Latest Quaternary mass-transport processes of fan-shaped body in the western margin of the Ulleung Basin, East Sea (Japan Sea)

In the western margin of the Ulleung Basin, a detailed analysis of cores with geophysical data from a fan-shaped body, just downslope of a submarine gully associated upslope with failure scars, reveals various modes of mass-transport processes. The arcuate failure scars occurs in water depths exceeding 600 m. The fan-shaped body, less than ca. 10 km long in radius, displays strong backscatter intensity in sonar images, and corresponds to the uppermost transparent mass in Chirp sub-bottom profiles. Sediment cores penetrating to the uppermost transparent mass consist mostly of various facies of mass-transport deposits (MTDs), causing the strong back-intensity in the sonar images. The interval of MTD facies comprises the upper and lower units without hemi-pelagic muds between them, implying that the fan-shaped body was probably deposited during a single event separated in at least two stages without a significant time break. The lower unit shows brittle to plastic deformation of soft muds (slides/slumps), whereas the upper units exhibits fully fragmented soft mud clasts (low viscous debris flows). Both the upper and lower units involve same original lithology (i.e., soft hemi-pelagic mud) prior to failures, suggesting that the lithology could not significantly affect depositional processes. The fully fragmented soft mud clasts of the upper unit are probably indicative of more shearing than the brittle to plastic deformation of soft muds in the lower unit. Considering the small dimension of the failure scars/gully and the same original lithology, the more shearing of the upper unit was most likely caused by longer transport distance than that of the lower unit. The rare turbidites with absence of channellevee systems in the fan-shaped body and the failure scars confined in the upper to middle slopes suggest that the submarine gully probably formed by slope failures, not by erosion of turbidity currents.