GEOCHRONOLOGY OF THE NORTHEAST COAST OF THE HARIMANADA IN THE EAST OF SETO INLAND SEA, SOUTHWEST JAPAN

Abstract

In order to establish a geochronological standard during the late Quaternary in the inner zone of Southwest Japan, this study investigates the marine and fluvial terraces along the northeast coast in the eastern part of Seto Inland Sea, i.e., Harimanada. The results obtained are summarized as follows: 1. Marine terraces with six different levels develop: they are Meimi I, Yamatedai, Kane-gasaki, Uozumi, Nishiyagi, and Maiko terraces in descending order. Fluvial terraces having two levels also develop: Meimi II and Ikawadani terraces (Fig. 6). The deposit of each marine terrace is about 10m thick and suggests that the marine terraces in this area were formed at the transgressional period during the Late Quaternary (Fig. 5). 2. The Meimi I terrace is considered to have been built during interstadial about 170ka (1 ka=1, 000yr B. P.) on the basis of the absolute age of the volcanic ash embedded in the terrace deposits (Fig. 14). The Yamatedai, Kanegasaki, Uozumi and Nishiyagi terraces were built during the last interglacial about 120ka and subsequent interstadials in the Late Pleistocene. The estimation of these ages is based on the characteristics of their terrace deposits and the sequence of geomorphological development of this area. The Maiko terrace is presumed to be the Holocene marine terrace judging from the height distribution. 3. The fan shaped development of the Meimi II terrace indicates that the terrace was formed during the regressional period toward the low stand of sea level at Riss Glacial, i. e., during the period between 170ka and 120ka. Ikawadani terrace is a river terrace which is dated back to about 10ka in 14C age.