[HDS12-P02] Deep-seated gravitational slope deformation process deduced from composition and age of sediments accumulated in a linear depression of Nishifuruike on the ridge near Kamikochi, Northern Japan Alps
Keywords:deep-seated gravitational slope deformation, Northern Japan Alps, tephrochronology
Kamikochi is located about 1,500 m in altitude on a wide riverbed of the Azusagawa River. The ridges extending NE-SW direction from Mt. Ohtakiyama (2616 m) to Tokugo Pass (2140 m) on the left side of Azusagawa River have wide crests with low relief and also have 2 to 3 lines of narrow depressions parallel to the ridges, which were regarded to be formed by the deep-seated gravitational slope deformation. The depressions are, in part, pond, swamp, and wet lowland with no trees unlike the surrounding coniferous forests, and are filled with fine-grained sediments. We have drilled 7 cores in Nishifuruike, one of the depressions, in order to clarify the development history of the depression. The cores are common in sedimentary facies, and are composed of the upper dark-brown silt and mud rich in organic materials covering the lower alternating coarse-grained volcanic sands, laminated silt and fine-grained sand, and silt. The sediments were sampled with about 5 cm-thick intervals, and their very fine-grained fractions have been observed under microscope, most of which consist of eolian volcaniclastic materials: volcanic glasses and minerals. The facts that the volcanic glasses are mainly composed of microlite- and pumice-type glasses and the sediments are coarse-grained volcanic materials such as granule-size clasts indicate the sediments were derived from the Yakedake volcano about 8 km west from the Nishifuruike. The last large-scale magmatic eruption of Yakedake volcano is recorded by the 2.3 ka Nakao pyroclastic flow deposits; this indicates the depression had been formed at or before 2.3 ka.