Stratigraphy and Correlation of the Younger Ontake Tephra Group: An Implication to the Late Pleistocene Chronology in Central Japan

Ontake Volcano was one of the most active volcanoes in Japan, producing many extensive sheets of tephras during the late Pleistocene. These sheets are very useful for the cor- relation and chronology of the late Pleistocene deposits distributed in central Japan. The authors established the stratigraphy of the Younger Ontake tephra group by careful investigation along the Kiso valley and on the eastern foot of Ontake volcano. The Younger Ontake tephra group is classified into 17 formations by buried soils and intervening unconformities. In addition, the tephra ages are estimated using the radiometric ages of related sediments, the stratigraphic positions of widespread marker tephras, and the geologic histories of well-investigated areas in central Japan. The following are the new results obtained from this investigation. 1) Among the many marker tephras originating from the Ontake volcano, Katamachi pumice layer (On-KtP) extends to the Takada coastal plain on the Sea of Japan, Ina pumice layer (On-Inp) to southern Kanto, Nagawa pumice layer (On-NgP) to the northern part of Gunma Prefecture and Tatsuno pumice layer (On-Ttp) to the Nobi plain. Therefore, these extensive pumice layers make it possible to correlate the geomorphic surfaces and the late Pleistocene deposits which are distributed in areas separated by great distances. 2) The upper part of the Atsuta formation in the Nobi plain is correlated to the upper part of the Katamachi sand bed in the Takada plain. These two beds were deposited during the marine regression stage from 90, 000 to 60, 000 years ago, after the culmination of the last interglacial transgression. 3) Kisogawa mudflow deposits from the Ontake and Komaki gravels in the Nobi plain are considered to have been accumulated about 45, 000 years, which makes them much older than the previously estimated age of 27, 000 years. 4) The maximum phase of the Kumazawa glacial stadial in the Central Japan Alps is estimated to be older than 57, 000 years.