- Republished from: International Caver 2000, pp 24-31.
- PDF: /pdf/seka_pdf9536.pdf
Abstract: Ha Long Bay is distinguished by the hundreds of small limestone islands that rise steeply or vertically from its shallow waters. Its dramatic and beautiful landscape is deservedly famous as one of the world’s outstanding natural sights, but it is also a UNESCO World Heritage Site of international geomorphological significance (Fig. 1). The bay lies on the northeastern coast of Vietnam, immediately east of the Red River delta . It is bounded on the north by the mainland hills either side of Ha Long City (also known as Hong Gai), to the south by the open waters of the Gulf of Tonkin, to the west by Cat Ba Island, and to the east by islands of sandstone (Fig. 2). Ha Long Bay has an area of about 1500 km2, and contains nearly 2000 limestone islands.
The caves described here were all visited during an assessment of the bay’s geomorphology with respect to its position as a World Heritage Site. Records of other caves in Ha Long Bay are sparse. A British team led by Howard Limbert mapped the Hang Hanh stream cave in the mainland limestone along the north shore of the bay; and a French team led by Marc Faverjon explored caves in the islands east of the bay, and also a few in Ha Long Bay itself.
- Republished from: Acta Carsologica 2005, 34 (2), 277-316
- PDF: /pdf/seka_pdf9534.pdf
Underground drainage systems and geothermal flux
Abstract: The paper presents an analysis of the interaction between the geothermal flux and the water or air- deep drainage networks. The problem of geothermal power intercepted by deep structures and, in general, the temperature field calculations, is converted to classical thermo-engineering problems in terms of shape factors. It is shown that the fluid flow in a conduit perturbs the whole deep rock temperature field until the geothermal flux of a large area is focalised onto the conduit. It is shown that either small water masses flowing into a mountain are able to perturb the rock temperature up to the surface, on sizes that do not depend on water mass dimension, but on its depth, and then on enormous volumes. The introduction of the ?geothermal cross section? of an underground drainage structure allows us to improve the classical formula of minimum provenance depth of geothermal water. Enlarging factors are applied to the classical estimation in dependence of the ratio between the actual average discharge and the critical discharge Qc, which depends on the conduit geothermal cross section. The geothermal ?umbra cones? created in the overlying rock by deep underground structures are described.
It is shown that the geothermal flux can play a significant role in the underground drainage phenomenology.
- Republished from: Geologica Carpathica 2002, 53 (6), 399-410
- PDF: /pdf/seka_pdf4507.pdf
Bosák P., Bella P., Cilek V., Ford D. C., Hercman H., Kadlec J., Osborne A. and Pruner P.
Abstract: Ochtiná Aragonite Cave is a 300 m long cryptokarstic cavity with simple linear sections linked to a geometrically irregular spongework labyrinth. The limestones, partly metasomatically altered to ankerite and siderite, occur as lenses in insoluble rocks. Oxygen-enriched meteoric water seeping along the faults caused siderite/ankerite weathering and transformation to ochres that were later removed by mechanical erosion. Corrosion was enhanced by sulphide weathering of gangue minerals and by carbon dioxide released from decomposition of siderite/ankerite. The initial phreatic speleogens, older than 780 ka, were created by dissolution in density-derived convectional cellular circulation conditions of very slow flow. Thermohaline convection cells operating in the flooded cave might also have influenced its morphology. Later vadose corrosional events have altered the original form to a large extent. Water levels have fluctuated many times during its history as the cave filled during wet periods and then slowly drained.
- Republished from: Acta Carsologica 2005, 34 (2), 317-348
- PDF: /pdf/seka_pdf4510.pdf
Condensation corrosion: a theoretical approach
Wolfgang Dreybrodt, Franci Gabrovöek and Matija Perne
Abstract: Condensation of water from warm, humid air to cold rock walls in caves is regarded to play a significant role in speleogenesis. The water condensing to the cave walls quickly attains equilibrium with the carbon dioxide in the surrounding air, and consequently dissolves limestone or gypsum forming various types of macro- ,meso-, and micromorphologies. In this paper we present the basic physical principles of condensation and give equations, which allow a satisfactory estimation of condensation rates. Water condensing to a cooler wall releases heat of condensation, which raises the temperature of the wall thus reducing the temperature difference ΔT between the warm air and the cave wall. Furthermore one has to take into account the heat flux from the air to the cave wall. This defines the boundary conditions for the equation of heat conduction. For a constant temperature of the air initial condensation rates are high but then drop down rapidly by orders of magnitude during the first few days. Finally constant condensation rates are attained, when the heat flux into the rock is fully transmitted to the surface of the karst plateau. For spherical and cylindrical conduits these can be obtained as a function of the depth Z below the surface. When diurnal or seasonal variations of the air temperature are active as is the case close to cave entrances, condensation rates can become quite significant, up to about 10-6 m/year. The theoretical results are applied also to corrosion of speleothems and the formation of »röhrenkarren« as described by Simms (2003). To convert condensation rates into retreat of bedrock the saturation state of the solution must be known. In the appendix we present experiments, which prove that in any case the solution flowing off the rock is saturated with respect to limestone or gypsum, respectively.
- Republished from: Environmental Geology 1997, 31 (1/2), 1-20
- PDF: /pdf/seka_pdf9537.pdf
Abstract: Most of exogenous epigenetic sulphur deposits are clearly associated with intensely karstified carbonate and sulphate rocks. This paper demonstrates, using the Pre-Carpathian region as an example, that karstification is one of the most important processes guiding the formation of sulphur deposits. This is determined by a coincidence of some major prerequisites of these two processes.
In the Podol'sky and Bukovinsky regions the Miocene aquifer system is well drained by erosion valleys; the giant network caves known here in gypsum formed under past artesian conditions. In the region of sulphur deposits, associated with the same karstified gypsum strata, true artesian conditions still prevail. Hydrogeologic data show that abundant cavities detected in the vicinity of sulphur deposits can be interpreted as having the same origin as the relict caves of the Podol'sky and Bukovinsky regions. The widespread belief that the gypsum/anhydrite bed in the region is an aquifuge separating the Miocene aquifers is inadequate. This belief caused much controversy with regard to the genetic interpretations of sulphur deposits in the region. Cave systems formed by the upward water flow through the gypsum/anhydrite bed govern the water exchange between the aquifers within the aquifer system.
A new karst model for the formation of sulphur deposits is suggested. It agrees well with the hydrogeological features of the Miocene sequence and with biogeochemical mechanisms of sulphur origin in low-temperature diagenetic environments.