Late Quaternary sedimentation in the Ulleung Interplain Gap, East Sea (Korea)

The Ulleung Interplain Gap (UIG) is a deep (2300–2700 m) passage which has served as a conduit for deep-water circulation between the Ulleung and Japan basins. A detailed analysis of Chirp (2–7 kHz) subbottom profiles (ca. 6270 line-km) and nine sediment cores (8.6–11.4 m long) together with age data of tephra layers and four AMS 14C from the UIG and the adjacent areas reveals complex sedimentation caused by an interaction between bottom currents and mass flows during the last- and post-glacial periods. From high-resolution subbottom data, rock basement, slide/slump/rock-fall deposits, mass-flow chutes/channels, mass-flow deposits, bottom-current deposits, and a large-scale bottom-current channel system are recognized. Core sediments consist of various deposits of turbidites, muddy contourites, manganiferous contourites, and pelagic/hemipelagic sediments. Based on vertical distribution of sedimentary facies together with a chronostratigraphic framework, core sediments can be divided into Units I (<~15 ka) and II (>~15 ka).The extensive mass-flow deposits with slope failures on the entire slopes of topographic highs around the UIG and the dominant turbidites in Unit II (>~15 ka) suggest that a relatively large amount of sediment was delivered into the UIG by frequent mass flows (recurrence intervals of ca. 250–500 years in the upper Unit II) during the last-glacial period. Erosion or hampered sedimentation by bottom currents is indicated by the truncated reflectors of channel walls and muddy/manganiferous contourites in the Ulleung Interplain Channel (UIC) along the UIG. Interbedded turbidites in the UIC floor reflect that some large-scale mass flows intermittently entered into the UIC.The UIC has an asymmetric channel-flank geometry. The southeastern flank shows a gentle, wide mound morphology of mass-flow deposits derived from large-scale slope failures on the slopes of the Oki Bank, reflecting a dominance of downslope gravitational processes over alongslope bottom currents. In contrast, the northwestern flank is characterized by a narrow, steep geometry of mass-flow deposits, where a relatively small amount of sediment derived from the slopes of the South Korea Plateau could not overcome bottom-current activity.The dominant muddy and manganiferous contourites with rare turbidites in Unit I (<~15 ka) reflect intensified bottom currents and infrequent slope failures (recurrence intervals of ca. 1700–5000 years) during the post-glacial period. These conditions facilitated the formation of a thin, elongate mound of bottom-current drifts overlying mass-flow deposits on the southeastern UIC flank, and sustained erosion or hampered sedimentation in the UIC