Silicification of cave corals from some lava tube caves in the Jeju Island, Korea: Implications for speleogenesis and a proxy for paleoenvironmental change during the Late Quaternary

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Abstract

Cave corals, formed by groundwater seepage, are present in some lava tubes (Socheon, Jingaemot, Susan, Mosimoru, and Bilemot caves) of the Jeju Island, Korea. The cave corals in Socheon Cave consist of calcite, whereas those in Jingaemot and Mosimoru caves are composed of opal-A, calcite, and/or aragonite. Cave corals in Susan and Bilemot caves consist of opal-A. Most show similar morphology to those in limestone caves, but some have morphology resembling frostworks. Internal texture of the cave corals in Jingaemot Cave shows alternating layers of aragonite and calcite, spherulitic-fibrous, and isopachous-fibrous microstructures, respectively. Calcite layers were more susceptible to silicification, and the replacing opal-A commonly retains relic crystals of pre-existing carbonate minerals. The corroded nature of replaced carbonate minerals indicates that the growth of carbonate minerals and silicification were controlled by the pH conditions of intermittent cave water. The chemical conditions of the fluids responsible for the precipitation of aragonite or calcite and for silicification appear to be influenced by the presence of overlying carbonate sediments, the degree of basalt weathering, and the amount of rainfalls, ultimately controlled by local climatic changes in the past.

Introduction

Lava tube caves may form by lava flowing down along slopes after the eruption of magma. They develop as hard crusts on the surface of lava flow due to rapid cooling, while inside the lava remains liquid, draining downslope and leaving an empty tube behind it (Woo, 2005). Numerous lava tube caves are in Hawaii, Iceland, western USA, Japan, Australia, Azores (Portugal), Canary Islands (Spain), and many other places in the world. So far, more than 120 volcanic caves including the lava tubes have been discovered in the Jeju Island formed during the Late Quaternary volcanic activities (Son, 2003; Hwang et al., 2005).

Lava tube caves are known to form in basaltic lava. Lava speleothems in these caves commonly show similar mineralogy and chemical composition to those of surrounding basalts (for more details, see Woo, 2005). However, secondary minerals (the speleothems formed by groundwater after the formation of lava tubes) have been regarded as a common product in many lava tubes (e.g. Forti, 2004). Especially, cave corals, defined as a variety of nodular, globular, botryoidal, or coral-like speleothems (Hill and Forti, 1997), grow on the wall and floor in many lava tube caves. It was suggested that subaerial cave corals are generated by capillary-film water (Hill and Forti, 1997). In Jeju, white speleothems growing on the wall and floor of several lava tubes have been called “siliceous flower” by Korean cavers and geologists because these cave corals are thought to be composed only of silicate minerals. However, their mineral composition and the mode of formation are virtually unknown.

Various types of secondary speleothems in lava tube caves have been reported in many parts of the world, e.g., quartzose flowstone in Azores, Portugal (Hill, 1976; White, 1976; Webb, 1979; Cody, 1980; Forti, 2004). Siliceous speleothems were also reported in sandstone caves in South America, North America and South Africa (e.g. Wray, 1997a). Despite their wide abundance, only very small quantities usually can be found in most caves, probably due to the low supply rate of fluids bringing silica into caves and/or the physico-chemical conditions of the fluids responsible for precipitation (Young, 1987; Watchman, 1990, Watchman, 1992, Watchman, 1994; Young and Young, 1992; Hill and Forti, 1997). Siliceous minerals are sometimes intercalated with carbonate minerals (White et al., 1966; Lassak, 1970; Urbani and Szczerban, 1974; Urbani, 1976; Zawidzki et al., 1976). This alternation is suggested to result from the change in the chemical compositions of supplying water, environmental changes of surface area overlying caves, and probably diagenesis (Krauskopf, 1956; Hill and Forti, 1997). Various shapes of siliceous speleothems can be found in different caves of the world (Hill, 1976; White, 1976; Webb, 1979; Cody, 1980; Hill and Forti, 1997). For example, siliceous cave corals were reported in lava tube caves in USA (Anderson, 1930), and branching-type siliceous cave corals were also discovered from granite and sandstone caves in Australia (Webb and Finlayson, 1984; Wray, 1997a, Wray, 1997b, Wray, 1999). They were mostly composed of opal as well as other minerals such as opal-CT, chalcedony, limonite, and calcite (Lassak, 1970; Webb, 1979; Wray, 1997b). These speleothems were suggested to be formed by evaporation for silicate and oxide minerals or by degassing of carbon dioxide for carbonates after groundwater entered into the caves (Hill and Forti, 1997).

The objective of this study is to delineate the mineralogy and the origin of the cave corals in some lava tube caves in Jeju Island. This study also shows that diagenetic processes were responsible for the genesis of cave corals. In addition, this study has implications for paleoenvironmental changes on Jeju Island based on textural and geochemical data.

Section snippets

Geologic setting

Jeju Island, 90 km south of the Korean peninsula, was formed by several stages of volcanic activity from about one million to a few thousand years ago. It has an elongate shape, 74 km east–west and 39 km north–south (Fig. 1). The island is almost entirely composed of volcanic rocks, mainly trachyte, trachy-andesite, andesite, alkali basalt, and volcanoclastic rocks (Won, 1976; Sohn and Chough, 1992). Mt. Halla, a shield volcano, is located near the center of the island. The eastern and western

Methods

The cave corals were collected from Mosimoru, Jingaemot, Socheon, Bilemot, and Susan caves in Jeju Island to investigate the mineralogy, texture and geochemical compositions. Cave coral samples were impregnated using epoxy resin and cut into two slabs. One-half was thin-sectioned and examined with a polarizing microsope, and the other half was stained with Alizarine Red S and Feigl's Solution to determine the carbonate mineralogy (Friedman, 1959). The mineralogy of cave corals was also

Morphology of cave corals

Cave corals in this study are mostly white or translucent, and vary from 0.5 to 6 cm in size. They commonly occur on the cave ceiling, walls, and floor where water seeps through rocks or is present in pools. Six morphologic types can be identified (Fig. 2). Type A (spherulitic crystalline type) is white or translucent and about 1–2 cm long. This type is composed of acicular crystals with a radiating growth pattern from a single point, and resembles the frostworks found in limestone caves (Fig. 2A

Discussion

Siliceous cave corals discovered in some lava tubes of Jeju Island show several morphologic types (Fig. 2). The morphology appears to be controlled by the original mineralogy (aragonite or calcite), crystal growth habit and mode of replacement by opal. In terms of mineral composition and texture, there are three types of internal microstructure within the cave corals: (1) the cave corals composed of aragonite and calcite, which show selective partial replacement of the calcite by opal

Conclusions

Six types of cave corals are present in some lava tubes of Jeju Island, and their formation processes and diagenetic influences are different. In terms of mineral composition and texture, three types of internal microstructure are described: (1) the cave corals composed of the repetitive aragonite and calcite layers, which show selective partial replacement of only the calcite layers by opal (Jingaemot Cave); (2) the cave coral composed of calcite, which was replaced by opal (Susan and Mosimoru

Acknowledgments

This work was supported by the Korea Polar Research Institute (KOPRI Grant PE07010). Special thanks to the Jeju Self-Governing Province to give the permission for entering the caves. The authors are also grateful to the Jeju Island Cave Research and the Korean Research Institute of Biospeleology.

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    Present address: Property Division, Jeju Special Self-Governing Province 690-700, Republic of Korea.

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