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Siliceous crust-type (SCT) formations are pervasive silicified horizons associated with fluoritebarite-polymetallic sulfide mineralization in carbonate rocks. They almost invariably show a peculiar grid-work texture formed by microcrystalline quartz. The mineralogical and textural features of several SCT samples from Italy, Spain, France, China, and Brazil were investigated by optical and electron microscopy and X-ray and neutron powder diffraction. The resulting {001} pole figures exhibit two main components of orientation of the crystallographic c axis with respect to the normal to the sample surface, one located at about 35 ∞ and the other in the range 60‐85∞. The observed textural features are seen in all samples and they appear to be independent of the age and geographical location of the SCT formation. The observed grid-work texture is tentatively interpreted as forming by nucleation of quartz seeds on specific faces of surrounding crystals. The observed morphology and the regularly oriented populations of crystals are consistent with simultaneous growth of quartz crystals having well-defined reciprocal orientations. The grid-work texture of quartz in SCT horizons is similar to the type reported for Magadi-type cherts, which are commonly interpreted as formed by magadiite-mediated silica precipitation processes. Since the genesis of SCT deposits is totally unrelated to alkaline-lake genetic processes, it is concluded that cherts showing strongly textured patterns of microcrystalline quartz may have a different origin than Magadi-type deposits.

During recent humid episodes, stromatolites were built along a paleolake margin, some 60 m above the modern water level of lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently coating pebbles and boulders eroded from the older ones. Therefore, three distinct lake level rises are distinguished, ranging in age from middle Pleistocene to Holocene. The stromatolitic incrustations indiscriminately cover the whole hard substrates, including the bedrock surfaces as well as some vegetal remains. They show a variety of growth forms from oncolites, tabular and cylindrical encrustations to various type bioherms. Through detailed morphologic and pétrographie examination of the stromatolites it was possible to establish a precise littoral zonation. Several parameters of the physical environment, such as depth, hydrodynamism and light intensity have a strong influence on the stromatolite gross morphologies. Both blue-green algae and bacteria, growing in mats or colonies, precipitate calcite which is the main component of the stromatolites. The most common stromatolitic microstructure appears to be the repetition of a doublet composed of a dark-colored micritic lamina and a light-colored lamina of radial fibrous calcite. This doublet represents an ecological cycle of the microbial mat growth and is interpreted in terms of seasonal contrast., Au cours d'épisodes humides récents, des stromatolites se sont formés en bordure d'un paléolac, dont le rivage se situe 60 m au-dessus du niveau actuel des lacs Natron et Magadi. Trois générations de stromatolites sont décrites, les plus récentes encroûtant généralement les formes érodées plus anciennes. Elles correspondent à trois hauts niveaux lacustres différents, dont les âges se distribuent du Pléistocène moyen à l'Holocène. Les stromatolites recouvrent l'ensemble des substrats durs, parmi lesquels figurent de nombreux supports végétaux. Oncolites, encroûtements plans et cylindriques, ainsi que diverses formes de bioherms sont les principales morphologies rencontrées. Une analyse pétrographique et morphologique détaillée a permis d'établir une zonation littorale précise. Les principaux facteurs de l'environnement, tels que la profondeur, l'hydrodynamisme ou l'intensité lumineuse, influencent directement la morphologie des stromatolites. La phase carbonatée est de la calcite précipitée conjointement par des cyanophycées et des bactéries sous forme de tapis ou de colonies. La microstructure la plus fréquente résulte de la répétition d'un doublet composé d'une lamine sombre micritique et d'une lamine claire sparitique. Ce doublet représente un cycle écologique de la croisance du tapis microbien ; il est interprété en termes de contraste saisonnier., Casanova Joel. Stromatolites et hauts niveaux lacustres pléistocènes du bassin Natron-Magadi (Tanzanie-Kenya). Pleistocene stromatolites and high lake levels of the Natron-Magadi basin (Tanzania-Kenya). In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 135-153.

