The Harakawa and the Jogashiro Formations of the Nijo Group and volcanism of Sanukitoid

Abstract

The Nijo Group of middle Miocene, is divided into three units, as the Donzurubo, the Harakawa and the Jogashiro Formation. The first consists of mostly garnet-biotite rhyolite lava and pyroclastic rocks. The Harakawa Formation is subdivided into four members, as the Tajiri, the Myojin, the Tomesho-yama and the Higashiyama Member because of local variety of sedimentary facies. This Formation is interbedded with sanukitoid lavas and dike rocks. These sanukitoid rocks are aphyric andesite or/and olivine-pyroxene andesite, hornblende-bearing aphyric andesite, olivine- or hornblende- tearing or -free two pyroxene andesite, olivine-augite basalt and two pyroxene andesite in ascending order. Numerous large irregular boulders of granitic rocks occur at two stratigraphic horizons. The Tajiri Member yields fresh water diatoms and warm fossil plants. The Jogashiro Formation is subdivided into the Sekiya and the Tanabe Member. These consist of conglomerate, including biotite dacite gravels. Biotite dacite lava and tuff erupted in this time. The Jogashiro Formation unconformably covers the Harakawa Formation.