著者

高波 紳太郎

出版者

公益社団法人 日本地理学会

雑誌

地理学評論 Series A (ISSN:18834388)

巻号頁・発行日

vol.92, no.3, pp.175-190, 2019-05-01 (Released:2022-09-28)

参考文献数

54

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薩摩半島の川辺盆地周辺において,基盤地形に応じて階段状に堆積した阿多溶結凝灰岩に発達する遷急点群の平均後退速度を明らかにした.特に永里川における後退期間の大きく異なる2遷急点での結果から,最終氷期中の降水量減少が岩盤侵食速度を低下させたかどうかを考察した.現地露頭調査や掘削資料に基づく阿多火砕流堆積時の原地形の復元により,遷急点形成当初の位置を推定し,その現在までの後退距離および長期的な平均後退速度を求めた.短期的な後退速度の算出には人工の曲流短絡で生じた遷急点を用いた.結果,遷急点の平均後退速度は11万年間で2.0~2.6 cm/年,最近315年間では0.8~2.0cm/年と推定され,氷期を含む長期的な速度が温暖期に限られた短期的な速度をやや上回った.過去11万年間の薩摩半島では,気候変動に伴う河川流量,すなわち降水量の減少による岩盤侵食速度の極端な低下はなかったことが,地形学的に強く示唆された.

著者

高波 紳太郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.131, no.3, pp.317-338, 2022-06-25 (Released:2022-07-08)

参考文献数

59

被引用文献数

2

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Ata welded ignimbrite (110 ka) lies beneath Ito non-welded ignimbrite and Osumi pumice fall deposit (30 ka) in the Kimotsuki Plain, southern Kyushu. Previous geomorphological studies of the Kimotsuki Plain focused on landform development after deposition of Ito ignimbrite. Landform development of Kimotsuki Plain since the last interglacial, especially until just before deposition of the Osumi pumice fall, is reconstructed using geological data collected from outcrop observations and borehole records. The basal-surface of the Osumi pumice fall deposit obtained shows that Ata welded ignimbrite had been dissected by the Kimotsuki River and its tributaries in response to the last glacial sea-level drop before the Osumi pumice fall was deposited. Longitudinal profiles along the Kushira River in 110 ka and 30 ka indicate recession of the Tanida waterfall, which formed at the edge of the Ata welded ignimbrite plateau. These profile changes imply that the Tanida waterfall retreated 2.4-6.0 km upstream between 110 ka and 30 ka. The Kushira formation, Marine oxygen Isotope Stages (MIS) 5e marine deposits under Ata welded ignimbrite, was found below the present sea-level at multiple locations in the Kimotsuki Plain. This vertical distribution of the Kushira formation indicates that the Kimotsuki Plain has been in a tectonically stable or subsidence area since the MIS 5e, in contrast with the Onejime and Natsui areas, which have been tectonically uplifting. The depositions of the two ignimbrites had significant impacts on filling the Paleo-Shibushi Bay (Sea) and the development of the Kimotsuki Plain under sea-level lowering during the last glacial period. The top-surface of basement rocks is more than −120 m below sea level at the floors of paleo valleys, even though it is adjacent to mountains composed of the basement. Further investigation of lower alluvium and its basal-surface is required for an understanding of valley incision and delta evolution of the Kimotsuki Lowland after deposition of the Ito pyroclastic flow.