Deformation-induced right-side-up pseudo-stratigraphy of the early Paleozoic Joseon Supergroup in the southeastern Danyang area, South Korea

Abstract

The formations in the early Paleozoic Joseon Supergroup (mainly carbonates with subordinate siliciclastics) within the Taebaeksan Basin in the Danyang area, South Korea, show an apparent right-side-up homoclinal stratigraphy without repetition or omission of any formation, and it was therefore thought that the NW-dipping formation boundaries are primary depositional contacts. Our detailed examination revealed, however, that the formation boundaries are reverse-slip shear zones parallel to the second-generation foliation (S₂) crenulating transposed S₀//S₁ foliation and that WNW-dipping bedding planes (S₀) are only locally preserved. The most penetrative regional planar structure in the area is the NNW-dipping first-generation foliation (S₁) that is defined by compositional layering and is parallel to the axial planes of isoclinal F₁ folds. Isoclinal to close F₁ folds occur on both the mesoscopic and macroscopic scale, whereas tight to close F₂ folds occur locally only on the mesoscopic scale. D₁ deformation presumably involved a NNW-SSE horizontal contraction and resulted in the buckle folding and transposition of S₀. D₂ deformation involved a NW-SE contraction, probably at a high angle to the S₀//S₁ transposed layers, and produced passive shear folding of S₀//S₁ with S₂ crenulation cleavages as discrete shear surfaces. Shearing deformation was strongly localized along some S₂ foliation planes, resulting in reverse shear zones that represent the current ‘pseudo’-formation boundaries. Based on zircons U-Pb ages, we suggest that the Okdong Fault was initiated from an unconformable boundary between the Precambrian basement gneiss and overlying sediments of the Paleozoic Taebaeksan Basin, probably during the Middle Jurassic.