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

Abstract

On the southeastern continental shelf of Korea, within the Korea Strait Shelf Mud deposit, seafloor undulations trending north-northwest‒south-southwest have been identified offshore of Ulsan. This study utilized multibeam echo sounding, two- and three-dimensional (3D) high-resolution CHIRP seismic surveys, and piston core analysis to investigate the formation and timing of these undulations on the muddy continental shelf. The multibeam data revealed irregular seafloor undulations aligned with the bathymetric contours, and the sea-bottom reflected amplitude of 3D CHIRP profiles identified elongated mounds within this region. The surface undulations extend approximately 30 km at a depth of 46‒56 m, running parallel to the bathymetric contours. These features vary laterally in width from 50 to 400 m and including small, elongate mounds. Core 20MAP-P05, which is located closer to the coast, contains coarse-grained typhoon deposits from the high-energy environment above a storm wave base, dating to 225‒287 cal yr BP. However, this interval is absent in core 20MAP-P04, which was sampled from the undulating seafloor at depth of 51.2 m. This core contains vertical burrows, 10‒20 mm in diameter, dating to around 183 cal yr BP at a depth of 220 cm, suggesting the rapid redeposition of resuspended sediments. These results indicate the formation of elongate mounds in an area with seafloor undulations off Ulsan, and that sudden re-sedimentation on the muddy continental shelf occurred around 183 cal yr BP due to the influence of contourite currents beneath the storm wave base.