Slump scars and slump deposits on the fan-delta slope, Late Miocene rocks in Saigawa Anticline area, Northern Fossa Magna Region, Japan

Abstract

The study area is situated on the right side of the middle of Sai-river, the southern part of the Northern Fossa Magna region. There are large cliff exposures which consist of an upper Miocine sedimentary rocks, Aoki Formation. The sedimentary environment of these rocks was fan-delta slope in the progradational stage of fan-delta system. Exposures show discordances which concave upward, and large slump structures being followed for about 100m. Discordances are interpreted as slump scars formed by downslope sliding or slumping of fan-delta slope sediments. The large slump structure resulted from a slumping sidewall of scar into scarsurface. In this case, slumping and sliding occuerred frequently on the fan-delta slope. Then, a lot of clust were released down-slope and formed lobe shaped sediments. This is characteristic of fan-delta slope sedimentation during the progradational stage and may reflect hightstand of the relative sea-level.