A detailed chronostratigraphic framework established by the mapping of tephra key beds and application of oxygen isotopic data allows assessment of the synchroneity and diachroneity of depositional systems formed in coastal and deep-water environments. This framework also allows estimation of the timing of active delivery of coarse-grained sediments beyond the shelf margin in relation to relative sea-level changes. The depositional processes of deep-water massive sandstones (DWMSs) are still enigmatic; their formation is a result of active delivery of sands in association with the supply of organic carbon into deep-water environments. DWMSs are also important as reservoirs for hydrocarbon explorations. This study investigated the origins of DWMSs in the upper Umegase, Kokumoto, and Chonan formations (in ascending order) of the Pleistocene Kazusa Group on the Boso Peninsula, central Japan. Each formation contains several packets of DWMSs that are interpreted to have formed in response to the progradation of gravelly shelf-margin deltas or fan deltas during the falling and lowstand stages of relative sea-level changes controlled primarily by glacioeustasy. The development of DWMSs and associated sandstone beds is interpreted to have been induced by hyperpycnal flows, in association with sediment gravity flows that were initiated by breaching and/or collapse of sandy substrates on the shelf-margin deltas or fan deltas. The timings of the initial and final deposition of the packets vary within and between the formations, and are considered to have been controlled by the interaction between allogenic and autogenic processes operating in the gravelly shelf-margin deltas or fan deltas. A muddy horizon that contains the Lower–Middle Pleistocene Subseries boundary (the base of the Chibanian Stage) in the Kokumoto Formation is also underlain and overlain by the packets and represents a deposit formed in a condensed section in an upper slope environment. This depositional setting may have favored the development of the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene Subseries boundary in the formation.

We classify volcanic products of the 1914 1915 eruption of Sakurajima Volcano into four types according to vesicularity and the texture of plagioclase microlite. Type 1 is white Plinian fall pumice with high vesicularity (> 60 vol%) and low modal content (< 1.0 vol%) and number density (< 10 14 m ) of plagioclase microlite. Type 1 and type 2 pumices were mainly deposited within the lower and middle parts of the Plinian fall deposit, whereas type 3 pumices were mainly deposited in the upper part. Homogeneous chemical features of preeruptive melts of all types of pumices indicate that the observed variations in number density and modal content of plagioclase microlite, as well as the decreasing vesicularity from type 1 to type 3 pumices, are related to a decrease in the decompression rate of magma from early to late stages of the Plinian phase. Residual water contents and degree of vesicularity of Plinian fall pumices suggest a change in the degassing process during the Plinian phase of eruption from minor degassing during the eruption of type 1 pumices to effective degassing during the eruption of type 3 pumices. Differences in microlite texture between type 1 and type 2 pumices of the early to middle stages of the Plinian phase and late stage type 1, 2 and 3 pumices reflect changes in the decompression rate, perhaps related to differences between the central and marginal parts of conduits. A systematic increase in the An content of plagioclase microlites from Plinian fall pumice to Taisho Lava reflects a chemical change in the preeruptive melt from silicic to basic composition related to progressive magma mixing.

1This 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.

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