Depositional History Of The Upper Part Of The Seongdongri Formation, Janggi Basin, Controlled By Tectonics And Volcanic Activity

In the Janggi Basin, 8 cores were drilled for underground carbon dioxide storage. In this study, sedimentary response to tectonics, sediment supply and volcanic eruption was speculated on the upper part of the Seongdongri Formation, using 5 drill cores. Depositional unit S-6 consists of a thick (20 - 30 m in thickness) lacustrine shale (FA4) and overlying mouthbar and swamp sediments with coarsening-upward trends (FA3 and FA5). The lower boundary of depositional unit S-7 is represented by thick volcaniclastic deposits (FA6) formed by dacitic volcanic activity and resedimentation. The volcaniclastic sediments were overlain by meandering stream sandstone with fining-upward trends (FA2) and swamp or floodplain mudstone (FA5). The lower boundary of the depositional unit S-8 is characterized by a thick (6 - 10 m in thickness) volcaniclastic tuff and is overlain by sandstone with fining-upward trend (FA2), deposited in meandering river, and homogeneous mudstone with organic matter (FA5), indicating swamp or floodplain environment. The lower boundary of depositional unit S-9 comprises a thick (similar to 10 m in thickness), white volcanic tuff, and is overlain by floodplain or swamp mudstone (FA5) with intercalating meandering river sandstone (FA2). In the middle part, thick volcaniclastic sediments commonly occur, which are overlain by braided stream conglomerate (FA1), suggesting increase in sediment supply. In the Janggi Basin, the Janggi Conglomerate and the lower part of the Seongdongri Formation are dominated by braided stream and lake deposits with westward increase in bed thickness, showing a wedge-shape distribution pattern in cross-section. The upper part of the Seongdongri Formation is, however, dominated by fine-grained meandering river and floodplain deposits without significant lateral variation in unit thickness, suggesting that, during the late stage of Janggi Basin development, small-scale subbasins were merged, forming a single basin.