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タイトル: 凝灰岩層の粒度組成と鉱物組成の均一性について
その他のタイトル: Homogeneous Characteristics of the Particle Size Distribution and Mineral Composition of a Tuff Bed
著者: 村井, 勇
著者(別言語): Murai, Isamu
発行日: 1962年11月15日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第40冊第2号, 1962.11.15, pp. 333-355
抄録: Tuff beds intercalated in sedimentary formations have been adopted as convenient key beds by field geologists. Pyroclastic materials, such as volcanic ash and dust, blown up in the air by an explosive eruption are spread over an extensive area and settle down in a short time, resulting in a continuous sheet of ash fall deposits. When such a continuous sheet of pyroclastic materials is intercalated as a tuff bed in a pile of sedimentary strata, it is useful as a horizon-marker which represents a certain horizon in a geologic section. It shows good continuity and homogeneity within its length and width, regardless of facies change of adjacent strata. A tuff bed is usually homogeneous in the characteristics of the particle size distribution and in mineral composition. Therefore, the identification of a certain tuff bed buried in a pile of different kinds of deposits may be carried out on the basis of its homogeneous characteristics of texture and composition. In fact, ash fall deposits produced by a volcanic explosion show regular variations in texture and composition according to the distance from the source, and the effect of an eolian differentiation during transportation. Such regular variations may be recognized pronouncedly when an extensive examination of the deposits is performed. A small part of the whole stretch of the deposits, however, may exhibit no significant variation in texture and composition. Especially, the deposits far from the source may show practically homogeneous characteristics over a fairly long length. Consequently, it is considered that a tuff bed intercalated in sedimentary formations holds its homogeneous characteristics of the texture and composition within a certain limited area. The present paper shows the result of an examination on such homogeneous characteristics of the particle size distribution and mineral composition of a tuff bed. The writer took up a tuff bed in the Pliocene formations distributed on the Boso and Miura peninsulas as an example. 13 samples were collected from its 20 km length in E-W direction extending in the western half of the Boso Peninsula. They showed quite similar characteristics of the particle size distribution and mineral composition. Three extra samples were collected from the E-W 25 km stretches of that bed distributed in the eastern half of the Boso Peninsula and in the Miura Peninsula. They also showed no significant variation in texture as well as in composition. Consequently, it has been clarified that the tuff bed maintains homogeneous characteristics along its 70 km stretch in E-W direction. In addition, for the purpose of comparison the writer collected 5 samples from other tuff beds which are situated in the horizons adjacent to the above- mentioned tuff bed, and which appear to have similar characteristics of texture and composition. But he could find none having practically similar characteristics to that tuff bed.
URI: http://hdl.handle.net/2261/12054
ISSN: 00408972


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