| 作成者 |
|
|
|
|
|
| 本文言語 |
|
| 出版者 |
|
|
|
| 発行日 |
|
| 収録物名 |
|
| 巻 |
|
| 号 |
|
| 開始ページ |
|
| 終了ページ |
|
| 出版タイプ |
|
| アクセス権 |
|
| JaLC DOI |
|
| 概要 |
Itoshima Lowland extends from Imazu Bay west-southwest ward to Kafuri Bay. It is a long and narrow lowland below 5 meters above sea-level, and is about 3 km in width and about 10 km in length. The Qua...rternary deposits except for the reclaimed soil and younger dune sand do not expose in this lowland. 33 boring-data were examined in order to clarify the subsurface geology. In addition to the detailed survey at the four sites of excavation hand-boring was carried out at four points in the narrowest part of the lowland. The Quarternary deposits unconformably overlay a granitic basement, and another unconformity is observable between the Holocene and Pleistocene Series. The Holocene Series is stratigraphically classified into 3 lower, middle and upper units. The middle part yields abundant marine molluscs which reveal an embayment condition. The other two parts are composed of non-marine deposits. The carbon-14 isotopic ages of the shell and wood samples from the upper part of the marine deposits are respectively about 3, 300y. B. P. and 3, 500y. B. P., Granitic basement is present at only 3 meters below in the narrowest part and at 17 meters below in the both end of the lowland. Distribution. of the marine deposits can not be traceable in the narrowest part of the lowland. We were not able to produce evidence of a narrow channel which has been considered to have been existed there above 1, 000 years ago. Fossil molluscan assemblages were collected by block-sample method at Mizusaki and Yubi in the lowland. The dominant species of each assemblage were determined by comparing the relative abundance data and the species rank calculated one according to MACARTHUR'S model. As a result, seven species of gastropod assemblage and two species of pelecypod assemblage of Mizusaki, three species of gastropod assemblage and one species of pelecypod assemblage of Yubi were recognized as dominant species and palaeoenvironments were discussed on these species. The environments suggested by these fossil assemblages are commonly an inner inlet condition but slightly differ from each other, i.e. tidal creek of bay center for the both assemblages of Mizusaki and innermost bay for the gastropoda assemblage of Yubi. Two methods were used in order to investigate the degree of the modification of the primary information of dead shells of the original populations of the dominant species. Analyses of size-frequency distribution and coexistence index of both valves revealed that the fossil shell assemblages are autochthonous except for the gastropod assemblage of Mizusaki. The latter is considered to be critically reworked before the final burial. Judging from the altitude and ages of the uppermost part of the marine shell bed, the highest sea level is considered to have attained to 1 to 2. 5 meters above the present sea level in about 3, 300y. B. P. in the Itoshima Lowland. This conclusion is incompatible with the current concept that the highest sea level attained to 3 or 4 meters above the present sea in about 6,000y. B. P., The mentioned fact is an interesting problm from a viewpoint that the local relative changes of sea-level may be effected by local tectonic movements.続きを見る
|
Mendeley出力
