近畿地方中部に分布する後期更新世〜完新世の火山灰層について

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Late Pleistocene to Holocene sediments in central Kinki district, which are mainly distributed in separated basins, i.e. Osaka, Kyoto, Nara and Omi Basins, are composed of unconsolidated gravels, sands and clay with frequent intercalations of volcanic ash layers. In this paper, the volcanic ash layers found in these sediments are lithologically and petrographically investigated, and correlation of volcanic ash layers is discussed based on the stratigraphic position, and lithologic and petrographic properties. The study has revealed that five volcanic ash layers are widely traceable and are available as stratigraphic time-markers. They are Heianjingu and Kitoragawa volcanic ash layers in the Lower Terrace deposits (late Pleistocene), and Sakate, Minato and Yoko-oji volcanic ash layers in the Alluvial deposits (latest Pleistocene to Holocene), in ascending order. Heianjingu volcanic ash layer which is one of the most useful key beds, is intercalated in the upper part of the Lower Terrace deposits and its correlatives. This layer about 10 cm thick is composed of yellowish white to pinkish white and medium to coarse grained ash. It consists chiefly of glass (mainly H-type shards with index of 1.498-1.501; mode 1.500), with trace amounts of plagioclase, orthopyroxene (γ=1.702-1.735), amphibole and clinopyroxene. Kitoragawa volcanic ash layer which occurs about 10 cm above Heianjingu ash layer, is composed of white and fine to medium grained ash. The average thickness is about 1 cm. This ash consists of phenocrysts and T-type glass shards with index of 1.502-1.504 (mode 1.503). The commonest phenocrysts are of plagioclase, followed in order of abundance by amphibole and orthopyroxene (γ=1.700-1.710; mode 1.704-1.708). Sakate volcanic ash layer intercalated in the lowermost part of the Alluvial deposits, is about 10 cm thick, and is composed of yellowish white and medium to fine grained ash. This ash consists of phenocysts, and C- and T-type glass shards with index of 1.498-1.504 (mode 1.500). The phenocrysts are of plagioclase and amphibole (mainly green hornblende), with a few crystals of biotite. Minato volcanic ash layer intercalated in the lower part of the Alluvial deposits, is about 2 cm thick, and is composed of white to grey and fine to medium grained ash. It consists of T-type glass shards with index of 1.521-1.526 (mode 1.522-1.523) and phenocrysts. The phenocrysts are of feldspar, with a few crystals of biotite, amphibole and clinopyroxene. Yoko-oji volcanic ash layer which is one of the most useful key beds, is intercalated in the middle part of the Alluvial deposits. This layer about 5 cm thick is composed of light red to yellowish brown and medium grained ash. It consists mainly of pale brown H-type glass shards with index of 1.507-1.515 (mode 1.510-1.512), with trace amounts of plagioclase, orthopyroxene (γ=1.700-1.717; mode 1.705-1.711), clinopyroxene and amphibole.
地学団体研究会の論文
1986-01-25

近畿地方中部に分布する後期更新世〜完新世の火山灰層について

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