Karstification Processes on Minamidaito Island in the Nansei Archipelago, Southwest Japan

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  • 南大東島におけるカルスト化過程の一考察
  • ミナミダイトウジマ ニ オケル カルストカ カテイ ノ イチ コウサツ

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Abstract

Minamidaito Island is a karstified uplift coral island, located on the Philippine Sea Plate. The karstification periods can be divided into three. The first period was at the end of Tertiary Pleiocene, the second period during the Pleistocene, and the third period during the Holocene. The first period of karstification can be observed as an unconformity border with fossil soil layers called rainbow stone in the lower Daito layers composed of dolomitized limestone. The ages of the dolomite have been analysed as 5 Ma and 4 Ma. The long karstification seems to have occurred as a result of sea level change caused by the cooler period of the palaeoclimatic conditions. After that period of karstification processes, the island subsided again and dolomitized limestone accumulated until about 2 Ma. This is found in the upper Daito Layers. After that, the island underwent karstification continuously, because of its location in the uplifted zone of the plate. The time of karstification which formed the higher land, Hagu, characterized by limestone walls, and the basin-shaped lowland, Hagu Shita, is calculated as 1.6 Ma according to a previous study by the present author in 2009. The limestone walls and the basin-shaped lowland together with the doline lakes were formed during Pleistocene. The coast of the island is formed by coral limestone dated as 5e, and has an altitude about 12 m a.s.l. Based on this value, the rate of uplift was calculated as 0.05 m/ka by Ota et al. This island has been uplifted without tilting. In the present study, the corrosional terrain was measured on the Kaigunbo coast. It was found that, on landforms of the same age, different corrosional terrains could be observed on the bench and platform, which were formed during the period with high sea levels during the Holocene. However, the bench had been formed in the lower Daito Layer. On the bench are found kamenitzas at a height of about 3.5?4 m a.s.l. The depth of the kamenitzas is usually 20?25 cm, and their diameter is about 50 cm on the bench. On the platform at about 2 to 3 m a.s.l., the kamenitzas have a depth of 20?50 cm, and diameter of 20?30 cm. The density of these kamenitzas is very high, and the sizes of the pinnacles are 20?30 cm. The solution rate is very fast on the platform. This phenomenon supports the different lithologies of the two terrains. On the platform, congromerate with young coral limestone cover the lower Daito Layer well.

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