Controls of contour currents on intra-canyon mixed sedimentary processes: Insights from the Pearl River Canyon, northern South China Sea

Contour currents are ubiquitous in global oceans, the role of which in shaping submarine morphology and controlling deep-water sedimentary processes has long been underestimated. This study uses 3D seismic and piston-core data to investigate the seismic geomorphology and lithofacies within the upper segment of the Pearl River Canyon (PRC), northern South China Sea, to better understand the mixed sedimentary processes within the submarine canyons. Within the upper segment of the PRC, various morphosedimentary features are recognized according to their seismic expressions in map and section views, including gravity flow-dominated (slope failures, turbidity channels and gullies) and contour current-dominated features (contourite drifts, contourite channels, contourite terrace, and sediment waves). The piston cores contain abundant shelf-derived coarse sediments within several contourite intervals, implying that the contourites were very likely sourced from pre-existing gravity-flow deposits. AMS 14C dating results further indicate that the contourites were mostly deposited during and after the last glacial maximum (LGM), suggesting active gravity flows during the same period. Silt-free sandy contourites are preferentially observed at the canyon thalweg, which may be related to intense contour currents. All the above evidence implies that the study area was influenced by mixed sedimentary processes, including gravity flows and vigorous contour currents. Further analyses suggest that the South China Sea Branch of Kuroshio (SCSBK) and Intermediate Water Current (IWC) greatly influenced the late Quaternary sedimentary processes within the PRC. Specifically, the SCSBK was responsible for the formation of the upper-slope contourites and associated contourite channels near the shelf edge. The SCSBK also promoted the occurrence of upper slope failures and gravity flows when the sea level was much higher than the shelf edge, and contributed to the LGM and post-LGM gravity-flow deposits. In contrast, the IWC was responsible for the formation of contourite drifts along the southwestern flank of the PRC. The IWC intensified when flowing along the canyon thalweg and fault scar, contributing to the formation of contourite channel. Under the influence of winnowing processes from the intense IWC, silt-free medium sands are exposed at the canyon thalweg. The mixed sedimentary processes that are highlighted in this study may have been a potentially underestimated mechanism in explaining the formation of high-quality deep-water sands within submarine canyons.