Abstract
The Middle Pleistocene Tsumagoi Group, western part of Gunma Prefecture in central Japan is composed mainly of lavas, pyroclastic deposits and lacustrine, fluvial deposits and intercalated with numerous air fall tephra beds. In this study, the three pumice fall beds (KRP, SKP, KKP) which include hornblende mineral so much could be found from the Tsumagoi Group on the southeastern foot of the Azumaya Volcano. The KRP, SKP, KKP were examined in order to correlate with the StPm and KbPm- I, II, III from the Older Ontake Volcano in central Japan, based on their petrographic properties, heavy mineral assemblages and chemical compositions of hornblende measured by microprobe analyses (EDS). Conclusively, the KRP and SKP were correlated with the StPm or KbPm- I, KbPm- II or III in ascending order. The correlation of tephra beds has revealed that the StPm and KbPm- I, II, III are widely distributed in the northeastern side of the Older Ontake Volcano. The ages of these 4 tephra beds were inferred from the stratigraphic relationships with the many dated lavas on the foot of the Older Ontake Volcano. Therefore these tephra beds became valuable marker tephra beds for geochronological studies of the middle Pleistocene in central Japan.
