Multistage unidirectionally migrating canyons and the evolution of their trajectories in the canyon zone in the Baiyun Sag, northern South China Sea: Insights into canyon genesis

Abstract

In deep-sea slope areas, canyons provide an ideal space to preserve sediments and provide a window to explore the deepwater deposition process, such as turbidity flow and contourite currents. We have evaluated results of the study of the northern continental slope of the South China Sea characterized by the presence of mostly straight canyons. After evaluating core samples and interpreting the corresponding seismic data, we static the sedimentary parameter and identified two types of canyons with different migrating trajectories: “convex downward curve-shaped” trajectories and “convex upward curve-shaped” trajectories. The convex downward curve-shaped canyon trajectory is distinguished by a lower layer of coarse-grained sediment and an upper layer of fine-grained sediment, whereas the convex upward curve-shaped canyon trajectory features a lower layer of fine-grained sediment and an upper layer of coarse-grained sediment. Combining the grain size of the core sample and the scale of the sedimentary structure, we restore the turbidity flow rates and the corresponding turbidity flow behaviors. Coarse-grained turbidity flows are characterized by lower vertical erosion rates and higher lateral abrasion rates, whereas fine-grained turbidity flows exhibit the opposite characteristics. Thus, the convex downward curve-shaped migration trajectory is mainly formed by coarse-grained turbidity flow erosion in the first stage (the late migration stage) and fine-grained turbidity flow deposition in the second stage (the vertical aggradation stage). In contrast, the convex upward curve-shaped trajectory forms through the opposite pattern of sedimentary evolution.