伊豆半島の完新世における相対的海水準変化
スポンサーリンク
概要
- 論文の詳細を見る
Izu Peninsula is situated at the northern tip of the Philippine Sea Plate, which is being subducted beneath the Japanese island of Honshu at the Suruga Trough along the west coast of the Peninsula. Because the tectonic setting is different from other areas, the record of coastal uplift in the Quaternary might also be expected to differ.The purpose of this paper is to clarify Holocene relative sea-level change and Holocene tectonic movement in the Izu Peninsula. These are estimated on the basis of analysis of sedimentary facies, molluscs and diatoms, 14C dates of wood and peat samples, and examination of emergent coastal landforms.Peat-coring and core boring by mechanized drilling were carried out in four coastal lowlands, at Ito, Shimoda, Toji and Nishina.At Ito, the apparent marine limit is about 0m amsl. Radiocarbon dates of Holocene deposits range from ca. 9, 000yrs BP at the basal part to ca. 4, 000yrs BP, on the peat above the marine deposits. At this locality, marine deposits are probably truncated by fluvial erosion; the original marine limit is estimated to be slightly above the present sea level, and younger than ca. 4, 000yrs BP. At Shimoda, the upper limit of marine deposits is 1.4m amsl, with 14C age of ca. 2, 400yrs BP. This is similar to the data obtained from the Okamo and Kisami areas (Ota et al., 1986).Examination of emergent coastal landforms indicates that they can be divided into two zones. The height of the upper zone is +2.5-+3.0m, and that of the lower zone is +1.5-+2.0m. These heights indicate that there were two discontinuous uplifts.The data imply that the upper limit of the marine deposits is significantly younger than in other areas, where the postglacial transgression culminated at ca. 5, 000-7, 000yrs BP, and that the study area subsided until ca. 2, 000-3, 000yrs BP, followed by a slight, probably episodic uplift.