The Plio–Pleistocene Saji Formation in Central Tottori, Southwest Japan: Implications for the Late Neogene–Quaternary Stratigraphy of the Eastern San'in Region

Abstract

The late Neogene–Quaternary stratigraphic framework of central Tottori area in the eastern San'in region, southwest Japan, has long been based on lithostratigraphic schemes established before robust geochronologic control, leaving unresolved inconsistencies between mapped units and independently derived ages. In the Saji area of central Tottori Prefecture, a semi-consolidated terrestrial clastic–volcaniclastic succession has previously been assigned either to the Lower Miocene Tsukudani Conglomerate Member or to the Togo Formation of the Misasa Group. Detailed geological mapping and zircon U–Pb dating conducted in the present study show, however, that this succession is Plio–Pleistocene rather than Miocene. A volcaniclastic bed within this succession yielded a weighted-mean age of 2.58 ± 0.04 Ma, which is consistent with the whole-rock K–Ar age of 2.29 ± 0.03 Ma for the overlying andesite lava. Recognition of this unit, designated here as the Saji Formation, together with published radiometric ages of associated volcanic rocks, highlights the need to reassess the stratigraphic framework of the Misasa Group and younger volcanic units. This recognition also contributes to refining the tectono-volcanic evolution of the eastern San'in region on the back-arc side of southwest Japan.