Mode of occurrence and origin of the kaolin mineral-rich gravelly sediments at Rendaiji, Yuzawa City, Akita Prefecture, NE Japan

This paper describes the mode of occurrence of the Rendaiji Gravel Bed that is newly exposed in the quarry of Rendaiji, Yuzawa City, Akita Prefecture. The bed is of unknown attribution, >10 m thick, and comprises 15 or more flow units. Constituent gravels characteristically contain kaolin minerals and lesser amounts of cristobalite and/or smectite, together with iron oxides and hydroxides formed by weathering of iron sulfides. Most of the gravels rich in kaolin minerals are plastically deformed, have lenticular or thin platy morphology, and are accompanied by sparse larger gravels of kaolin mineral-bearing tuffaceous rocks. Individual flow units are poorly sorted and non-stratified with a sharp base. In many cases, they show inverse grading in the basal part and finer-grained laminations in the uppermost part. Associated with minor cristobalite and/or smectite, kaolin minerals commonly replace volcanic glass shards. In addition, crossbedding and flame structures suggest a paleoflow from the southeast. The gravelly deposits are interpreted to have been settled from lahars that were derived from one of the geothermal areas in the Sanzugawa Caldera that have been active since 2 Ma. Lenticular and/or platy, plastically deformed gravels were likely concentrated and emplaced parallel to one another from viscous laminar flows.