How do supercritical turbidity-current bedforms transition? Insights from seismic data interpretation in the South China Sea

The transition of supercritical turbidity-current bedforms has been studied in the flume experiments and outcrops, whereas similar bedform transitions in deep-sea cases are rare. To better understand the mechanism behind bedform transitions in natural environments, we investigated the tempo-spatial transition of supercritical turbidity-current bedforms in the lower continental slope to abyssal plain in the northeastern South China Sea, by high-quality single-channel seismic data analysis coupled with simple numerical modeling. Quaternary bedforms were delineated at >3400 m water depth, covering an area of ~20000 km². These bedforms are characterized by long wavelength (0.4-5 km), low wave height (1-15 m), and large aspect ratio (80-730), which are identified as supercritical-flow bedforms. Four types of bedforms were further identified based on the morphology and internal structure, which are (I) upslope-migrating cyclic steps characterized by asymmetrical morphology with thick backsets and long wavelength; (II) upslope-migrating antidunes (UMAs) featured by nearly symmetrical morphology and relatively short wavelength; (III) downslope-migrating antidunes (DMAs) typified by gentle and sigmoid foresets and large aspect ratios; (IV) upper-stage plane beds (UPBs) consisting of low-relief wavy to subhorizontal reflections. Slope variations are highlighted to induce flow energy changes and facilitate bedform transitions. A slight slope decrease from 0.5 to 0.1° and 0.3 to 0.1-0.2° would respectively lead to the transition from UMAs to UPBs and from cyclic steps to UMAs, due to the hydraulic jump and flow acceleration. In contrast, an increased slope from 0.1 to 0.2° can contribute to the transition from UMAs to cyclic steps or DMAs by re-accelerating flows. Over time, the bedforms evolve from DMAs to UMAs and cyclic steps with growing wavelengths and wave heights, possibly caused by the inherited development of bedforms and increasing aggradation rates linked with progressively rising Taiwan uplifting rates. These bedforms consist of three contiguous fields fed by inter-seamount pathways and Manila Trench, comprising a supercritical-flow submarine fan apron that is far from the shelf edge and lacks submarine channels. This research was supported by the National Key Research and Development Program of China (Grant Number 2022YFF0800503).