奄美群島 徳之島・沖永良部島・与論島・喜界島の地質 (2)
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This paper is the second part on the geology of the four islands of the Amami Gunto, Kagoshima Prefecture, Japan, and describes Kikaijima and discusses the Quaternary correlation. The stratigraphical sequence of Kikaijima is shown in Table 1. The Somachi Formation consisting of sandstone, siltstone and an alternation of sandstone and siltstone, occurs along the terrace scarps as the basement of the flattop island. Columnar sections of the Somachi Formation are shown in Figs. 2 and 3. Abundant fossil foraminifers of benthonic and planktonic species were yielded from throughout the Somachi Formation. Those collected by the present author were investigated by Huang (1966) and the result of their faunal assemblages indicate the Early Pliocence age. The general structure of the Somachi is homoclinal, dipping westwards with a gentle anticline in the northeastern part and faults of N-S direction in the central and western parts of the island. The Somachi is covered with unconformity by the Hyakunodai and Wan formations of the Ryukyu Group and younger terrace deposits. The Hyakunodai Formation consisting largely of limestone is distributed in the east central part of the island between sea level and 224 m above it. The limestone, which is an accumulation of foraminifers, corals, molluscs, calcareous algae and other calcareous organic remains, is well consolidated but poorly compacted. A strong recrystallization is frequently observed in the limestone of the Hyakunodai Formation. The Hyakunodai Terrace which represents the summit plane of Kikaijima is thought to have been developed as the depositional plain of the Hyakunodai Formation, and together with the Somachi Formation is faulted. The Nagamine Terrace is developed as the erosional terrace and is distributed locally along the marginal part of the Hyakunodai Terrace. The Wan Formation is distributed in the northeastern, west central and southwestern parts of the island. It consists mainly of foraminiferal sand with corals, molluscs, brachiopods, calcareous algae, echinoids, bryozoas and other calcareous organic remains. A coral reef facies is found in the Wan Formation in the Hasato-Kawamine area of the western part of the island. Except for the sea cliff, along which case-hardening is common, the calcareous organic sediments of the Wan Formation are not consolidated and the organic remains are well preserved. The Wan Formation occurs between sea level and about 70 m above it, and the Kawamine Terrace is thought to have been developed as its depositional surface. The lower terraces with conglomeratic deposits are distributed locally. Emerged wave cut benches are well developed around Kikaijima and are partly accompanied by the coral reefs. They occur between sea level and 10 m above it and the levels of 7 m and 3-5 m are prominent. Absolute dates of 6630±150 and 4370±130 yr. B. P. (GaK-454 and 452) of the sediments of the higher level and 2740±100 yr. B. P. (GaK-453) of the lower level were measured by the radio-carbon method (Mii and Kigoshi, 1966). On the emerged benches, sand dunes and beach ridges are developed in parallel with the coast. They are accumulations of broken and abraded tests of foraminifers with the fragments of other calcareous organic remains and pumice grains. In the Wan-Suitengu-area of the western part of the island, another sand dune is buried beneath the sand accumulation mentioned above. It extends inland in NNW-SSE direction and the buried soil can be observed at the top of its section. A shell of a land snail collected from the buried soil was dated to 4360±140 yr. B. P. (GaK-451 ; Mii and Kigoshi, 1966) by the radiocarbon method. The present beach deposits consist largely of foraminiferal sand with coral and other fragments. Cobble size or smaller grains of pumice are also found frequently in the beach deposits. Table 2 shows the correlation of the Quaternary formations and terraces in the Amami Gunto with those of the other islands of the Ryukyu Islands. The criteria to correlate the terraces and their formations is based on the uniformity of the levels and sequence in development of the graded plains and on the similarity in lithology of the formations throughout the islands discussed here. The higher limit of about 200 m above sea level in the distribution of the Ryukyu Group and the terraces built by it can be applied to the whole Ryukyu Islands. The Itokina Terrace of Tokunoshima, Shimoshiro Terrace of Okierabujima, and Hyakunodai Terrace of Kikaijima represent this highest level and their correlatives can be found on Okinawajima, Amami-Oshima, and Iriomotejima. In the southern part of Okinawajima, the Naha Limestone is the highest terrace formation. The Kinoko Terrace of Tokunoshima is characterized by the step-like berms developed along the margin of the highest Itokina Terrace and by its formation consisting of foraminiferal sand with organic and nonorganic detrital deposits. The erosional terraces between the Shimoshiro and Shinjo Terraces of Okierabujima, the Kawamine Terrace of Kikaijima, and probably the Gusuku Terrace of Yoronto can be correlated with the Kinoko Terrace. Concerning the relation to the Serikaku Terrace, the Shinjo Terrace of Okierabujima can be included in the youngest and most widely developed member of the correlatives of the Kinoko Terrace. The graded plain represented by the Kametsu Terrace of Tokunoshima is widely distributed at 50-70 m above sea level in the Amami Gunto and 20-70 m above sea level in the Ryukyu Islands. Its formations, the Kametsu Formation of Tokunoshima, the Serikaku Formation of Okinoerabujima, the Nama Formation of Yoronto, and the Wan Formation of Kikaijima are characterized by unconsolidated, not recrystallized limestone with coral reef facies. Their correlatives can be found on Takarajima, Amami-Oshima, Okinawajima, Kumejima, Miyakojima, Taramajima, Ishigakijima, Iriomotejima, Haterumajima, Yonagunijima and other small islands of the Ryukyu Islands. The two marked levels of the graded plains with their formation, which are represented by the Itokina and Kametsu Terraces, can be found in the main parts of the Japanese Islands, they are the Kunohe and Shimosueyoshi Terraces. The sequence in the development of the terraces in Amami Gunto corresponds closely with the Quaternary changes in sea level in the Japanese Islands (Naka-gawa, 1961, 1965, 1967). The most important difference in the character of the terraces between Amami Gunto and the main part of the Japanese Islands is the nature of the marine terrace formations caused by the difference in climate. Some problems on structural geology and geomorphology will be discussed in part 3.
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- 1969-08-25
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