Abstract
This paper describes the formation and development of slip surfaces of the landslides that have occurred in the Triassic Nariwa Group. The formation of the slip surfaces is controlled by the characteristics of lithology and pre-existing shear plane. The non-marine facies of the Nariwa Group is characterized by the fining-upwards cycles that start with sandstones at the bottom and end with mudstones and coals at the top. The slip surfaces formed in coaly beds are intercalated between sandstones and mudstones. A difference in rock strength between sandstone and coaly beds might cause the formation of a pre-existing shear plane. The Nariwa Group was folded by the flexural-slip folding. This type of folding creates shear planes parallel to beddings. The shear planes are inferred to be formed in coaly beds. Resulting from slope instability, the shear planes in coaly bed developed into the slip surface that is associated with sheared coaly bed under a landsliding.
