Tertiary tectonics of the Yellow Sea: Formation and evolution of the Gunsan Basin, Korea

Tertiary geology of the Yellow Sea is characterized by extensional tectonics. Several rift basins
concurrently developed including Bohai Bay Basin, North Yellow Sea Basin(West Korea Bay Basin), South Yellow Sea Basin(Gunsan and Heuksan Basins), and East China Sea Basin(Jeju Basin). The Gunsan Basin which consists eastern part of the northern South Yellow Sea Basin displays graben and horst structure typical of rifts formed by basement involving fault block movement. A series of fault bounded depressions and highs were created by fault block rotation. The basement is deep near the bounding fault and becomes shallow towards graben shoulder. Some basement highs are explained to be the results of hinged structure that stayed intact during extension probably due to rigidity. Transfer faults which are strike-slip faults crosscut basin-forming normal faults. Transfer faults release the stress through rupture between basements having differences in elastic property. The basin is bounded by the normal fault in the south. The normal fault dips northward and extends deep into the underlying basement at low angle, which could be large-scale structural lineation. The Gunsan Basin was rifted in Late Cretaceous-Early Tertiary, and experienced rapid filling and subsidence by volcanics and lacustrine and alluvial sediments during Paleocene and Eocene, and deformed under compression prior to Miocene. The subsequent basin filling and subsidence continued
at much slower rate. The synrift sedimentary sequences are separated from postrift sediments by the base Miocene regional unconformity. The regional unconformity is observed in adjacent seas as well, and Neogene and Quaternary sedimentation is not restricted in the basin, and occurs all over the Yellow Sea.