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廣島大學地學研究報告 3 号
1953-11-20 発行
讃岐山脈中部の地質
On the Izumi Group of the Central Part of the Sanuki Mountain-Range, Japan
中野 光雄
Abstract
The Izumi group, separated from the crystalline schists of the Outer Zone by the Median Tectonic Line, lies unconformably on the granitic rock mass of the Inner Zone and consists of numerous alternation of beds of sandstone and shale, partly including conglomerate, liparitic tuffaceous sandstone and siliceous fine-grained sandstone. The group seems to be characterized by epicyclic features of sedimentation.
The stratigraphic sequence of the group in this district can be arranged in the ascending order as follows:
1) Korobishi conglomerate and sandstone: Fossils are scarcely found. 640 meters thick.
2) Nakato shale: The wedge-shaped layers of sandstone and shale in alternation (sandstone predominant) are intercalated in the middle part. Fossils are scarcely found. 2,770 m. thick.
3) Horita sandstone: Alternation of sandstone and shale (sandstone pre-dominant), yielding Archaeozostera. 1,420 m. thick.
4) Katsuura shale: Shale with interbedded sandstone, liparitic-tuffaceous sandstone and siliceous fine-grained sandstone; 1,000m.± thick. Remarkable layers of liparitic tuffaceous sandstone and siliceous fine-grained sandstone are intercalated at the upper horizons.
5) Kawahigashi sandstone: Thick series of alternating sandstone and shale (sandstone predominant); 1,570 m. thick. Fossils are scarcely found in shale of the upper part.
6) Ishibotoke shale: Nodules and fossils are sporadically found. 410 m. thick.
7) Nakano sandstone: Sandstone and shale in alternation (sandstone predominant). 500 m.+ thick.
The correlation table of the Izumi group in the Awaji Island and this district is referable to the Japanese text. (See page 8)
In general view, the Izumi group forms a synclinorium structure, and the axial direction is about E—W, pitching toward east.
In the northern and southern foot regions of the range, the group thrusts up on the Plio-Pleistocene bed. The thrust faults between both members are covered with the Pleistocene bed or talus, and are considered to have been formed in relation to the movement of the Median Tectonic Line at the Shobudani Phase
The stratigraphic sequence of the group in this district can be arranged in the ascending order as follows:
1) Korobishi conglomerate and sandstone: Fossils are scarcely found. 640 meters thick.
2) Nakato shale: The wedge-shaped layers of sandstone and shale in alternation (sandstone predominant) are intercalated in the middle part. Fossils are scarcely found. 2,770 m. thick.
3) Horita sandstone: Alternation of sandstone and shale (sandstone pre-dominant), yielding Archaeozostera. 1,420 m. thick.
4) Katsuura shale: Shale with interbedded sandstone, liparitic-tuffaceous sandstone and siliceous fine-grained sandstone; 1,000m.± thick. Remarkable layers of liparitic tuffaceous sandstone and siliceous fine-grained sandstone are intercalated at the upper horizons.
5) Kawahigashi sandstone: Thick series of alternating sandstone and shale (sandstone predominant); 1,570 m. thick. Fossils are scarcely found in shale of the upper part.
6) Ishibotoke shale: Nodules and fossils are sporadically found. 410 m. thick.
7) Nakano sandstone: Sandstone and shale in alternation (sandstone predominant). 500 m.+ thick.
The correlation table of the Izumi group in the Awaji Island and this district is referable to the Japanese text. (See page 8)
In general view, the Izumi group forms a synclinorium structure, and the axial direction is about E—W, pitching toward east.
In the northern and southern foot regions of the range, the group thrusts up on the Plio-Pleistocene bed. The thrust faults between both members are covered with the Pleistocene bed or talus, and are considered to have been formed in relation to the movement of the Median Tectonic Line at the Shobudani Phase
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