the Late Miocene, turbidity gullies from the shelf break have developed on the modern seafloor of the study area at present (Figs. 1a and 3d)23,39. Evidence of turbidity and contour currents, as well as the heterogeneous lithologies within the logged sequence, further indicate that the depositional system in our study area was dominated by distinct processes at different stages.

On the northern continental slope of the SCS, turbidity currents are more active during periods of sea-level lowstands or drops, when the distance between the shoreline and shelf edge decreases24,40,41,42. In the study area, the evidence of turbidity currents, such as the gullies before 5.0 Ma and the canyons after 3.8 Ma, demonstrates the increase of turbidity current activity during sea-level lowstands and drops (Figs. 2c and 3a). Although the turbidity gullies from shelf break are observed on the modern seafloor of study area (Figs. 1a and 3d), the northward location of shelf break before present, as well as the initiation of these turbidity currents since LGM23,39, suggest these gullies should be inactive during the deposition of logged sequence (Fig. 1b). On the other hand, the coarse-grained (sandy and silty) turbidites (Sites P1-P7 in Fig. 1a) from these gullies can indicate the CGLs in FCGU and UCGU as the records of turbidity currents on the paleo-seafloor (Fig. 1b). In parallel, fine-grained (silty and clayey) sediments are acquired around the study area and identified as the deposition of contourite drifts on the modern seafloor (Sites 18, 69 and 138 in in Fig. 1a)43. Considering the establishment of modern water circulation patterns since the Late Miocene and the development of moats and drifts in the study area at the same time (Fig. 2d,e), the FGLs in LCGU and UCGU are the deposition of contour currents.

Although both the features of turbidity and contour currents were absent in the study area between 5.0 and 3.8 Ma, the continuously generated contourite drifts in the northern and southern SCS reveal a long-term existence of contour currents since the late Miocene36. Contour currents with low velocity or weak activity can form fine-grained contourites without prominent erosional features, such as plastered drifts that dip oceanward and gradually thicken downslope, characterized by continuous, sub-parallel internal reflections44. The oceanward-dipping FGU with continuous, subparallel reflections and gradually downslope thickening structures (Fig. 2e,f) is therefore interpreted as a relatively contourite-dominated interval formed under weak contour current activity and absence of turbidity currents. The interbedded fine- and coarse-grained LCGU and UCGU, on the other hand, represent a relatively turbidite-dominated mixed depositional system with active turbidity and contour currents.

Formation mechanism of the weak layer

Features indicative of focused fluid flow are identified within the FGU. The pipes terminate at the top of FGU or just below. The constant diameter of the pipes suggests that there is no lateral fluid escape45,46,47. The roots of the pipes are located at the top of the LCGU. Based on these observations and permeability contrasts, we infer that the LCGU was sufficiently overpressured for pipes to form in the overlying FGU. The positive correlation between the density of pipe occurrence below 1500 ms TWT and the thickness of FGU on the lower slope shows that the continuous deposition of low-permeability sediment between 5.0 and 3.8 Ma, when a contourite-dominated depositional system developed, likely was a key contributor to generating overpressure in the LCGU, driving focused fluid flow (Figs. 3f and 4b). The pipes enabled the fluids to bypass the FGU and migrate into the upper coarse-grained layer, in which fluids migrated laterally and upwards in the pore space. The overlying low-permeability contourites (FGL) acted as a seal to further upward migration, preventing dissipation of excess fluids and leading to excess pore pressure buildup within this CGL (in particular in its upper part near the interface to the sealing FGL). Overpressure at the interface (H1 in this study) ultimately promoted the development of the glide plane during submarine landslides (Fig. 4c,d).

Fig. 4: Schematic model for the development of a weak layer during long-term sea-level rise.
figure 4

a Sea-level lowstand: turbidite-dominated lower coarse-grained unit (LCGU) with interbedded turbidites (CGL) and contourites (FGL); b Sea-level rise: contourite-dominated fine-grained unit (FGU) forming thick drifts without input of turbidites. Focused fluid flow and pipes develop within this low-permeability unit; c Sea-level drop: upper coarse-grained unit (UCGU) with a permeable CGL overlain by an FGL as a seal, accommodating fluid overpressure in long term. d During submarine landslides, the permeability contrast at the CGL–FGL interface localizes the glide plane. Perm. – permeability.

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Due to the limits in vertical seismic resolution, we cannot pinpoint the exact location of the glide plan,e and we cannot exclude that an external trigger was necessary. Failure could have taken place (i) along the upper part of the fluid-charged coarse-grained layer, which may be prone to earthquake-induced liquefaction, (ii) at the interface to the overlying low permeable layer, which may also represent a zone of contrast in shearing resistances, which act as preferential shearing planes, or (iii) in the lowermost part of the overlying low permeable layer, given that cohesive clay-rich sediments behave mechanically weak when fluids are provided10,48,49.

