Rift basins in their early stage of development: Examples from the Makgadikgadi-Okavango-Zambezi Basin and the southern Tanganyika Basin. Geochemical composition of sediments as tracers of climate change and tectonic evolution
Résumé
We present the results of a geochemical study of Quaternary sediments from two rift basins in the East African Rift, the Makgadikgadi-Okavango-Zambezi (MOZ) basin and the Mpulungu sub-basin of Lake Tanganyika. The early initiation stage in the development of a continental rift is characterized by the development of shallow half-graben basins where nascent faults exert a primary control in the evolution of drainage catchment formation. This stage is illustrated by the present Okavango fan, located within the larger Makgadikgadi-Okavango-Zambezi basin (MOZ) in NW Botswana. The immediate successive stage concerns the development of basins where shallow lacustrine environments occur, as the result of fault growth represented by the larger MOZ basin. A more mature stage concerns the development of well-defined, actively subsiding half-graben basins, where propagation and interaction between fault segments lead to basin linkage illustrated by the southern Mpulungu sub-basin of Lake Tanganyika. The use of geochemical compositions of sediments in diagnostic diagrams has been shown to be useful in determining tectonic settings although the field defined for passive margins (PM) include a wide range of tectonic environments, from rifted continental margins of the Atlantic-ocean type, sedimentary basins near to collision orogens and inactive or extinct convergent margins. In this work, based on the major element geochemistry, we define two new fields within the PM setting being 1)alluvial fan (AF) and 2) lacustrine basin (LB).The provenance of the sediments of both the Okavango Delta and Mpulungu sub-basin sediments was determined in this study. For the Mpulungu sub-basin, our results indicate that the immediate source material is a quartzoze sedimentary rock represented by the sandstones of the Mbala formation. The ultimate source material was a felsic source represented by the basement granites. These results agree very well with the current hydrological conditions in the basin, where the Lufubu River traverses these two source rocks. For the Okavango Delta sediments, the interpretation is more complex as the sediment source includes a felsic rock source and a pyroxene and olivine rich mafic-ultramafic rock complexes which are mixed with sand and diagenetic carbonates to produce the Okavango sediments. Geochemical data was used as proxies for climate change in both the Lake Ngami (SW Okavango) and the Mpulungu sub-basin of Lake Tanganyika. The Lake Tanganyika data shows a remarkable excursion coinciding with the Younger Dryas (YD) event, reflecting the input of previously chemically weathered material into the basin due to a change of vegetation