Geological Heritage Values of the Yongcheon Cave (Lava Tube Cave), Jeju Island, Korea

Abstract

Yongcheon Cave is a typical lava tube cave, which formed by multiple flows of lava during the Late Quaternary. However, it is also a lime-decorated cave with numerous splendid carbonate speleothems along two parts of the passage. This paper deals with these special unique features of the cave together with their formation processes and the geoheritage significance based on detailed investigation of overlying carbonate sand dunes and speleothems. The cave is about 3360 m long, composed of 2590-m-long main passage and about 370-m-long branch passages. Because the cave contains unexplored passages at the downstream end, it is estimated that the total length can be at least about 400 m longer. The cave contains various microtopographic characteristics such as multiple and various forms of lava coatings on the surface as well as dented features by thermal erosion. They include pahoehoe and aa lavas, rockfalls, lava falls, lava benches, gutters with overflowed lava levees, tube-in-tubes, and lava benches on the floor. Lava flowlines are prominent on the wall, and also, anastomosing branches, lava bridges, and multiple leveled floors are other distinctive morphological features. Carbonate sand dunes were formed by transportation of beach sands nearby and partially overlie two parts of the cave passage. Carbonate sediments are medium sand-sized and are mostly composed of mollusk and coralline red algae fragments up to 60~70% with a minor contribution of benthic foraminifers, echinoderms, bryozoans, and volcanic rock fragments, which may well represent typical temperate carbonate constituents elsewhere. Beach sediments, the source of carbonate dune sands, also show the similar texture and composition. This suggests that the dune sands were transported onto the area by wind along the valleys. Previous age dating results of four carbonate sand dunes including several paleosol layers imply the formation ages of the dunes at each site since the deposition of carbonate sands in shallow seas between 4700 and 6000 years BP due to full deglaciation after Last Glacial Maximum. Carbonate speleothems began to grow in the cave after the deposition of dunes. Detailed mapping of the cave and dune fields strongly suggest that the dune distributions are directly related to the formation of carbonate speleothems. A variety of carbonate speleothems are found in Yongcheon Cave such as soda straws, stalactites, stalagmites, columns, rimstones, flowstones, cave corals, draperies (curtains and bacon sheets), cave pearls, shelfstones, and moonmilks. However, many carbonate speleothems show erratic morphologies that cannot be seen in any limestone caves elsewhere. It is because the formation of erratic speleothems is directly influenced by distribution of plant roots which helped to transport rainwater into the cave and to precipitate calcite mineral. Their unique abundance of carbonate speleothems providing splendid scenic values as well as Jeju’s special geological conditions for their formation is considered to be the outstanding universal value for the World Heritage site.