Petrographic study of the Miocene Mizunami Group, Central Japan; Detection of unrecognized volcanic activity in the Setouchi Province

Sasao, Eiji   

Petrographic study of sandstones from the Miocene Mizunami Group distributed in Central Japan has been performed using core from a single borehole to evaluate the provenance of the sedimentary rock. Sandstones studied are divisible into three types based mainly on their heavy mineral and plagioclase compositions; one type is characterized by biotite and plagioclase in the range from albite to oligoclase, the second is characterized by dominance of amphibole and labradorite, and the third type is characterized by dominance of pyroxene and andesine with lesser labradorite, bytownite and anorthite. The first type was derived from basement granite whereas the others were derived mostly from volcanic ash. The volcanic activity, which supplied the volcanic ash to the Mizunami sedimentary basin, occurred in two phases, distinguishable by variations in mineralogical and geochemical compositions. This is an indication of change in the character of the volcanic activity.