千曲川上流地方の第四紀地質(その2)

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The surveyed area is situated in the north-eastern foot of Yatsugatake Volcano on the upper stream of the Chikuma river. The loam in this district is devided by uncomformity into four; the upper and the lower parts of the older loam, the middle loam, and the younger loam. The lower part consists mainly of the eolian sediment of the Oishigawa scoria, the older ejecta of Yatsugatake Volcano. On the contrary, the lake deposit of the scoria is called the Oishigawa formation. Moreover, the scoria falls on the Kanto district and the lower part of the Tama loam is composed of it. The white pumice of the lower part covers widely from this district to Hanno city, 45 km north-west to Tokyo and it is, therefore, a useful key bed. The upper part of the older loam consists not only of the ejecta of Yatsugatake Volcano, but also of the volcanic ejecta from Central Japan, west to Yatsugatake Volcano. These materials make also the upper part of the Tama loam in north-western Kanto. Especially, the three pumice beds (B_1 B_2, B_3) of the upper part, though the ejecta of volcanoes in Central Japan, are widely deposited to northwest Kanto and are therefore useful as key beds. After these volcanic activities, Yatsugatake Volcano was erupted pyroclastic flows several times and on all such occassion to Chikuma river was dommed up, forming lake and marsh. By this study, the older loam in the foot of Yatsugatake Volcano was correlated to the Tama loam in the Kanto district, which has been regarded as type area of the Quaternary tephrochronology in Japan, and consequently the tephrochronology and geological history of the early Pleistocene in the foot of Yatsugatake Volcano was clarified.
地学団体研究会の論文
1968-03-25

千曲川上流地方の第四紀地質(その2)

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