Yoshihisa Hiroki

Sedimentary facies and total organic carbon and total sulfur concentrations of mudstones of the lower Miocene Nojima Group confirm that the Nojima Group has been deposited in a fresh-water environment before a major marine incursion into the Japan Sea. We identified nine sedimentary facies, which were grouped into seven facies associations, and recognized three depositional environments : lacustrine, deltaic, and fluvial. Total sulfur concentrations are much lower than 0.3wt%. Total organic carbon concentrations are up to 5.61wt%. The carbon/sulfur ratios show typical values in fresh-water sediments. The sedimentary facies and geochemical analysis indicate that the most of Nojima sedimentary basin was filled with lacustrine, deltaic and fluvial fresh-water deposits. Although no sample was obtained from the upper most part of the Nojima Group (Minamitabira Formation) because of lack of exposure, marine fauna in the uppermost part of the group reported by previous researchers possibly indicate a transgression caused by a combination of a global sea-level rise and an active tectonic activity during the opening of the Japan Sea.