Geomorphology of a bended submarine canyon in Wanhu Seamount region, northern South China Sea: Insights from manned submersible observation and measurement

Abstract

Submarine canyons can transport large volumes of terrigenous material from the continent to the deep-sea environment and are the primary conduits of source-to-sink systems. The large-scale morphologic features of canyons have been well studied using seismic reflection and the bathymetry data. However, detailed analyses of the small-scale elements of canyon morphology are relatively few because of the limited resolution of traditional field investigations. In this paper, the multi-scaled geomorphology of a bended submarine canyon (canyon C4) located in Wanhu Seamount region (northern South China Sea) is described and discussed based on the in-situ high-definition video data, high-precision near-bottom acoustic data, and push-cores obtained using the manned submersible vehicle. The NNW-trending submarine canyon diverted to 122.5° at the foot of Wanhu Seamount, and the fresh erosional surfaces were recorded using the video data at the canyon axis bend. A total of eight sediment waves were generated at the overbank of the bending region. These waves contain wavelengths and wave heights decreasing in an upslope direction from the canyon axis towards the Wanhu Seamount. The C-M pattern (coarsest one percentile grain size versus median grain size values), grain cumulative probability curves, and grain size gradations derived from the sediment cores suggested that these sediment waves were induced by a turbidity current. The bending of the canyon resulted from the deflection and reflection of the turbidity currents, which were constrained by Wanhu Seamount. The abnormal geomorphologies of some sediment waves were related to the back-squeezed underflow of the turbidity currents. The turbidity current behavior was compared and found to be in agreement with the experimental and modeling simulation results mentioned in the literature. However, the morphologic setting in the natural case was more complex than that observed from the experiments, with the flow behaviors being more complicated. Future studies are required for more detailed comparison between the natural case and simulations to better understand the turbidity current behavior in a bended submarine canyon.