Occurrence of submarine canyons, sediment waves and mass movements along the northern continental slope of the South China Sea

In this study, we reveal a series of newly discovered submarine canyons, sediment waves, and mass movements on a flat and smooth seafloor using high-resolution, multi-beam bathymetry and shallow seismic surveys along the northern slope of the South China Sea. We also describe their geomorphology and seismic stratigraphy characteristics in detail. These canyons display U-shaped cross sections and are roughly elongated in the NNW–SSE direction; they are typically 8–25 km long, 1.2–7 km wide, and form incisions up to 175 m into Pliocene–Quaternary slope deposits at water depths of 400–1000 m. Slide complexes and the sediment wave field are oriented in the NE–SW direction and cover areas of approximately 1790 and 926 km2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {km}^{2}$$\end{document}, respectively. Debris/turbidity flows are present within these canyons and along their lower slopes. Detailed analysis of seismic facies indicates the presence of six seismic facies, in which Cenozoic strata located above the acoustic basement in the study area can be roughly subdivided into three sequences (1–3), which are separated by regional unconformities (Tg, T4, and T3). By combining these data with the regional geological setting and the results of previous studies, we are able to determine the genetic mechanisms used to create these canyons, sediment wave field, and mass movements. For example, frontally confined slide complexes could have been influenced by high sedimentation rates and high pore pressures. A series of very large subaqueous sediment waves, which record wavelengths of 1.4–2 km and wave heights of 30–50 m, were likely produced by interactions between internal solitary waves and along-slope bottom (contour) currents. Canyons were likely initially created by landslides and then widened laterally by the processes of downcutting, headward erosion, and active bottom currents and debris/turbidity flows on canyon floors. We therefore propose a three-dimensional model to describe the development of these mass movements, the sediment wave field, and canyons. The four stages of this model include a stable stage, followed by the failure of the slope, and subsequent formations of the sediment wave field and canyons.