Rivers in the Oshima Peninsula can be classified into four types: A, B, C and D types. A-type rivers are steep ones less than 10 km in length and flow rapidly into the sea. B-type rivers are relatively steep ones in length from about 10 to 20 km and develop river beaches and terraces. C-type rivers are sluggish ones more than 20 km in length and develop alluvial plains with meanders. D-type rivers are along wetland in various lengths and flow very slowly into the sea. A-type rivers constitute rapids, falls and pools, then are recognized as “upstream” type. This type has frequently experienced natural disasters including debris flows, avalanches and bridge washouts. In turn, B-type rivers are recognized as “upstream-to-midstream” type. Upstream zones of this type are similar to those of A type, whereas midstream zones develop meanders, river beaches and terraces. Upstream zones of this type have frequently experienced natural disasters such as debris flows, whereas midstream zones have frequently been suffered by roads destructed, houses destroyed and fields inundated. C-type rivers can be divided into three zones, namely, upstream, midstream and downstream zones. Upstream zones of this type are similar to those of A type. Midstream zones of C type develop meanders, river beaches and terraces, whereas downstream zones develop many paddy fields with banks. Upstream zones of this zone have frequently experienced natural disasters such as debris flows. Midstream zones of this type have frequently experienced natural disasters such as roads destructed. Downstream zones of this type have frequently experienced natural disasters such as homes flooded. D-type rivers have frequently experienced natural disasters including homes flooded and fields flooded.
Diatom stratigraphy was established for the Quaternary Tomikawa Formation in the Hokuto area, southwestern Hokkaido, northern Japan. As the results, three diatom zones of Neodenticula koizumii, Actinocyclus oculatus and Proboscia curvirostris, totally spanning 2.43 to 0.3 Ma were recognized. Geologic age given to the lower horizon of the formation estimated as 1.93 — 1.71 Ma (CN13a) based on the calcareous nannoplankton assemblage was consistent with the diatom one. According to the lithological and faunal correlation by the previous works, the Tomikawa Formation has been thought to be isochronal with the Setana Formation regarded as the type deposits for the middle Pleistocene in the southwestern Hokkaido area. However, our study suggests a disagreement in chronology of the formation with previous works because the basal horizon was revealed to extend geologic ages backward to early Pleistocene (Gelasian). Dominance of ice-related diatoms confirmed from the upper horizons of the Tomikawa Formation strongly suggests the localized impact by the cold coastal water on the area.
Magnetite was found from hydrothermally altered granite, or hydrothermal syenite, in the Yasu Granite pluton, Shiga Prefecture, central Japan. The hydrothermal syenite consists of several types of rock facies. Hydrothermal secondary silicate minerals in the syenite are dominantly of potassium feldspar and albite, and subordinately of quartz and an assemblage of chlorite, hydrobiotite and biotite. The syenite is associated with mineralization forming sulfide minerals such as sphalerite and pyrite. The magnetite was found in the hydrothermal syenite facies of dark green olor. This report describes the occurrence, chemical compositions and XRD data of the magnetite, which was found for the first time at least in Japan from hydrothermal syenites.
Adata set of the microfloral (diatoms and calcareous nannoplanktons) assemblages was obtained to establish biostratigraphy of the Quaternary Tomikawa Formation in the Hokuto area, southwestern Hokkaido, northern Japan. For details, see Shimada et al. (2021) published also in this issue.