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.
This study investigated downslope variations in sedimentation rate, grain size, magnetic susceptibility, and clay mineral composition of hemiplelagic siltstones in the Kd8 (ca. 1.2 Ma) and O7 (ca. 0.9 Ma) volcanic ash key bed horizons in the Pleistocene Kazusa Group on the Boso Peninsula, Japan. In general, sedimentation rates decrease in the downslope directions in association with the decrease in grain size, except for local increases in sedimentation rates and grain size as a response to additional supply of fine-grained sediment particles from turbidity currents. In contrast, although the magnetic susceptibility shows a downslope variation in harmony with that in grain size in the O7 volcanic ash key bed horizon, it is also influenced by local development of authigeneic magnetic minerals in the Kd8 volcanic key ash bed horizon. Regardless of the spatial variations in sedimentation rates, grain size, and magnetic susceptibility, clay mineral composition does not show any distinct spatial variation and can be used as a proxy of temporal variation in hinterland geology and paleoclimates in a one-dimensional stratigraphic record.