Sedimentary characteristics and processes of submarine mass-transport deposits in the Ulleung Basin and their relations to seismic and sediment physical properties

Three successive Pliocene MTD units (M2, M3, and M4 in ascending order) in the Ulleung Basin are characterized by continuous strong-amplitude negative basal reflections with transparent to chaotic internal seismic fades. Mapping of these MTD units in a dense 2-D grid of seismic reflection data reveals that each MTD unit generally thickens upslope to a maximum of 113 m and wedges out downslope against a structurally uplifted pre-existing surface. Although the source failure scars cannot be traced on the slope, the minimum runout distances of the MTD units from the base of the upper slope near the 1000 m isobath are estimated to be > 120 km. Each MTD unit is characterized in the logging-while-drilling (LWD) data by a gradual increase in bulk density, P-wave velocity, and resistivity values at the basal part and an abrupt decrease at the lower boundary. Nine sedimentary facies are identified in the MTD units, which indicate a variety of mass-transport processes such as sliding with brittle to plastic deformations and high-to low-viscosity debris flows. Vertical distribution of sedimentary facies shows a repeated generalized pattern in all MTD units. The pattern is interpreted to represent initial deep failures involving sand-prone consolidated sediments near the shelf break and successive shallow failures involving soft heterogeneous muddy sediments that were deposited in a deeper downslope area. The presence of sand-prone mass-transport deposits accounts for the basal densification and strong-amplitude negative basal reflections. Absence of such sand-prone basal parts in MTDs at other locales implies unpredictability of lithologic characteristics in MTDs with similar seismic reflection signatures.