900,000-year cyclostratigraphic record from a 600m core, GS-NB-1, the Nobi Basin, central Japan
Authors: Toshihiko Sugai (Univ. Tokyo), Yuichi Sugiyama and Kiyohide Mizuno
Abstract：A 601-m core named GS-NB-1 (Geological Survey of Japan Nobi Basin Boring No.1) was collected from the western part of the Nobi tilted basin in 1998. A comprehensive core analysis comprising lithostratigraphic classification, magneto- strtigraphic and tephrostratigraphic studies and 14C dating has revealed the subsurface stratigraphy of the Nobi basin. Magnetic stratigraphy indicates the Matsuyama/Brunhes transition (ca. 783 ka) at the core depth of around 545 m. It also indicates five within-Brunhes excursions (or short polarity events) including Blake, alpha, and beta.
The core contains volcanic ash layers of Sakura (another name is Kobayashi-Kasamori, ca. 520 ka) and Azuki (ca. 850 ka), which are repre- sentative widespread tephras in Japan. Variations in grain size of sediments in NB-1 core clearly reflect eustatic sea-level changes. The core contains ten gravel layers more than 5 m thick, which deposited during the glacial periods of MIS (Marine Oxygen Isotope Stage) 2, 6, 8, 10, 12, 14, 16, 18, 20 and 22, respectively. Silt and clay layers, in contrast, deposited during the interglacial periods. Large accommodation space produced by continuous basin subsidence of about 1 m/ky has been filled with coarse fluvial sediments and with fine marine and/or lacustrine sediments alternatively. The cyclostratigraphy of the basin indicates that effects of sea-level change have been quickly transmitted to the basin by rivers through active fluvial processes.
Fig. 1. Location map of the GS-NB-1 drill site
Fig. 2. Stratigraphic column of the GS-NB-1 core from the Nobi Plain (right) and plot of magnetic inclinations through the core.
Fig. 3. Correlation of late Quaternary stratigraphic units from the three largest metropolitan areas in Japan: Nobi, Osaka and Tokyo (south Kanto),and relation to the marine oxygen-isotope record(Bassinot et al.,1994).
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