紀伊半島西部中津村周辺の日高川層群美山累層 : 紀伊半馬匹万十累帯の研究(その11)
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概要
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The Miyama Formation around Nakatsu-mura is in fault contact with the Ryujin Formation on the south (Takatsuo-Kawarago Fault), and is divided into two members, that is, the Yakendo and Misoigawa Members, by the longitudinal fault named the Yakendo Fault. The Yakendo Member, more than 1,000 meters thick, is composed mainly of sandstone and sandy alternations of sandstone and shale accompanied with shale, shaly alternations, acidic tuff, chert and greenstones. The Misoigawa Member, 1,100 meters thick, consisits mainly of shaly alternations and slump deposits associated with sandstone, sandy alternations, shale, acidic tuff, chert and greenstones. The strata dip northward or southward at high angles and repeat a few times by longitudinal reverse faults, forming imbricated structures. The Yakendo and Misoigawa Members are assigned to Coniacian to Santonian in age on the basis of the abundant radiolarian fossils, Artostrobium urna Assembalges contained in black shale. Meanwhile, late Valanginian to Turonian radiolarian assemblages are found from chert and red shale. Chert and greenstones often occur as small blocks or large bodies concordantly embedded in argillaceous beds with sedimentary contact. The age of chert is older than that of the surrounding strata in general. These argillaceous beds which contain various size and lithology of chert and greenstones is traceable for a long distance in the strike direction. Judging from the occurrence and radiolarian age, chert and greenstones are considered to be exotic in origin and to form a chert-greenstones olistostrome, accompanied with red and green shale, acidic tuff, and angular fragment-bearing shale. The Miyama Formation, being thus characterized by chert-greenstones olistromes, flysch sediments, slump deposits, and imbricated structures, seems to be deposited on or near the trench floor in an arc-trench system of Late Cretaceous age.
- 1986-07-25
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