남해대륙붕 분지의 탄성파층서, 퇴적환경 및 지구조 진화

Abstract

This study focuses on the seismic stratigraphy, tectonic evolution, and depositional environments of the sedimentary basins in the southern continental shelf of Korea, which were analyzed using 2D seismic data (about 3,150 line-km) obtained by the Korea Institute of Ocean Science and Technology (KIOST) and supplementary data of three drilling wells from the previous hydrocarbon exploration. In the study area, 3 sedimentary basins, Socotra, Jeju and Domi basins, are defined by large-scale basement highs. The basin-filling sedimentary sequences are divided into 4 mega-sequences bounded by the top surface of acoustic basement and major unconformitic surfaces recognized both in drilling wells and seismic profiles. Chronostratigraphy of the seismic sequences is established on the basis of biostratigraphic data from hydrocarbon exploratory wells and the stratigraphic framework of Chinese research data which constrain the age of the lowermost sequence to be approximately the Eocene. In the eastern part of the Scotra Basin, the boundary fault systems gave rise to various-scale grabens or half-grabens immediately above the acoustic basement, which strongly suggestive of a rifting accompanied by extensional fault movement. In the Jeju Basin, however, the acoustic basement is dominantly characterized by monotonous surface topography instead of high-relief faulted basement blocks or wedge-shape syn-rift deposits. Therefore, the Jeju Basin is interpreted as a sort of sag basin where the basin opening was mainly controlled by basement subsidence without significant horizontal extension of the crust. Depositional environments of mega-sequences, interpreted based on thecorrelation with biozones in drilling wells and partly on the seismic facies analysis, are summarized as follows: (1) terrestrial lowland for the upper part of MS1 (Eocene), (2) lower coastal and oceanic environments respectively for the lower and upper parts of MS2 (Oligocene), (3) open lowland and middle coastal environments respectively for the lower and upper parts of MS3 (Miocene), and (4) shoreface and marine environments respectively for the lower and upper parts of MS4 (Plio-Pleistocene). On the other hand, the Eocene and Oligocene sequences (MS1 and MS2) in the Socotra Basin have experienced with compressional deformation giving rise to folds and thrust faults, and the upper part of the deformed sequences has been eventually eroded forming an angular-unconformity at the base of the Miocene sequences (MS3). In western part of the Jeju basin, Eocene, Oligocene and Miocene sequences were also regionally deformed. More than three events of the compressional tectonism are indicated by distinct unconformable surfaces at the top of each mega-sequence units. To conclude, four stages of basin opening and evolution history are suggested in the Socotra Basin and the western part of the Jeju Basin. Initial rifting began in the Late Cretaceous building a series of grabens and hlaf-grabens in the Socotra Basin, whereas basement subsidence without significant structural deformation occurred in Jeju Basin at the same time (stage 1). Regional uplift and folding (Yuquan movement) in Late Eocene - Early Oligocene terminated the initial rifting in Socotra Basin and subsidence in Jeju Basin regions. In the Early Oligocene, the rifting and subsidence started again in the Socotra Basin and Jeju Basin respectively (stage 2), which were interrupted by uplift and folding in Early Miocene, marking the transition to the postrift phase. Regional subsidence continued in Early Miocene - Late Miocene (stage 3). The tectonic inversion (Longjing movement) occurred in the Late Miocene, interrupted the subsidence in Jeju Basin region. Subsequently, the entire area entered a stage of regional subsidence again (stage 4).