関東-中部地方の第4紀テフラ研究--とくに中期更新世テフラの重要性 (テフラ--第4紀研究に果たす多様な役割特集号)
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概要
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The purpose of this paper is twofold. One is to review tephra studies in the Kanto and Chubu areas of central Japan, and the other is to identify important problems that remain to be studied. In these areas, there are marny tephra layers, well preserved fluvial and marine terraces, and glacial deposits, that are stratigraphically related to the tephras. Such favorable conditions have stimulated Quaternary researchers and volcanologists, and consequently there have been many tephra studies done in the Kanto-Chubu areas. Tephra study is more advanced in this region than in any other areas of Japan.The tephrostratigraphy of the late Pleistocene is relatively well established (Figure 1). This results from many studies, ranging from those carried out in the 1960's during the time of the KANTO LOAM RESEARCH GROUP (1965), to those being carried out at present. In particular, studies of widespread tephras throughout Japan, initiated by MACHIDA and ARAI's (1976) work on the AT ash in the 1970's, contributed to the determination of stratigraphic relations between distant tephra layers.Now it is necessary to construct a middle Pleistocene tephrochronology of the Kanto area. Widespread tephras should play a significant role. Here, the authors present new data on several recently discovered widespread tephras. The Omachi APm tephra beds (A1Pm-A5Pm), originating in the Hida Mountains at 300-350ka, are widely distributed, from northern Chubu to the north Kanto. A4Pm and A5Pm sandwich a vitric ash layer composed mainly of bubble-walled glass shards, identified as Nuka 1 Ash from the eastern foot of Yatsugatake Volcano. This vitric ash layer probably correlates with the Kakuto pyroclastic flow deposit (Kt-U) in southern Kyushu. Moreover, another vitric ash layer, the Sanada Ash (SnA), is found above A5Pm. A tephra similar to SnA is found on the Oiso Hills in the southern Kanto, and is called the Beige Ash, with an estimated age of 240ka. SnA and the Beige Ash are similar to the Torihama pyroclastic flow deposit (Th) in south Kyushu, suggesting their correlation.