Submarine fan sequences of the lower Kazusa Group, a Plio-Pleistocene forearc basin fill in the Boso Peninsula, Japan

Ito, M.

Sedimentary Geology 122(1-4): 69-93

1998


ISSN/ISBN: 0037-0738
DOI: 10.1016/s0037-0738(98)00099-2
Accession: 020154883

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Abstract
The Kazusa Group (up to 3000 m thick) is the infill of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The lower part of the group is represented by thick successions of submarine fan deposits associated with deep-sea basin-plain and slope deposits. The submarine fan deposits are characterized by repetitions of sandstone-dominated and siltstone-dominated intervals (cycle thickness as much as 270 m thick). Each pair consists of a sandstone-dominated and a siltstone-dominated interval and is a depositional sequence (ca. 0.2-0.05 m.y. cyclicity). In general, sandstone-dominated intervals consist of channel and overbank deposits and age-equivalent sheet-like sandstone beds interbedded with siltstones in down-fan directions, indicating lowstand systems tract deposits. The uppermost part of sandstone-dominated intervals commonly contain chaotic deposits and fine upward to the lower part of siltstone-dominated intervals. These fining-upward successions are age-equivalent to transgressive shelf and slope deposits in up-slope areas and may have developed in response to reduction in active supply of coarse-grained clastic sediments to a deep-sea environment during transgressive stages. The remainder of siltstone-dominated intervals are characterized by thin- to medium-bedded sandstones and interbedded siltstones. Bed thickness and grain sizes of intercalated sandstones increase up-section and indicate restoration of active supply of coarse-grained terrigenous clastic sediments to a deep-sea environment in response to progradation of shallow marine depositional systems during highstand sea-level stages. The distinctive associations of lowstand, transgressive, and highstand systems tracts in submarine fan successions of the lower Kazusa Group are interpreted to reflect the development of point-sourced, small sand-rich submarine fans in the Kazusa forearc basin. This basin is inferred to be characterized by a narrow shelf, a steep slope, and high sediment supply. Coarse-grained terrigenous clastic sediments in such a setting were supplied to submarine fans during every stage of a relative sea-level cycle. Lithofacies organization of depositional sequences in submarine fan successions of the lower Kazusa Group can represent one type of variation in sequence-stratigraphic models for settings controlled by forearc tectonics that were overprinted by glacioeustasy during the Pliocene through Pleistocene.