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

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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.