The lower part of the Josoji Formation, Shimane Peninsula, contains clues for figuring out changes in deep-water characteristics during the opening of the Japan Sea. The foraminiferal assemblage includes early to middle Miocene biostratigraphic index taxa such as planktonic foraminiferal Globorotalia zealandica and Globorotaloides suteri. The occurrence of these two species, together with the absence of praeorbulinids, suggests that the lower part of the Josoji Formation is assigned to the top of planktonic foraminiferal Zone N7/M4 (16.39 Ma). The benthic foraminiferal assemblage, which is characterized by Cyclammina cancellata and Martinottiella communis, clearly suggests that the lower Josoji Formation was deposited at bathyal depths, and that it developed in association with the abrupt appearance of deep-sea calcareous forms. Such bathyal taxa are the main constituents of the Spirosigmoilinella compressa–Globobulimina auriculata Zone of the Josoji Formation and also of the Gyrodina–Gyroidinoides Zone at Ocean Drilling Program Site 797 in the Japan Sea. The base of these benthic foraminiferal zones can be correlated with the base of the nannofossil Sphenolithus heteromorphus Base Zone (= CNM6/CN3); thus, its estimated age is 17.65 Ma. This biostratigraphic information suggests that the lower Josoji Formation was deposited from shortly before 17.65–16.39 Ma in upper limit age. Evidence that fresh to brackish and shallow-water basins formed in the rifting interval of 20–18 Ma in the Japan Sea borderland suggests that the abrupt appearance of deep-sea calcareous foraminifera occurred about 1 my earlier in this area than in other sedimentary basins and suggests that a significant paleoceanographic change occurred in the proto-Japan Sea at 17.65 Ma.

中文翻译：

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日本本州西南部岛根半岛下常总寺组的地质时代：日本海早期开放阶段突然转变为深水的意义

岛根半岛的 Josoji 组下部包含确定日本海开放期间深水特征变化的线索。有孔虫组合包括早至中新世生物地层索引分类群，如浮游有孔虫Globorotalia zealandica和Globorotaloides suteri。这两个物种的出现，加上无长龙科的缺失，表明常总寺组的下部被指定为浮游有孔虫区 N7/M4（16.39 Ma）的顶部。底栖有孔虫组合，以Cyclammina cancellata和Martinottiella community为特征，清楚地表明下常总寺组沉积在深海深处，并且它与深海钙质形式的突然出现有关。这种深海分类群是Josoji 组的Spirosigmoilella compressa-Globobulimina auriculata区以及日本海大洋钻探项目 797的Gyrodina-Gyroidinoides区的主要成分。这些底栖有孔虫区的基部可以与纳米化石Sphenolithus heteromorphus的基部相关联基区（= CNM6/CN3）；因此，它的估计年龄是 17.65 Ma。该生物地层信息表明，下常总寺组是在上限年龄 17.65-16.39 Ma 之前不久沉积的。有证据表明，在日本海边界 20-18 Ma 裂谷区间形成了淡水到微咸水和浅水盆地，这表明该地区深海钙质有孔虫的突然出现比其他沉积盆地早约 1 my表明原始日本海在 17.65 Ma 发生了显着的古海洋学变化。