Supercritical turbidity-current bedforms in the South China Sea: An overview

The South China Sea (SCS) is one of the largest marginal seas on Earth, located at the convergence of the Eurasian, Pacific, and Indo-Australian plates. The basin formed through seafloor spreading during the Oligocene to Middle Miocene and has since been undergoing eastward subduction beneath the Philippine Sea Plate. This process has led to the gradual enclosure of the SCS since the Late Miocene. The region's dynamic tectonic setting, coupled with a tropical typhoon-prone climate, has contributed to the formation of numerous supercritical turbidity current (TC) bedforms within the SCS.

Supercritical bedforms have been documented at more than 20 sites in the SCS, including bedform trains found along canyon/channel thalwegs, as well as bedform fields in unconfined environments like levees, overbank or interfluvial areas, and fans at canyon mouths. The bedform trains along thalwegs typically consist of 5-19 bedforms and vary in length from a few kilometers to ~100 km. The unconfined bedform fields may contain several to over 30 rows of bedforms, covering areas ranging from 176 to 20,000 km². Both the confined and unconfined bedforms are considered supercritical based on their diagnostic morpho-depositional characteristics, including upslope migration, and backset dominant bedding, and erosional truncations on the lee side. The turbidite-dominated sediment components, crest orientation parallel to local isobaths, and their occurrence in canyons/channels and related environments, all suggest formation by TC rather than contour or other bottom currents.

Individual supercritical bedforms in the SCS can be identified as one of the two end members: cyclic steps and antidunes. Cyclic steps show upslope or downslope asymmetry, step-like morphology, and typical downstream-thinning backsets. Antidunes have symmetrical cross-sections, convex-upward structures, downstream-thickening backsets, and, when occurring with cyclic steps, smaller dimensions and aspect ratios. Occasionally, antidunes are superimposed on the stoss side of cyclic steps, identified as chutes-and-pools, representing a transitional type between antidunes and cyclic steps. Both confined and unconfined bedforms exhibit a wide range of wavelengths and wave heights, varying from smaller bedforms with wavelengths of 200-300 m and wave heights of several meters, to larger bedforms with wavelengths typically in the range of kilometers and wave heights reaching tens of meters or more. Confined bedforms are dominated by erosional to partially depositional cyclic steps, or by partially depositional antidunes. Unconfined bedforms are predominantly composed of fully to partially depositional cyclic steps and antidunes, with erosional cyclic steps and chutes-and-pools forming locally. In both cases, depositional bedforms are the most prevalent, likely due to high sedimentation rates resulting from the combined effects of rapid post-rift subsidence and ample sediment supply in the SCS. The widespread presence of supercritical bedforms highlights the important role that supercritical TCs play in SCS’s deep-water sedimentation.

This work was supported by the National Key Research and Development Program of China (2022YFF0800503) and the National Natural Science Foundation of China (91028003, 41676029, and 41876049).