東京湾北部沿岸域の沖積層と堆積環境

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タイトル別名
  • The Late Pleistocene -Holocene Deposits and the Sedimentary Environment in the Northern Coastal Area of Tokyo Bay, Central Japan
  • トウキョウワン ホクブ エンガンイキ ノ チュウセキソウ ト タイセキ カンキョウ

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説明

The author carried out research on the variations in the sedimentary environment of the late Pleistocene-Holocene deposits at Tokyo Port and the surroundings area in the Tokyo Bay, based on geological and geotechnical data. Changes in the sedimentary environment were clarified using facies analysis, engineering properties, diatom analysis, and age determination by radiocarbon dating for the latest Pleistocene-Holocene valley-fill deposits in the Paleo-Tokyo Valley, the Paleo-Kandagawa Valley, and the Paleo-Urayasu Valley. The three buried valleys were filled with the Nanagochi Formation, which is overlain by the Yurakucho Formation, although the sizes of the basins, river bed gradient, geology of the hinterland, and the thickness of the deposits are different.<br>The Nanagochi Formation and the Yurakucho Formation can be distinguished in the light of diatom analysis and engineering properties of the soil. The basal part of the Nanagochi Formation in the Paleo-Tokyo Valley is river gravel, and the middle to upper parts are sandy to muddy layers showing the facies of a river delta to brackish water environment. This formation was deposited rapidly between 15, 000 and 12, 000cal yrs BP. In the Paleo-Kandagawa Valley and the Paleo-Urayasu Valley, however, the basal horizon consists of sand and mud layers, and the upper horizon of the Nanagochi Formation consists of mud layers in the facies of brackish and marine water. The boundary between the Nanagochi Formation and the Yurakucho Formation is -45 to -50m A. S. L., corresponding to 12, 000 and 10, 000cal yrs BP. Changes in sedimentation rate and sedimentary facies around the boundary suggest temporal stagnation and minor lowering of sea level.<br>The Yurakucho Formation is composed of two zones, a lower zone characterized by sandy clay layers and an upper zone (a lower subzone of mud layers and an upper subzone of sand layers). On the basis of diatom analyses, the lower sandy clay layer of the Yurakucho Formation indicates environments of estuary to inner-bay in the stage of rapid transgression and enlargement of the bay between 10, 000 and 7, 000cal yrs BP. The dominant diatom species changes from Paralia sulcata of the lower to Thalassionema nitzschioides of the upper zones in a marine water environment. The ages of the boundary between the two zones may be about 6, 000 to 5, 000cal yrs BP. The mud layer (the lower subzone) of the Yurakucho Formation, showing the marine water facies, was deposited rapidly after the maximum transgression, between 5, 000 and 3, 000cal yrs BP. Since about 2, 000cal yrs BP, the upper sand layers (upper subzone) has shown advancement of a delta front in the east part of the Paleo-Urayasu Valley, where the deposits are characterized by the diatom assemblage of brackish to river water.

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  • 第四紀研究

    第四紀研究 43 (4), 297-310, 2004

    日本第四紀学会

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