Missing geologic unit recorded on the gravel of the Oda coast in the Izumo area, San’in district, southwest Japan

Abstract

Whole-rock geochemical compositions and the K-Ar age of welded tuff gravels of the Oda coast, San’in district, southwest Japan were examined. The welded tuff gravels are mostly massive vitrophyre characterized by strong welding and an eutaxitic texture. Phenocrysts are mainly plagioclase sometimes corroded, and quartz grains of which are strongly corroded. The estimated age of the welded tuff gravel sample is 52.3 Ma. The welded tuff gravel samples show SiO₂ contents varying 73-76 wt%. These rocks of welded tuff are not distributed upstream of the Oda River and the surrounding areas. They might have been eroded from their source area and transported by fluvial processes to the river mouth. Now they remain at the coast due to their endurability and high hardness.