Tephra stratigraphy of Toki Sand and Gravel Formation on the Nenoue highland; Correlation of widespread tephra deposits by major element chemistry of glass inclusions in quartz

Furusawa, Akira*; Yasue, Kenichi; Nakamura, Chisato*; Umeda, Koji

Distinguishing strongly weathered tephras is very difficult because major-element chemistry of volcanic glasses is affected by weathering. On the other hand, it recently became clear that major-element analysis of glass inclusions in quartz and plagioclase is an effective method of investigating strongly weathered tephras. We studied the weathered tephras in the Toki Sand and Gravel Formation (TSG) located on the Nenoue highland, in the hanging wall of the Byobuyama fault. The TSG consists mainly of gravel beds with intercalated clay-sand beds in the lower and middle horizons. Glass inclusions in the quartz occur only in the weathered clay-sand beds. We found that the glass inclusions in the quartz in the middle horizons of the TSG provide the best correlations with those in the Minamidani-1 tephra deposits in the Kinki, Chubu and Shin-etsu Districts.