姫川中流域の飛騨外縁構造帯

特に,ジュラ系来馬層群について

抄録

The northern half of the Itoigawa-Shizuoka Tectonic Line extends along the River Hime-kawa. On the west of the River, rocks belonging to the Hida Marginal Tectonic Belt of the South-west Japan are widely distributed. The Belt in this region is called the Ohmi-Renge Belt, where the early Jurassic Kuruma Group is distributed in fault contacts with the Ohmi-Renge crystalline schists, unmetamorphosed Permo-Carboniferous system and serpentinite forming a sequential serpentinite melange on the whole. Unconformably overlying these rocks, the Cretaceous and the later strata are distributed on the higher topographic position. The Kuruma Group along the River Hime-kawa consists of the alternation of conglomerate, sandstone and shale. On the basis of two major cycles recognized in vertical sequence, the Group is divided into the Gamawarazawa, Ohdokorogawa and Yoshinazawa Formations in ascending order. Among them, the Gamawarazawa Formation and the lower part of the Ohdokorogawa Formation is correlated with the Johgodani Formation (Kobayashi et al., 1957) and the upper part of the Ohdokorogawa Formation and the Yoshinazawa Formation are with the Kitamatadani Formation respectively. They are the sediments at the initial stage of the formation of the basin. The Gamawarazawa and the Ohdokorogawa Formations consist of conglomerate at the lowermost part of the Formation and show fining upward sequence to shale at the upper-most part. The Yoshinazawa Formation is composed from the alternating sandstone dominant part and shale dominant part. Gravels of the conglomerate in the Kuruma Group are mostly granule to pebble in size, they are well-rounded and well-sorted. They consist mostly of pyroclastic rocks such as tuff and welded tuff, volcanic rocks as well as sedimentary rocks such as sandstone, shale and green rocks, and rarely contain those of granite and metamorphic rocks. The sand-stone of the Group contains abundantly lithic fragments to belong to lithic wacke or felds-pathic wacke. The shale is carbonaceous and intercalates coal seams as-well as abundant yield of well-preserved plant fossils. There are some horizones of the yield of marine shell fossils in the upper part of the Ohdokorogawa Formation. The sediments were supplied from the south or the southeast into the sedimentary basin of the Kuruma Group, namely the central part of the basin had been migrated to the northward. The Akahageyama Formation, which unconformably covers the Kuruma Group, is the middle to late Cretaceous in age. The Formation consists of coarse grained quartzo-felds-pathic sandstone belonging to quartz arenite or arkose. The gravels contained in the conglomerate are almost granitic rocks with orthoquartzite gravel. It is safely inferred that the sedimentary rocks and volcanic rocks were extensively, but plutonic and metemorphic rocks were rarely distributed in the hinterland of the sedimentary basin of the Kuruma Group. On the contrary, in the hinterland of the Akahageyama Formation, granitic rocks might be widely distributed. It suggests that at the time between the sedimentation of the Kuruma Group and that of the Akahageyama Formation, there occurred not only the folding and faulting but also a great sift of the hinterland. This is called the Post-Kuruma Disturbance, which reveals a part of the widespread earth movement occurred in the central part of the Honshu Arc in Japanese Islands.