An<i>in situ</i>Vesicomyid-dominated Cold-seep Assemblage from the Lowermost Pleistocene Urago Formation, Kazusa Group, Forearc Basin Fill on the Northern Miura Peninsula, Pacific Side of Central Japan

Utsunomiya, Masayuki; Majima, Ryuichi; Taguchi, Kiminori; Wada, Hideki

doi:10.2517/2014pr024

Cited by 6 publications

(3 citation statements)

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“…Such coarse deposits are unusual in the lower part of the Kazusa Group, which is generally characterized by alternating beds of sheet-like turbidites and hemipelagic mudstones (Ito and Katsura, 1992;Tokuhashi, 1992;Utsunomiya, 2019), suggesting a supply from lapilli beds in the collapsed section of the Anno Formation owing to a submarine landslide. Another candidate for a sedimentary source would be ocean-current-induced pyroclastic-rich sand-ridge deposits formed on an upper slope, as seen in the Urago Formation (Utsunomiya et al, 2015), because it is possible to assume sand-ridge deposits were developed on the excavated upper slope. In either case, the pebbly sandstone would have been deposited rapidly, resulting in poor sorting.…”

Section: Major Erosion At Around 32 Ma and The Resultant Depositsmentioning

confidence: 99%

“…The lowermost part of the Kazusa Group, the Urago Formation, consists mainly of tuffaceous medium-to coarse-grained sandstones and tuffaceous muddy sandstones (Utsunomiya and Majima, 2012). Trough-and tabular-cross bedding is common in the tuffaceous sandstones, and this has been attributed to the migration of dunes under the in uence of bottom currents that varied in direction between northward and eastward (Utsunomiya et al, 2015).…”

Section: Miura Peninsulamentioning

confidence: 99%

“…5d). The absence of ne ash beds is a probable result of strong bottom currents, which are considered to have been dominant when the coarser facies was deposited (Utsunomiya et al, 2015(Utsunomiya et al, , 2017, and this can explain the absence of An157.5 and An158.5 in other areas.…”

Section: Tephra Correlationmentioning

confidence: 99%

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Basin-wide erosion and segmentation of the Plio-Pleistocene forearc basin in central Japan revealed by tephro- and biostratigraphy

Utsunomiya

Tamura

Nozaki

et al. 2023

Preprint

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The basement of the Tokyo metropolitan area consists of the Miocene–Pleistocene forearc basin fills that are well exposed around Tokyo Bay, especially on the Miura and Boso peninsulas. The forearc basin fills on these two peninsulas are called the Miura and Kazusa groups, and they were deposited during the late Miocene–Pliocene and Pliocene–middle Pleistocene, respectively. Because many biostratigraphic datum planes, paleomagnetic reversal events, and other chronostratigraphic tools are available for these deposits, they provide the “type stratigraphy” of other equivalent sedimentary sequences on the Japanese islands and in the northwest Pacific. However, the use of such stratigraphic markers has not been fully applied to understand the architecture of a basin-wide unconformity between the Miura and Kazusa groups called the Kurotaki unconformity. For our study, we made correlations among the Pliocene vitric tephra beds based on their stratigraphic levels, lithologic characteristics, the chemical compositions of glass shards, and calcareous nannofossil biostratigraphy. As a result, we were able to correlate tephra beds Ng-Ky25 just above the C3n.3n normal subchronozone (4.7 Ma), IkT16-An157.5 and IkT19-An158.5 near the top of the Mammoth reverse polarity subchronozone (3.21 Ma), and Ahn-Onr (2.6–2.7 Ma) across Tokyo Bay on the Miura and Boso peninsulas. We were able to recognize erosional surfaces and coeval mass-transport deposits immediately below the top of the Mammoth reverse polarity subchronozone, which suggests that submarine landslide(s) may have produced the lack of stratigraphic horizons (4.5–3.2 Ma) in the Miura and eastern Boso regions. Basal pebbly sandstone beds pervasively cover the erosional surfaces, and they show lateral variations into the thick (up to 60 m) mass-transport deposits and overlying turbidite sandstones. The lateral variations in sediment thickness of the post-failure deposits suggest that the basin-wide erosion was associated with the initial growth of a basin-bounding structural high that separates two distinct sub-basins in the forearc basin, which resulted in the subsequent onlapping deposition in the earliest stage of the Kazusa forearc basin. The basin-wide erosion marks the initiation of tectonic reconfigurations that led to segmentation of the forearc basin around the Tokyo Bay region.

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Section: Major Erosion At Around 32 Ma and The Resultant Depositsmentioning

confidence: 99%

Section: Miura Peninsulamentioning

confidence: 99%