RESUMENSe presentan los resultados de nuevos trabajos realizados en la cueva del Medio, Ultima Esperanza, Chile. Algunos de los principales procesos de formación de la cueva y de acumulación de sus sedimentos han sido identificados. Nuevas excavaciones mostraron evidencias de fauna extinta utilizando la cueva desde aproximadamente 14,000 años radiocarbónicos AP. Las ocupaciones humanas finipleistocenas han sido confirmadas con nuevos hallazgos y fechados, aunque la importancia de la fauna extinta en la subsistencia humana parece ser inferior a lo sugerido con anterioridad.PALABRAS CLAVE: Cueva del Medio, Última Esperanza, Pleistoceno, geomorfología, tafonomía ABSTRACT New studies at cueva del Medio, Ultima Esperanza, Chile, are presented. Some of the main cave formation and sediment accumulation processes were identified. New excavations indicated that extinct fauna was using the cave at least since 14,000 radiocarbon years BP. Human occupations at the end of the Pleistocene were confirmed with new findings and radiocarbon dates. The importance of extinct fauna in the human subsistence was less important than previously sustained.
International audienceAbstract Breakthrough tailing has been observed during dye-tracing recovery tests in the Norville aquifer system (chalk), France. Karst-conduit flow and transport parameters were assessed using two different interpretative methods: the linear graphical method and the Chatwin method (implemented in the Qtracer2 program). The linear graphical method was used to model the observed tailing effects, which was explained by a second smaller delayed breakthrough curve. By comparing the results of tracertest interpretation for the two methods, it was possible to relate the area of this second curve to the importance of turbulent flow in spring discharge. The more turbulent the flow, the less important the contribution of the second breakthrough curve and the tailing effect. The observed tailing could possibly be controlled by hydrodynamics to a greater extent than usually expected, the tailing effectsbeing mostly attributed to diffusion phenomena. Tailing effects were expected to increase with discharge and the piezometric level, which would have resulted in overpressure in conduits, fissure flooding, etc. Instead, breakthrough tailing tended to disappear with increasing aquifer discharge, which would support the hypothesis of there being mostly hydrodynamic-controlled tailing effects instead of matrix- or fissure-diffusion
This paper presents an extensive review of the process of ghost-rock karstification and highlights its role in the formation of cave systems. The process integrates chemical weathering and mechanical erosion and extends a number of existing theories pertaining to continental landscape development. It is a two stage process that differs in many respects from the traditional single-stage process of karstification by total removal. The first stage is characterised by chemical dissolution and removal of the soluble species. It requires low hydrodynamic energy and creates a ghost-rock feature filled with residual alterite. The second stage is characterised by mechanical erosion of the undissolved particles. It requires high hydrodynamic energy and it is only then that open galleries are created. The transition from the first stage to the second is driven by the amount of energy within the thermodynamic system. The process is illustrated by detailed field observations and the results of the laboratory analyses of samples taken from the karstotype area around Soignies in southern Belgium. Thereafter, a series of case studies provide a synthesis of field observations and laboratory analyses from across western Europe. These studies come from geologically distinct parts of Belgium, France, Italy, and the United Kingdom. The process of ghost-rock karstification challenges a number of axioms associated with the process of karstification by total removal. On the basis of the evidence presented it is argued that it is no longer acceptable to use karst morphologies as a basis with which to infer specific karstogenetic processes and it is no longer necessary for a karst system to relate to base level as ghost-rock karstification proceeds along transmissive pathways in the rock. There is also some evidence to suggest that ghost-rock karstification may be superseded by karstification by total removal, and vice versa, according to the amount of energy within the thermodynamic system. The proposed chemical weathering and subsequent mechanical erosion of limestone suggest that the development of karst terrain is related far more closely to the geomorphological development of aluminosilicate and siliceous terrains than is generally supposed. It is now necessary to reconsider the origin of many karst systems in light of the outlined process of ghost-rock karstification.
Figure 2. Residence time distribution for each tracer test (peak areas = 1). Residence time distributions ordinates are H(t) = tracer flux/recovered mass (Reprinted from Journal of Hydrology, 275, Massei N, Wang HQ, Dupont JP, Rodet J, Laignel B, Assessment of direct transfer and resuspension of particles during turbid floods at a karstic spring, 109-121, Copyright (2003), with permission from Elsevier).
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