Anatomy of Niger and Benue river sediments from clay to granule; grain-size dependence and provenance budgets

This study explores in detail the complexity of textural/compositional relationships in fluvial sediments. To this aim, fifteen size fractions (from clay to granule) of three sediment samples characterized by virtually identical size distribution from the Niger and Benue rivers in central Nigeria were separately analysed by multiple methods (optical microscopy, manual and semi-automated Raman spectroscopy, X-ray diffraction, elemental geochemistry, Nd isotopes). The independent mineralogical and geochemical datasets thus obtained allowed us to investigate processes of sediment generation for five diverse size modes (clay, fine cohesive silt, very coarse frictional silt, very fine sand, coarse sand) derived in different proportions from different sources (wind-blown dust, soils and paleosols, fine-grained and coarse-grained siliciclastic units, igneous and metamorphic bedrocks). Controls on the size distribution of detrital minerals (settling equivalence, size inheritance, weathering, mechanical durability, and chemical durability through multiple sedimentary cycles) were examined, specifically focusing on tectosilicates and on the long-standing petrological problem of feldspar-grain size relations. Different factors determine the composition of different size modes: kaolinite-dominated clay derives from both deeply weathered soils or paleosols and distant Saharan sources; cohesive silt is largely recycled from soils formed in sedimentary basins. The proportion of detritus derived first-cycle from basement rocks increases from very coarse silt to very fine sand, whereas the coarse sand mode is quartz-dominated with minor plagioclase and amphibole and local occurrence of garnet, staurolite, monazite, or xenotime reflecting a combined influence of size inheritance from igneous (pegmatite) and metamorphic sources, mechanical and chemical durability, and recycling from coarse-grained siliciclastic units. Sediment budgets based on mineralogical, geochemical, and geochronological signatures consistently indicate dominance of Benue sediment supply, although contributions from the Niger mainstem to the Niger Delta are inferred to have been notably greater in the wetter past, before clastic fluxes dropped in response to the aridification of the Sahel.