Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-GL Geology

[S-GL38] Lower-Middle Pleistocene Boundary GSSP in the Kazusa Group

Mon. May 25, 2015 11:00 AM - 12:45 PM 102B (1F)

Convener:*Makoto Okada(Department of Earth Sciences, Faculty of Science, Ibaraki University), Yusuke Suganuma(National institute of Polar Research), Osamu Kazaoka(Research Institute of Environmental Geology, Chiba), Chair:Yusuke Suganuma(National institute of Polar Research), Osamu Kazaoka(Research Institute of Environmental Geology, Chiba)

12:39 PM - 12:42 PM

[SGL38-P04] The Tokyo Bay Unconformity and the Mandano Ice Age

3-min talk in an oral session

*Hisashi NIREI1, Osamu KAZAOKA2, Hideto KIMURA3, Takeshi YAOSHIDA2, Wataru NIREI4 (1.Ibaraki Univ. ; IUGS GEM, 2.Res. Inst. of Environmental Gology, Chiba, 3.Tohocjhisui Co., 4..Tokyo Univ. of Infor. Sci.)

Keywords:Tokyo Bay Unconformity, Mandano Ice Age, Kazusa Group, Shimohusa Group, Kanto Continental Shelf

The Kanto fore-arc basin in the Kato plain is an extremely deep submarine basin that was formed during the early Pleistocene. The sediments in the basin change from deep-sea sediments to lacustrine-alluvial sediments, the Mandano formation which is up to 95 m in thickness and overlays the Tokyo Bay unconformity that is widely distributed under Tokyo bay area. The formation consists of three parts. The lithofacies in the lower part on the unconformity gradually change, with decreasing depth, from sand to gravel. The lithofacies of the middle part are muddy. The upper part changes with increasing depth from gravel to sandy shilt by transgression.
The lower part and lower half of the middle part comprise sediments characteristic of a topset fan delta (Nirei H., 1997) in the regression stage. The upper half of the middle part and upper parts are composed of transgression sediments.
The lower half of the middle part contains sediments from the ice age regression stage, evidence of which is provided by the cold-index plant remains,Picea maximowiczii, Tsuga diversifolia,Fagus crenata, Cryptomeria japonica, etc., present in the uppermost part of the lower half.
Geological analysis reveals the Kanto continental shelf to extend widely under the Kanto Plain; it is also underlaid by a range of bottom set sediments in distinct formations and forest sediments, also in distinct formations, in ascending order under the conformity.