Abstract

Plio-Quaternary deposits of the Ulleung Basin, East Sea consist of nine seismic units (Unit 1 to Unit 9) based on seismic stratigraphy analysis using multi-channel seismic reflection data. According to chronostratigraphy, the geological ages of the Plio-Quaternary deposits are assigned as follows: Unit 1 (Pliocene) and Units 2 to 9 (Quaternary). Unit 1 is the thickest and composed of debris-flow deposits throughout the basin. Whereas, Units 2 to 9 are dominated by gradual changes from the debris-flow deposits to hemipelagic sediments basinward. Detailed analysis of dimensional characteristics of the debris-flow deposits, including mean thickness and run-out distance from the shelf-edge, divides the nine seismic units into four groups. Group 1 (Unit 1) is characterized by predominance of the debris-flow deposits which suggest that margin-wide slope failures were actively generated due to regional uplift along the southwestern shelf of the basin during the Pliocene. Group 2 to Group 4 (Units 2 to 9) is characterized by rapid retreat of the debris-flow deposits landward caused by decrease in tectonic movement during the Quaternary. At this time, the sedimentation was mainly controlled by sea-level fluctuations. The debris-flow deposits were stacked due to slope instability and gas hydrate dissolution during sea level lowering period, whereas the hemipelagic sediments were deposited under the stable slope environment during sea level rising period.Keywords: seismic stratigraphy, debris-flow deposit, sea level fluctuations, tectonic movement, Ulleung Basin

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