士別市犬牛別川地域の空知層群および下部蝦夷層群の砕屑岩組成とその意義  [in Japanese] Composition of clastics of the Sorachi Group and the Lower Yezo Group in the Inushibetsu-gawa area, Hokkaido, Japan, and their significance  [in Japanese]

空知-蝦夷帯に属する犬牛別川地域(士別市)に分布する空知層群および下部蝦夷層群の岩相や化石年代, 空知層群と下部蝦夷層群との層序関係, 両層群の砕屑岩組成を検討した。空知層群下部は, 幌加内オフィオライトの上部をなし, 同層群上部は, 火山岩源砕屑岩や緑色頁岩から構成される。下部蝦夷層群は, 陸源のタービダイト砂岩よりなる。この地域においては, 幌加内オフィオライトから空知層群上部をへて, 下部蝦夷層群までが整合関係で累重している。空知層群上部の砕屑岩類は, ソレアイトを主とする島弧的な後背地より供給された。その後背地は, 礼文-樺戸帯であった可能性が高い。下部蝦夷層群の主な供給源は, 渡島帯のジュラ系付加体や花こう岩類であったと推定される。空知層群上部の堆積から下部蝦夷層群の堆積に移行する段階に, 火成弧が背後に退き, 渡島帯が広域的に上昇した。

The Inushibetsu-gawa area in the Sorachi-Yezo Belt, Hokkaido, isoccupied mainly by the Horokanai ophiolite and Upper Jurassic-Lower Cretaceous sedimentary strata. The upper part of the Horokanai ophiolite is assigned to the lower part of the Sorachi Group. The sedimentary strata form the upper part of the Sorachi Group and the Lower Yezo Group. The upper part of the Sorachi Group is made up of volcanogenic sandstone/siltstone, tuff and green shale. The Lower Yezo Group comprises alternating beds of sandstone and black shale with acid tuff layers. Fossil evidence and field observations suggest that the upper part of the Sorachi Group conformably overlies chert of the Horokanai ophiolite and is overlain conformably by the Lower Yezo Group. Geochemical signatures and a microscopic examination of sandstone, tuff and clino-pyroxene fragments in sandstones are indicative of vigorous, and esitic to basaltic volcanism in the provenance area from which the upper part of the Sorachi Group were derived. The provenance area must have been an Early Cretaceous volcanic zone, the Rebun-Kabato subbelt in the Oshima Belt. On the other hand, the sandstones of the Lower Yezo Group are considered to have been derived from a provenance area composed of older sedimentary, granitic and volcanic rocks. The lithological features of the provenance area of the Lower Yezo Group correspond to those of the Oshima Belt which is constructed of Jurassic accretionary complex, and Early Cretaceous granitic and volcanic rocks. The abovementioned results indicate that abroad upheaval of the Oshima Belt took place concurrently with the shifting of depositional stages from the upper part of the Sorachi Group to the Lower Yezo Group.