Total organic carbon（TOC）and total sulfur concentrations of 147 mudstones adjacent to the boundary of the Koura and Josoji（K/J）Formations（16 to 16.5Ma）have been analyzed in order to reveal the characteristics of the Japan Sea transgression in the Middle Miocene．The samples were collected at Kitaura and Houda，eastern Shimane Peninsula，where the K/J boundary section is well exposed．
TOC concentrations of mudstones from the lower to middle Koura Formation are generally less than 0.3％，and their TOC to total sulfur（C/S）ratios are more than 5．The upper Koura mudstones（Shichirui Member）have comparatively high sulfur concentrations of about 1％ and low C/S ratios of less than 5，which show a significant variation．These results indicate that most of the Koura Formation was deposited in freshwater，but the depositional environment of the upper Koura Formation fluctuated between fresh to brackish and marine conditions．
The lowermost Josoji mudstones close to the K/J boundary are characterized by higher sulfur concentrations of more than 4％．However，the sulfur concentrations of Josoji mudstones stratigraphically higher than 100m above the K/J boundary drastically decrease to about 1％ which is similar to the average sulfur concentration of open marine mudstones．This suggests that the depositional environment of the lower Josoji Formation was highly reducing，due to the abrupt invasion of sea water into the terrestrial environment where the ferric iron is abundant．An open marine environment was subsequently formed in a geologically short time due to the rapid transgression of the Japan Sea．The Middle Miocene transgression in this area is not only due to eustatic sea level rise， but is also controlled by regional tectonic subsidence of the basement．
Key words : Josoji Formation，Koura Formation，Middle Miocene，Shimane Peninsula，total organic carbon，total sulfur，transgression of the Japan Sea，