2007 Volume 113 Issue 12 Pages 628-635
The Konan terrace along the middle reaches of the Arakawa River, northwestern Kanto Plain, has been believed to have formed as an alluvial fan during MIS 5e or a subsequent regressive stage. Recently, we detected a tephra layer from a tuffaceous clay bed above the terrace gravel bed in the Konan terrace. Petrographic and EPMA analyses reveal that this tephra layer is assigned to the lower part of the Iizuna-Nishiyama Tephra Group (In-Ny).
The examined tephra layer is characterized by coarse- to granule-sized yellowish-white pumice grains, and contains quartz, plagioclase, cummingtonite, and iron minerals. Of them, the cummingtonite shows the refractive index of 1.663-1.670 (mode: 1.666) and the Mg value of 63.3. These characteristics coincide with those of the lower part of the In-Ny Tephra Group obtained from several localities close to the Iizuna Volcano such as Takayama, Furuma, Sugadaira, and Yokokawa.
The In-Ny Tephra Group is considered to have fallen at ca. 180 ka (MIS 7-6 transition) on the basis of its stratigraphic position between the SgP.2 and Tt-D tephra layers. As the Konan tephra is intercalated in the base of the tuffaceous clay bed thought to be of eolian deposit origin, the formation age of the Konan terrace should be same as that of the In-Ny Tephra Group and older than the previous interpretation (MIS 5e). Then, the slip rate of the Konan Fault displacing the Konan terrace is reestimated at 0.06 m/ky.
