Abstract Neogene sediments on western Hokkaido have been divided into eight formations which are organized into three depositional facies. Facies 1 consists of volcaniclastic sandstone (lithic wacke) interbedded with volcanic ash and mudstone. Ash is normally graded from coarse granule to fine sand, whereas siltstone is organized as a normally graded sequence of coarse siltstone overlain by clayey siltstone and capped by clay stone. Deposition occurred by ash fall and turbidity currents on a marine slope. Facies 2 consists of mudstone and fine-grained ash containing slump blocks ranging from 20 cm to 5 m in diameter, load casts and reworked fossils. Facies 2 was derived from a volcaniclastic source and deposited on a marine slope where slumping was common. Facies 3 consists of coarsening-upward sequences from mudstone through thick, cross-stratified sandstone, produced by a prograding delta. The lithic wackes contain more than 85 percent volcanic clasts and other lithic clasts. Clay minerals include illite, montmoriJJonite, chlorite and mixed layer illite-montmorillonite. These facies were deposited in a small intraslope basin comprising part of an early Neogene active continental margin. Deposition occurred on slopes by turbidity currents and slumping. Deltaic progradation occurred during a relative lowering of sea level. It appears that slumping dominated deeper-water episodes of basin evolution, whereas sand influx occurred during lower stands of sea level associated with deltaic progradation. The primary criteria for recognition of this ancient slope sequence include paleoecology, dominance of fine-grained sediments organized into normally-graded cycles, and abundant slump features.