鉱物組成からみた沖積層の堆積環境

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There has been little information about mineralogical or geochemical characteristics of Alluvial deposits in Japan. We once studied the sedimentary environment of the sediments distributed in subsurface of Tokyo area for the investigation on the cause of the accidents due to oxygen deficiency in underground work. In this report, recent knowledges on sedimentary environment of Alluvium and Pleistocene series are reviewed from mineralogical standpoint chiefly using the data in our investigation. Iron minerals are given particular attention in field of sedimentology. According to specific environmental conditoin, different minerals of the iron family are deposited in sediments. Although various parameters such as salinity, temperature, microbial activity, pressure and so on, determine the development of the prospective end-members, the Eh-pH relationship is the most decisive factor in low temperature and low pressure environments at or near the surface of the earth. Sedimentary rocks may be divided into four major facies-oxide, silicate, carbonate and sulfid~on the basis of the dominate iron minerals. This facies classification has been theoretically established by the distribution of stability fields of the iron species expressed most effectively in terms of Eh and pH. The facies are a direct reflection of sedimentary environments. Clay minerals record the physical and chemical conditions under which they were formed. They, once formed, are sensitive to subsequent changes in physical and chernical environments of their host rocks. Each stage of diagenesis is characterized by the structural change of clay minerals and their polytype. Clay mineral is one of the indicators of each stage of diagenesis. According to the crystallo-chemical mechanisms of the diagenetic change of clay minerals, they are transformed by movement of ions from the intersitial solutions towards the crystalline lattices and complex cation exchanges in the octahedral and tetrahedral layers. At the end of this evolution, micas and chlorites are the common stable layer silicate. The compositions of interstitial water may be a function of chemical and physical characteristics of the mineral phases with which it is in contact. An important indicator of each stage of the diagenesis is organic matter which as a rule closely correlates with the alteration of layer silicates. It is generally thouht that as a result of interaction between minerals and interstitial water, formation of new minerals and redistribution and recrystallization of substances in sediments take place during diagenesis.
日本地質学会の論文
1972-12-25

鉱物組成からみた沖積層の堆積環境

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