長野県における後期更新世の降下火山砕屑物層序

Abstract

In this paper, the results of a detailed stratigraphical study on air-fall pyroclastics (the Osakada Loam Formation, the Hata Loam Formation and the Omachi Loam Formation) are outlined. Especially valuable as marker beds are nine Ontake volcano pumice falls in the Osakada Loam Formation, six Ontake scoria falls in the Hata Loam Formation and two pumice falls that are inferred to originate from the Tateyama volcano which is intercalated in the Omachi Loam Formation. The Ontake Pm-1', Pm-1A, Pm-2A, Pm-2B and Pm-3D beds and the Tateyama Dpm and Epm beds are each subdivided into several sedimentary units which are widely recognized by their grading structure or lithofacies.The stratigraphic positions of fine-grained ash beds are determined by stratigraphical survey of subaquatic sediments that intercalate the upper Pleistocene air-fall pyroclastics. KIGOMA ash and vitric N-Tf, K-Tf, O-Tf, and T-Tf ashes are well sorted, and the thickness of these beds are constant. It is assumed that these ashes were supplied by anomalous volcanoes.Tracing these wide-spreading pumice or scoria beds together with the ashes as sets of marker beds made it possible to correlate stratigraphic successions of separated areas. It appears that the Osakada Loam Formation, the Upper Omachi Loam Formation II, the Middle and Upper Saku Loam Formation (originating from the Yatsugatake volcano) and the Nojiri and Kaisaka Loam Formations (originating from the Kurohime and Myoko volcanoes) have accumulated during the 50, 000-10, 000 years before the present.