2001 Volume 40 Issue 1 Pages 29-41
The Iizuna-Kamitaru Tephra Group, which consists of three distinctive fallout tephras (Iz-KT a to Iz-KT c in descending order), erupted from the Iizuna volcano, a Quaternary stratovolcano of the Myoko Volcano Group in central Japan. The purpose of this paper is to describe the characteristic properties, distribution, and stratigraphic position in several areas of this group, to discuss its age, and to show the possibility of applicative studies. Iz-KT a (9.8km3), the most widespread tephra of the group, is a plinian deposit distributed in the boundary area between Nagano and Niigata prefectures, north part of Gunma and Tochigi prefectures. Iz-KT a is petrographically identified by min eral composition, characteristic shape of orthopyroxene, refractive indices of orthopyroxene (γ: 1.707-1.716) and hornblende (n2: 1.673-1.687), and chemical composition of titanomagnetite (FeO: 82.0-82.5%, TiO2: 5.9-6.0%). Stratigraphic positions of the Iizuna-Kamitaru Tephra Group in the tephra sequence of distal areas and radiometric ages by the K-Ar dating method carried out in proximal areas show that the deposition of the group occurred between 150ka and 125ka of the transition from the marine isotope stage 6 to 5. This group is useful for identification of climatic fluvial terraces formed in the glacial stage of marine isotope stage 6, and provides a datum plane for eruptive histories of several Quaternary volcanoes in this area.
