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Nishida N.,Geological Survey of Japan | Kazaoka O.,Research Institute of Environmental Geology | Izumi K.,Japan National Institute of Environmental Studies | Suganuma Y.,Japan National Institute of Polar Research | And 8 more authors.
Quaternary International | Year: 2015

The Chiba composite section of the Kokumoto Formation in central Japan is a remarkable, thick marine succession and is a candidate for formal recognition as the Global Boundary Stratotype Section and Point (GSSP) for the base of the Middle Pleistocene stage. Although there have been numerous studies of the Kokumoto Formation, the sedimentology of the formation, particularly in the Chiba composite section, has been unclear. We use field observations and laboratory analyses to elucidate the sedimentary processes and depositional environments of the formation. The Chiba composite section includes the Matuyama-Brunhes paleomagnetic reversal, which is the primary guide for the Lower-Middle Pleistocene boundary, along with multiple tephra deposits that provide isochronous correlational tie points. One bed, Byk-E (also known as Ontake-Byakubi Tephra), immediately underlies the paleomagnetic reversal and hence provides a useful widespread marker for the boundary. The predominant silty beds of the Chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites. However, minor sandy beds intercalated within the silty section, particularly in its lower part, are either (1) fining upward sequences with faint planar or ripple laminations and an erosional base, or (2) sequences containing abundant mud clasts in a poorly sorted sand matrix with a sharp base and an irregular top surface. The silty beds are interpreted to be hemipelagite formed by deposition of fine-grained suspended material under stable and calm bottom-water conditions. The sandy deposits are interpreted to be sediment gravity-flow deposits (i.e., turbidites and debrites). These sedimentary processes and the trace fossil assemblage indicate that the Chiba composite section was formed in a continental slope environment. The depositional environments of the Chiba composite section, together with tephrostratigraphy, make it suitable for high-resolution studies on the paleoenvironment, paleoclimate, and paleoceanography of the northwest Pacific Ocean across the Lower-Middle Pleistocene boundary; it is therefore a suitable candidate to become the GSSP for the Middle Pleistocene. © 2015 Elsevier Ltd and INQUA.

Kazaoka O.,Research Institute of Environmental Geology | Suganuma Y.,Japan National Institute of Polar Research | Suganuma Y.,Graduate University for Advanced Studies | Okada M.,Ibaraki University | And 9 more authors.
Quaternary International | Year: 2015

Major climatic reorganizations, including changes in the nature of glacial-interglacial cycles through the Pleistocene, are a key issue for improving the understanding of Earth's climate system. Highly resolved marine sedimentary records are essential to reconstruct the details of these past climatic changes and investigate the mechanisms responsible for them. The Kazusa Group, located in the central part of the Japanese island chain, is well-exposed and contains a remarkably continuous and thick, deep- and shallow-water marine sedimentary succession. This group also contains well-preserved marine microfossils, pollen, paleomagnetic reversal events, geochemical signatures, and a large number of tephra beds. These features allow us to establish a robust chronological and stratigraphic framework for the Kazusa Group, and provide a rare opportunity to study oceanic and terrestrial climatic and environmental changes at high resolution especially through the Lower and lower Middle Pleistocene. In the Boso Peninsula, Chiba Prefecture, the Kazusa Group is deeply incised, yielding spectacular river-cut exposures. The Matuyama-Brunhes paleomagnetic reversal is observed immediately above the widespread Byakubi-E (Byk-E) tephra in the Chiba composite section within the Kokumoto Formation (Kazusa Group). Because the Matuyama-Brunhes boundary customarily serves as the primary guide for the Lower-Middle Pleistocene Subseries boundary, the Chiba composite section is considered an excellent candidate for its global boundary stratotype section and point (GSSP). This study reviews the published data for the Kazusa Group (hitherto almost exclusively in Japanese) in order to place the Chiba composite section within its broader depositional and chronostratigraphic context. © 2015 Elsevier Ltd and INQUA.

Suganuma Y.,Japan National Institute of Polar Research | Suganuma Y.,Graduate University for Advanced Studies | Okada M.,Ibaraki University | Horie K.,Japan National Institute of Polar Research | And 14 more authors.
Geology | Year: 2015

The youngest geomagnetic polarity reversal, the Matuyama-Brunhes boundary (MBB), provides an important datum plane for sediments, ice cores, and lavas. Its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records, and is supported by 40Ar/39Ar dating of Hawaiian lavas using recent age calibrations. Challenging this age, however, are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high-sedimentation-rate marine records, and cosmogenic nuclides in marine sediments and an Antarctic ice core. Here, we present a U-Pb zircon age of 772.7 ± 7.2 ka from a marinedeposited tephra just below the MBB in a forearc basin in Japan. U-Pb dating has a distinct advantage over 40Ar/39Ar dating in that it is relatively free from assumptions regarding standardization and decay constants. This U-Pb zircon age, coupled with a newly obtained oxygen isotope chronology, yields an MBB age of 770.2 ± 7.3 ka. Our MBB age is consistent with those based on the latest orbitally tuned marine sediment records and on an Antarctic ice core. We provide the first direct comparison between orbital tuning, U-Pb dating, and magnetostratigraphy for the MBB, fulfilling a key requirement in calibrating the geological time scale. © 2015 Geological Society of America.

Nirei H.,Geo pollution Control Agency | Furuno K.,Research Institute of Environmental Geology | Osamu K.,Research Institute of Environmental Geology | Satkunas J.,Geological Survey of Lithuania
Episodes | Year: 2012

Patterns and rates of deposition, migration and retention of pollutants in man-made strata depend on the depositional history and physical and chemical characteristics of the constituent materials. A sound understanding of the spatial and chronological relationships of materials is required for effective evaluation of solid, liquid and gaseous geo-pollution and design and understanding and interpretation of investigations of potentially contaminated land. Description of materials and boundaries requires clear terminology. A proposed terminology has been developed based on experience from a variety of sites in Japan including the experimental infilling of an old quarry. The terminology is commended for further discussion. geo-pollution and designing and undertaking investigations of potentially contaminated land. Discrimination of materials and boundaries in terms of time and properties requires a clear set of descriptive terms. This paper sets out a proposed terminology.

Kagawa A.,Research Institute of Environmental Geology | Furuno K.,Research Institute of Environmental Geology
IAHS-AISH Publication | Year: 2010

The Kanto Plain, in Japan, is underlain by the Kanto groundwater basin. The basin is composed of Plio-Pleistocene marine sediments. Chiba Prefecture is located in the southeast part of the Kanto groundwater basin and, as of 2009, land subsidence has been monitored by 148 observation wells and 1159 benchmarks. Based on continuous monitoring, the mechanism of land subsidence is now reasonably well understood. The monitoring system is important for the sustainable use of groundwater. Copyright © 2010 IAHS Press.

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