(δ²H et δ¹⁸O) values of hydrothermal springs from the Lake Magadi -Lake Natron area show a linear relationship with a 4.7 slope indicative of evaporated waters. The springs are considered to be mixtures of three components : (1) deep saline groundwaters of the rift floor (¹⁸O~-7‰ ; δ²H ~-50‰); (2) shallow dilute groundwaters from the basaltic and Precambrian plateaus (δ¹⁸O~-4‰ ; δ²H~-20‰) ; and (3) interstitial brines from the lake sediments (δ¹⁸O~8‰δ²H ~-24‰) . No apparent traces of deep rock-water isotopic exchange are observed. 13C and 14C contents of the inorganic dissolved carbon are considered to represent deep carbon inputs (δ¹³C, Les sources hydrothermales du bassin Magadi-Natron présentent des compositions isotopiques (δ²H et δ¹⁸O) groupées autour d'une droite de pente 4,7 indiquant une évaporation assez importante. Elles sont interprétées comme le produit d'un mélange de trois composantes : la nappe profonde du rift (¹⁸O~-7‰ ; δ²H ~-50‰), la nappe superficielle des plateaux basaltique et précambrien (δ¹⁸O~-4‰ ; δ²H~-20‰) et les saumures interstitielles des sédiments lacustres (δ¹⁸O~8‰ ; δ²H ~-24‰). Le circuit géothermal de l'eau n'a laissé aucune trace isotopique apparente. Les teneurs en I3C et en l4C du carbone inorganique dissous reflètent des apports de carbone profond (δ¹³C, Hillaire-Marcel Claude. Hydrologie isotopique des lacs Magadi (Kenya) et Natron (Tanzanie). Isotopic hydrology of the lakes Magadi (Kenya ) and Natron (Tanzania ). In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 111-120.

The Lake Natron-Lake Magadi basin has been occupied by extensive lakes on several occasions during the late Pliocene and Pleistocene times. During the most recent lacustrine episodes, stromatolites were built at the lake margins, some 50 m above the modern water level. Three distinct phases of encrustation were identified by detailed morphologic and pétrographie examination of these algal limestones. The more recent phases frequently cover pebbles and boulders eroded from the older phases. More than 30¹⁴C measurements on the third generation yielded ages ranging from approximately 12,000 to 10,000 yrs B.P. Their ¹³C values (≥ 2.6 ‰) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO₂, thereby lending credence to the reliability of the ¹⁴C ages. I80 and 13C measurements on these algal limestones when compared to values on modern waters and carbonates, give some indication about the paleohydrology. Long residence time of the paleolake waters and less seasonally contrasted regimes have been inferred for the corresponding humid episodes. The ²³⁰Th/²³²Th ratio of the detrital component was determined by Th/U measurements on the recent stromatolites. Using this values we then calculated a ²³⁰Th/²³⁴U chronology for the older stromatolites. Age ≥240,000 yrs and 135,000 ± 10,000 yrs were obtained respectively for the first and second generations. A humid episode seems therefore to characterize eastern Africa during each glacial-interglacial transition period., Trois générations de stromatolites marquent l'extension récente de paléolacs dans le bassin Natron-Magadi, à la frontière tanzano-kenyenne. Leurs teneurs en ¹⁸O et en ¹³C, relativement élevées, reflètent un long temps de résidence de l'eau des paléolacs favorable à l'établissement d'un équilibre entre le CO₂ atmosphérique et le carbone inorganique dissous malgré les forts apports de carbone profond lié au volcanisme carbonatitique régional. De ce fait, les âges ¹⁴C obtenus sur les stromatolites les plus récents (12 00 à 10 000 B.P.) sont considérés comme représentatifs. Les stromatolites plus anciens ont donné des âges U/Th de 135 000 ans et de ca. 240 000 ans. Un épisode humide semble donc caractériser l'Afrique de l'Est au cours de chaque transition entre les périodes glaciaires et interglaciaires., Casanova Joel, Hillaire-Marcel Claude. Chronologie et paléohydrologie des hauts niveaux quaternaires du bassin Natron-Magadi (Tanzanie-Kenya) d'après la composition isotopique (18O, 13C, 14C, U/Th) des stromatolites littoraux. Chronology and paleohydrology of former lake levels in the Natron-Magadi basin (Tanzania-Kenya) from isotopic composition (18O, 13C, 14C, U/Th) of shoreline stromatolites. In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 121-134.