Abstract The Carboniferous to Early Triassic Pyeongan Supergroup overlies the Early Paleozoic Joseon Supergroup unconformably. A great unconformity occurs between these two sedimentary sequences ranging from Middle Ordovician to Lower Carboniferous in age. The Pyeongan Supergroup in the Samcheog coalfield is mainly distributed along the axis of the Baegunsan Syncline, between Yemi area and the northern part of the Taebaeg city. Modal analysis of medium- to coarse-sized sandstone is used to infer the provenance tectonic setting of the Pyeongan Supergroup and further to infer the geologic events during the unconformity period during Paleozoic. Upward changes in detrital modes of the Pyeongan Supergroup sandstones are observed. The sandstone types from the lowermost Manhang to the Dosagog Formation are quartzarenite, and sublith- to litharenite, whereas the Gohan and the uppermost Donggo sandstones contain feldspars and are classified as feldspathic litharenite and arkose, respectively. Using the various ternary diagrams of Dickinson and Suczek (1979), the provenance of the Manhang to the Gohan Formation is suggested to be a "recycled orogeny setting". Some Gohan sandstones are plotted within the "arc-related" field. The Donggo sandstones are plotted within both the "continental block" and "recycled orogeny" fields. Quartz grain petrography using a diagram of Basu et al. (1975) shows that sadstones from the Manhang to the Gohan formations are composed mainly of slightly to strongly undulose mono- and polycrystalline quartzs. The quartz types of the Donggo sandstones are composed mainly of non-undulose monocrystalline quartzs. All quartzs but in the Donggo sandstones indicate low-rank metamorphic source rocks whereas quartzs in the Donggo sandstones indicate medium- to high-rank metamorphic and plutonic source rocks. The ratio of mono- to polycrystalline quartz increases toward the uppermost Donggo Formation. Minor amounts of volcanogenic quartzs exist in the Gohan and the Donggo formations. Considering the sandstone composition and the paleocurrent data (Kim, 1976, 1978), the Pyeongan Supergroup is a molasse foreland basin sediment which was derived from the syndepositional thick-skinned fold-thrust belt located to the east of the present Pyeongan Supergroup distribution area. This study indicates the presence of the Middle to Late Paleozoic orogeny, whose presence needs to be studied further.

Keywords : Provenance, Paleozoic, Unconformity, Sandstone, Petrofacies, Continental block, Foreland, Fold-thrusting, Paleocurrent, Orogeny