Geomorphic Development of the Pohang Coastal Plain

Abstract

Pohang coastal plain, located along the Yeongil Bay, southeast coast of Korea Peninsula (Fig. 1), has marine and fluvial terraces, beach ridges, natural levee and back marsh. The writer tried to clarify the geomorphic development of this area.Marine terraces along the coast of Yonghan-Ri are classified into three levels according to their altitude and continuity, namely surfaces I, II, and III in deceding order. These surfaces are originally abrasion benches cut into Tertiary bedrock, and covered with terrace veneers of gravel and sand bed of marine origin. Surface I is dissected by shallow channels and its surface soil is a little reddishly weathered. But surfaces II and III are scarcely dissected.Fluvial terraces along the Neng-Cheon River are classified into three levels according to their relative height above river bed and continuity, namely surfaces I, II and III in desending order. Surfaces II and III are submerged by alluvium near the coast.Two rows of beach ridges are recognized along the shore line. The outer one is continuous and relatively higher and the inner one is uncontinous.Three groups of the natural levees are recognized along the Hyungsan River. The first group is scattered at the north side of river. The second group is well continued at the south side of river. The third one has been formed by the present course of Hyungsan River.The writer measured radiocarbon dates of 7 samples from boring core and alluvium outcrops (Fig. 5), and analyzed grain side distribution of deposit from boring core No. 13 (Fig. 6). Based on the radiocarbon dates and grain size distribution, the writer drew the transition curve of sedimental environment in Pohang area (Fig. 7).On the consideration of land classification and transition curve of sedimental environment, experienced the geomorphic development this coastal plain as follows:1) The marine terrace surface I was formed at the high sea level during Last Interglacial. Surfaces II and III were formed at any stable period of lowering sea level to Würm or at the relatively high sea level in Würm Glacial.2) The fluvial terrace surface I was formed at the same period of marine surface III formatioon. Surfaces II and III were formed at Würm maximun or at any stable period of rising sea level of Post Glacial.3) Concerned with the Post Glacial word wide transgression, the sedimental environment in Pohang coastal plain changed as follow:There was a stabe period about 10, 000 y. B. P. in rising sea lfvel of Post Glacial transgression, when the old valley of Neng-Cheon River was submerged and deposited by lower gravel bed (MG) shown at 20m below sea level in boring column.The sea level about 6, 000 y. B. P. was fairly close to the present sea level, and since then it has been relatively stable with slight oscillation until the present, and natural levee and beach ridge have been formed during that time.