Mineralogical studies on two types of Miocene sedimentary rocks (shale and siliceous shale) in the Oga Peninsula, northern part of Japan indicate that (1) weathered shale contains gypsum, while weathered siliceous shale does not, and (2) carbonates are abundant in unweathered shale, but they are poor and feldspar is abundant in unweathered siliceous shale.
The calculations on the change in groundwater chemistry due to the dissolution of calcite and feldspar indicates that gypsum is formed by the calcite dissolution but it is difficult to be formed by the feldspar dissolution. The mineralogical studies and the dissolution calculations suggest that the mineralogical composition of original sedimentary rocks is important factor controlling gypsum formation, together with pyrite abundance in the original rocks and humidity.