Implications for weak layer in low-latitude oceans

Climatic changes have controlled the depositional processes on several continental slopes and resulted in lithological contrasts50. Our study reveals that the weak layer of the giant Baiyun Slide Complex in the South China Sea was controlled by the alternating deposition of coarse (turbidite) and fine (contourite) grained units during sea-level variations. Focused fluid flow and overpressure likely reduced the resistive forces at lithological contrasts, promoting the development of the glide plane during submarine landslides.

On glaciated continental margins, mostly in mid- to high-latitude oceans, sedimentation shows pronounced alternations between coarse, stiff glacial deposits (e.g., diamictic debris and basal tills) and fine, soft interglacial sediments (e.g., hemipelagic, contourites and biogenic ooze)4,5,6,51. These distinct lithological contrasts, coupled with overpressure from gas hydrate dissociation, rapid loading, or diagenesis, promote the development of both ‘inherited’ and ‘induced’ weak layer4,10,47.

Climate changes also influence the depositional processes on non-glaciated (low latitude) continental margins through the effects of sea-level variations that change shorelines and river-shelf connections, resulting in the alternation of coarse-grained turbidites/debris and fine-grained hemipelagites/contourites. In the failed southeastern Brazilian margin, for example, coarse-grained turbidites were transported through the continental shelf and deposited on the slope during sea-level lowstand, resulting in fine-grained contourites deposited during sea-level rise52,53. In the northern Gulf of Mexico, large amounts of coarse-grained turbidites and sandy debris were delivered across shelf and deposited on the continental slope and in the deep ocean during glacial periods, whereas fine-grained deposition is more prominent in the inter-glacial periods54. A similar process has also been recorded in Western Sahara and SCS, where turbidites frequently passed through the shelf and canyons during sea-level lowstand but became absent during sea-level highstand42,55.

Compared with the glacigenic sediments on high-latitude margins, the turbidite deposits during sea-level lowstand result in more gradational lithology changes on non-glaciated low-latitude margins12. Nevertheless, recurrent giant submarine landslides, such as the Cap Blanc Slide and Mauritania Slide Complex offshore northwest Africa, and the Baiyun Slide Complex in this study, reveal fluid overpressure as an ‘universal’ factor to reduce the strength of specific intervals and generate the ‘induced’ weak layers on non-glaciated margins as well28,55,56 (Fig. 4). Considering the wide distribution of (non-glaciated) low-latitude continental margins, the formation mechanism of weak layer proposed in this study can serve as an analog for preconditioning factors responsible for the occurrence of submarine landslides in low-latitude oceans. Moreover, a similar mechanism could also be valid on non-glaciated continental margins in mid- or high-latitude oceans, where sea-level variations can alternate the sediment input and depositional systems, along with the existence of overpressure.

Data and Methods

Seismic and bathymetric data

This study utilized ~1600 km2 of high-resolution 3D seismic data and a 2D seismic profile, complemented by multi-beam and GEBCO bathymetric data covering the northern SCS (Fig. 1a). Multi-beam bathymetric data were collected using differential GPS positioning and processed using the software CARIS HIPS®. Horizontal and vertical resolutions for the bathymetric data are 100 m and 3–6 m, respectively. Seismic data were acquired and processed by the China National Offshore Oil Corporation (CNOOC), with bin spacing ranging from 12.5 to 25 m. The seismic data have a frequency bandwidth of 30–45 Hz, providing an average vertical resolution of 8–10 m at the depth of the studied strata. Seismic interpretation was conducted using Schlumberger Petrel®, including the identification and auto-tracking of key seismic reflections, as well as the computation of thickness and attribute maps. Variance maps of key horizons were generated to highlight areas where seismic facies exhibit discontinuities, often correlating with the presence of faults, pipes, MTDs and canyons.

Well-logs of Well BY7-1-1

Located on the upper slope of the Baiyun Slide Complex, Well BY7-1-1 was drilled in 1987, from the modern seafloor at a water depth of 500 m, penetrating sediments from Late Miocene at 600 meters below the seafloor (mbsf) (Fig. 1). Porosity, permeability, and grain size were estimated by CNOOC from well-log data, which can be found in the Supplementary Data 1. Biostratigraphic correlation was performed by CNOOC, providing dates to key seismic horizons through core-log-seismic integration20. The top of the logged sequence is situated at a depth of ~300 mbsf, corresponding to a two-way travel time (TWT) of ~1000 ms, and is dated to ~2.4 Ma (Fig. 1b)20. Other key seismic reflections have been tied to dated cores as well, including Horizons H1, H2, H3, and H4, to 3.4 Ma, 3.8 Ma, 5.0 Ma, and 6.3 Ma, respectively (Fig. 1b)17,20. Surface samples from sites P1-P7 were acquired by the Guangzhou Marine Geology Survey57, while other surface samples plotted in Fig. 1a, including sites 18, 29, 69, 138, and 145, were obtained by the South China Sea Institute of Oceanology, Chinese Academy of Sciences43.