The Ohtani Volcanic Ash Bed, also referred to as the Znp-Ohta Tephra Bed, is a prominent marker tephra from the early Pliocene in central Japan. This study investigates the depositional processes of the ash bed within the framework of lahars and reconstructs the formation processes of soft-sediment deformation structures observed in its lower portion. The findings indicate that the ash bed was deposited by flows that underwent progressive transformations in vertical, forward, and backward directions, involving debris flow and hyperconcentrated flows during downstream transport. The rapid and thick accumulation of lahar sediments generated excessive pore water pressure in the underlying strata, inducing liquefaction and fluidization, which led to the development of clastic dikes and load structures. In addition, drag shear stresses caused by the rapid lahar flow resulted in the formation of overturned folds, thrusts, and S-(Z)-shaped flexural structures in the lowermost part of the lahar deposits. These soft-sediment deformation structures provide a record of the overloading and shear stresses exerted by the lahar, providing valuable insights into the dynamic processes involved in their formation.

In order to restrict the source of hornfels gravels, geochemical analyses of whole-rock major, trace and rare earth elements of the hornfels gravels in the upper part of the Hanno Formation distributed on the western part of the Kanto Plain were conducted. The same analyses on samples from the area around the Ichinose River, a tributary of the Tama River, which flows in the Shimanto Belt, and the Nakatsu River, a tributary of the Ara River flowing in the Chichibu Belt were conducted. Therefore, it is presumed that the source of the hornfels gravels in the Hanno Formation is in the Shimanto Belt. It is presumed that the supply route was either the Tama River system with the upper reaches of the Shimanto Belt or the river flowing through the Chichibu depression zone.

Various well-preserved fish scale fossils were found in laminated black shale from 11 Lower to Middle Miocene Josoji Formation sites on the Shimane Peninsula in southwestern Japan. Based on the observed morphologies, five families – Clupeidae, Myctophidae, Halosauridae, Sparidae, and Scombropidae – were identified for the first time in this study. To reconstruct the depositional paleoenvironment of the study sites, analyses of fossil benthic foraminifera and total organic carbon, nitrogen, and sulfur contents were also conducted. The sea bottom paleoenvironment of the lower part of the Josoji Formation was highly reductive with oxygen-deficient water masses, allowing only neritopelagic clupeid fish scales to be preserved at the sea bottom. During the deposition of the lower to middle part, the paleoenvironment shifted to a slightly oxidative sea bottom in the middle bathyal zone, enabling mesopelagic myctophid fish and demersal halosaurid fish to live in deep seas. In the upper part, the paleoenvironment transitioned to an oxygenated sea bottom in the middle bathyal zone, resulting in a high diversity of fish fauna. Based on the correlation with the paleoenvironments of other contemporaneous formations in the coastal area of Japan Sea, transgression and deepening are inferred to have occurred earlier at the study site.

Siliceous mudstones of the lower part of the Middle to Upper Miocene Yagen Formation are locally exposed and have undergone hydrothermal alteration at the northeastern part of Mt. Hiuchi-dake, a Quaternary volcano of the Shimokita Peninsula, Aomori Prefecture. This study aims to clarify the effects of the hydrothermal system of Mt. Hiuchi-dake on the concentration and composition of organic matter and the lithological composition of the mudstone. Mudstone samples were collected from three outcrop locations, all of which are situated at nearly the same stratigraphic level: (1) National road outcrop, (2) Oakagawa outcrop and (3) Altered continuous outcrop, and analyzed for organic / inorganic geochemical, and mineral investigations. The results showed a clear trend of decreasing total organic carbon (TOC) and hydrocarbon contents closer to Mt. Hiuchi-dake. The C/N and C/S ratios varied more widely closer to Mt. Hiuchi-dake, which indicate that hydrothermal alteration caused pyrolysis of the organic matter in mudstones, decomposition of the amino groups, generating asphaltenes and the dissolution and reprecipitation of indigenous pyrite. It is found that hydrothermal flow promoted hydrocarbon discharge, but the initial relative composition of n-alkanes was almost preserved. The estimated heating temperatures of the hydrothermal fluid were 100-110 °C at the National road outcrop, 200-240 °C at the Oakagawa outcrop, and 150-250 °C at the altered continuous outcrop (Ueda and Sampei 2024). Mudstones of the Oakagawa outcrop correspond to the transition zone between Opal CT and quartz, and preserve high porosity. In the altered continuous outcrop, there is a correlation between SiO₂ concentration and porosity, and rock density increases with silicification. According to mineral composition, the altered continuous outcrop was altered by high sulfidation hydrothermal system with a high contribution from volcanic volatiles. On the other hand, the Oakagawa outcrop experienced weakly acidic to neutral hydrothermal alteration originated from heated groundwater. The age of alteration may have been after 0.1 Ma in both outcrops.

Here we report foraminifera and calcareous algae discovered within calcareous clastic gravels of the Sekinoiriya Formation in the Yokozemachi Group of the Neogene Chichibu Basin. The fossil assemblages are comparable to those of the Takaoka Formation in the eastern margin of the Kanto Mountains and the Cretaceous deposit of the Sanchu Graben. The fossils indicate that the Cretaceous system existed to the east of the Chichibu Basin as a source of clastics during the deposition of this formation.