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 -

通过对奥贺半岛古近纪至新近纪最下部的门前群火山岩相分析，明确了门前群的古环境和水体特征，门前群的一些喷发中心和流向由补给岩脉、粗层状粗碎屑沉积物和熔岩流结构所决定。卡莫火山的许多喷发中心位于半岛西海岸的沿着地区。库古里瓦熔岩的一些玄武质岩石从西北方向流出，火山岩的产状和岩相表明，门前期火山作用发生在浅水-陆缘条件下，与晚中新世Subari安山岩的面貌相似，古近纪至中新世最早期火山作用发生在晚中新世（赤岛和门前时代）仅限于日本海沿着的山区。17 ~ 14 Ma（大岛-西黑泽期），群岛火山作用向东扩展，穿过基干山区，火山场东缘到达北上山脉西部。此后，北上山脉火山活动停止，东缘向西迁移，靠近第四纪火山前缘。

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.