Sedimentary processes controlling ultralong cells of littoral transport: Placer formation and termination of the Orange sand highway in southern Angola
| Autor: | João Huvi, Pieter Vermeesch, Mara Limonta, Sergio Andò, Marta Padoan, Eduardo Garzanti, Annette Hahn, Pedro Dinis, Giovanni Vezzoli, Alberto Resentini, Martin Rittner |
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| Přispěvatelé: | Garzanti, E, Dinis, P, Vermeesch, P, Andò, S, Hahn, A, Huvi, J, Limonta, M, Padoan, M, Resentini, A, Rittner, M, Vezzoli, G |
| Jazyk: | angličtina |
| Rok vydání: | 2018 |
| Předmět: |
Moçamedes Desert and Cunene River sand
Provenance Placer mining 010504 meteorology & atmospheric sciences Stratigraphy Raman counting of deep-sea silt 010502 geochemistry & geophysics 01 natural sciences Detrital-zircon geochronology Paleontology Continental margin Littoral zone 0105 earth and related environmental sciences garnet and pyroxene chemistry Canyon geography sedimentary petrology geography.geographical_feature_category heavy mineral Geology longshore sediment transport grain density and geochemistry of placer deposit Black sand Sedimentary rock Sediment transport |
| Popis: | This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal Namibia to Angola. All four dunefields are terminated by river valleys, where aeolian sand is flushed back to the ocean; and yet sediment transport continues at sea, tracing an 1800Âkm long submarine sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths via canyon heads connected to river mouths. Garnetâmagnetite placers are widespread along this coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the rest in similar proportions from the Cunene River and from the Swakop and other rivers draining the Damara Orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, clinopyroxene grains and bimodal zircon-age spectra with peaks at ca 0·5ÂGa and ca 1·0ÂGa, is lost abruptly at Namibe, and beach sands further north have abundant feldspar, amphibole-epidote suites and unimodal zircon-age spectra with a peak at ca 2·0ÂGa, documenting local provenance from Palaeoproterozoic basement. Along with this oblique-rifted continental margin, beach placers are dominated by FeâTiâCr oxides with more monazite than garnet and thus have a geochemical signature sharply different from beach placers found all the way along the Orange littoral cell. High-resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of kilometres, providing essential information for the correct reconstruction of âsource to sinkâ relationships in hydrocarbon exploration and to predict the long-term impact of man-made infrastructures on coastal sediment budgets. |
| Databáze: | OpenAIRE |
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