Sand-spits systems from Benguela region (SW Angola). An analysis of sediment sources and dispersal from textural and compositional data

Sand spits are important coastline features in western Angola, but only limited knowledge on their recent evolution and sediment sources were obtained so far. The present study is focused on the Baía Farta and Lobito sand spits of coastal Benguela that develop to the north (i.e. downdrift) of the Coporolo and Catumbela river outlets. We used grain-size distributions, heavy-mineral suites and clay-mineral assemblages of sediments in the Coporolo-Baía Farta and Catumbela-Lobito coastal stretches to characterize the main depositional units and investigate sediment provenance. From the combined grain-size and mineralogical variability in mud and sand samples it is possible to infer sediment sources and dispersal in the two coastal stretches. Kaolinite is mainly derived from the Angola hinterland, and is particularly common in finer grained floodplain sediments from the Catumbela River. Expansive clays (smectite and illite-smectite mixed layers) are inferred to be mainly sourced by Meso-Cenozoic units of the Benguela Basin, being abundant in coarser grained fluvial deposits and in lagoonal deposits near Baía Farta. Sand supplied by the sedimentary units from Benguela Basin and their basement rocks tends to be enriched in epidote associated with blue-green hornblende. The Coporolo River sand is progressively diluted during the longshore northward transport by sand supplied by coastal units. Conversely, beach deposits in the Catumbela-Lobito coastal stretch are mainly sourced by the Catumbela River. A divergent longshore transport from Catumbela river-mouth occurs at Catumbela delta. Sand spit morphology and evolution reflect the patterns of dispersal of bedload and suspended load in settings of contrasting orography and human influence

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 α Al E 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≈Rb, anomalously high or low α 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, α Al Na 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 α Al Na for sands, α Al Mg 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

Dynamic uplift, recycling, and climate control on the petrology of passive-margin sand (Angola)

The subequatorial Angolan continental margin offers excellent conditions to test textbook theories on the composition of passive-margin sediments generated in different climatic and tectonic regimes. We use here comprehensive petrographic, heavy-mineral, geochemical and zircon-geochronology datasets on modern fluvial, beach, shelfal, and deep-marine sands and muds collected from hyperarid northern Namibia to hyperhumid Congo to investigate and assess: a) how faithfully sand mineralogy reflects the lithological and time structures of source rocks in a tectonically active rifted margin; b) in what climatic and geomorphological conditions the mark of chemical weathering becomes strong and next overwhelming; and, c) to what extent the effect of weathering can be isolated from quartz dilution by recycling of older siliciclastic strata and other physical controls including hydraulic sorting and mechanical wear. A new refined classification of feldspatho-quartzose and quartzose sands and sandstones is proposed.First-cycle quartzo-feldspathic to feldspar-rich feldspatho-quartzose sand eroded from mid-crustal granitoid gneisses of the Angola Block exposed in the dynamically uplifted Bié-Huila dome is deposited in arid southern Angola, whereas quartz-rich feldspatho-quartzose to quartzose sand characterizes the lower-relief, less deeply dissected, and more intensely weathered rifted margin of humid northern Angola. Pure quartzose, largely recycled sand is generated in the vast, low-lying hyperhumid continental interiors drained by the Congo River. The progressive relative increase of durable minerals toward the Equator results from three distinct processes acting in accord: active tectonic uplift in the arid south, and progressively stronger weathering coupled with more extensive recycling in the humid north. The quartz/feldspar ratio increases and the plagioclase/feldspar ratio decreases rapidly in first-cycle sand generated farther inland in the Catumbela catchment, reflecting stronger weathering in wet interior highlands. Discriminating weathering from recycling control is difficult in northern Angola. Although textural features including deep etch pits even on relatively resistant minerals such as quartz and microcline or rounded outline and abraded overgrowths provide valuable independent information, recycling remains as a most elusive problem in provenance analysis of terrigenous sediments