Abstract
Three major treads of fluvial terraces (lower, middle, and higher terraces) are well developed along the Kabura-gawa river, tributary of the Tone-gawa river, in the southern part of Gunma prefecture, northern Kanto region. We detected cryptotephras in overbank and aeolian deposits covering the middle terrace gravels, and examined refractive indices and chemical compositions of some of amphiboles in the cryptotephras. We also investigated refractive indices and chemical compositions of amphiboles in the Iizuna-Kamitaru tephra (Iz-KT) and the Iizuna-Nishiyama (Iz-NY) tephra extracted from outcrops near the Iizuna volcano, northern Nagano prefecture. Based on these data, we identified possibly Iz-KT and Iz-NY tephra at the bottom of overbank and aeolian deposits of the middle terrace of the Kabura-gawa river. This indicates that the middle terrace was formed through aggradation at the glacial age of marine oxygen isotope stage (MIS) 6 and subsequent degradation at interglacial of MIS 5e. The Kabura-gawa river basin is located out of the previously reported distribution area of Iz-KT. This fact implies that Iz-KT is distributed more widely in the northern Kanto and southern Tohoku regions, and that Iz-KT is useful as marker tephra of the middle Pleistocene in these regions.
