On "The Kurotaki Unconformity"

Koike, kiyosi

doi:10.5575/geosoc.57.143

Cited by 17 publications

(9 citation statements)

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“…The Boso forearc basin is divided in the Miocene to Pleistocene Miura Group and Kazusa Group. The boundary between the two groups is the Kurotaki Unconformity, 13) a large-scale geological boundary distributed from east to west. The Kazusa Group comprises alternation of sand and mud without hydrothermal metamorphism and includes many volcanic tuff layers.…”

Section: Experimental Materialsmentioning

confidence: 99%

Strain Softening of Siltstones in Consolidation Process

et al. 2020

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Strain softening is the mechanical behavior of soil and rock materials and is important in understanding soft rock foundation. To investigate the mechanical behavior of siltstone, a sedimentary soft rock, consolidation tests using constant-strain rate loading were conducted using the consolidation ring to constrain lateral deformation. Using Quaternary siltstones distributed in the Boso Peninsula, central Japan as specimens, strain softening in the consolidation process was confirmed in some formations using two test machines at Kyoto University and Nagoya Institute of Technology. Just before the yielding, stress decreased suddenly at increasing strain. The stress at the time of the softening differed even for specimens taken from the same formation. Furthermore, micro-focus X-ray CT images taken before and after the tests indicated that the specimens had no macro cracks inside. This suggests that strain softening is not due to brittle failure in local areas but due to the softening of the framework structure of the siltstone itself.

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Section: Experimental Materialsmentioning

confidence: 99%

Strain Softening of Siltstones in Consolidation Process

et al. 2020

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“…On the Boso Peninsula, the unconformity between the Kazusa and Miura groups is distinctly angular, representing a temporal gap from ca. 3 to 2.4 Ma (Kameo and Sekine, 2013), but on the Miura Peninsula there is no signi cant angular relationship between the two groups (Koike, 1951;Akamine et al, 1956). No signi cant temporal gap corresponding to the Kurotaki unconformity has been identi ed in the Inubo Group, exposed on the Choshi Peninsula, based on biostratigraphy and tephrostratigraphy (Sakai, 1990;Tamura et al, 2014).…”

Section: Forearc Basin Lls: the Miura And Kazusa Groupsmentioning

confidence: 99%

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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“…The boundary between the Miura and Kazusa Groups is marked by the Kurotaki Unconformity (Koike, 1951), which represents a depositional hiatus and submarine erosion at~3 Ma when the convergence direction of the Philippine Sea Plate changed (M. Takahashi, 2006). Lithostratigraphy differs between the eastern and western Boso Peninsula (Figure 2 Kamiya et al (2017).…”

Section: Geological Settingmentioning

confidence: 99%

“…The orientations of structures (e.g. bedding and faults) differ between the Kazusa and Miura Groups (Koike, 1951), and the stratigraphy differs between the eastern and western parts of the basin. The Miura Group has been substantially eroded in the eastern part, with the uppermost part of the group being absent.…”

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confidence: 99%

Vitrinite reflectance and consolidation characteristics of the post‐middle Miocene Forearc Basin in central and eastern Boso Peninsula, central Japan: Implications for basin subsidence

et al. 2020

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To understand the sedimentary development of the Boso Forearc Basin, central Japan, since~3 Ma, we investigated paleothermal structure and consolidation trends in the central and eastern parts of the forearc basin through vitrinite reflectance measurements and consolidation tests. Vitrinite reflectance (R m) was in the range 0.33 % to 0.61 % for the Miura Group in the central part of the forearc basin and 0.34 % to 0.41 % for the Miura and Kazusa Groups in the eastern. These values suggest a roughly uniform vitrinite reflectance for the Miura Group from the central to eastern parts. No significant vitrinite reflectance difference is observed across the~3 Ma Kurotaki Unconformity in the eastern part of the basin. The consolidation yield stress (p c) was calculated as 27.5 MPa and 32.2 MPa for the Kiyosumi and Amatsu Formations of the Miura Group in the eastern part, respectively. Both the p c values are consistent quantitatively with represent the trend of the maximum overburden pressure estimated from the thickness and density of overlying sediments, and the difference in p c is expected by the maximum burial depths of the strata at the sampling localities. Values of p c in the eastern part of the basin increase with thickness of overlying sediment, showing no break across the Kurotaki Unconformity. Considering the eroded thickness of the Miura Group, the continuous trends in vitrinite reflectance and consolidation between the Miura and Kazusa Groups in the eastern part reflect the greater deposition of the eastern part of the Boso Forearc Basin since~2.3 Ma.

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On "The Kurotaki Unconformity"

Cited by 17 publications

References 0 publications

Strain Softening of Siltstones in Consolidation Process

Strain Softening of Siltstones in Consolidation Process

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

Vitrinite reflectance and consolidation characteristics of the post‐middle Miocene Forearc Basin in central and eastern Boso Peninsula, central Japan: Implications for basin subsidence

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