中国・四国地方の第四紀テフラ研究

広域テフラを中心として

抄録

A tephrochronological study has been carried out in the Chugoku and Shikoku Districts, paying special attentions to the widespread marker tephras from Mts. Sanbe and Daisen, as well as tephras from volcanoes in Kyushu. Chemical analysis of the major element composition of glass shards and/or magnetite grains collected from each tephra layer, using the electron microprobe technique, has been quite useful in identifying the individual tephra. The stratigraphy, chronology, and distribution of tephras, and their implications for Late Quaternary events are summarized as follows: (1) Aira Tn (AT; erupted about 25, 000BP), and Kikai Akahoya (K-Ah; erupted about 6, 300BP) ashes, of Kyushu origin, occur as the most distinctive markers. Both are easily identified by the major element composition of their glass shards. The Aso-4 ash is another maker tephra of Kyushu origin, distinguishable by the major element composition of its magnetite. (2) Of the many tephras erupted from Sanbe volcano, the Sanbe Kisuki (SKP), Unnan (SUn), and Ikeda (SIP) pumice layers are the most extensive and representative. The SUn and SIP pumice layers are identified in a marine core taken in the Japan Sea to the north of Tottori, 200km away from Mt. Sanbe. (3) Of the Daisen tephras, the Matsue (DMP), Kurayoshi (DKP) and Kusadanihara (KsP) pumice layers occur most extensively. KsP, of ca 18ka, was recognized in a marine core from the Japan Sea to the west of Sakata City, 600km away from Mt. Daisen. This correlates with the San'in 2 ash. (4) The followings are evident from the relationship of tephrochronology and geomorphic development in the Chugoku and Shikoku Districts: maximum sea level regression in the Late Pleistocene had occurred before the deposition of AT ash in the Izumo Plain; there were three stages of dune sand formation in the San'in District: the first was prior to the deposition of DMP, the second was between the eruptions of DMP and Aso-4, and the third was after the deposition of K-Ah; (5) Some of the unidentified tephras were examined and showed that an ash layer sampled from Pleistocene gravels near Ozu City, Ehime Pref., and one from the lowermost part of the Oetakayama pyroclastic formation in the Tsunozu Group are similar to the Guminoki ash and Yellow III of the Osaka Group respectively, from the viewpoint of the major element composition of glass shards or glassy matrix.