Chemical Composition of Neogene Muddy Rocks from Chikubetsu Area, Northwetsern Hokkaido, Japan

Abstract

Sedimentary rocks cover the large part of Hokkaido Island and they reserve various kinds of natural resources, such as coal, petroleum, natural gas and porcelain raw materials. But, until today, we have few systematic data on the chemical composition of muddy rocks which are interbedded in the sediments. This paper deals with the chemical and expansion characteristics of the Neogene siltstones from Chikubetsu area, northwestern Hokkaido, and also the geochemical study of depositional environment of the geologic sequence of the area. To get the basic informations on the geochemical and sedimentological studies, seven siltstone samples were obtained at the surface of the ground and additional one sample of fire clay was obtained from the underground mining site. Major elements and expansion properties of these samples were determined. The data were compared with those of average sedimentary rock and average shale reported by F.W. CLARK in 1920, and the following results were obtained; average value of sedimentary Chikubetsu siltstone rock and shaleSiO2% low high P2O5% high low Fe2O3% high low MnO % high low CaO% high low K2O % high low The geochemical study of the depositional environment for each formation was tried using the value of Fe2O3/FeO ratio, and the followings were obtained. fire clay in Haboro Formation…fresh water Kotanbetsu Formation…brackish water Chikubetsu Formation…brackish water Haboro Formation…brackish water other formations intercalating tuffaceous siltstones…volcanic ash containes a large amount of Fe2O3 and could not determine the depositional The value of expansion ratio of a siltstone sample from Haboro Formation reaches up to 5 to 6, therefore, this newly discovered expansion siltstone may be suitable for the practical usage.