西南北海道・島牧村大平山周辺の中生界

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The Mesozoic system, newly named Tomarigawa Group, is distributed in Shimamaki area, Southwest Hokkaido. It is divided into following two formations, namely the Kanayamagawa and the Garozawagawa Formations in ascending order. The Kanayamagawa Formation is divided into three members, that is the Lower, Middle and Upper Members. The Lower Member is composed mainly of shale and shaly flysh (alternations of sandstone and shale), associated with chert, limestone and basic tuff. The Middle Member is characterized by the dominance of chert. The Upper Member consists of flysh, associated with chert, pebbly shale, conglomerate and thick arkosic sandstone. This formation yeilds some Upper Jurassic radiolarians and Upper Triassic conodonts from cherts. The total thickness exceeds 1100 meters. The Garozawagawa Formation, conformably overlying the Kanayamagawa Formation, is divided into the Upper and Lower Members. The Lower Member is composed mostly of limestone, basic tuff and chert, associated with alternations of chert and shale. The Upper Member consists of greenstones and chert, associated with limestone, shale and arkosic sandstone. Greenstones are made up largely of reworked hyaloclastite (hyaloclastic or epiclastic volcanic breccia, sandstone and siltstone), but include small amounts of pillow lava and massive lava. Some fossils such as calcareous algae, hexacorals and Upper Triassic conodonts have been obtained from limestones of this formation. However, these limestones are largely conglomeratic, or blocks of various size, which are included in greenstones. Such feature described above suggests that limestones of this formation include partly allocthonous ones. Based on radiolarian fossils obtained from chert and shale, the age of this formation seems to range from Upper Jurassic to early Cretaceous. The total thickness exceed 1400 meters. The sediment of this group seems to have been transported from following two provinances, namely, granitic hinterland and shallow seamount composed of greenstones and limestone. This group was deposited well away from granitic hinterland, but in the vicinity of shallow seamount. The Tomarigawa Group, which includes allocthonous seamount materials, may be comparable to the submarine terrace sediments on the continental slope or trench sediments, but further detailed study is necessary before we come to our final conclusion.
地学団体研究会の論文
1984-11-25

西南北海道・島牧村大平山周辺の中生界

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