島根県美郷町にみられる古第三紀川内コールドロンの南東部の構造について

The Oligocene Kawauchi Group in central Shimane Prefecture consists of andesitic to rhyodacitic lavas and pyroclastic rocks. However, neither detailed lithological nor stratigraphic descriptions of these rocks have been reported to date.
From the base upward, the Mimata subgroup consists of a lower andesite lava, rhyolitic lapill tuff, rhyolitic welded tuff, an upper andesite lava, and rhyodacite tuff. Acidic to intermediate dikes and plutons occur inside the cauldron or along the cauldron rim.
Autobrecciated structure is observed in the andesite‘sills' reported by Kano et al.（2001）, and thus these‘sills’are considered to be lavas. The Mimata subgroup is in fault contact with the basement rocks. These faults strike N-S and E-W. Accordingly, the cauldron rim seems not to be arcuate but is rectilinear in plan view, although Matsuda（1985）described a circular collapse structure. This suggests that the Kawauchi cauldron may be a polygonal cauldron.