Abstract
The Nakaguri landslide of Pliocene mudstone in Niigata Prefecture is a reactivated landslide. Chemical and mineralogical analyses of the drilled cores from the Nakaguri landslide area clarified the following facts:
The degree of chemical weathering of the movement layer is highest near the surface and declines with increasing depth. In the oxidation layer, sulfuric acid is produced by oxidizing pyrites, the solid soil is made acidic, and this acidification process reaches the layer in the subordinate position.
Moreover, there is more smectite in soils in contact with the assumed slip surface and the narrow band near it than in the soil outside of these parts. It is assumed that minerals change in quality and become smectite as the movement layer acidifies. Chemical weathering of the mudstone is dominated by the oxidation of pyrite and the dissolution of rock-forming minerals by the resultant sulfuric acid, which leads to the deterioration of the mudstone. The increase of smectite is assumed to be one of the main causes of the movement of the landslide.
