2001 Volume 107 Issue 4 Pages 243-257
The Cretaceous Abu Group and related intrusive rocks are widely distributed in the central part of Yamaguchi Prefecture, and form the Chomonkyo igneous complex. A restudy of the volcanostratigraphy of this complex revealed that it is divided into the following three formations and nine members ; Ato Formation (Kane Welded Tuff Member and Tokusagamine Sandstone-Mudstone Member), Ikumo Formation (Yunose Welded Tuff Member and Usutaniyama Rhyolitic Lava), and Sasanami Formation (Somakidani Welded Tuff Member, Mitsuke Siltstone Member, Kitakoma Welded Tuff Member, Kobukidao Felsic Intrusive Rocks, and Komatsugatani Dacitic Lava) in ascending order. A new cauldron, named Sasanami cauldron composed of the latter five members, was discovered. The cauldron has an elliptical outline, 17 km in diameter from east to west and 14 km from north to south. The Somakidani Welded Tuff Member and Kitakoma Welded Tuff Member are distinct from the Kane Welded Tuff Member and Yunose Welded Tuff Member with respect to phenocryst mode, and particularly the Nb content of the bulk rock chemistry.The Sasanami cauldron was formed by two subsidences, synchronous with the eruptions of the Somakidani Welded Tuff Member and Kitakoma Welded Tuff Member. The collapsed block was destroyed over a distance of several kilometers and bounded by a discontinuous ring fracture, and these features approximately correspond to a piecemeal collapse. This collapsed block is intruded by a ring dike (Kobukidao Felsic Intrusive Rocks) and by a dacite lava dome (Komatsugatani Dacitic Lava) in the central part of the cauldron. The cauldron has a basin structure with dip of up to 30°, and the caldera collapse appears asymmetric with a maximum subsidence of about 500 meters along the ring fracture judging from the thickness of the intra-caldera tuffs.
