Jurassic synorogenic basin filling in western Korea: sedimentary response to inception of the western Circum-Pacific orogeny

This is the first sedimentologic and stratigraphic attempt to demonstrate Jurassic subduction-induced basin-filling processes in the early stage of the western Circum-Pacific orogeny. The Chungnam Basin in western Korea was filled with a Lower to Middle Jurassic nonmarine succession, the Nampo Group, whose deposition postdated the Triassic final assembly of Chinese continental blocks. The Nampo Group consists of two repeated, fining- to coarsening-upward alluvio-lacustrine sequences, separated by an interval of thick breccia–gravel progradation deposits and its related strong proximal unconformities. No temporal variation in the degree of chemical weathering, along with the predominance of coals and a tropic to subtropic paleoflora, reveals little or no climate fluctuations during deposition of the Nampo Group. The observed relationships provide a record of sedimentation most likely controlled by temporal variations of tectonically driven sediment flux. Such syntectonic sedimentation of the Chungnam Basin occurred at a convergent margin of continental-arc setting during the Daebo orogeny, synchronous with the early subduction of the western paleo-Pacific ocean that resulted in formation of an accretionary complex along the East Asian continental margin during Jurassic time. Hence, synorogenic deposition in the Chungnam Basin is interpreted as sedimentary response to subduction–accretion of the western paleo-Pacific plate.