| 概要 |
Tertiary chaotic rocks are extensively distributed in the southern part of the Muroto Peninsula of eastern Shikoku, which belongs to the southern part of the Shimanto Terrane of the Outer Zone of Sout...hwest Japan. They consist of angular to subrounded clasts and blocks dispersed in more or less scaly cleaved argillite. The size of clasts and blocks ranges from a few millimeters to more than three kilometers. The clasts are composed of various kinds of rocks such as graywacke, arkose, glauconitic sandstone, conglomerate, siliceous shale with radiolarian remains, limestone, tuff, greenstone, and so on. In these chaotic rocks, various types of slump structures are ubiquitously developed. This fact shows that the brecciation and the mixing of clasts were produced by the sliding process. From the style and degree of tectonic deformation, chaotic rocks of this area are generally divided into three units. In the most intensely deformed area the argillite matrix is pervasively sheared and the contained clasts are represented as tectonic lenses. Both the clasts and the argillite matrix are sharply cut by numerous minor thrusts. The cleavages, minor thrusts and tectonic lenses imply the deformation was taken place under the unconsolidated condition. The chaotic rocks are referable to an olistostrome, and their strongly cleaved structure indicates that they were converted into a melange of olistostrome origin by secondary deformation. The writer has found many mega-fossils such as shallow marine molluscs, balanus and vertebrate bone from the olistoliths and the sourrounding argillite matrices. These fossils indicate that the chaotic rocks are of Late Oligocene to Early Miocene in age. From the ages of fossils and the regional geologic framework, it is concluded that the olistostrome was formed in latest Early Miocene time.続きを見る
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