«Stalactites extérieures » dans les karsts tropicaux humides : dépôts stalagmitiques de tufs calcaires

Résumé En Fr

" Outside Stalactites " in humid tropical karst -stalactitic deposits of calcareous tufa. Friable and porous stalactitic deposits composed of calcareous tufa -rather than sparry calcite characteristic of normal cave stalactites -are often encountered in the entrances of caves and plastered to cliffs in the humid tropics. Tufaceous stalactitic outside deposits are frequently mentioned in literature but are typically dismissed in a few sentences, even in review articles dedicated to calcareous tufa. Mostly based on field work in the Mariana Island, we have identified a variety of depositional settings where stalactitic tufa occurs. These settings can be grouped into spelean, transitional, epigean, and littoral realms. Centimeter to tens of meters in scale, their overall shapes can be quite irregular, with crooked, bulbous, pendant-like, light-oriented and other deflected forms exceedingly common. The outside surfaces of these " stalactites " invariably lack the crystalline luster of cave speleothems and feel wet and pasty, or powdery and earthy when dry. They are often covered by organic coatings. Stalactitic tufas are generally lightweight, porous, and friable, and many small specimens are weak enough to be plucked by hand. Composed of layered microcrystalline material, sometimes reminiscent of chalk, these "stalactites" exhibit a bewildering variety of fabrics, which can be classified as encrusted, amorphous, and laminated. In addition, they contain much organic material, microbial structures, and detrital grains. A wide array of biota is associated with these features, and they are thought to form by biogenic mechanisms superimposed on abiotic physico-chemical precipitation from karst water. Biologic processes involved in the formation of stalactitic tufa are numerous and appear to involve hundreds of species. While it is now clear that stalactitic tufas are a result of abiotic and biogenic deposition, an additional possibility remains to be considered. It is not improbable that tufa-like stalactites could form by decay and diagenesis of true cave speleothems, if the latter are exposed at the land surface conditions. Stalactitic tufas represent a unique, subaerial variety of calcareous tufa rarely deliberated in karst literature.

Les entrées de grottes et les falaises des milieux tropicaux humides sont souvent enduites d'un dépôt stalagmitique friable et poreux. Ce dépôt se présente généralement sous un faciès de tuf calcaire plutôt que de calcite sparitique caractéristique des stalactites de grottes. De dimension centimétrique à décamétrique, les formes de ces «stalactites extérieures » peuvent être irrégulières, courbées, avec des bulbes, des pendants, voire orientées par la lumière par exemple. Leur surface extérieure est poudreuse ou terreuse et fréquemment couverte pas un enduit humide organique. Composée de couches de matériel microcristallin se rapprochant de la craie, ces «stalactites » montrent une grande variété de fabriques parfois difficiles à distinguer. On distingue les fabriques d'encroûtement des dépôts amorphes ou laminés. De plus, ces dépôts contiennent une grande quantité de matière organique, de structure microbienne et de grains détritiques. On trouve ces stalactites dans un grand nombre de biotopes. On suppose que leur formation est liée à des processus biogéniques surimposés aux processus abiotiques de la précipitation/dissolution physico-chimique des eaux karstiques. Les stalactites extérieures représentent des formations sub-aériennes spécifiques rarement abordées dans la littérature karstologique.