Hiroshi Sato

The Ohnuma Moor in the eastern part of the Chugoku Mountains, western Japan, is located about 80-km west of Daisen Volcano and more than 100-km west of Sambe Volcano. The moor has thick sediments more than 65 meters which are composed of finer peat and organic clay and silt above about 17 meters in depths, and of coarser sand and gravels below. The finer part contains four tephra layers that were correlated with Kikai-Akahoya Volcanic Ash Bed (K-Ah), Daisen-Misen Pumice Bed (MsP), Daisen Shitano-hoki Volcanic Ash Bed (Sh), and Aira-Tanzawa Volcanic Ash Bed (AT) in descending stratigraphic order. We have newly found two volcanic ash layers between the K-Ah and MsP, and below the AT in the finer sediments. Based on the petrologic characteristics of these two tephra layers, the upper tephra layer is correlated with the Sambe Ukinuno Volcanic Ash Bed (SUk), and the lower is correlative to the Daisen Nise-hoki Volcanic Ash Bed (Nh). The eruptive ages of five tephra layers except for the K-Ah are estimated on the basis of 22 AMS-^<14>C dates obtained for the moor deposits above 17 meters in depths, with the assumption of a constant sedimentation rate between horizons where the AMS-^<14>C dates are measured. As the results, the estimated eruption ages of the SUk, MsP, Sh, AT, and Nh are 16,740±150 ^<14>CyBP, 24,000±150 ^<14>CyBP, 24,320±150 ^<14>CyBP, 24,830±100 ^<14>CyBP, and 35,660±200 ^<14>CyBP, respectively. The estimated ages are in good agreement with the results of the previous studies, although the MsP is considered to have erupted at the age by 5,500 to 7,500 years older than the previous estimated age of 16,500 to 18,500 ^<14>CyBP. This may be resulted from mismatching of tephra correlation and/or pollution of materials used for the ^<14>C-age determination in the previous studies.