Geomorphological Development of Cenozoic Sedimentary Basins Since the Last Glacial Maximum: A Case Study of the Songcheon River and Pyonghaenamdaechon River Drainage Basin

The estuary and surrounding area of the Pyeonghaenamdaechon and Songcheon Rivers on the east coast of Gyeongsangbuk-do are regions where a Paleogene-Neogene sedimentary basin was formed after tectonic movements during the Cenozoic era. Local subsidence and the formation of sedimentary basins during the Paleogene~Neogene period, as well as changes in sea level, have significantly influenced the landform of the coastal and nearby river. This study identifies the depth and extent of the incised valleys formed during the low sea level periods of the glacial period and the depth and characteristics of the sediment layers deposited during the post-glacial period. The Songchon River has incised valleys up to 55 m below the current sea level, with valley widths ranging from approximately 2 to 4 km. Additionally, the plain area north of the Songchon River contains valleys formed approximately 10m below the current sea level. Overall, the estuary area of the Songchon River exhibits a deep incised valley formed around an altitude of -40 m. In the Pyeonghae area, deep incised valleys at altitudes of -36 m and -34.7 m have been identified, with sediment layer depths in the upstream direction ranging from 15 to 20 m. Similarly, in the Pyeonghaenamdaechon River, the locations of deeply incised valleys align with the current river valleys. The estuaries and adjacent bays of these two rivers are relatively large compared to the scale of the rivers, likely influenced by underlying geological structures. These valleys were transformed into bays due to rising sea levels during the post-glacial period and were later filled with riverine sediments. During the early post-glacial period, a high sedimentation rate was observed, characterized primarily by coarse sediments. Over time, this was followed by a lower sedimentation rate dominated by fine sediments. In the Geumcheon River, a tributary of the Pyeonghaenamdaechon River, the presence of a convex longitudinal river profile in the downstream section may be attributed to differences in rock types; however, it is also plausible that changes in the erosional base level, such as the subsidence of the Pyeonghae Basin and sea level fluctuations in the East Sea.