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

Layers in deep-sea slope area can serve as indexes of various sedimentary dynamic evolutions, despite their complicated processes. The sedimentary architecture evolution in a slope can exhibit different stratigraphic records, especially in terms of canyon migration trajectories. We use the northern continental slope of the South China Sea as a case study, where the main straight canyons in this area have developed. After describing the core samples and performing the corresponding well seismic data calibration and sedimentary parameter statistics, we identified two different patterns of canyon trajectory evolutions: the “exponential curve-shaped” canyon migration trajectory path and the “logarithmic curve-shaped” path. The “exponential curve-shaped” canyon migration trajectory path is distinguished by a lower layer of coarse-grained sediment and an upper layer of fine-grained sediment, and the “logarithmic curve-shaped” canyon migration trajectory exhibits the opposite trend, i.e., fine-grained sediment on the bottom and coarse-grained sediment on the top. Based on the above phenomenon, we quantitatively calculated the flow and deposition rates of turbidites and determined their genesis. We infer that due to the different grain size characteristics of different types of turbidites, the turbidites show different flow patterns. Coarse-grained turbidites are characterized by lower vertical erosion rates and higher lateral abrasion rates, while fine-grained turbidites have exactly the opposite characteristic. Thus, the “exponential curve-shaped” migration trajectory is mainly formed by the coarse-grained turbidity current erosion in the first stage (later migration stage) and the deposition of fine-grained turbidites in the latter stage (vertical aggradation stage). In contrast, the “logarithmic curve-shaped” turbidites are developed by the opposite sedimentary process.