Bulletin of the Mizunami Fossil Museum

Bulletin of the Mizunami Fossil Museum No. 48

Petrographic examination of tuffs in the Miocene Mizunami and Iwamura Groups has been carried out to provide basic data for characterization of each tuff beds. This paper presents the descriptions of 16 and 4 tuff samples from the Mizunami and Iwamura Groups, respectively. Specifically, the following properties have been analyzed; mineral and heavy mineral compositions, shape of volcanic glass, and refractive indices of volcanic glass and plagioclase. Although T-type shards of volcanic glass were dominated in most of the tuffs studied, the study shows that it is possible to characterize each tuff based on these petrographic properties. This means that petrographical properties of tuff in the Miocene series can be utilized as the fundamental data for stratigraphic correlation in this region. Heavy mineral and plagioclase compositions of tuffs studied have a close similarity to those of host tuffaceous sedimentary rocks in the Mizunami Group. Tuffaceous material in the sedimentary rock has been derived from volcanic ash generated from unrecognized volcanic activity occurred around the Mizunami sedimentary basin. Thus, it is considered that tuffs in the Mizunami and Iwamura Groups have been formed by a volcanic activity around the Mizunami sedimentary basin.