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).