Depositional features of co-genetic turbidite-debrite beds and possible mechanisms for their formation in distal mass-flow lobes beyond the base-of-slope, Ulleung Basin, East Sea

Abstract

A detailed analysis of the MR1 (11−12 kHz) sonar images, chirp (2−7 kHz) profiles,core sediments and 14C ages from the latest Quaternary mass-flow lobes on the deep(>2,100 m water depth) basin plain of the western Ulleung Basin reveals various depositional processes of mass flows in the distal setting beyond the base-of-slope and changes in slope-failure style. Eight mass-flow lobes are present in the basin plain, ca. 30−60 km beyond the base-of-slope, and were generally deposited retrogressively. Lobes 1−4 (distal lobes) are longer (>27 km) and wider (15−25 km) than lobes 5−8 (proximal lobes) having the length of 8.8−31.5 km and the width of 1.2−12 km. Lobes 1 and 2 consist dominantly of the muddier mass-flow deposits deposited between 19.5 and 20.0 cal. ka B.P. after the Last Glacial Maximum. The relatively huge masses with the high content of muds failed on the muddy upper slopes, deeper than 350−400 m water depth. On other hand, the sandier, smaller lobes 6 and 7 were deposited between 16.5 and 17 cal. ka B.P., probably by sediment failures on the sandy uppermost slope, shallower than 350−400 m water depth. Lobe 1 consists of a lower sandy-mud turbidite grading upward into an upper mud-matrix debrite having small, rounded mud clasts. In contrast, the mass-flow bed of lobe 6 is characterized by a gradual upward change from a lower clay-posin reveals various depositional processes of mass flows in the distal setting beyond the base-of-slope and changes in slope-failure style. Eight mass-flow lobes are present in the basin plain, ca. 30−60 km beyond the base-of-slope, and were generally deposited retrogressively. Lobes 1−4 (distal lobes) are longer (>27 km) and wider (15−25 km) than lobes 5−8 (proximal lobes) having the length of 8.8−31.5 km and the width of 1.2−12 km. Lobes 1 and 2 consist dominantly of the muddier mass-flow deposits deposited between 19.5 and 20.0