Volcanostratigraphy on the lower green tuff, Oga Peninsula, northeast Honshu, Japan - Comparative study with the Neogene volcanic sequences distributing in the Dewa Mountains and Ou Backbone Mountains -

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The volcanic facies analysis of the Monzen Group, the Palaeogene to the Lowest Neogene, has been done in the Oga Peninsula to make clear the paleoenvironments and character of aqueous area appeared in them.Some eruption centers and flow directions of the Monzen Group are determined by the feeder dikes, crude-bedded coarse air-fall deposits, and structures of lava flows. Many of the eruption centers of the Kamo Lavas were in the area along the west coast of the peninsula. Some basaltic rocks of the Kuguriiwa Lavas flowed from northwest, and the pyroclastic rocks of the Shiosenomisaki Sandstone and Conglomerate were supplied from south.Occurrence and facies of the volcanogenic rocks show the volcanism of the Monzen age took place under shallow water to terrestnal condition with a similar appearance of the Upper Miocene Subari Andesite.The Palaeogene to the earliest Miocene volcanism (Akashima and Monzen age) was limited in the mountain area along the Japan Sea. During the period from 17 to 14 Ma (Daijima-Nishikurosawa age), the archipelagic volcanism spread east ward, across the backbone mountain area, and the eastern margin of the volcanic field reached to the western part of the Kitakami Mountains. After this period, the volcanism in the Kitakami Mountains ceased, and the eastern margin moved westward, near to the Quaternary volcanic front.