Submarine landslides on a carbonate platform slope changing transport pathways of deepwater gravity flows: Insights from the Xisha Islands, South China Sea

Abstract

Landslides frequently block rivers, forming a barrier lake, resulting in a dam break event in the terrestrial environment. However, reports about the submarine landslides are scarce. Herein, the periplatform submarine landslides changing transport pathways of deepwater gravity flows within the southeastern region out of Xuande Atoll on the Xisha Islands, South China Sea, are described. Submarine landslides, including SL I and SL II, with a total area of 170.9 km2, were identified using up-to-date geophysical data, including multi-beam bathymetric and seismic data and sediment gravity cores. SL I directly connects the Sansha Canyon forming a large-scale negative relief with an area of 151.8 km2, developing the terraces, and transporting blocks and remnant blocks. SL II covers an area of 19.1 km2 and was mainly deposited on the deepwater plain with a toe part that is higher than the initial seafloor. The negative relief created by SL I on the seafloor forms the dominant pathways for the gravity flows, creating the new active channel system directly flowing into the canyon. The turbidity currents flowing along the steep lateral margin of SL II formed the cyclic steps. The remnant submarine landslide deposits on the western canyon margin indicate that SL I previously blocked the canyon and formed a submarine barrier lake, which was then eroded by the gravity flows flowing along with the channel system within SL I and the upstream canyon. Finally, the canyon was formed again, whose axis was “pushed” eastward by SL I. These observations update the evolution model of submarine landslide on a carbonate platform slope and the transport system from the slope to the submarine canyon or the deep basin and clarify the stability of submarine engineering and island-and-reef systems.