the basis of the petrographic and stratigraphic studies of these tephras, it is concluded

Cretaceous Taiheizan composite granitic pluton occurs in the Taiheizan mountains, central Akita prefecture, Northeast Japan. The rocks are divided into the East and West masses by the Kyowa Hitachinai mylonite zone, which resulted in right lateral movement. The East mass is composed of mainly weakly foliated medium to coarse grained hornblende biotite granodiorite and tonalite. They contain magnetite, but these magnetic susceptibilities average 8.08 10 3 SI units, which are relatively lower than those of typical magnetite series Kitakami granitic rocks. On the other hand, the West mass is composed of mainly foliated granodiorite and tonalite, which belong to the ilmenite series due to the absence of magnetite. Both rocks have different chemical features. The magnetite series East rocks have lower contents of Al 2 O 3 , Na 2 O and higher contents of MgO, K 2 O than the ilmenite series West rocks. Trace element contents show that the East rocks have higher contents in Cr, Ni, V, but lower in Zr. However, the chemistries of both rocks resemble those of Abukuma granitic rocks as compared with those of Kitakami rocks. The reported K Ar ages of the East and West masses also show the same ages from Abukuma rocks. Therefore, I conclude that the Taiheizan granitic rocks belong to the Abukuma belt, and the Kyowa Hitachinai mylonite may coincide with the boundary of low magnetic susceptibility magnetite series rocks and ilmenite series rocks in the Abukuma granitic rocks.

No abstract

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 mineralbearing 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.