Tectonic implications of quartzite-shale and phyllite beds in the Seochangri Formation (Okcheon group), Bonghwajae section, mid-Korea

摘要

This study focuses on the depositional and deformational features of quartzite-shale and phyllite beds in the lower part of the Seochangri Formation, Okcheon Group, mid-Korea. Each quartzite unit (25–320 cm thick) is massive, whereas the overlying laminated shale unit (0.5–10 cm thick) is either homogeneous or slightly laminated. Although these interbeds were strongly deformed under brittle-ductile conditions (ca. 300°C), the quartzite units show a thickening-upward trend. The quartzite-shale interbeds represent deposition from turbidity currents, i.e., Bouma-A and-D/E divisions. The thickening-upward quartzite-shale units are suggestive of progradation of sand lobes in the middle-outer fan. The overlying phyllite beds with thin quartz interlayers most likely represent outer fan and basinplain environments. The Seochangri Formation in the Bonghwajae section forms a thrust front against the limestone and dolomite beds of the Joseon Supergroup in which the quartzite-shale units are overlain by the phyllite beds and, in turn, underlain by the folded limestone beds. The phyllite and limestone beds were strongly deformed, showing upright isoclinal to tight folds, sheath folds, and boudins. The fold axes generally trend north-northeast with a low plunge angle. The thrust front represents part of the restraining bend of a dextral fault, the South Korean Tectonic Line. It is the major structural discontinuity between the Okcheon Group (Proterozoic) and the Joseon Supergroup (Lower Paleozoic). The collisional offset between the Gyeonggi Massif (South China Block) and the Yeongnam Massif (Sino-Korean Block) most likely occurred in the Jurassic prior to the emplacement of the mafic dyke and amphibolite.