八溝山地鶏足山塊の中生界 Mesozoic Strata of the Keisoku Mountain Block in the Yamizo Mountains, Central Japan
The Yamizo Mountains consist of four mountain blocks: the Yamizo, Torinoko, Keisoku and Tsukuba Mountain Blocks, from north to south. The northern three mountain blocks are composed mainly of Mesozoic sedimentary rocks such as chert, shale, and sandstone. The sedimentary complex in the Keisoku Mountain Block is subdivided into the following four lithostratigraphic units: the Kasama, Kunimiyama, Takatori, and Ayuta Units based on the lithology and geologic age. The Kasama Unit is composed mainly of alternations of sandstone and shale with a small amount of chert and tuff layers. The Kunimiyama Unit consists of massive sandstone and alternations of sandstone and shale. The Takatori Unit is characterized by the chert-clastic sequence comprising "Toishi-type shale, " bedded chert, siliceous shale, and clastic rocks in ascending order. The Ayuta Unit is composed mainly of alternations of sandstone and shale. These units are in fault contact with each other.<BR>Bedded chert of the Takatori Unit yields Middle Triassic to Early Jurassic radiolarians belonging to the Triassocampe deweueri, Triassocampe noua, Canoptum triassicum, Parahsuum simplum, and Hsuum hisuikyoense Assemblages. Middle Jurassic radiolarians, which are comparable to the Tricolocapsa plicarum and Tricolocapsa conexa Zones, occur from siliceous shale of the Takatori Unit. Late Jurassic radiolarians belonging to the Tricolocapsa yaoi, Mirifusus baileyi and Pseudodictyomitra primitiva Assemblages are obtained from clastic rocks of the Keisoku Mountain Block. Furthermore, late Early Triassic conodont fauna referable to the Neospathodus homeri Zone was discriminated from the "Toi shi-type shale" of the Takatori Unit.<BR>Based on the lithological and chronological characteristics, Mesozoic sedimentary complex of the Keisoku Mountain Block is correlated to those of the Tamba, Mino and Kiso areas in southwest Japan. Mesozoic strata distributed in the Keisoku Mountain Block is regarded as an accretionary wedges formed mainly by of fscrape-accretion during Late Jurassic time.