The northeastern South China Sea margin created by the combined action of down-slope and along-slope processes: Processes, products and implications for exploration and paleoceanography

Abstract

Processes and products associated with the interplay of down-slope and along-slope processes are geographically widespread and yet poorly documented. Using a high-quality 2D database consisting of bathymetry, 2D seismic, piston cores, faunal, and radiocarbon data, six major depositional systems are recognized along the northeastern South China Sea margin, and from the upper slope to abyssal plain, they are erosional features, mass-flow systems, sediment gravity-flow (SGF) systems, mixed contourite-SGF systems, contourite systems and hemipelagic systems.Sedimentary processes on the studied margin show considerable spatial complexity, yielding a depositional model that must incorporate the interplay of down-slope and along-slope processes. Erosional features and mass-flow systems are common on upper and middle slopes, respectively, where high mass flows probably dominate over bottom currents. In the lower slope and continental rise where mass flows transform into SGFs, yielding SGF system. In the lower segment of the Taiwan canyon, there is a strong interplay of SGFs and bottom currents, forming the mixed contourite-SGF system. On the abyssal plain where SGFs are volumetrically overwhelmed by contour currents, contourite depositional systems are well developed. Our results highlight the complex interaction between down-slope and along-slope processes on continental margins, thus helping to better understand the deep-water sedimentation.Bottom-current reworked sands lacking ‘typical turbidite signatures’ are recognized, and are interpreted to be created by the interplay of SGF and bottom currents (including contour currents, bottom currents associated with internal solitary waves and deep-marine tidal bottom currents), which could potentially yield excellent hydrocarbon reservoirs after burial, affording an alternative for interpreting deep-marine non-turbidite reservoirs. Fine-grained bottom-current sediment waves are first recognized in the abyssal plain of the South China Sea and are interpreted to be created by contour currents resulting from the North Pacific Deep Water (NPDW–CCs), providing solid evidence for the intrusion of NPDW into the abyssal plain of the South China Sea. Preliminary bedform-velocity analysis suggests that NPDW–CCs have a maximum flow velocity up to 3～7 cm/s.