The Sesong Formation (middle Guzhangian to middle Jiangshanian), Taebaek Group revisited and redefined

The Sesong Formation spans from the middle Guzhangian to the middle Jiangshanian in age and is traditionally known to consist mostly of dark gray mudstones with interbedded sandstone and limestone layers. While the formation has been intensively studied in terms of paleontology, the full lithostratigraphy of the formation remains incompletely appreciated. This study reports, for the first time, the complete lithostratigraphy of the Sesong Formation (about 114 m thick) where both the lower and upper boundaries with the underlying Daegi and the overlying Hwajeol Formations occur in a single section. We utilized the TB-17A core drilled in 2017 by Korea Institute of Geoscience and Mineral Resources (KIGAM) at the Hyeoldong, Taebaek, supplemented by an outcrop section located 2 km to the southeast. The Sesong Formation is composed of five lithofacies of sandstone, shale, packstone to grainstone, hematite-coated grainstone, and lime pebble conglomerate. The renewed lithostratigraphic framework informally divides the formation into three members: the lower member (42 m thick) composed of shale with intermittent carbonate facies in the lower half, and very fine to fine laminated sandstone with minor shale and carbonate facies in the upper half, the middle member (45 m thick) characterized by dominance of bioturbated and massive sandstone facies, and the upper member (27 m thick) with fine laminated and bioturbated sandstones with alternating minor carbonate facies. Correlation with previous studies shows that up to 45-m-thick upper middle member and lower upper member has not been reported in previous studies, which fills the lithostratigraphic gap between the Jikdong and Sagundari sections. We suggest that this newly reported interval might contain previously unrecognized trilobite biozones in the Taebaek Group, corresponding to the Changshania–Irvingella and Maladiodella Zones in North China (Sino-Korean Craton).