[Abstract]

Two volcaniclastic deposits--a debris-avalanche deposit (NgDA) and a lahar deposit (NgL) were discovered along the Nigorikawa River on the western slope of Aso central cones, southwestern Japan. The Nigorikawa debris-avalanche deposit, having a maximum thickness >3 m, contains numerous plastically deformed debris-avalanche blocks (<3.7 m) of volcanic ash and soil layers in a poorly-sorted silty to clay matrix. The Nigorikawa lahar deposit contains many subangular to subrounded lithic clasts (<0.7 m) and shows clast-supported and matrix-rich depositional structures. We obtained a 14C age of 2,230+-0 years BP from a wood fragment in NgDA, which corresponds to 400-100 cal BC (2 sigma). The date is consistent with the age of cultural remains (the Yayoi period: 300 BC òû300 AD) underlying the debris-avalanche deposit. We obtained a 14C age of 4,100+/-60 years BP (2880-2480 cal BC) from a wood fragment in NgL. These 14C ages indicate that major volcaniclastic flows have inundated the Nigorikawa River multiple times in the past 4,000 years. Although the source and cause of the volcaniclastic flows remain unsolved, this discovery provides important information about volcanic hazards in the western part of Aso central cones. E-mail: yasuo@affrc.go.jp