Sea Level Changes and Tectonics Inferred from the Quaternary Deposits and Landforms of Boso Peninsula, Central Japan. Depositional Dynamics of the Middle and Upper Pleistocene Shimosa Group, Japan.

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The Shimosa Group, cropping out in the Shimosa Upland, was deposited in the middle and late Pleistocene Paleo-Tokyo Bay (ca. 450-80ka). Paralic and shallow marine depositional systems (fluvial, estuarine, delta, shoreface-beach, barrier island) are recognized in each formation and constitute transgressive-regressive cycles (depositional sequences). These sequences are interpreted to have developed during stage 12-4 of the oxygen isotope curves and to have been primarily controlled by glacio-eustatic sea level changes. Spatial distribution in the depositional system is directly restricted by the topography of the basin, resulting from tectonic movement in the Shimosa Upland. The tectonic movement can be detected from the direction of gradient shown by the base contour lines of each formation of the southern part of the Shimosa Upland. The direction turned from northward to northwestward during the deposition of the Kamiizumi Formation, which indicates a shift in the uplift zone, from the Mineoka zone to the Kashima-Boso zone. Additionally, spatial variation of tectonic movement in the Kashima-Boso zone had resulted in the development of different depositional systems in the Shimosa Upland.
日本第四紀学会の論文