Weathering geochemistry and Sr-Nd fingerprints of equatorial upper Nile and Congo muds

This study investigates processes of sediment generation in equatorial central Africa. An original, complete and integrated mineralogical-geochemical database on silt-sized sediments derived from different parent rocks (basalt, granite, gneiss, metapsammite, sandstone) along the East African Rift from 5°S in Tanzania to 5°N in Sudan is presented, and used to assess the incidence of diverse factors controlling sediment composition (source-rock lithology, geomorphology, hydraulic-sorting, grain-size, recycling), with particular emphasis on chemical weathering. Kaolinite abundance, CIA and αAl values consistently indicate less intense weathering along the steep inner flank and drier axis of the rift hosting Lakes Kivu and Tanganyika than in hot-humid forested highlands east of the Nile-Congo divide, where slopes are gentler and time for weathering longer. The observed order of bulk-sediment mobility (Na≥Ca>Sr>Mg>K>Ba≥Rb>Cs) roughly corresponds to the degree into which these elements are partitioned in unstable plagioclase versus K-feldspar and phyllosilicates. Weathering-limited erosion characterizes the Rwenzori massif and the Lake Albert graben where sediments are recycled from syn-rift deposits. 87Sr/86Sr ratios, 143Nd/144 Nd ratios and Sm-Nd tDM model ages of river muds proved instead to be insensitive to weathering, and provided a faithful integrated signature of the geology of each drainage basin. The comparison of CIA and WIP indices offers a key for discriminating compositional modifications due to weathering and recycling, a most challenging problem in sedimentary geochemistry. Integrated mineralogical-geochemical databases are essential to improve our understanding of weathering processes as they occur in the natural environment, and to distinguish their effect from other factors controlling sediment composition