Tectono-stratigraphic evolution of the SW margin of Ulleung Basin since the late Middle Miocene

Multichannel seismic reflection data were interpreted to reveal structural and stratigraphic developments of the late Middle Miocene to Holocene successions in the southwestern margin of the Ulleung Basin, East Sea. Based on the density of deformational structures such as thrust faults and folds, the study area can be divided into southeastern deformed zone and northwestern undeformed zone. Uplift and coeval erosion created pronounced unconformities in the defomed zone and allowed large amounts of sediments to accumulate in the undeformed shelf region. Depositional units I, II, and III are recognized in deformed zone based on three erosional unconformities formed by regional compression during the Late Miocene (?12.5-6.3 Ma). Erosional areas marked by the unconformities progressively migrated southwestward. Depocenter of individual depositional unit (I, II, and III) also migrated from the northeast to the southwest. On the other hand, seismic facies boundaries between neritic and shelf, and shelf and slope moved northeastward (basinward). The sequential transition of erosional areas, depocenter and seismic facies of each unit suggests that uplift and corresponding erosion in deformed zone occurred progressively from the northeast to southwest. High rates of sediment supply from the uplifted region are responsible for the northeastward migration of seismic facies boundaries. Depositional units IV, V, and VI, formed during the PlioPleistocene, have a close relation to the formation of the anticlinal flexure. The anticlinal flexure uplifted after the deposition of Unit IV, forming a small-scale basin in the southwestern part of the present shelf area. Deposition of Unit V was confined within this small basin. After filling the small basin, Unit VI was deposited in the entire shelf region except for the crest area of the anticlinal flexure. The Late Pliocene deformation had been quite stable. Uplifting of the anticlinal flexure and normal faulting in the northeastern and southwestern parts of the study area are still active. The tectonic movement, however, is insufficient to change the overall depositional patterns.