Abstract
The Miocene Tottori Group in southwestern Japan was formed in association with the expansion of the Japan Sea as a back-arc basin along the margin of East Asia. Strata of this group record processes involved in the opening of this back-arc basin. However, despite previous stratigraphic studies, the geochronology of the Tottori Group is uncertain, and the sedimentology of conglomerate strata of the group exposed along the Hakuto Coast in eastern Tottori Prefecture has not been examined.
Here, we present results of geological investigation and K-Ar dating of andesite from the Kawabara Volcanic Member of the Yazu Formation, Tottori Group, exposed along Cape Keta, as well as sedimentary characteristics of unclassified conglomerate strata of the Iwami Formation, Tottori Group, exposed on Okinoshima Island off the Hakuto Coast. Plagioclase in the groundmass of two-pyroxene quartz andesite of the Kawabara Volcanic Member has a K-Ar age of 18.3±0.6 Ma, supporting the interpretation of previous studies that the Kawabara Volcanic Member is correlated with the Yoka Formation of the Yabu Subgroup, Hokutan Group. The unclassified conglomerate strata contain sediments that record a series of environmental changes in a fluvial system, including sedimentation due to intense volcanic activity and subsequent slope deformation caused by riverine downcutting, followed by Gilbert-type fan-delta deposition and finally alluvial-fan deposition. Paleo-current directions of the fan-delta and alluvial-fan sediments indicate a northeasterly to southwesterly orientation, implying the existence of land to the northeast of the study area during the sedimentary period. Findings of the study will be useful for reconstructing the paleogeography of the Sea of Japan during the Miocene.
