福島県磐梯火山のテフラ-ローム層序と火山活動史

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The petrography and stratigraphy of tephra and loam deposits at Bandai volcano, Fukushima Prefecture have been studied. The tephra-loam association in this area consists of the Hayama and the Mineyama Loam Formations. A widely developed erosional contact between the two units indicates local topographical change. Twenty five layers of tephra are recognized in the Mineyama Loam Formation, and nineteen layers in the Hayama Loam Formation. Among the forty four combined Mineyama and Hayama Loam Formation's tephra deposits, eighteen layers are from distant volcanic eruptions in Central and Southern Japan, Kyushu, and nearby Sunagohara, Numazawa, and Adatara volcanoes. Most of those tephras contain hornblende phenocrysts and acid volcanic glass shards, with or without biotite, and quartz. In contrast, the twenty-six remaining tephra deposits from Bandai volcano contain no hydrous minerals. Based on published age data, for those tephra, the age of the Loam Formations range from 0.5 Ma to the present. The age of the boundary between the Mineyama and the Hayama Loam Formations is about 0.07 Ma. The depositional rate of the loam shows substantial correlation against time. The depositional rate is 12 mm/ky during the middle Pleistocene, and 18 mm/ky (r^2=0.999) in the late Pleistocene. The variation in rate can be attributed to the compaction effect or difference in accumulation rate. The rate of the sedimentation is nearly constant, susuesting that the loam sedimentation is a type of secular phenomena, such as aeolian dust deposition. Tephra from Bandai volcano is characterized by the following two stratigraphic associations; 1) scoria followed by basic ash in the Mineyama Loam Formation, and 2) pumice followed by andesitic ash in the Hayama Loam Formation. At least two repeated associations are both present in the two periods. The ejecta of the repeated associations within each Formation is petrographically similar. It suggests that the magma differentiation system have been changed between the boundary of the Formations. The change seems to be related to tectonic movement which have resulted in widely developed erosional contact between the two Formations At least five debris flow deposits are intercalated in the tephra-loam associations. The debris flows might have been triggered by volcanic activity. Because the debris flows horizons are concentrated at the beginning and end of each eruption phase.
地学団体研究会の論文
1994-05-25

福島県磐梯火山のテフラ-ローム層序と火山活動史

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