Depositional characteristics and processes of alongslope currents related to a seamount on the northwestern margin of the Northwest Sub-Basin, South China Sea

• A seamount-related contourite depositional system is reported.
• Bottom current sedimentation changes during the progressive burial of the seamount.
• The bottom current is part of South China Sea Intermediate Water circulation.
• The contourite depositional system has formed since the early Late Miocene.

This study infers, from 2D seismic data, the presence of a seamount-related deep-water contourite depositional system, located on the northwestern margin of the Northwest Sub-Basin of the South China Sea. Alongslope aligned erosive features and contourite drifts developed in water depths between 1000 and 1500 m on the present seafloor. A moat formed north of the seamount as a result of bottom current intensification after being deflected by an obstacle morphology, indicating a major eastward flowing current. Due to the Coriolis deflection, an elongated-mounded drift developed to the north of the moat and a modest plastered drift to the south (onlapping the north side of the seamount). Bottom currents away from the seamount are less or minimally intensified, generating contourite channels and furrows over the drift to the north of the moat. Combined with the known oceanographic setting, the bottom currents could belong to the anticyclonic South China Sea Intermediate Water circulation. From the seismic data, the architecture of contourite deposits during progressive burial of the seamount is documented. The first appearance of contourite channels in seismic Unit 1 is inferred to be the onset of the contourite depositional system, which could be traced back to the early Late Miocene. The following aggradational pattern of this system, from the base of seismic Unit 2 onwards, indicates relatively enhanced and stable South China Sea Intermediate Water circulation occurring on the northwestern margin of the northern South China Sea. Subsequently, the behaviour and depositional processes of bottom currents upstream and downstream the seamount are discussed. The depositional model suggests that a complex contourite channel system will replace the former moat–drift system once the seamount is buried. Finally, our findings may provide new challenges for investigating the South China Sea Cenozoic palaeoceanography, which are helpful to understand the sedimentary dynamic processes related to the South China Sea deep-water circulation.