福岡平野における縄文海進の規模と第四紀層

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  • The Qua ternary beds in the Fukuoka Plain with special reference to the magnitude of the Jomon transgression
  • 福岡平野における縄文海進の規模と第4紀層
  • フクオカ ヘイヤ ニ オケル ジョウモン カイシン ノ キボ ト ダイ4キソウ

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Both the exposed and subsurface Quaternary deposits were investigated in the Fukuoka Plain, and the following 4 points became obvious. / 1. The Quaternary deposits of the Fukuoka Plain can be divided on the basis of the nature of the sediments and sedimentary environments. The Holocene deposits are classified into the artificially reclaimed soil, the Uminonakamichi Sand Member, the Hakozaki Sand Member, the Hakata-bay Silt Member and the Sumiyosi Member. The Pleistocene deposits are divided into the Suzaki Member, the Nameri Gravel Member, the Jigyo Mud Member and the Nakabaru Gravel Member. The Uminonakamichi Sand Member and the Hakozaki Sand Member consist of beach sand and eolian sand, and they comprise the so-called "New sand dune". The Nata Sand Member is correlated to the socalled "Older sand dune". Eolian dust of loess origin and widespread tephras are intercalated in it. The Hakata-Bay Silt Member and the Jigyo Mud Member are of marine origin and contain many animal fossils of inner bay. The former deposited in the Hakata Bay during about 10,000 years in the past, and the main part represents the Jomon transgression. The latter was formed in old Hakata Bay about 100,000 years ago. The Sumiyosi and Suzaki Members consist of delta and flood plain deposits. They were composed of clayey beds with humic material and alternation of sand and gravel with cross-lamination. The Hibaru Gravel, the Otsubo Sand and Gravel and the Nameri Gravel Member are composed of ancient fan deposits. / 2. The main part of the subsurface Quaternary beds of the Fukuoka Plain is occupied by the Suzaki and Nata Sand Members. The Suzaki Member fills up irregularities on the eroded surface of the Nakabaru Gravel Member and the basement in Tenjin area and its neighborhood in Fukuoka City. The Nata Sand Member composes the subsurface geology of Uminonakamichi. The Holocene beds are widely distributed, but are of very thin veneer. In other words, the frame-work of the present topography of the Fukuoka Plain was already formed about 70,000 years ago. / 3. The Jomon transgression and the succeeding small regression in the Holocene developed shore sand beds and back marsh sediments. Examination of a number of boring data clarifies fluctuation of shore-line accompanied by the transgression and regression. The inner limit of the shore line of the climax of the Jomon transgression is thus estimated only ab叫3or 4km landward from the present coast line. Maximum altitude of Holocene marine deposits is 2.2m above the present sea level. The thickness of the Holocene marine deposits often reaches 20m on the north side of the Hakata Bay and in the south part of Uminonakamichi. These parts must represent valley filling. / 4. The shell beds in the Jomon sequence were formed in a few separate periods of shell accumulation. At least three high sea-levels are suggested in the Jomon transgression by the paleo-environmental condition revealed from the molluscan assemblages and the 14^C dates. They are dated about 6,000y.B.P., about 4,700y.B.P. and about 3,100y.B.P., respectively

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