SEDIMENTOLOGY AND GEOCHEMISTRY OF LACUSTRINE SEQUENCES OF THE UPPER PLEISTOCENE AND HOLOCENE IN INTERTROPICAL AREA (LAKE MAGADI AND GREEN CRATER LAKE) - PALEOCLIMATIC IMPLICATIONS

Sedimentological and geochemical analyses have been carried out on lac ustrine deposits of East Africa, at Lake Magadi (2-degrees-S, 36-degre es-E, Kenya) and at Green Crater Lake (0-degrees-S, 36-degrees-E, Keny a), to determine the parameters controlling climatic and environmental dynamics during late Pleistocene and Holocene. These sedimentary sequ ences were collected with a stationary piston corer. At Lake Magadi (F ig. 1), sedimentary and geochemical control show three phases of lake level variation which corresponds to climatic change occurring during the last 40 thousand years. These phases were defined by three lithost ratigraphic units. Laminated deposits of Lake Magadi were formed durin g a wet period. Analysis of these laminae define two microfacies: a da rk lamina, characterised by lacustrine organic matter and a light lami na enriched in detritus, carbonates (CaCO3) and magadiite (NaSi7O13(OH )3, 3H2O). The formation and preservation of each couplet was favoured by climatic contrast, lake stratification and various origin of the s ediments (autochthon and allochthon) in the drainage basin. Therefore a relative chronology can be derived from laminae counting and the dur ation of deposition of each couplet. Spectral analysis applied on vari ation of the laminae thickness, shows the existence of three main peri ods, 4-7 years, 8-14 years and 18-30 years, respectively (Fig. 2). The se cyclicites of the lacustrine environment precise former determinati ons established on more recent lacustrine sequences from East Africa. They are related to the global climatic cycle (quasi-biannual oscillat ions, El Nino Southern Oscillations and the sun spot cycles). At Green Crater Lake, the study of the sedimentary sequence was completed by p hysico-chemical analysis of the waters and interface sediments which d emonstrate the carbonate, sodium, bicarbonate composition and the ther mal and chemical stratification of the modem lake. The sedimentary seq uence is characterized by volcanic deposits overlain by physico-chemic al analysis of the lake waters and interface sediments which demonstra te the carbonate, sodium, bicarbonate composition and the thermal and chemical stratification of the modem lake. The sedimentary sequence is characterized by volcanic deposits overlain by silt and clays deposit ed before 7400 years B. P., followed by lowering of the lake level at 3000 years B. P. Results from lake Magadi document the occurrence of a wet period starting at about 12,000 years B. P. The methodology appli ed on modern Green Crater lake provides base of interpretative models for other Holocene sequence lacustrine systems of intertropical zones.