THE GEOMORPHIC HISTORY OF THE SOUTHEASTERN COAST OF THE KOREAN PENINSULA

Abstract

The study area, from Janggigok to Dangweol, is located in the southeastern part of the Korean Peninsula (Fig. 1). Marine terraces distributed along the coast of the area were classified into three levels according to their altitude and continuity (Fig. 3); 1) the high terrace (Gampo surface), 2) the middle terrace (Seakcheonri surface), and 3) the low terrace (Sanhari surface). The author investigated these terraces in order to clarify the geomorphic history of the area. The main results are as follows; 1) The high terrace (elevation 60 to 80m) is scattered throughout the area. This terrace surface is mostly an abrasion bench in origin occurring on the top of low hills and ridges which are composed of Cretaceous and Tertiary series (Fig. 2), and are partly covered with highly weathered gravel beds whose facies are of marine origin. At the type locality of Gampo (Fig. 4), its surface consists of weathered gravel beds more than 3m thick of wellsorted fiattish and rounded pebbles. Red soils and red weathered crusts, indicating past warm climatic conditions, are found on the surface of this terrace. 2) The middle terrace is the most widely and continuously distributed among the terraces in the area. The former shoreline on this terrace is 30 to 50m in height. The terrace surfaces is composed of well-sorted gravel beds filling small relief of bed rocks and in places overlying unconformably the weathered gravel beds of the high terrace. These gravel beds are mainly of marine origin, but at some places, their facies change from fluvial to marine from lower to upper. Soils on this terrace is pale orange in color. 3) The low terrace (elevation 10 to 20m high) is distributed as continuously as the middle terrace, especially conspicuous on headlands along the coast. The surface of this terrace is covered with veneers of fine-medium pebbles overlying the lower gravel beds of the middle terrace. 4) From the displacement of the former shoreline on the middle terrace, it was inferred that the Bulgooksa fault, whose axis was NNW-SSE from Bulgooksa to Ulsan Bay, was active after the formation of the middle terrace, which was dislocated by about 10m at Ulsan Bay. However, it was inactive after the formation of the low terrace (Fig. 6). The geomorphic history of the study area can be summarized as follows; a) The surface of the high terrace was formed during the period when the dissected low hills and ridges were submerged. b) By the transgression succeeding to the regression, the surface of the middle terrace was formed. Red soils and red weathered crusts on the surface of the high terrace were formed in this period. After the formation of this terrace, the Bulgooksa fault was active and the middle terrace was dislocated at Ulsan Bay. c) During a temporary cessation of the regression, the surface of the low terrace was formed by marine abrasion. d) The submerged delta (40 to 70m deep) located in Yeonil Bay (Fig. 4) was formed by the extended Hyeongsan River when sea level dropped below 70m deep during the Worm glacial stage. e) The Holocene terrace and beach ridges (elevation 1 to 3m) which were distributed poorly along the coast were formed by the post-glacial transgression.