韓国洛東江下流沖積平野の地形発達

The Nagdong River Basin occuping the area of 24, 000km² with 525km long trunk stream is located in the southeastern part of the Korean Peninsula. The alluvial plain along the lower Nagdong is divided into two areas, i. e. the down-reaches basin and the river-mouth delta.<br>This study intends to clarify geomorphic characteristics and geomorphic development of this plain in relation to post-glacial sea-level change. The investigation is made up of micro-landform classification, stratigraphic analysis, and restoration of depositional environment.<br>The main findings are as follows:<br>1) There are buried valleys below the 50-70m thick alluvium in th delta, and below the 15-10m thick one at Woegwan located about 210km upstream from the river mouth.<br>2) The alluvium of the delta is classified into the following six different layers in ascending order; Basal Gravel (BG), Lower Silt-Clay (LC), Middle Sand (MS), Middle Silt-Clay (MC), Upper Sand (US) and Upper Clay (UC). The LC and MC are considered to be the bottom-set bed deposited in the transgression phase. The MS and US are considered to be the foreset or topset bed formed in the regression phase.<br>3) The above mentioned sequence of alluvial deposits shows that there are two large sealevel fluctuations in the post-glacial time in this plain as demonstrated in the plains of Central Japan (Ikeda, 1964).<br>4) In the longitudinal profile, the LC and MC show a wedge-shape structure which is sharp upstream. These layers were traced up to 50km (LC) and 150km (MC) from the river mouth.<br>5) In the cross-section of down-reaches basin, silty clay lenses are recognized on the both sides of main stream and are identified as back marsh deposits which formed in submerged environment during the transgression.<br>6) Geomorphic development of this alluvial plain are summarized as follows:<br>a) The down-reaches of Nagdong River was drowned in the postglacial transgression. This drowned valley was consequently filled continuously with stream deposits supplied from the neighbouring mountain sides and with flood deposits provided from the main Nagdong River sides. The area of water surface was gradually decreased. The back marshes such as Upo, Samolpo etc., are the latest remnants of water surface.<br>b) The delta plain formed on the river mouth is made up of three different geomorphic units, that is, the main delta, the lower delta and the marginal marsh. In the bay, the main delta was formed by bifurication of main stream resulting in the formation of interdistributary islands. By contrast in the bay mouth which is affected by open sea, the lower delta consisting of beach ridge, swale, and barrier islands has been developed. The development of main delta provided the enclosure of water surface and resulted in the formation of marginal marsh.