Abstract
An outcrop of manganese oxide about one meter thick, inter-bedded with several volcanic ash layers, is found at the Yunotaki hotspring, Hokkaido. The manganese wad is precipitated from hydrothermal water. On the basis of the previously known age of the inter-bedded volcanic ash layers, linked to well-known eruptions, it has been possible to determine the chronology of the manganese oxide precipitation. The ash layers were identified by the chemical composition of the separated volcanic glasses. Since the lowermost manganese oxide layer occurs immediately above the mud flow of Mt. Ponmachineshiri without the presence of any intermediate soil layer between them, it's age of formation could be near the age of the mud flow, i.e. 4500 - 5000 years BP. Manganese oxide layers occurring below 50 cm is hard and has a diagenetically altered texture reflecting change in the filament structure to a grain aggregate structure. These oxides have lost their structural water and the mineralogy has begun to change from 10A phase to 7A phase during the diagenetic process.
