Depositional process of the Pliocene Anno Formation, the uppermost part of the Awa Group, Boso Peninsula, central Japan-A case study on a turbidite depositional system of a forearc-basin filling-

与四郎, 石原; 秀一, 徳橋

doi:10.5575/geosoc.111.269

“…In the western part of the peninsula, the Anno Formation generally shows an upward-coarsening succession from mudstone, sandy mudstone, and muddy sandstone to sandstone (Nakajima, 2005). The Anno Formation represents submarine fan deposits (Nakajima et al, 1981;Ishihara and Tokuhashi, 2005) or a slope base system (Saito and Ito, 2002). Ishihara and Tokuhashi (2005) recorded the lateral distribution and successive changes in the turbidite facies and paleocurrent directions.…”

Section: Boso Peninsulamentioning

confidence: 99%

“…The Anno Formation represents submarine fan deposits (Nakajima et al, 1981;Ishihara and Tokuhashi, 2005) or a slope base system (Saito and Ito, 2002). Ishihara and Tokuhashi (2005) recorded the lateral distribution and successive changes in the turbidite facies and paleocurrent directions. They identi ed six sedimentary facies associations: channel ll deposits of pebbly sandstone and conglomerates; natural levee deposits of alternating thin turbidites and hemipelagic mudstones; thick-bedded lobe deposits; marginal deposits of alternating thin turbidites and hemipelagic mudstones; marginal deposits of alternating hemipelagic mudstones and tephra beds; and extensive slump deposits.…”

Section: Boso Peninsulamentioning

confidence: 99%

“…They identi ed six sedimentary facies associations: channel ll deposits of pebbly sandstone and conglomerates; natural levee deposits of alternating thin turbidites and hemipelagic mudstones; thick-bedded lobe deposits; marginal deposits of alternating thin turbidites and hemipelagic mudstones; marginal deposits of alternating hemipelagic mudstones and tephra beds; and extensive slump deposits. The upper part of the Anno Formation consists of marginal deposits of alternating hemipelagic mudstones and tephra beds that coarsen upward, suggesting a shallower depositional environment or the winnowing of ne-grained sediment owing to bottom currents (Ishihara and Tokuhashi, 2005). There are many mass-transport deposits in the Anno Formation, of which "Ta slump" below An22 and "Sak slump" below An157 can be traced Fig.…”

Section: Boso Peninsulamentioning

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

¹

,

Tamura

²

,

Nozaki

³

et al. 2023

Preprint

0

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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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Neogene–Quaternary sedimentary successions

Itô¹,

Kameo²,

Satoguchi³

et al.

The Geology of Japan

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

Utsunomiya

¹

,

Tamura

²

,

Nozaki

³

et al. 2023

Prog Earth Planet Sci

1

0

View full text Add to dashboard Cite

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.

show abstract

Depositional process of the Pliocene Anno Formation, the uppermost part of the Awa Group, Boso Peninsula, central Japan-A case study on a turbidite depositional system of a forearc-basin filling-

Cited by 6 publications

References 20 publications

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

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

Neogene–Quaternary sedimentary successions

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

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