An ancient tufa in South Korea associated with topographic evolution from a stable cave environment to a near-surface environment

초록

Buried freshwater carbonates in the Moorungri region (Jeongseon-gun, South Korea) were discovered in 2003 during a local excavation for natural ornamental stones. Although the local government recently designated the Moorungri freshwater carbonates (MR-FC) as a potential natural monument based on its surficial beauty, the origin, processes, and depositional environments of the MR-FC are still unknown. This study aimed to determine the origin of the MR-FC based on petrographic and isotopic analyses. On the outcrop scale, the MR-FC overlies typical speleothems (a stalagmite-flowstone sequence) and can be divided into four units (Units 1-4), each of which comprises a pair of carbonate crystalline and clastic beds. Although each clastic bed in the individual units is mainly composed of detrital components, including residual red clays as a weathering product and coarse to medium sand-sized grains from the surrounding carbonate bedrock, several thin layers of authigenic carbonate minerals are observed in the free surfaces and pore spaces of each clastic bed. These features indicate that water supersaturated with calcium carbonate (CaCO3) was present throughout the deposition of the MR-FC, although the clastic beds were most affected by episodic pluvial events. Scanning electron microscopy showed that the MR-FC contains not only inorganic components, such as rock fragments and large calcite crystal arrays, but also various biogenic materials such as tree fragments, pollen, spores, fungal filaments, and diatoms. The abundance of biogenic components indicates that the MR-FC was influenced, at least indirectly, by the air outside the cave, and by sunlight. The mean delta C-13 values of the crystalline beds in Units 1, 2, and 4 were -8.71 parts per thousand, -8.62 parts per thousand, and -8.11 parts per thousand, respectively. These values clearly show that the carbonic acid forming CO2 was mostly from meteoric and soil sources. The relationship between the delta C-13 and delta C-18 values in each crystalline bed suggests that the rapid CO2 degassing effect gradually increased relative to the evaporation, implying that the parent water of the upper units in the MR-FC came into abrupt contact with the air outside the cave, which had a lower level of pCO(2). This circumstance would arise through cave decay near the discharge location and/or upstream, because removal of the covering bedrock allows for more vigorous degassing of CO2 through numerous openings created by surface denudation. Thus, we concluded that the MR-FC is a tufa deposit that formed during the topographic evolution from a stable cave environment to a near-surface environment.

키워드