[HGM03-P09] The Relationship Between the Coastal terrains and Geological Layers, in the Western part of Iki Island, Nagasaki Prefecture
Keywords:Rock control, Holocene sea-level change, Sea cave, Breccia, Basalt, Natural bridge
Around the research area, the layers of basalt, which were formed around 2.1-2.5 Ma (Sano 1995) and layers of breccia form the base rock. The layers of breccia are weak of erosion and are collapsed rapidly from bigger rock brocks. Basalt is harder and that forms overhang sea cliff and upper natural bridge of Onino-Ashiato. From these results, it can be said that sea caves began to be formed at the layer of breccia. Onino-Ashiato is a horseshoe shaped sea cave, with 54 m in length, 33m in width, 26 m in depth, and the height of natural bridge is 9.4 m. The unique shape might be the result of the flow of wave in the sea cave as the result of observation. The relationship of sea caves in shape and distribution of them can be a link of the rock control theory. Also, the height of sea cave is around 10 m, and there is a sea cave, which exist 5.2 m higher than today’s sea level. This shows the possibility of sea level changes. The height of sea cave’s existing point, 5.2 m is similar height of the tide in Holocene sea level change in Genkai-Nada (Shimomura 1994). From these evidences, 2 factors, Holocene sea level change and rock control, the possibility of relationship in forming coastal terrain is shown.
・Takashi Sano 1995. Geology of Iki Volcano Group: Lava Flow-stratigraphy Mainly Based on K-Ar Dating. Bulletin of the Volcanological Society of Japan 40-4: 329-347.
・Shoichi Shimomura 1994. Shorelines and Trend of Local Movements since the Peak of
the Jomon Transgression in North Kyushu, West Japan. The Quaternary Research 33-5: 351-360.
・Mitsuhisa Watanabe 1990. Comments on the Tectonics of the Tsushima Strait Region,
Northwestern Kyushu. The Quaternary Research 29-3: 227-279.