Mineral assemblage and geochemistry of sandstones of the miocene Mizunami group, central Japan; A Key to reconstructing the miocene volcanic activities

Sasao, Eiji   

The Miocene Mizunami Group is located in the eastern part of Setouchi Province, in the Inner zone of Southwest Japan. One of the geological characteristics of the Setouchi Province is the weak and locally limited volcanic activity. Recently pyroclastic flow deposits were found in the Mizunami Group. If there had been volcanic activity in the Setouchi Province, reconsideration of Miocene tectonics would be needed. This study aims to reconstruct the volcanic history recorded in sandstones of the Mizunami Group by describing vertical mineralogical and chemical changes using core from a single borehole. As a result, all sandstones studied contain volcanic glass and/or their alteration products. Distinctively, biotite is dominant in some sandstone in the Toki Lb and Hongo Formations. The second dominant heavy mineral in the Hongo and Toki Lb Formations is amphibole. Pyroxene is the dominant heavy mineral in the uppermost part of the Hongo Formation and in the Akeyo Formation. These facts imply that these heavy minerals were derived from volcanic ash. If so, the change in chemistry and heavy mineral compositions indicates an extended, though not necessarily continuous, history of volcanic activity that supplied volcanic ash to the Mizunami sedimentary basin.