The role of large-scale mass wasting processes in changing the sediment dispersal pattern in the deep-water Central Canyon of the northwestern South China Sea

Abstract

Large-scale mass transport complex (MTC) (usually covering hundreds to thousands of km2 with a volume of tens to hundreds of km3) resulting from slope failures, can change the overall sediment dispersal pattern in deep-water canyon. However, the causal mechanisms remain enigmatic. We investigate the large-scale MTCs in the Central Canyon that are imaged in 3D by high-resolution seismic reflection data from the northwestern South China Sea. Our results indicate that a total volume of 435.3 km3 of sediments related to large-scale mass wasting processes were dumped into the Central Canyon from its side direction between 5.7 and 4.2 Ma. This led to adjustments of the geomorphology of the thalweg slope, which changed from a simply stepped slope (2 steps separated by 1 ramp) into a complexly stepped slope (3 steps separated by 2 ramps). The adjustments are argued to be caused by the thickness variations of MTCs1-4 that also ponded the axial turbidity flows at their up-dip directions. And then, the adjusted thalweg slope controlled the formation of a multi-stage submarine fan, which is proved to be an important hydrocarbon exploration target within the Central Canyon. Besides the first step in the canyon where sediments were mainly transported downslope due to the initially large velocities of turbidity flows, the following ramps were also considered as sediment bypass zones due to the further accelerated flows. Whereas, the subsequent steps became sediment deposition dominated zones as turbidity flows decelerated abruptly. Slope overstepping resulting from high sediment supply that was controlled by the East Asia summer monsoon, together with seismicity caused by the structural inversion of the Red River Fault were responsible for the occurrences of the large-scale mass wasting processes. We highlight how the large-scale mass wasting processes change the sediment dispersal pattern in the deep-water canyon for the first time. This study is of great significant to identify the locations of hydrocarbon reservoirs and evaluate the spatial variability in depositional reservoir qualities within the deep-water canyon.