松本盆地南部と伊那谷における小坂田ローム層中の浮石層 : 松本盆地の形成過程に関する研究(2)

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  • Pumice Beds in the Osakada Loam of the Southern Part of the Matsumoto Basin and of the Ina Valley, Central Japan

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The Quaternary volcanic ash formations from Mt. Ontake are scattered wide in the central and southern parts of Nagano Prefecture, central Japan, and are called collectively the Shinshu Loam. Among them the Osakada Loam of the Late Quaternary age, once studied and named by K. KOBAYASHI, was reexamined by the present writers, and the results are described here and are compared with those of the former investigators. (1) Twelve pumice beds are distinguished in the Osakada Loam, and: are named, in ascending order, Pm-1', Pm-1A, Pm-1B, Pm-2A, Pm-2B, Pm-3A, Pm-3B, Pm-3C, Pm-3D, Pm-3E, Pm-3F and Pm-3G. (2) Curie temperatures (Tc) of ferromagnetic minerals from eleven pumice beds, excepting Pm-1', are measured. The results are as follows. Pm-1A : 450℃ Pm-1B : 60°, 450℃ Pm-2A : 425℃ Pm-2B : 60°, 415℃ Pm-3A : 60°, 400℃ Pm-3B～3G : 400℃ (3) A pumice bed, called Pm-3 by K. KOBAYASHI and H. SHIMIZU (1962), is really a composite bed, in which four pumice beds, i. e. Pm-3D, Pm-3E, Pm-3F and Pm-3G, are discriminated from each other. Where they were deposited under the lacustrine water intercalating silt and sand beds are distinct. (4) The Ono Conifer Bed, reported as 35,700 ± 1,400 ^<14>C years B. P. by K. KOBAYASHI and H. SHIMIZU (1966), is embedded in the peaty silt beds between Pm-3E and Pm-3F. (5) In the Matsumoto Basin the accumulation of thick gravel beds began at the time of Pm-3A which marked the late stage of the Osakada Loam.

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