Microbiostratigraphy and Paleoenvironment of the Daijima and Nishikurosawa Stages distributed in the Akita Area.

我们研究了秋田地区与中新世日本海开放有关的微生物地层学(钙质超微化石和遗迹化石)以及分布在秋田地区的大吉岛和西黑泽期的古环境。分布在Uyashinai和Gongenyama地区的Nishikurosawa期岩相呈块状，没有纹层，表明底水为高溶氧状态，没有动物群和细菌活动。在那个阶段发现的钙质超微化石组合由暖水物种组成。翁长川期以硅质硬岩为特征，钙质超微化石很少或根本没有。该阶段的岩相主要由块状泥岩和含球粒陨石的弱层状泥岩(遗迹化石)组成，但两个地区的翁长川期中段均有层状泥岩沉积，表明沉积在极少或无氧溶解的底水下。翁长川时代的古日本海为无底流入侵的砂砾盆地。因此，与块状或弱层状泥岩的存在相比，层状泥岩的存在指示了更深的水深。根据层状泥岩的分布模式和遗迹化石，重建了贡山地区西北部深于南部和东部的翁长川期的古环境。

We have studied he microbiostratigraphy (calcareous nannofossils and trace fossils) and the paleoenvironment of the Daijima and the Nishikurosawa stages distributed in the Akita area related to the opening of the Japan Sea in the Miocene. The lithofacies of the Nishikurosawa Stage distributed in the Uyashinai and Gongenyama areas are massive without lamina which indicates the high oxygen dissolution bottom water without faunal and bacterial activities. Calcareous nannofossil assemblages found in that stage are composed of warm water species. The Onnagawa stage is characterized by the siliceous hard rock and few or no calcareous nannofossils are found. The lithofacies of this Stage is mainly composed of massive mudstone and weakly laminated mudstone with Chondrites (trace fossil).However, laminated mudstone which indicates the deposition under the few or no oxygen dissolved bottom water is found in the middle part of the Onnagawa stage in both areas. The paleo-Japan Sea in the Onnagawa age was the silled basin without the invasion of the bottom water current. Therefore, thepresence of laminated mudstone indicates the deeper water depth compared with the area presence of the massive mudstone or weakly laminated mudstone. On the basis of the distribution patterns of the laminated mudstone and the trace fossils, paleoenvironment in the Onnagawa age is reconstructed that the northwestern part was deeper than the south and/or eastern part in the Gongenyama area.