We carried out the U-Pb dating on detrital zircons obtained from tuffaceous sandstone in the Nyunokawa Formation of the Shimanto Belt in the Kii Peninsula to clarify its depositional age. The youngest peak age from the Nyunokawa Formation indicates 63.0±1.8 Ma (2σ). On the other hand, the age of detrital zircons from the Otonashigawa Group indicates 50.8±1.0 Ma (2σ) corresponding to radiolarian ages obtained from this group. Therefore, there is a major gap between the depositional ages of the Nyunokawa Formation and the Otonashigawa Group. In addition, the age from the Ryujin Formation distributed along the thrust bounded between the Nyunokawa Formation and the Otonashigawa Group shows 55.8±2.5 Ma (2σ). Basically, the depositional ages become younger as tectono-stratigraphically downward (north to south), and the tendency corresponds to the characteristics yielding an accretionary complex.
Event deposits were identified in the Shonai Sand Dunes, which are situated in northeastern Japan on the coast of the Sea of Japan. The deposits reach a maximum height of 37.9 m at the Old Dune. Using carbon-14 dating with the known volcanic ash “To-a” (AD 915) as a key bed, two distinct events were identified: a northern event that occurred between the late 700s and the 800s, and a southern event (including the Sakata-kita port event) that occurred between the 1000s and the first half of the 1100s. A number of structures peculiar to tsunami deposits are observed in these beds, indicating that they formed by tsunamis. The main muddy sections of the event deposits settled during the period of standing water after the run-up flow, while the sandy sections were deposited by the return flow. Historical records indicate that the northern event corresponds to the AD 850 “Dewa earthquake”. Although the southern event does not have a matching historical record, it may correspond to an earthquake that caused deformation shortly after AD 915 to some remains in the Shonai Plain. The event deposits were well protected by the New Dune, because a sudden environmental change after the events resulted in rapid deposition of eolian sands. The growth of the New Dune of the Shonai Sand Dunes began at the time of these events and continues today; it is now the country's largest class of coastal sand dunes.
Internal load cells can be used to measure the axial load applied to a sample during a triaxial experiment precisely, which is key to obtaining reliable mechanical data. However, the output from an internal load cell contains an offset, which is a function of the confining pressure, pore pressure, and temperature. For an apparatus without a split-piston, the sample column has a non-zero axial load supported by O-rings even before a hit-point. Thus, a simple zero setting before the hit-point would cause an error in the measured axial load. In this technical note, we present a method that can be used to estimate the offset and correct the output by using outputs from both internal and external load cells. This method does not require additional experimentation.
We identified a Permian manganese-bearing carbonate layer within the siliceous shales of the Nishiki Group in the Akiyoshi Belt, Southwest Japan. The Nishiki Group is an accretionary complex composed of massive sandstones, mudstones, and Permian radiolarian-bearing siliceous shales and bedded cherts. In this paper, we present the initial descriptions of the newly discovered Permian manganese carbonates. They contain well-preserved early Permian radiolarians, including Albaillella asymmetrica and Pseudoalbaillella cf. ishigai. Although Jurassic radiolarian-bearing manganese carbonate nodules have been documented in the accretionary complexes of Japan, those of Permian age have not been described until now. In addition, we present U-Pb ages of detrital zircons extracted from the Nishiki Group sandstones, which yield a weighted mean age of 268.7±2.1 Ma, slightly younger than the age of the radiolarians in the Nishiki Group.