北海道大夕張地域の白亜系層序
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概要
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The Cretaceous System along the Shuparo and Hakkin-zawa rivers in the Oyubari area, central Hokkaido, is characterized by deep-water muddy and turbiditic facies, and is divided into four groups, the Sorachi, Lower Yezo, Middle Yezo and Upper Yezo groups, in ascending order. The Sorachi Group is composed exclusively of siliceous rocks within this area. The Lower Yezo Group is subdivided into two formations, the Tomitoi and Shuparogawa formations. The former is dominantly sandstone with lesser alternating beds of mudstone, while the latter is dominantly mudstone, with alternating beds of sandstone, and including allochthonous limestone blocks. The Middle Yezo Group is subdivided into three formations, the Maruyama, Hikagenosawa and Saku formations. The Maruyama Formation is dominantly sandstone, alternating with beds of tuff and mudstone, the Hikagenosawa Formation is laminated mudstone, and the Saku Formation is also mudstone-dominated, with alternating beds of thickening-upward sandstone and mudstone. The Upper Yezo Group is composed of one formation, the Kashima Formation, characterized by monotonous mudstone. Ammonoids and inoceramid bivalves are abundant, and they suggest that the uppermost part of the Shuparogawa Formation to the Kashima Formation are correlated to the Upper Albian to Coniacian stages.
- 2000-06-22