PRELIMINARY STUDY OF MARINE TERRACES AND ITS APPLICATION TO ACTIVE TECTONIC EVENTS OF SOUTHEASTERN KOREA

The southeastern coast of Korean peninsula shows well-developed flat surfaces. Geographers and geologists have investigated flat topographic surfaces along the coastline since the 1970s and divided them into three to six steps. In this study, platforms of marine terraces distributed along the 20km coast from Gampo to Suryom village were classified to four steps by using GPS. At Suryom to the south of Gampo, former shoreline angles of four steps are 60m, 46~48m, 17~23m and 4~5m high in order of 4 to 1. Paleo-shoreline angles at Gampo are slightly lower than to the south. On 2nd and 3rd terraces lie unconsolidated beach sediments more than 2m thick. The 3rd terrace is more highly weathered than the 2nd terrace. The late Tertiary bedrock is unconformable below the cover. The highest terrace is not well developed. C14-age of charcoal from sediments above the 2nd terrace was yielded to 12,060±600 (Kim, 1977) and 30,170±160 to 35,730±300 yrs (Chwae et al, 2000). OSL age of deposits on the 3rd terrace has wide range of 17,600±7,900 to 90,000±9,000 yrs (Kwon et al, 1999, Chwae et al, 2000, Cheong, 2001). On the basis of age data, the 3rd terrace seems to have formed during the Last interglacial period (5e). Graphical correlation method indicates ages of 2nd and 1st terraces as 75,000 and 1,500 BP. Uplift rate is 0.34m/ka. At the 3rd terrace of Suryom area, a top-up-to-the-west reverse fault was reported as reactivated fault and cuts unconsolidated beach sediments. Disagreement exists as to whether or not the fault is capable. However, the fault does not offset the lower terrace. Therefore, the reactivated age of the fault is constrained between the last interglacial period and 75,000 yr.