Multistage geomorphic evolution of the Central Canyon in the Qiongdongnan Basin, NW South China Sea

Seismic reflection data were used to investigate the multiple stages of headward erosion, incision, and sedimentation on the initiation and evolution of the Central Canyon, a Late Miocene-Pliocene continental slope-parallel canyon on the northwestern margin of the South China Sea. Secondary channels of the early stage generally display deep, V-shaped cross-sectional morphologies, indicating that vertical erosion was dominant in the early stage of canyon formation, while later-phase secondary channels have shallow, U-shaped cross sections, indicating that sedimentation dominated in the late stage. Influenced by gradual sea-level rise, headward erosion shifted the Central Canyon head from the Qiongdongnan Basin to the southeastern margin of the Yinggehai Basin. The length of headward erosion is up to 140 km, equivalent to one-fourth of the total length of the Central Canyon (525 km). The thalweg depth, height, and width of the Central Canyon increases down canyon from 2723 m, 164 m, and 2 km to 5318 m, 1023 m, and 16 km, respectively. This increase is gradual on gently-sloping palaeoseafloor, and more abrupt on steep-gradient palaeoseafloor. Canyon segments on gently-sloping palaeoseafloor are also characterised by a good correlation between canyon height and width. The sediments fill in the Central Canyon differ across the canyon’s segments. Turbidites and debrites infill the upstream segment of the Central Canyon. Turbidites and Mass-transport deposits are widespread in the middle reaches of the Central Canyon. The typical feature of the Central Canyon’s downstream segment is under-compensation, and canyon wall collapse are common in the upper fill of the downstream segment. We reconstruct the multistage evolution of the Central Canyon as comprising primary erosion, burial, renewed erosion, infilling, and abandonment stages. This multistage evolution was mainly controlled by relative sea-level fluctuations of the Qiongdongnan Basin.