支笏・クッタラ両火山起源のテフラに関する加速器質量分析(AMS)法による^<14>C年代

北海道南西部の支笏・クッタラ両火山から後期更新世に噴出した数多くのテフラの中で, Kt-1, Kt-TkおよびSsfa・Ssflの3つのテフラに関して, 3点の加速器質量分析(Accelerator Mass Spectrpmeter, AMS)法による14C年代値を得た. これらの年代値と, これまでに得られたAMS法による14C年代値を整理し, 両火山を起源とするテフラの噴出年代を, Kt-1が約4万年前, Kt-Tkが4.4～4.8万年前, Kt-3が4.7～5.1万年前, Ssfa・Ssflが4.7～5.3万年前であると推定した. また, Kt-1直上に層位するSpfa1の噴出年代は3.8～3.9万年前と, Kt-1とKt-Tkの中間に層位するZ-M(山縣ほか, 1989)の噴出年代は4.2～4.4万年前と, それぞれ推定された.
The Shikotsu and Kuttara volcanoes in southern Hokkaido often erupted violently during the Late Pleistocene and provided many time-marker tephra layers which are widely distributed to the east of them. These layers are important for the Quaternary tephrochronological studies in the Ishikari Lowland, Hidaka District, and Tokachi Plain. The nomenclature of the tephra layers here are based on Yamagata (1994). We obtain there new radiocarbon dates by accelerated mass spectorometry (AMS-14C dates) : 39,290±970, 45,500±2,000, and 49,800±3,100 yrs BP for Kt-1, Kt-Tk, and Ssfa • Ssfl, respectively. By correlating these and other AMS-14C dates in previous studies, we estimate the eruption ages of tephra layers from the two volcanoes as follows : ca.40 ka for Kt-1, 44 - 48 ka for Kt-Tk, 47 - 51 ka for Kt-3, and 47 - 53 ka for Ssfa • Ssfl. In addition, the dates of Spfa 1 and Z-M are estimated 38 - 39 ka and 42 - 44 ka based on their stratigraphical relationships with the above tephra layers.