Depositional features of co-genetic turbidite-debrite beds and possible mechanisms for their formation in distal lobated bodies beyond the base-of-slope, Ulleung Basin, East Sea (Japan Sea)

摘要

A detailed analysis of the MR1 (11-12 kHz) sonar images, chirp (2-7 kHz) profiles, core sediments and ~(14)C ages from the latest Quaternary lobated bodies (LB) on the deep (>2100 m water depth) basin plain of the western Ulleung Basin reveals depositional features and context of co-genetic turbidite-debrite beds in the LB on the distal setting and a plausible mechanism for generating these beds. Eight LB are present in the basin plain, ca. 30-60 km beyond the base-of-slope, and were generally deposited retrogressively. Older and more distal LB 1-4 have large dimensions (>27 km long, 15-25 km wide). In contrast younger and less distal LB 5-8 are small (8.8-31.5 km long, 1.2-12 km). The muddier, larger LB 1 and 2 were most likely originated from the relatively large-scale sediment failures on the muddy upper slopes (>350-400 m water depth) between 18.5 and 20.0 cal. ka B.P. On other hand, the sandier, smaller LB 6 and 7 were deposited between 17.0 and 17.5 cal. ka B.P., probably by the relatively small-scale sediment failures on the sandy uppermost slope, shallower than 350-400 m water depth. In LB 1, a lower sandy-mud turbidite is transitional upward into an upper mud-matrix debrite having small, rounded mud dasts. In contrast, LB 6 exhibits a gradual upward change from a lower clay-poor, sandy turbidite to an upper clay-rich, sand-matrix debrite with large, interlocking mud clasts. Each mixed bed in LB 1 and 6 can represent a co-genetic (or linked) turbidite-debrite bed formed by the down-dip flow transformation from turbidity current to debris flow at a point during the same event The abundant large-scale (up to 20-25 m deep, 3-5 km long) erosions of fine-grained substrates near the base-of-slope area suggest that the down-dip transformation was driven by incorporation of mud via erosional bulking by turbidity currents reaching the base-of-slope area The different clay/sand content and size/shape of mud clasts in the co-genetic turbidite-debrite beds between LB 1 and 6 imply that the composition (sandy or muddy) and dimension of initial turbidity current approaching the base-of-slope, the volume of eroded masses from the finegrained substrates, and the transport distance of sediment gravity flows from the erosional areas of muddy substrates can be all important factors controlling the depositional styles of co-genetic turbidite-debrite beds.