2007 Volume 46 Issue 1 Pages 47-61
The Plio-Pleistocene Gotsu Group is widely distributed in central and western Shimane Prefecture, southwest Japan. The chemical compositions of volcanic glass shards and constituent minerals in tephras collected from seven sites in the Gotsu Group were examined for correlation of the Plio-Pleistocene successions. Rhyolitic glass shards (SiO2=77.0-79.8wt.%) were found in four tephra beds. Common hornblende and zircon occur in all tephras. These tephras were divided into four groups based on their association of mafic minerals : (1) olivine, (2) orthopyroxene and biotite, (3) cummingtonite and biotite, and (4) cummingtonite.
Combining the assemblages and chemical compositions of constituent minerals with fission track ages, two tephra beds belonging to group (2) dated at 1.7±0.2Ma, and two tephra beds from group (3) with ages of 1.1±0.1Ma can be correlated. Tephra beds in group (3) may be derived from the Plio-Pleistocene Oe-Takayama volcano.
The tephras described with clear characteristics and reliable radiometric ages will be valuable for correlation of the Plio-Pleistocene successions in southwest Japan outside Shimane as well as in the Gotsu Group.
