Karsts en roches cristallines silicatées non carbonatées.. Cas dans les métagabbros : grotte de Guessédoundou, Niger occidental

Since the Seventies, a séries of phenomena developed in non carbonated silicated rocks question the traditional vision of karst. The term of karst is used in the meaning of "any feature of the classical karst morphology (caves, sink-holes, lapiaz...) where dissolution plays the main genetic action and this in any kind of rocks".
The cave of Guéssédoundou illustrâtes a karst in métagabbros. It opens at the top of a small hill, in a dépression twenty meters long and three to four meters deep. The walls of this dépression are subvertical and the bottom is partially occupied by angular pluridecimetric blocks. The cave is opened on several meters in the southern side of the dépression and is prolonged in the form of too narrow conduits to be explore. The bedrock is a metagabbro with fine grains, slightly weathered, crossed by decimetric quartzose veins. Gabbroïc texture is preserved, as well as part of magmatic plagioclases. The remainder is transformed into a mixture of white feldspar and clinozoisite. Pyroxene are pseudomorphosed in actinote and chlorite.
The site dépression was created along a N 10° shear zone where rocks suffered important fracturation and fluid transfers, as shown by its silification and ferruginisation.. Moreover, a East-West schistosity, former to shearing, structures the gabbro. The présence of angular blocks of variable size and the absence of human activity trace on the site show that the dépression was formed naturally by collapse of the roof of a more vast cavity whose current cave would be the visible residual prolongation. The absence of flow traces and mechanical abrasion means that this cavity is due only to chemical érosion .
At the cave entry and on the subvertical walls of the dépression were formed by alveoli a few centimètres to a few décimètres in diameter. Thèse alveoli of wall are known in other lithologies, in particular in limestones. The absence of salts within the rock draws aside a formation by sait weathering. The smooth aspect and non fractured walls of the cells cannot resuit from a corrosion to a fracturing node. The closed aspect of the dépression and the absence of flow trace eliminate the assumptions from formation by water swirls or eolisation. The genesis of the cave must be earlier to the installation of a weathering cover since the rock is little deteriorated and the alveoli are not developed on a quite particular level which would correspond to a précise pedogenic horizon. Only a generalized dissolution of the métagabbros, in relation to a slow solution circulation, being carried out with the favour of the fracturing plans of the shearing zone makes it possible to explain the formation of the alveoli. The inversion of relief results from a weaker érosion of the rocks made more résistant to this place by the quartz seam mainstay and by the ferruginisation along the shearing zone. In the other hand, the crushed rocks, become porous, drained the infiltrations of meteoric water, facilitating the dissolution processes. The study of the site of Guéssédoundou shows that karstic phenomena can also exist in silico-aluminous formations of crystalline rocks.