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.

This study examined depositional processes of muddy deposits intercalated in coarse-grained sandridge deposits of the Middle Pleistocene Mandano and Ichijiku formations on the Boso Peninsula, central Japan. We identified these intercalated muddy deposits as fluid-mud deposits on the basis of the combination of lithofacies, geometry, bioturbation, and clay fabric. The fluid-mud deposits are interpreted to have formed in response to storm events. Thus, muddy intercalations, which may have formed as fluid-mud deposits, are interpreted to be more common than those having been identified in coastal and shallow-marine sandy successions.

Sand ridges are present in modern storm-dominated and tide-dominated shallow-marine environments worldwide, but the number of published ancient examples is limited. The role of ocean currents in the formation of sand ridges is also poorly understood. This study investigated the lithofacies architecture of a lenticular sandstone body (ca 70 m thick and ca 30 km long) in the upper Mandano Formation (ca 0.6 Ma) on the Boso Peninsula, Japan, to establish criteria for identifying sand ridge deposits built by storm and ocean currents. The sandstone body is unconformably underlain by coastal and valley-fill deposits and fines upward to outer-shelf muddy deposits. It consists mainly of medium-grained to very coarse-grained sandstones and pebbly sandstones that are moderately to intensely bioturbated and are represented by three-dimensional compound dune deposits. The body consists of six 10 to 20 m thick units (units 1 to 6 in ascending order), which are each defined by an erosional base and locally capped by storminduced fluid mud deposits. Each unit also contains low-angle (ca 5°) large stratification inclined obliquely to palaeocurrents, indicating lateral accretion surfaces. Units 1 and 2 are characterized by south-eastward migration (locally north-westward), smaller dune deposits, storm-induced sedimentary structures and cold-water molluscs indicating a palaeowater depth of up to 50 m. The other units are represented by east/north-eastward migration, larger dune deposits, warm-water molluscs indicating a palaeowater depth of up to 200 m, and a general lack of storm-induced sedimentary structures. Temporal changes in the migration directions and lithofacies features of the units indicate that sand ridge formation was initially controlled by storminduced currents that were subsequently replaced by the northward or north-eastward intruding palaeo-Kuroshio Current in response to the expansion of the shelf during an overall transgression. The initiation and development of the sand ridges may have been supported by the supply of clastic sediments eroded from the underlying coastal and valley-fill deposits.