Hemipelagites are widespread fine-grained sediments found from shelf margin to deep-water environments, and they are considered to have been formed as background sedimentation by vertical settling and slow lateral advection of clastic and biogenic fine-grained particles. Downflow variation in hemipelagic sedimentation, however, is not yet clearly understood, and was investigated in the lower Pleistocene Kiwada and Takamizo formations in the Kazusa forearc basin on the Boso Peninsula, Japan. Mapping of three tephra key beds enabled detailed correlation of two hemipelagite beds over a distance of ca 30 km from shelf-margin to basin-plain environments. The sedimentation rates decreased gradually in the downslope direction, but increased locally in response to turbidite sedimentation. This downslope variation indicates that hemipelagic sedimentation is not necessarily constant and varies in response to progradation and retrogradation of deepwater depositional systems, as well as additional input of fine-grained sediments from turbidity currents. In contrast, the grain size, magnetic susceptibility, clay-mineral composition, total organic carbon contents relative to total nitrogen contents (carbon/nitrogen), and d 13 C values of organic carbon do not exhibit distinct downslope variation. This downslope consistency indicates that hemipelagites document temporal perturbations of geological and palaeoclimatic conditions in hinterlands, regardless of spatial and temporal variations in deep-water depositional environments. The concentrations of total organic carbon, total nitrogen and five biomarkers were lower values in turbiditic mudstones and hemipelagites that formed above a turbidite bed than in those of the other hemipelagites. This variation suggests that an additional supply of fine-grained sediments from turbidity currents likely diluted the organic matter in the hemipelagites. In addition, hemipelagites for which deposition was affected by turbidity currents exhibit slightly lower d 13 C values, and may also document an additional contribution of terrestrial organic matter from turbidity currents.

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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.