日高帯中の川層群北部ユニットの堆積層序,堆積環境および堆積年代  [in Japanese] Sedimentary stratigraphy, environment and age of the northern unit of Nakanogawa Group in the Hidaka belt, central Hokkaido, Japan  [in Japanese]

日高帯, 中の川層群北部ユニットは, 尾田メランジ, 坂下層およびヤオロマップ川層に区分される。ヤオロマップ川層の堆積年代は, その放散虫化石群集から暁新世〜始新世前期, 尾田メランジは暁新世(前期?)と推定される。坂下層から得られたFT年代は, 始新世前期に対比されるが, この年代値は他の日高累層群から報告されている始新世の放散虫化石年代とは矛盾しない。ヤオロマップ川層と尾田メランジを比較した場合, 後者の方が中〜塩基性火山岩片, 砕屑性単斜輝石, 普通角閃石が卓越する傾向がある。しかし, 基本的な岩片組成の構成要素は類似している。このことは, 両者の供給源は主として古千島弧およびその周辺であったことを示唆する。両者の砂岩組成の違いは古千島弧の開析度の差によるものと解釈できよう。尾田メランジは古千島弧側への付加体, ヤオロマップ川層は尾田メランジ付加後に形成された海底扇状地堆積体と推定される。

The early Paleogene Nakanogawa Group distributed along the eastern foot hills of the Hidaka mountain range belongs to the Hidaka belt, northern Japan. We obtained some new fossil ages and fission track ages for the group. This group is divided into two tectonic units; northern and southern units bounded by the Nupinaigawa fault. On the basis of data of sedimentology, structural geology, sedimentary petrology and geochronology, the northern unit is further divided into three sedimentary bodies; Oda Melange, Sakashita and Yaoromappugawa Formations according to east to west. These sedimentary bodies are bounded by faults. The Sakashita and Yaoromappugawa Formations are made up solely of coherent (turbidite) facies, often interbedded with acidic tuff layers, on the other hand the Oda Melange is made of broken formation with hemipelagic red and green mudstones. These sediments were provided undissected to transitional arc region (Paleo-Kuril arc?) with basic to intermediate volcanic rocks, grahitic rocks, ultra basic rocks and sedimentary rocks from accretionary complex. Radiolarian fossils from the Oda Melange suggest the (early?) Paleocene age, equivalent to the age of the Hiroo Complex. Radiolarian fossils from the Yaoromappugawa Formation suggest the Paleocene to early Eocene age, younger than the Hiroo Complex. Fission track ages from Sakashita (northern unit) and Kamitoyoni Formations (southern unit) suggest the early Eocene (48〜50 Ma). Oda Melange must have accumulated at first near the Paleo-Kuril trench and later incorporated into an accretionary complex in the subduction zone. Then, Yaoromappugawa Formation must have been formed as a submarine fan near the Paleo-Kuril trench area later.