西南日本秋吉帶石炭系高山(Ko-yama)灰巖群的褶皺構造

西南日本秋吉帶石炭系高山(Ko-yama)灰巖群的褶皺構造

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Folded Structure of the Carboniferous Ko-yama Limestone Group,Akiyoshi Belt, SW JAPAN

Noriyuki INADA1), Shigeyuki SUZUKI1), Keisuke ISHIDA2), Shinji YAMASHITA1)
1)Dept. Earth Sci., Okayama University,700-8530,Okayama, Japan;
2)Laboratory of Geology, Institute of SAS, Tokushima University,770-8502,Tokushima, Japan

The study area is located in Kawakami-cho,Okayama Prefecture, which is occupied by Lower Carboniferous to Middle Permian Ko-yama Limestone Group (Yokoyama et al., 1979), Permian Yoshii Group(Sano et al., 1987) and Triassic Nariwa Group (Teraoka, 1959). The Nariwa Group unconformably covers the Paleozoic successions (Otoh, 1985). These successions belong to the Akiyoshi Belt. The Ko-yama Limestone Group is composed mainly of massive limestone, with basic volcanic rocks, acidic tuff and chert. The group was dated by foraminifer and fusulinid as Lower Carboniferous to Middle Permian (Yokoyama et al., 1979).

The Akiyoshi Belt was folded during the Middleto Late Permian (Suzuki et al., 1990). Strata of the Ko-yama Limestone Group generally strike E-W and dip to the north. In the Hoya section, about 300 m thick strata comprise limestone, basic tuff, chert and acidic tuff, limestone, basic lava and tuff in ascending order. Ishida et al., (2012) studied conodont faunal succession in this section, which indicates a nearly complete succession ranging from the upper Visean to the lower Moscovian. The faunal succession indicates the age becomes younger southward and the strata are overturned. A folded strucuture of tight overfold with north-dipping axial plane is reconstructed. Similar style of small scale overfolds of banded chert and turbidite are observed in the overlying Yoshii Group.

folded structure; Ko-yama Limestone; Carboniferous; Akiyoshi Belt

10.3975/cagsb.2012.s1.13

E-mail: sc19506@s.okayama-u.ac.jp.