Correlation of the Middle Pleistocene Tephra Beds from the Older Ontake Volcano with Those in the Southern Part of Matsumoto Basin, Central Japan

Abstract

The Matsumoto Basin in central Japan is covered by thick gravel beds, among which the Nashinoki Gravel Formation (NGF) is the oldest. Although the age of NGF is crucial to elucidate the start age of sedimentation in the Matsumoto Basin, its age remained undetermined. In order to determine the age of the NGF, 10 tephra beds (NAT1-7, KRT1-3) intercalated in the NGF and the overlying Nashinoki Loam Formation were investigated. Based on the occurrences and petrography of these tephra beds, along with the chemical compositions of hornblende and orthopyroxene in them, these tephra beds were correlated with some of the tephra beds supplied from the Older Ontake Volcano, the age of NGF being inferred to be 0.78-0.64Ma from the stratigraphic relationships with the dated lavas on the foot of the Older Ontake Volcano.
The NGF can be correlated also with the upper part of the Kokumoto Formation-Chonan Formation or the base of the Kasamori Formation in the Kazusa Group, from the tephra correlation. This correlation suggests that the deposition of the NGF took place during the marine isotope stage (MIS) 19-17 or MIS19-16. Conclusively, the NGF is not the resulted of a single transitional stage from the interglacial stage to the glacial stage, but was formed by the actualization of the geographical feature contrast between the Matsumoto Basin and the Hida mountain range.