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建立于1928年，直到最近才被许多地质学家接受，没有大的变化。它是小贺县和秋田市地区地层的合成序列：按升序排列的翁长川组、文川组、天户寺组和佐佐冈组。因此，两个地区之间的地层对比是这一方案的基础。位于小贺半岛的翁长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组进行了硅藻生物地层学研究。结果表明，分布于今秋田海岸东侧内陆地区的翁长川组对应于Oga组的下部和中部，内陆地区的Funakawa组对应于Oga半岛同一组的上部，秋田市地区的Tentokuji组和Sasaoka组与Oga的Kitaura组和Wakimoto组长期对比。通过对几个内陆地区田托库组和佐佐冈组的硅藻、浮游有孔虫和钙质超微化石的研究表明，这两个组可以与欧贺半岛的船奈川组对比，而不是与上覆的北村组和和本组对比。根据这些结果，秋田油田可以在构造上划分为现今秋田海岸沿线划分的主要区域，这里运行着北百里冲断系统。冲断的东侧在佐佐冈组沉积过程中迅速抬升，最终露出。相反，在逆冲的西侧，包括Oga半岛，继续存在一个海洋盆地，并沉积了厚厚的半深海Kitaura和Wakimoto组。这些结果可能需要对秋田油田构造开发的传统概念进行未来的修订。

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.