Study of Widespread Tephras Derived from Northeast Japan

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The purpose of this study is to recognize widespread tephras derived from Northeast Japan for providing time-markers to the precise Quaternary chronology. Research was done by collecting many samples in distal and proximal areas and describing characteristic properties of them. Results are shown as follows.(1) Two ignimbrites were newly identified in Hakkoda area. Stratigraphy of these ignimbrites and Hakkoda 1^<st> and 2^<nd> pyroclastic flow deposits previously identified was established and characteristic properties of these were determined. Moreover, paleo-magnetic investigation revealed that Hakkoda 1^<st> pyroclastic flow deposits have normal polarity and Hakkoda 0 and Ose pyroclastic flow deposits have reverse polarity. This result is concordant with the correlation of the distal tephra associated with Hakkoda 1^<st> pyroclastic flow.(2) A widespread tephra named Seiganji-Toga Tephra (0.6-0.7 Ma) previously recognized in Boso peninsula is newly recognized in Northeast Japan, and this tephra derived from Kyushu will be a favorable time-marker in this study. Chemical composition of rare earth element in volcanic glass shards shows reliability of the correlation of this tephra from Kyushu to Boso and Oga peninsulas.(3) To investigate widespread tephras from Northeast Japan, comparisons of chemical compositions among tephras including vitric tephras layers and ignimbrites in Northeast Japan were carried out. No vitric tephra correlative to ignimbrites in Hokkaido was found. It is clarified that several properties of Ose pyroclastic flow deposits in Hakkoda area and Ashino pyroclastic flow deposits in Shirakawa area are similar to those of U8 tephra in Boso peninsula. It is difficult to judge which is the tephra correlative to U8 tephra. This problem is left for a further study.