Miocene foraminiferal assemblage occurred from the lower part of the Josoji Formation, Shimane Peninsula, southwest Japan, and its geological implication

The lower–middle Miocene biostratigraphy of the Shimane Peninsula is difficult to interpret due to limited fossil occurrences in the lower Josoji Formation. In contrast, the middle–upper Josoji Formation and the underlying Koura Formation are rich in deep-sea foraminifera and non-marine to brackish fossils, respectively. We identified a diverse microfossil assemblage in the lower Josoji Formation that indicates deposition in dee-water environment that transitioned to the oceanic conditions. The microfossil assemblage consists mainly of agglutinated foraminifera such as Martinottiella communis and Spirosigmoilinella compressa , and calcareous hyaline forms such as Ammonia honyaensis , Valvulineria masudai , Globobulimina auriculata , and Nonionella miocenica . We therefore define this as the Spirosigmoilinella compressa –G lobobulimina auriculata Zone. The assemblage indicates upper to middle bathyal paleodepths and a poorly oxygenated depositional environment. The first occurrences of Spirosigmoilinella compressa and Martinottiella communis were identified in ODP Site 797 (Japan Sea). These species co-occurred just before the appearance of calcareous-rich foraminifera, similar to their occurrence in the lower Josoji Formation. The indicated geological age of the lower S. compressa – G. auriculata Zone is about 17 . 75 Ma, based on the biochronology of Miocene calcareous nannofossils of Backman et al. ( 2012 ). The biostratigraphy indicates progressive deepening of the sedimentary basin during deposition of the Koura to Josoji formations. This resulted in deep brackish waters during the earliest depositional period of the Josoji Formation, which explains the lack of foraminifera in the lower section. The appearance of both benthic and planktonic foraminifera along with nannofossils is interpreted as indicating a large-scale inundation of seawater. The basin, which was initially filled with brackish water, was then completely filled by sea water. This geological event, indicating abrupt water exchange, has important implications for the initial opening of the Japan Sea.