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  • Depositional sequence of th...
    Nanayama, Futoshi; Yamaguchi, Tatsuhiko; Nakanishi, Toshimichi; Tsuji, Tomohiro; Ikeda, Michiharu; Kondo, Yasuo; Miwa, Michiko; Sugiyama, Shinji; Kimura, Kazunari

    Chishitsugaku zasshi, 2020/09/15, Letnik: 126, Številka: 9
    Journal Article

    Here, the characteristics of post-LGM (Last Glacial Maximum) incised valley fills and their depositional sequence were examined by analyzing the SKM core collected in the Sukumo coastal lowlands, southwestern Shikoku Island, which has been subject to extensive seismic subsidence due to the large Nankai Trough earthquakes. Here we provide results of sedimentological, radiocarbon dating, and paleoenvironmental analysis. Sediments of the SKM core show clear indications that the succession was influenced by post-glacial sea level change. The Matsuda River incised valley formed during the LGM (26.5-19 ka), and was infilled by fluvial sand and gravel in the late Pleistocene. Following postglacial transgression, sea level rose by 30 m (a.s.l.) at 9.8 ka, and the incised valley became an estuarine environment. As sea level continued to rise, the estuarine environment was replaced by an inner bay mud bottom, and maximum water depth was reached at 7.5 ka. The 7.3 ka Kikai caldera eruption in southern Kyushu Island caused heavy K-Ah ash fall in southwestern Shikoku Island, and frequent large-scale lahars occurred immediately after the ash fall because of the proximity to the volcanic source. After the ash fall, the K-Ah secondary sediments were deposited rapidly in the inner bay environment and caused rapid sea level regression. After 7.0 ka, a delta region began to develop, which might have been due to the large K-Ah ash fall. At 5 ka, sea level reached +2.5 m (a.s.l.), estimated from the Sukumo midden, and is recognized as the Holocene marine limit in this area. This information on relative sea level change during the past 10,000 years suggests that the Sukumo Bay area has not subsided as a result of seismic-induced crustal deformation.