Spatial and temporal variations in depositional systems in the Kazusa Group: insights into the origins of deep-water massive sandstones in a Pleistocene forearc basin on the Boso Peninsula, Japan

Abstract

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.

Formal ratification of the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Stage and Middle Pleistocene Subseries of the Quaternary System: the Chiba Section, Japan†

“…The main part of the Chiba composite section, dominated by silty beds, is assigned to transgressive and highstand systems tracts of a fifth order depositional sequence as defined by Mitchum and VanWagoner (1991) during MIS 19 (Ito et al, 2016;Takao et al, 2020). The Byk-E tephra bed is correlated to the uppermost transgressive systems tract, almost a downlap surface (Takao et al, 2020). A detailed stratigraphic log and grain-size analysis of the Chiba section is shown in Fig.…”

“…beds of sediment gravity-flow origin are intercalated within the silty section, particularly in its lower part, the section is generally thought to represent a stable depositional unit (Nishida et al, 2016). The main part of the Chiba composite section, dominated by silty beds, is assigned to transgressive and highstand systems tracts of a fifth order depositional sequence as defined by Mitchum and VanWagoner (1991) during MIS 19 (Ito et al, 2016;Takao et al, 2020). The Byk-E tephra bed is correlated to the uppermost transgressive systems tract, almost a downlap surface (Takao et al, 2020).…”

Multiproxy sedimentological and geochemical analyses across the Lower–Middle Pleistocene boundary: chemostratigraphy and paleoenvironment of the Chiba composite section, central Japan

“…Because the Kokumoto Formation is well-exposed along the Yoro River of the Boso Peninsula, central Japan, there have been numerous studies on the formation based on litho-, bio-, tephro-, magneto-, and oxygen-isotope stratigraphy (as reviewed by Kazaoka et al 2015;Suganuma et al 2018). In addition, the sedimentological features of the Kokumoto Formation have also been extensively investigated (Katsura 1984;Ito 1992;Ito and Katsura 1992;Nakamura et al 2007;Nishida et al 2016;Takao et al 2020). However, the detailed chemostratigraphy (i.e., organic carbon isotope ratio, the contents of organic elements such as carbon and nitrogen, and major inorganic elements) of the CbCS, particularly near the Lower-Middle Pleistocene boundary, remains poorly understood.…”

Review of the Early–Middle Pleistocene boundary and Marine Isotope Stage 19

“…Deep-water massive sandstones in the middle and upper parts of the Kazusa Group represent hyperpycnal and sediment gravity flows originating from shelf-margin deltas or fan deltas. These flows were activated during the falling and lowstand stages of sea-level oscillations controlled primarily by glacioeustacy (Takao et al 2020).…”

“…14c) attests to continuous deposition without slumping. This muddy unit, underlain and overlain by deep-water massive sandstones, is a relatively condensed section formed in the uppermost part of a transgressive systems tract (Takao et al 2020) on the continental slope.…”