Development of shallow-water contourite deposits on the muddy continental shelf off southeastern Korea

We analyzed multibeam echosounder (MBES), 2D and 3D compressed high-intensity radar pulse sub-bottom profile (Chirp SBP) data, and piston core samples to investigate seafloor undulations and their distribution, depositional processes, and timing in the uppermost Korea Strait Shelf Mud (KSSM) deposits on the southeastern continental shelf of the Korean Peninsula. The seafloor undulations align with the bathymetric contours, trending from NNE to SSW offshore Ulsan, Korea. These undulations extend approximately 30 km in length and 600 to 1,100 m in width at water depths of 46–56 m in the KSSM depositional area. We observed elongated ridges (100–200 m in width and 1.5–2.5 m in height, and > 300 m in length) in shallower areas and gently eroded seafloors in deeper areas. Analysis of a piston core sample, collected from a ridge slope at a water depth of 51.2 m, indicates rapid deposition of reworked sediments around 183 calendar years before present (cal yr BP). The formation of these depositional features is likely influenced by the southwest-flowing Korea Strait Bottom Cold Water (KSBCW), which intrudes from the Ulleung Basin in the East Sea. This suggest that the seafloor undulations, including high elongated ridges and eroded lowlands, were shaped by shallow-water bottom currents on the southeastern muddy continental shelf offshore Ulsan, Korea. Our data further indicate that upward gas migration is confined to the undulating seafloor morphology highlighting the significance of bottom water currents in gas migration dynamics and the shaping of seafloor morphology in the southeastern shelf of the Korean Peninsula.