Actions éoliennes récentes aux abords du delta de l'Orénoque (Upper quaternary wind deposits near the Orinoco delta (Venezuela))

Abstract

Abstract. - On the western side of the Orinoco delta, some patches of quartz sands, which have been leeched from their ironoxydes, cover irregularly the Mesa Formation, of Upper Pliocene age. Most patches don't have any typical eolian land- form, but some of them are associated with longitudinal dunes. These sands are typically eolian well sorted sands (fig. 2 and table). They are fossilised on the edge of the Orinoco delta, by holocene and contemporary alluvium. They have been deposited before the end of the Flandrian transgression, during a low sea level stage. These observations suport and complete a previous paleoclimatic reconstruction of the Northern part of South America. Further W, in the Apure and Colombian Llanos, an extensive dune field has been shaped during the last andean cold period (Wurm, Wisconsin). At the end of it, the climate being wet again, the rivers have flooded the dune field and excavated their valleys through it, reaching again the Orinoco. In Eastern and Central Amazonia, the relatively dry conditions changed to the humid present ones during the begining of the Flandrian transgression. The present rain forest took the place of a former xeric vegetation. Climatic changes have been very broad in Northern South America, as broad as in Western Africa, but their pattern is quite different, as a result of a different structural frame, clearly visible in the distribution of sea and land. In Africa, the distribution of present and past climates is clearly zonal. In America, on the contrary, the zonal influence is strongly obliterated by orography and N-S atmospheric circulation.