Depositional processes of mass flows beyond the base-of-slope area and changes in slope failures in the SW Ulleung Basin (East Sea, Korea)

Abstract

17 AMS 14C ages reveals spatial and temporal variations in depositional style of mass-flow lobes in the Ulleung Basin. On MR1 sonar image, eight N&#1048576;S elongated mass-flow lobes are identified in the western basin plain (>2100 m). Lobes 1&#1048576;4, deposited retrogressively, have large dimensions (>27 km long and 15&#1048576;25 km wide) and occupy in the lower stratigraphic position. On the other hand, lobes 5&#1048576;8, deposited in the more proximal area,have small dimensions (8.8&#1048576;31.5 km long and 1.2&#1048576;12 km wide) and occur in the upper stratigraphic position. Lobes 1 and 2 are characterized by relatively strong back-scattering intensity with smooth surfaces on MR1 image,and show flat, sharp bottom echo and several distinct to diffuse internal reflectors in chirp profiles. Sediments near their edges consist of fine-grained (muddy) turbidites with minor massive clay-rich sand. However, they change to coarse-grained debrites and turbidites with the overlying fine-grained turbidites toward the proximal part. Lobes 1 and 2 deposited between ca. 20 and 18 cal. ka B.P. Lobes 3 and 4, overlying lobe 2, show weak to medium back-scattering intensity on MR1 image with slightly irregular to hummocky surfaces corresponding to small-scale (<100 m wide and <5 m high) hyperbolic bottom echoes in chirp profiles. Upper surfaces of the lobes are nearly flat to convex-up in cross Lobes 1&#1048576;4, deposited retrogressively, have large dimensions (>27 km long and 15&#1048576;25 km wide) and occupy in the lower stratigraphic position. On the other hand, lobes 5&#1048576;8, deposited in the more proximal area,have small dimensions (8.8&#1048576;31.5 km long and 1.2&#1048576;12 km wide) and occur in the upper stratigraphic position. Lobes 1 and 2 are characterized by relatively strong back-scattering intensity with smooth surfaces on MR1 image,and show flat, sharp bottom echo and several distinct to diffuse internal refl