Blue Lagoon, Afrique du Sud : une grotte à remplissage paléokarstique permien et à concrétions d'aragonite

Blue Lagoon, South Africa, a cave with permian paleokarstic fillings and aragonite speleothems. The authors describe a 7 km long phreatic maze they discovered and explored during the last decade of the 20th century in South Africa, developed in the late Archean dolostone in the Malmani Subgroup. This cave is of interest mainly for two aspects. Firstly the cave intersects paléokarst channels filled with bleached kaolinic residuals of Permian age. This paléokarst is most likely to have developed relatively shortly after the Gondwana glaciation in a cool, humid climate. Secondly the cave is remarkable by the abundance of aragonite speleothems. Particularly interesting are subaquatic aragonite formations : rafts, cones, volcanoes, sea urchins and pool floor crust. Aragonite rafts are always associated with more or less cakite, which seems to have formed first and was apparently essential in the initial formation of this speleothem. In the pool floor crust, a cyclical calcitearagonite deposition seems to correspond to alternation of humid and dry periods, calcite representing wet years. The amplitude of this cycle is possibly in the order of a few decades. Phosphate minerals which developed on cave soil, rock and carbonate spekothems in contact with bat guano, have been identified, in partkular the rare mineral collinsite.Blue Lagoon est une grotte phréatique de type labyrinthique située dans l'Ouest Transvaal (Afrique du Sud). Elle présente deux aspects remarquables. D'une part le réseau recoupe des galeries paléokarstiques permiennes remplies par des argiles kaoliniques blanches. Il est probable que ce paléokarst se soit développé en climat frais et humide après une période glaciaire. D'autre part la grotte est riche en concrétions aragonitiques de différents types, en particulier des dépôts formés en milieu subaquatique : aragonite flottante, cônes et concrétionnement des plans d'eau. Un dépôt cyclique calcite-aragonite semble correspondre à une alternance climatique humide-sèche, dont l'amplitude semble être de plusieurs décennies. Des minéraux de phosphates, formés par l'action du guano de chauve-souris sur la roche, le sol et les spéléothèmes, ont été identifiés, en particulier un minéral rare, le collinsite.Martini J.E.J., Moen H.F.G. Blue Lagoon, Afrique du Sud : une grotte à remplissage paléokarstique permien et à concrétions d'aragonite. In: Karstologia : revue de karstologie et de spéléologie physique, n°32, 2e semestre 1998. Grottes ornées de Bornéo (E Kalimantan) et Grotte de Blue Lagoon (Afrique du Sud) pp. 27-38

New age constraints on the tectonogenesis of the Kheis Subprovince and the evolution of the eastern Namaqua Province

Mapping and stratigraphic analysis of the rocks adjacent to the Kaapvaal Craton east of Upington has suggested that the area previously defined as the Kheis Subprovince was deformed and metamorphosed not during the so-called Eburnean "Kheis Orogeny" but as the result of a considerably younger event. U-Pb zircon SHRIMP ages on sedimentary as well as volcanic and plutonic rocks show that the arenitic successions that dominate the area were derived from a provenance similar in geochronological make-up to the Kaapvaal Craton and laid down no earlier than ∼1.8 Ga ago. Deposition of the volcanosedimentary Wilgenhoutsdrif Group is dated at ∼1.3 Ga, confirming its unconformable relationship to the arenites and its co-magmatic relationship with the similarly-aged Kalkwerf Gneiss. The regional dynamothermal event is concluded to be early Namaquan, and its age is revealed by its post-Wilgenhoutsdrif character and Rb-Sr muscovite ages clustering around ∼1.2 Ga. These findings confirm that the Kheis Subprovince contains no Eburnean orogenic signature and is, in fact, an integral part of the Namaqua Province. The post-tectonic Koras Group lies entirely within the subprovince and its <∼1.17 Ga age is consistent with the area's early Namaquan tectonic signature. It is separated from the ∼1.1 to 1.0 Ga old Gordonia Subprovince by the Trooilapspan shear zone, a terrane boundary thought to indicate significant lateral displacement