島根県桜江町から石見町にかけて分布する桜江層群の層序と構造

The Oligocene Sakurae Group，in which the Sakurae cauldron in central Shimane Prefecture is formed，is mainly composed of acidic to intermediate volcanic and plutonic rocks．The mapped area covers 9×4 kms on the southern bank of the Gonokawa river．The upper part of this group in the area investigated is characterized by densely welded tuff up to several hundred meters in thickness．A semi-circular collapse structure rimmed by a ring fault and an arcuate anticlinal axis cuts the general trend of the Sakurae Group．However，the inside of this collapse shows a half basin with a steeply-dipping inner margin and a gently dipping base．This suggests that the subsidence of the half-circular collapse was concurrent with the accumulation of the inner welded tuff，which represents a catastrophic pumice eruption．Many intermediate to acidic intrusions occur in the area．A granite body was emplaced mainly outside the half-circular collapse，whereas porphyry intrusions are concentrated along this collapse margin．Numerous unconsolidated andesitic fragments also occur in the welded tuff．Accordingly，the intermediate magma seems to have risen along the collapse margin during and after the pumice eruption．