奄美大島の海岸段丘と第四紀後期の地殻変動

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Amami-Oshima is located at the central part of the Ryukyu Arc which extends from Kyushu to Taiwan. In Kikai-Jima at about 20 kilo-meters south-east off Kasari Peninsula of Amami-Oshima, rapid uplifting (1.5-2 mm/y) during the past 130, 000 years was revealed by the altitudes of its marine terraces and their absolute ages derived from the calcareous terrace deposits (KONISHI et al., 1970). As the Ryukyu Arc has similar geological and geophysical settings to the outer arc of Southwest Japan, the rapid uplifting in Kikai-Jima was interpreted as earthquake related deformation (YONEKURA, 1975) which has been critically studied in the outermost belt of Southwest Japan. The purpose of this paper is to clarify the mode of the vertical deformation in the late Quaternary period in Amami-Oshima with relation to that in Kikai-Jima, and to present some data for comparison with above mentioned seismic crustal movements.<BR>The geomorphic surfaces of Amami-Oshima are roughly classified as follows : low, middle and high coastal terraces and the uppermost low-relief erosion surface. The low terrace (L) consist of Holocene marine terrace, wave-cut bench and alluvial plain, altitude of which does not exceed several meters high. The middle terraces (Ml and M2) are mostly developed in Kasari Peninsula at the height of 10 to 60 meters above sea level and scarcely observable in the other part of Amami-Oshima, except a minor wave-cut bench of 8 meters high above sea level with subrounded gravel and red soil observed at Honohoshi Beach, east of Koniya. The profiles of the middle terraces are, in general, continuous without any cliff or knick point in them and gently inclined seaward. However, in the east coast of Kasari Peninsula they can be divided into two levels : the higher (M1) and the lower (M2). The terrace deposits of M1 terrace are generally thin marine gravels covered with red soil, but valley fill deposits more than 40 meters thick exist inside some small isthmus such as at Taira, Kawauchi and Ogachi. The upper parts of these deposits consist of marine sands and gravels, and the basal parts of them consist of clayey deposits with a lot of breccia and fragments of plants into which a white tuff layer is inserted. M2 terrace is abrasion platform cutting M1 terrace with thin terrace gravels covered with red soil. The high terraces (H) are mainly developed in the northeastern part of Amami-Oshima (mostly included in the area of Fig. 2), which are highly dissected by V-shaped valleys but still preserve very flat original terrace surfaces. They are now at the height of 180 to 340 meters above sea level and probably can be divided into two or three levels. Terrace deposits are not observed on them. The low-relief erosion surface is widely developed in the western half of Amami-Oshima and its dependant islets at the height of 250 to 400 meters above sea level.<BR>From the evidences for a large transgression more than 40 meters deep and from the sequence of the middle and low terraces, M1 terrace is inferred to have been formed by the transgression in the last interglacial period, which is called “Shimosueyoshi transgression” in South Kanto, Japan, and dated 120, 000-130, 000 years B. P. (MACHIDA and SUZUKI, 1971).<BR>The mode of the vertical deformation during the last 120, 000 or 130, 000 years inferred from the distribution of altitudes of former shoreline of Ml terrace is as follows : (1) Kasari peninsula has been uplifting and tilting north-westward (Fig. 2). Hyakunodai terrace in Kikai-Jima 20 kilo-meters south-east off Kasari Peninsula, which is contemporaneous with Ml terrace in Amami-Oshima, is at the height of 220 meters above sea level. Therefore the tilted area is seemed to extend from Kasari Peninsula to this island.
社団法人東京地学協会の論文

奄美大島の海岸段丘と第四紀後期の地殻変動

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