Abstract
This study investigated downslope variations in vertical changes in grain sizes and sedimentary structures within sandstone and sandy siltstone beds, which are interpreted to have formed as hyperpycnites in front of a shelf-margin deltaic system. An example of this study is from the Middle Pleistocene Chonan Formation on the Boso Peninsula, Japan. In general, the hyperpycnites show an overall convex-up lenticular geometry and downslope decreases in grain sizes and thickness, except for medium- to very thick-bedded massive sandstones, which are associated with some bypass zones in a proximal area. Thin- to medium-bedded hyperpycnites in a proximal area are characterized by single or multiple inversely-to-normally graded bedding in association with tractinal structures, which pass upward to normally graded sandstones and sandy siltstones. In contrast, normally graded sandstones, which are gradationally overlain by sandy siltstones with local concentrations of plant fragments and can be described by the Bouma model, are more common in a distal area as a response to both the decrease in the density of hyperpycnal flows and the sedimentation from lofting plumes.
