地向斜の火山性物質に伴う堆積物

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As is exemplified by geosynclinal rock assemblages of various regions, home and abroad, chert and/or limestone occur in close association with submarine volcanic rocks. The chert and limestone, which are often underlain by the volcanics, are better developed when they are associated with palagonitized, amygdaloidal lapillituff and finer tuff of basaltic composition rather than with pillowed basaltic lava, coarse-grained tuff-breccia, and acid volcanic rocks. Regarding the source of silica and Ca-ions the writer's particular attention is focussed to the supply of a large amount of SiO_2, Ca^<++>, Na^+, etc. to the sea-water through the palagonitization of basalt glass. In the light of up-to-date knowledge it is noted that the explosive volcanic activity hardly occur in the deep sea below 500 m. The volcanic eruption would give rise to a row of submarine mounds and other oceanographic changes. The upwelling of the deep water would bring a large a mount of nutritious material for microorganisms. Thus, a certain kind of submarine volcanic activity, with its ejecta, probably provides favourable conditions, not only materially but also ecologically for flourishing of siliceous and probably also calcareous organisms, which, in turn, are primarily responsible for the accumulation of siliceous and a certain kind of micritic calcareous sediments. Several authors have interpreted that chert is formed by chemical precipitation of silica gel or gelatinous silica colloid by a rapid reaction between the sea-water and the hot magmatic material in the deep-sea at the time of eruption. This current hypothesis is not tenable in view of the recent knowledge on the nature of submarine basaltic activity, which, in turn, suggests that a chemical reaction between the two substances in most cases proceeds slowly and continuously after the chilling of the surface of the erupted material. On the basis of data obtained from the writers study of some Palaeozoic and Mesozoic cherts of Kyushu and Shikoku as well as those in published reports on the siliceous rocks of Phanerozoic ages and also on the recent sediments, the following conclusions are led: 1) Most of the cherts, normally occurring as bedded form, are primarily sediments of siliceous organic tests and their debris. 2) Nodular chert in limestone is of there placement products formed by the early diagenetic processes in carbonate sediments which contain disseminated siliceous organic remains. 3) The conversion of amorphous silica to quartz or chalcedony often destroys largely or almost completely the original structure of organic opaline tests and primary sedimentary textures, resulting in apparently non-fossiliferous, equigranular, microcrystalline chert. 4) Remarkable epidiagenetic recrystallization and deformation make the bedded structure of nomal chert obscure and finally give rise to a massive body of chert. 5) There is also massive chert of another type, although of local and minor occurrences, which seems to be related to chemical precipitation from silica-rich solutions under very restricted, special conditions. Although the writer expects a similar mode of formation for the so-called micritic limestone in a geosynclinal assemblage, a discussion in detail is omited in this paper. The conditional difference between the accumulation of siliceous organic remains and that of calcareous ones should be explained by further study.
日本地質学会の論文
1968-07-31

地向斜の火山性物質に伴う堆積物

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