Abstract
Defining the distribution of wide-spread Quaternary-Pliocene tephra deposits in the Chubu District has been enhanced by study of variations in mineral composition, refractive indices of volcanic glass and phenocrysts (orthopyroxene and hornblende), and major element chemical compositions of volcanic glass shards. The results have been used to differentiate the various tephras. But distinguishing strongly weathered tephras is very difficult because major-element chemistry of volcanic glasses is affected by weathering, thus hampering analysis.
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 sampled the weathered tephras in the Toki Sand and Gravel Formation located on the Nenoue Highland, in the hanging wall of the Byobuyama Fault. The Toki Sand and Gravel Formation consists mainly of gravel beds with intercalated clay-sand beds in the lower and middle horizons. Glass inclusions in the quartz and plagioclase occur only in the weathered clay-sand beds and not in the gravel beds.
The occurrences of the Toki Sand and Gravel Formation in several small, isolated basins in the Tokai District could not be correlated using lithostratigraphy of key tephras. However, when the major-element analysis of glass inclusions in quartz and plagioclase of the Toki Sand and Gravel Formation on the Nenoue Highland were compared with those of wide-spread Quaternary-Pliocene tephras in the Chubu District, we found that the Toki Sand and Gravel Formation on the Nenoue Highland were compared with those of wide-spread Quaternary-Pliocene tephras in the Chubu District, we found that the glass inclusions in the quartz in the middle horizons of the Toki Sand and Gravel Formation provide the best correlations with those in the Minamidani1 tephra deposits in the Kinki, Chubu and Shin-etsu Districts.
