北海道北部，天塩中川地域の知良志内川上流域における白亜系層序

概要

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A stratigraphy of the Cretaceous system distributed in the upper reach of the Chirashinai River in the Teshionakagawa area,northern Hokkaido,Japan,is revised. In the study area,the Cretaceous system is divided into the Sorachi Group and Yezo Supergroup.The former consists mainly of basaltic volcanic and volcaniclastic rocks, cherts, and siliceous mudstones containing felsic tuffs. The Yezo Supergroup is composed mainly of terrigenous rocks containing tuffs and is stratigraphically subdivided into the Kamiji, Moehoro and Shirataki formations in stratigraphically ascending order. The relationship between the Sorachi and Lower Yezo groups is conformity on the basis of a gradual shift from siliceous mudstones and felsic tuffs in the uppermost Sorachi Group to terrigenous mudstones and felsic tuffs in the lowermost Lower Yezo Group. Meanwhile, the Moehoro Formation occupies the lower portion of the Middle Yezo Group, and conformably overlies on the Kamiji Formation of the Lower Yezo Group.This result shows the stratigraphic similarity among the Nakagawa, Tomamae, Mikasa and Oyubari areas.However,a clear difference in the lithostratigraphy among these areas indicates that a differentiation or disorganization of the basin has taken place in the early stage of Cretaceous forearc basin formation.
2011-03-30