[H-QR03] 第四紀：ヒトと環境系の時系列ダイナミクス

[HQR03-P03] Beach-ridge development in the southern Sendai coastal plain, northeastern Japan

*堀 和明¹、丸山 愛太²、田村 亨³、石井 祐次³、清家 弘治³ (1.東北大学大学院理学研究科地学専攻、2.東北大学理学部地圏環境科学科、3.産業技術総合研究所地質調査総合センター)

キーワード：浜堤、前浜、光ルミネッセンス年代測定、海水準変動

Beach ridges, especially foreshore sediments that compose beach ridges, are important for reconstructing relative sea-level change on depositional coasts because they are formed close to the sea level. Several beach ridges occur commonly in the Sendai coastal plain, which was extensively subsided by the 2011 off the Pacific coast of Tohoku earthquake. The formation date of each ridge has been indirectly estimated based on radiocarbon dates of organic materials or molluscan shells obtained from swells between the ridges.

We analyzed four borehole core sediments (each 10 m in length), taken from the four ridges at the southern Sendai coastal plain and identified the foreshore sediments based on facies analysis. Additionally, IRSL dating of feldspars was conducted to determine the formation date of the ridges.
Foreshore/backshore, upper shoreface, and lower shoreface facies were identified from the core sediments based on facies analysis. IR₅₀ ages obtained from the foreshore sediments are approximately 6.6 ka for a ridge (here called conveniently as ridge A) located at the most inland and after 4.0 ka for other ridges (ridge B, C, and D) located at seaward. The lower elevation limit of the foreshore deposits in the ridge A appear to be higher than those in the other three ridges that formed after 4 ka. The difference may be attributed to hydro-isostatic deformation and/or tectonic movement since the middle Holocene.