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–Globobulimina 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 seawater. This geological event, indicating abrupt water exchange, has important implications for the initial opening of the Japan Sea.

The Choanji Formation is newly defined as an Upper Devonian (Frasnian‒Famennian) sandstone- and shale-dominanted formation, in the Choanji area, South Kitakami Belt, northeastern Japan. The Late Devonian brachiopods, Desquamatia (Seratrypa) and Cyrtospirifer occur within the middle and upper parts of the Choanji Formation, respectively. The former is described for the first time in Japan.

Devonian chert beds in the Permian Kozuki Formation of the Ultra-Tamba Zone are reported from Mimasaka City, Okayama Prefecture, Southwest Japan. The Kozuki Formation consists mainly of Permian clastic rocks and felsic tuffs with basalts, Carboniferous limestone, and Early Permian cherts, and is therefore interpreted as a Permian accretionary complex similar to the other formations of the Ultra-Tamba Zone. The Devonian chert beds are frequently intercalated with black layers composed of magnetite, in which a well-preserved radiolarian fauna occurs. These magnetite beds with radiolarian shells indicate the sedimentary origin derived from hydrothermal activity on the seafloor. The radiolarian fauna contains Holoeciscus foremanae and H. elongatus, as well as Ceratoikiscum avimexpectans and Archocyrtium sp., indicating the Fammenian Age of Late Devonian. These are the oldest fossils in the Chugoku District and the chert beds represent one of the oldest allochthons within the accretionary complexes of Japan, similar in age to the Devonian chert of the Nedamo Belt in the Tohoku District.

Dissolved gas concentrations, compositions, and carbon isotopic values are compared between times immediately prior to and following the 2014 Kamishiro Fault Earthquake at the Murono mud volcano (Tokamachi City, Niigata Prefecture, central Japan). Immediately following the earthquake, concentrations of methane, carbon dioxide, ethane, and propane increased, while carbon isotopic values of methane and ethane decreased, on average. Carbon isotopic values of methane and gas compositions (C₁/[C₂+C₃] ratio) indicate that the dissolved methane of the mud volcano originates from the thermogenic decomposition of organic matter. As the carbon isotopic values of methane and ethane are significantly more positive than those of natural gas from the Niigata Oil and Gas Field, the dissolved gases of the mud volcano are thought to be derived from the deepest source rocks of the Niigata Oil and Gas Field. However, the supply of ethane generated from less mature source rocks possibly increased following the earthquake. Our results reveal that a change in behavior of fluid deep below the mud volcano was likely caused by fluctuations in volumetric strain related to the earthquake. Theoretical strain at the mud volcano can be estimated as 687×10^-8, which is much higher than estimates from previous studies for the lower limits of strain triggered by anomalous activity at mud volcanoes. Murono mud volcano is an important case study for investigating the correlation between mud volcanoes and earthquakes, because it frequently experiences the large volumetric strain of earthquakes that are comparable to the 2014 Kamishiro Fault Earthquake.

Here U-Pb and reassessed fission track (FT) ages are presented for samples from the Miocene Mizunami and Iwamura groups in the southeastern Gifu Prefecture, central Japan. The U-Pb ages of three tuff samples coincide with the reassessed FT ages within the error range. These new U-Pb and reassessed FT ages are consistent with those reported in previous microfossil and paleomagnetic studies, which indicate the following depositional history: The Hongo and Akeyo formations of the Mizunami Group were deposited at ~19-18 Ma and ca. 18 Ma, respectively, and the lower part of the Toyama Formation of the Iwamura Group was deposited ca. 18 Ma.