2013 Volume 78 Issue 1 Pages 79-91
The tephra which spreads and deposits in wide areas by volcanic eruption is one of the most important time makers for the paleoenvironmental researches in and around the Japanese Islands. However, the source and eruption age of many tephras is still unknown (Machida 2011). The study area, off Joetsu of the Japan Sea, has great advantage to establish tephrostratigraphy because the various tephras from the Japanese Islands and Ulreung-do Island are probably distributed and because sedimentation rate in the study area is high since the area is situated close to the continental shelf and Japan Alps. This report revealed the tephrastratigraphy off Joetsu based on the identification and correlation of the tephras layers collected from the long core samples obtained by MD 179 survey.
The tephra samples were collected from the six piston cores and two gravity cores collected at geomorphologically different places off Joetsu, and at the submarine valley off Sado Island. Each sample was listed in the mineral composition and the morphologic feature of volcanic glass shards, and the chemical composition of 20 shards of volcanic glass measured by microprobe analyzer (SEM-EDS). The total of 95 tephra samples were classified into 32 kinds of tephras based on the shape and the chemical composition of volcanic glass along with heavy-mineral combination. Then 12 kinds of them were identified and correlated with the well-established tephras, although the source and age of the other kinds of tephra were uncertain.
