Climatic zonation and weathering control on sediment composition (Angola)

Complementary mineralogical and geochemical datasets on fluvial, beach and dune samples collected along the Atlantic margin of subequatorial southwestern Africa are used to investigate the relationships between provenance and climatic controls on sediment composition and to test the reliability of different geochemical and mineralogical weathering proxies as climatic indicators. The studied N/S-trending coastal region is characterized by strong latitudinal and inland climatic gradients, and thus represents an excellent natural laboratory in which to study the effects of climatic-induced weathering on sediment composition. Although the mineralogy and geochemistry of suspended-load muds closely reflects the different weathering intensities over both latitudinal and inland climatic gradients, the composition of mud and sand samples are strongly affected by sediment provenance. Consequently, weathering parameters such as the alpha E-Al values (estimating the degree of depletion in element E relative to the UCC standard), display complex patterns of variation especially for sand samples. By assuming a typical order of bulk-sediment mobility Na > Ca > Sr > Mg > K > Ba approximate to Rb, anomalously high or low alpha(Al) values placing a specific element off the expected mobility order are considered as an indicator of source-rock control on sediment composition. The composition of detritus recycled from Meso-Cenozoic strata reflects the cumulative effect of successive sediment cycles, with recycling processes affecting to a different extent the diverse weathering proxies. In particular, alpha Na-Al appears to be more strongly affected by recycling in muds than in sands. Among all mineralogical and chemical parameters, those that correlate best with rainfall in the drainage areas are alpha Na-Al for sands, alpha Mg-Al for muds and smectite content (only in areas of low rainfall). In the geological and geomorphological setting of SW Africa these proxies turn out to be better climate estimators than the classical weathering indices CIA or WIP. This case study reminds us to carefully consider source-rock control and mixing with recycled detritus when drawing inferences on climatic conditions based on weathering indices.