Late Cenozoic microbiostratigraphy and development of sedimentary basins in the Akita oil-fields.

秋田油田晚新生代地层方案自1928年建立以来，一直被许多地质学家所接受，直到最近才有较大的变化。它是大贺和秋田市地区的组合演替：女川、富川、天德寺和佐冈组按升序排列。因此，两个地区之间的地层对比是该方案的基础。位于大贺半岛的女川组和船川组在秋田油田得到了广泛的应用。对秋田油田几个地区的两个地层进行了硅藻生物地层学研究。结果表明，分布在今秋田海岸东侧内陆地区的女川组与大贺半岛同一组的中下段相对应，内陆地区的船川组与大贺半岛同一组的上段相对应。在秋田地区的天德寺组和佐佐冈组与大贺的北aura组和Wakimoto组相对应。对若干内陆地区天德寺组和佐冈组硅藻、浮游有孔虫和钙质纳米化石的研究表明，天德寺组和佐冈组可以与大贺半岛的船川组相对应，而不是与上覆的北aura组和Wakimoto组相对应。根据这些结果，可以将秋田油田在构造上划分为沿现在秋田海岸的一条线所分隔的主要区域，其中运行着北里逆冲系统。冲断构造东侧在佐佐冈组沉积时期迅速隆升，最终出现。逆冲构造西侧包括大加半岛继续发育海相盆地，沉积了厚厚的北aura组和Wakimoto组。这些结果可能需要对秋田油田结构开发的传统概念进行修订。

The late Cenozoic stratigraphic scheme of the Akita oil field was established in 1928 and has long been accepted by many geologists until recently without great changes. It is a composit succession of formations of the Oga and the Akita City areas: the Onnagawa, Fumakawa, Tentokuji, and Sasaoka Formations in ascending order. Thus the correlation of strata between both areas is the fundamental of this scheme.The Onnagawa and Funakawa Formations whose types are in the Oga Peninsula have been widely applied in the Akita oil fields. Diatom biostratigraphy was examined in the two formations in several areas in the Akita oil fields. The results demonstrate that the Onnagawa Formation distributed in the inland area, east side of the present Akita coast, is correlated with the lower to middle part of the same formation of Oga, and the Funakawa of the inland area with the upper part of the same formation of the Oga Peninsula.The Tentokuji and Sasaoka Formations, with their types in the Akita City area, have long been correlated with the Kitaura and Wakimoto Formations of Oga. The studies on diatom, planktonic foraminifera, and calcareous nannofossil of the Tenntokuji and Sasaoka Formations from several inland areas revealed that the two formations can be correlated with the Funakawa Formation of the Oga Peninsula, not with the overlying Kitaura and Wakimoto Formations.According to these results, the Akita oil fields can be structurally devided into major areas separated by a line along the present Akita coast, where runs the Kitayuri thrust system. The east side of the thrust was rapidly uplifted during the deposition of the Sasaoka Formation and finally emerged. In contrast, in the western side of the thrust including the Oga Peninsula, a marine basin continued to exist, and the thick bathyal Kitaura and Wakimoto Formations were deposited. These results may reqire a future revision of a traditional concept of the structural deveropment of the Akita oil fields.