Grottes hydrothermales dans le Nord-Ouest de la Namibie. Spéléogenèse et implication dans le développement des karsts en climat aride

Résumé En Fr

Hydrothermal caves in North-West Namibia. The authors investigated ten caves in Western Namibia, which is characterised by a semi-to hyper-arid climate. They seem to have formed in the past under hydrothermal conditions, which are evidenced by circular embayments, ceiling alveoles, avens, deposits of dog-tooth calcite and barite. The latter has been observed in one cave only. Fluid inclusions in calcite and barite indicate very low salinity and temperatures generally below + 70° C. It is proposed that the caves formed by mixing of hydrothermal solutions of deep origin with more surficial ground water in the vicinity of karst springs. Such ground water circulation patterns, close to the water-table, are suggested in several cases by the horizontal extension in caves, forming definite levels of passage networks cutting across the country rock stratigraphy. The alveolar avens developed upwards from these horizontal passages and seem to have formed subaerally by water evaporation from warm pools at the bottom, with condensation and corrosion above, against cooler rock. The suggested genetic processes are in agreement with models proposed by other authors. It is suggested that in arid climates, conditions are more favorable for development of this type of deep karst water circulation than under wetter conditions. It could possibly even be the predominant process of speleogenesis in very arid conditions. By extension, this concept — mixing of water of deep origin, not necessarily significantly hydrothermal, with surficial ground water — could explain the peculiar nature of most of the Namibian caves. The latter are typically characterised by the development of very large chambers and phreatic networks, but with restricted extension and not forming well integrated systems.

Une dizaine de grottes hydrothermales ont été étudiées par les auteurs dans les régions arides à semi-arides de l'Ouest de la Namibie. Elles sont caractérisées par des niches alvéolaires, des cheminées aveugles, des dépôts de calcite en dents de chien et, dans une seule grotte, de barytine. Les inclusions fluides indiquent une température de dépôt inférieure à + 70° C dans la plupart des cas. La formation de ces grottes résulterait du mélange, dans les aquifères karstiques, des eaux hydrothermales issues de la circulation profonde avec les eaux superficielles. Cette hypothèse pourrait expliquer le développement de cavités volumineuses, mais d'extension très réduite, caractéristiques des grottes de Namibie.