Journal of Cave and Karst Studies - ISSN 1090-6924
Volume 67 Number 1: 48-54 - April 2005

A publication of the National Speleological Society

Karst distribution and controls on Yoron-Jima, an emerged reef island in sub-tropical Japan
James P. Terry

Abstract

Yoron-jima is a small carbonate island located in the central Ryukyu Island Arc of southern Japan. The island was raised above sea level in the Quaternary period and most of the 21 km² land area is underlain by carbonate rock types associated with the regional Ryukyu Limestone Group. The island’s landscape is characterised by many surface depressions. This paper describes Yoron’s closed depressions and interprets their uneven spatial distribution. Some areas covered by unconsolidated Holocene deposits are almost absent of closed depressions. Elsewhere, depression clusters are observed on a variety of carbonate rocks. Small depressions (average long axis 76 m, area 2320 m²) tend to be shallow with regular elliptical morphologies, and are densely clustered. These have developed on a low elevation, emerged marine platform on the island’s western peninsula, where there has been minimal structural deformation of the coral limestone bedrock. In contrast, larger (average long axis 103 m, area 4060 m²) and deeper closed depressions (5-10 m), more often with irregular or star-shaped plans, have developed across the north and east-central region of Yoron, in association with 1. outcrops of rhodolith limestone geology, 2. major fault escarpments, and 3. carbonate/non-carbonate geological boundaries. Aggressive dissolution has also produced large elongated closed depressions trending along the north east coast of Yoron close the sea level, where tidal fluctuations control the salinity and surface height of the water table. Correlation with depressions elsewhere in the Ryukyu Islands on similar geology suggest that limestone surface denudation rates on Yoron may be 5–10 mm/1000 yrs.

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