Research paperThe Baiyun Slide Complex, South China Sea: A modern example of slope instability controlling submarine-channel incision on continental slopes
Introduction
Submarine landslides, turbidity currents and bottom currents are dominant sedimentary processes occurring along both passive and active continental margins (Vorren et al., 1998; Rebesco et al., 2014; Mosher et al., 2017). Downslope processes such as landslides and turbidity currents are driven by gravity and lead to the deposition of broad mass-transport deposits or turbidite systems within erosive channels (Moscardelli et al., 2006; Casalbore et al., 2010; Bourget et al., 2011; Li et al., 2015b). They can transport large volumes (>100 km3) of sediment sourced from the continental shelf and upper slope areas into the deep ocean (Georgiopoulou et al., 2010; Li et al., 2018), re-shaping the sea floor to influence subsequent sedimentary processes (Casalbore et al., 2018). They can also control the distribution of sand in deep-water environments (Haflidason et al., 2004; Mosher et al., 2017). Along-slope bottom currents result in extensive depositional (e.g. sediment drifts) and erosional (e.g. contourite channels) features on outer continental shelves and upper continental slopes (Hernández-Molina et al., 2006; García et al., 2009; Rebesco et al., 2013). A close interplay between downslope turbidity currents and alongslope contour currents is therefore expected when both processes occur on continental margins (Rebesco et al., 2002; Caburlotto et al., 2006; Brackenridge et al., 2013; Martorelli et al., 2016).
The continental margin offshore the Pearl River Mouth Basin (PRMB) is incised by deep-water submarine canyons and channels (Zhu et al., 2010). The most striking feature in the PRMB is the Baiyun Slide Complex, which has a large spatial coverage (~10,000 km2) and is composed of several intersecting slide scars and overlapping deposits (Li et al., 2014; Sun et al., 2018b) (Fig. 1). The total volume of sediment removed by the Baiyun Slide Complex is ~1035 km3 and comprises four major mass-transport deposits (MTDs) separated by basal erosional surfaces (Sun et al., 2018b). These MTDs retrograded upslope to reveal a decreasing time interval between events (Wang et al., 2017; Sun et al., 2018b). Two main instability events occurred in the headwall region of the Baiyun Slide Complex during the Quaternary (Li et al., 2014; Wang et al., 2017; Sun et al., 2018b), at ~0.79 Ma and ~0.54 Ma (Sun et al., 2018b). The older MTD (1570 km2) covers most of the headwall region, while the younger MTD is mainly limited to the northern area of the headwall region to reveal a relatively smaller area of ~840 km2 (Wang et al., 2017).
The study area is chiefly located in the headwall region of the Baiyun Slide Complex, at a water depth of 900 m–1800 m (Fig. 2a and b). This region is affected by alongslope bottom currents associated with a clockwise flow of intermediate water at a depth of 350 m–1350 m, and an anticlockwise flow of deep water at depths beyond 1350 m (Gong et al., 2013a,b; Chen et al., 2014) (Fig. 1). It provides a key opportunity to characterise how bottom currents, turbidity currents and submarine landslides influence the morphological and sedimentary evolution of the northern South China Sea margin. Hence, this work uses high-resolution bathymetric, 2D/3D seismic and borehole data are used to provide a detailed analysis of erosional and depositional features in and around the headwall region of Baiyun Slide Complex. The specific aims of this research are to:
- a)
investigate the seafloor morphology in and around the headwall region of the Baiyun Slide;
- b)
describe the internal seismic characters of the Baiyun Slide, and determine what are the main sedimentary processes in this area;
- c)
discuss the role of the Baiyun Slide Complex on the incision and development of submarine channels.
Section snippets
Geological setting
The South China Sea is one of the largest (and deepest) marginal seas in the western Pacific Ocean (Fig. 1). The South China Sea as observed at present results from the formation of a proto-South China Sea, likely floored by oceanic crust, that was subducted during the Mesozoic (Pubellier et al., 2003). The earliest phase of rifting in South China Sea started in the latest Cretaceous to Early Paleocene and, after ~30 Ma of rifting, continental breakup was initiated first in its Eastern
Data and methods
High-resolution multibeam bathymetric data, 2D seismic profiles and 3D seismic volumes are used in this work. The bathymetric data was acquired at water depths ranging from 230 m to 2600 m and was positioned by a differential GPS. It was processed using the software CARIS HIPS®. Its horizontal and vertical resolutions are ~100 m and ~1–3.3 m, respectively, enabling the identification and analysis of seafloor features generated by downslope and alongslope currents.
The interpreted seismic dataset
Seismic stratigraphy
The seismic stratigraphy of the study area was interpreted and tied to borehole data from Exploration Well L-13. Three main seismic units, named as Units A, B and C from top to bottom, were identified based on the differences in their internal reflection configurations (Fig. 4a and b).
Unit A is bounded by T0 at its base and its top coincides with the sea floor (Fig. 4a). Unit A is suggested to be Quaternary in age. On the upper continental slope, moderate-to high-amplitude reflections
Seafloor morphology
Seafloor morphology is uneven in the study area (Fig. 2, Fig. 5a). Different kinds of morphological features can be identified, with the most prominent feature being the Baiyun Slide scar (Fig. 2a).
Slide scarps and mass-transport deposits (MTDs)
The headwall and sidewall scarps of the Baiyun Slide Complex can be readily identified on the bathymetric map and seismic profiles (Fig. 2, Fig. 4, Fig. 7a). The slide scarps are steep and adjacent intact strata show obvious erosional truncations (Fig. 4, Fig. 7b). Most MTDs are located in Units A and B, especially downslope from the slide headwall where recurrent MTDs are observed (Fig. 4a and c). The uppermost MTD shows a thickness of ~75 m (Fig. 4c and d). Beneath this MTD, several
Importance of combined downslope and alongslope processes on a sediment-fed continental slope
We propose that the headwall region of the Baiyun Slide Complex was affected by both downslope and alongslope sedimentary processes since its inception as: (a) it is located at a water depth influenced by intermediate (350 m–1350 m) and deep-water currents (>1350 m); and (b) it is close to submarine canyons incising the continental slope in the same place where submarine slides and turbidity currents occurred frequently in the past. In this work, several types of depositional and erosional
Conclusions
High-resolution bathymetry and 2D/3D seismic data enabled us to investigate the headwall region of Baiyun Slide Complex on the northern South China Sea in terms of its morphology, associated sedimentary processes and its role on the initiation of submarine channels. The main conclusions of this study are as follows:
- (1)
The headwall region of Baiyun Slide Complex has a U-shaped morphology in plan view at a water depth between 1000 m and 1700 m. Sediment was almost completely evacuated from the slide
Author contribution section
Wei Li: Conceptualization, Writing- Original draft preparation, Funding acquisition. Tiago M. Alves: Conceptualization, Writing- Reviewing and Editing. Michele Rebesco: Writing- Reviewing and Editing.Jie Sun: Visualization. Jian Li: Visualization. Shuang Li: Visualization. Shiguo Wu: Writing- Reviewing and Editing.
Acknowledgments
We acknowledge China National Offshore Oil Corporation for their permission to release the seismic data. Dr. Neil C. Mitchell is thanked for his invaluable assistance and fruitful discussion, which improved this paper. This work was financially supported by the Innovation Development Fund of South China Sea Eco-Environmental Engineering Innovation Institute of the Chinese Academy of Sciences (ISEE2018PY02), National Scientific Foundation of China (41876054), National Key Research and
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