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＜departmental bulletin paper＞
The Qua ternary marine deposits and their morphogenetic processes in and around the Torikai Lowland, Fukuoka City, Japan
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| Abstract | The formative processes of the Quaternary marine deposits since the Late Pleistocene and morphogenetic processes after the Jo mon Transgression were investigated in terms of sea-level fluctuation in o...rder to clarify construction mechanisms of coastal plains in and around the Torikai Lowland (Paleo-Torikai Bay), Fukuoka City, North Kyusyu, Japan. Tephra identification, 14C-dating and paleoenvironmental analyses based on molluscan fossils and many boring data show that the sediments recorded some important evidence of paleogeographic and environmental changes, especially in sea-level and vegetation during the Jomon Transgression. The results of the study are summarized as follows. 1. High sea-level stands in the Late Pleistocene: Pleistocene marine beds beneath the bottom of the river mouth of the Komo River can be correlated with transgression stages of the Hikihashi, the Obaradai and the Misaki Stages in the South Kanto area based on two indicators of Aso-4 and Ata Tephras; the Submember C of the Nata Sand Member beneath the Ata Tephra, the Submember B of the Nata Sand Member and the Chigyo Mud Member beneath the Aso-4pfl Tephra and the main part of the Submember A of the Nata Sand Member and the Fukuhama Mud Member on the Aso-4pfl Tephra are correlated to the Hikihashi Stage, about 100,000y.B.P., the Obaradai Stage, about 80,000y.B.P., and the Misaki Stage, about 60,000y.B.P., in the South Kanto, respectively. 2. High sea-level stand during the Jomon Transgression: A curve showing the Holocene sea-level changes in Fukuoka City prepared on the basis of 14C dating and data of the uppermost level of the marine deposits at each stage indicates that the curve at the highest stage of the Jo mon Transgression does not present a single peak but three peaks with small regressions. The highest stage during the Jo mon Transgression corresponds to the second peak, about 4,700y.B.P., and the average of the highest sea-level is estimated as + 1.2m. The Submember B of the Hakata Bay Silt Member at Ohori Park formed at the first peak stage during the interval from 6,600y.B.P. to 5,400y.B.P. Sedimentary rates of the Hakata Bay Silt Member during the interval were estimated as 1.1m/ 1,000y. in the lower part and 5.4m/1,000y. in the upper. Such rapid sedimentation seems to reflect decays of old coastal dunes and fill-up of the Paleo-Torikai Bay into tidal flats at the first peak stage of the high sea-level stand of the Jomon Transgress1on. 3. From small regression stage of the Yayoi Period to the present: In the Yayoi Period, from B.C. 3rd to A.D. 3rd, the Torikai Lowland became the site of formation of lines of new coastal sand dunes facing the Paleo-Torikai Bay. The beach mound as the base of the new coastal sand dunes grew up by feeding river sands from the Naka River and the Muromi River as a building material. The Torikai Lowland kept the condition of tidal flat and swampy place before the Edo period, from A.D. 17th to A.D. 19th, because the deposition of the Hii River was very slow. Present topographic features of this area was almost completed by two artificial changes of the topography. The first change was due to the construction of the Fukuoka Castle and its surrounding town by the Kuroda Clan in 1605-1646 A.D. and the second one due to the construction of the exhibition grounds in 1927 A.D.show more |
| Table of Contents | I. はじめに II. 鳥飼低地周辺の地形 III. 鳥飼低地の地質 IV. 考察 V. まとめ |
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| Created Date | 2021.10.20 |
| Modified Date | 2022.06.20 |