Quantitative Analysis for Seismic Stratigraphy to Understand Evolution and Controls of Shelf-Margin Depositional System, Southwest Ulleung Basin, East Sea, Korea

Abstract

Geophysical and geological researches for hydrocarbon reservoirs and basin evolution in the Ulleung Basin have continued with continuous seismic acquisition and several drilling since the 1970s. Most of researches focused on understanding the structural evolution of the Basin, rather than the depositional process. This study follows recent trends of an integrated quantitative approach with conventional seismic interpretation to understand the development of shelf-margin and to figure out dominant controls for sequence development. The quantitative analysis allows us to identify five shelf-edge trajectory types based on the ratios of accommodation and sediment supply (A/S ratio), which are interpreted from stratal stacking pattern as well as the measured rates of progradation (Pse) and aggradation (Ase), the calculated shelf-edge gradient (αse), and the P/A ratio. These types are assigned to ten interpreted seismic units within eight different dip-oriented seismic profiles. This shelf-margin classification suggests five phases in shelf-margin development during the mid-to-late Miocene period (15 ~ 6.5 Ma) in the southwestern Ulleung Basin. The formation of Phase 2 and 4 are mainly influenced by the decrease of accommodation in the entire area, which most likely induced by the sea-level fall, whereas the various shelf-edge types of Phase 1, 3, and 5 in lateral and vertical directions probably are the result of complex controls, which were also not consistent with sea level fluctuation. Comparison of the southwestern Ulleung Basin to worldwide continental margins based on the quantitative dataset indicates that the Ulleung Basin formed under a relatively low rate of progradation (low sediment supply; 3.7 km/Myr) and high rate of aggradation (high shelf accommodation; 377m/Myr) condition from the middle-Miocene to late-Miocene period. This condition most likely formed under the significant of basin subsidence and limited sediment source area, which well represents a closing stage of back-arc basin.

Keywords: Ulleung Basin, Quantitative analysis, Seismic stratigraphy, Shelf-edge trajectory types, Progradation, Aggradation, Retrogradation