Abstract
The volcanic stratigraphy, geologic structure, petrography, and chemistry of the Abu Group were examined, along with its relationship with associated intrusive rocks, to investigate the geologic history of a Late Cretaceous caldera cluster in the Nagato-Hōhoku area of Yamaguchi Prefecture, Japan. The stratigraphy of the Abu Group in this area is divided into the lower Hōhoku and upper Nagato formations. The Hōhoku Formation is composed mainly of tuffaceous sedimentary rocks and is divided into the Awano Conglomerate and Sandstone, Hōzōyama Lapilli Tuff, Ōtōyama Tuff, and Asōgawa Shale and Tuff members. The Nagato Formation consists mainly of terrestrial volcaniclastic rocks and is divided into the Daibōgawa Rhyolite Tuff, Kumanodake Rhyolite Tuff, Kunihoro Andesite, and Funakoshi Rhyolite Tuff members. The volcanic rocks are in contact with the Kanmon Group along normal faults and with dikes intruded along the caldera faults, and they are intruded by felsites, granites, and diorites. In the west, the two formations have a half-basin structure that dips towards the Hibiki-nada Sea, whereas in the east the basin is closed. The two formations form the Nagato-Hōhoku caldera, which resembles a graben caldera. The caldera is elongated in an E-W direction and curves towards the NNE in the east, and is >34 km by >14 km. The ignimbrites of the Hōhoku and Nagato formations have subduction-related chemical compositions.
