Abstract

The Ulleung Basin is a presently tectonically inactive back-arc basin. It is a nonfan, slope-centered sedimentary basin in which up to 800 m of mostly fine-grained turbidites have accumulated. The precursors of these turbidity currents were slides, slumps, and debris flows on the slope and near the base-of-slope. An ideal mud-turbidite sequence identified in cores from the basin includes divisions of laminated mud (E 1 ), graded mud (E 2 ) and homogeneous mud (E 3 ), associated with convolute mud (E 4 ), bioturbated mud (F 1 ), bioturbated mud with authigenic pyrite (F 2 ) and indistinctly laminated mud (G). E 1 through E 4 muds are turbiditic in origin, as evidenced by an absence of bioturbation, preferred orientation of large-sediment particles, liquefaction, and high organic carbon content, all suggestive of rapid deposition from unidirectional flow. The indistinctly laminated mud (G) is characterized by poorly defined laminae, slight bioturbation, intermediate organic carbon content, and large amounts of skeletal debris in the silt fractions, in contrast to both the turbiditic and bioturbated muds. This mud is suggestive of deposition from bottom currents. The bioturbated mud (F 1 ) resulted from slow particle-by-particle settling in the postglacial oxic environment, whereas the bioturbated mud with pyrite filaments (F 2 ) is due to deposition in a suboxic environment when the basin was isolated from the Pacific during the last glacial period. Both bioturbated muds are low in organic carbon, poorly sorted, and characterized by random orientation of large-sediment particles.

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