Provenance versus weathering control on the composition of tropical river mud (southern Africa)
Introduction
Fine-grained sediments carried in suspension represent most of the solid load supplied by large rivers to the oceans (Hay, 1998). And yet, provenance of muds and mudrocks is a field of sedimentary geology that has not seen major advances in decades (Blatt, 1985), largely because silt and clay can hardly be studied by classic optical or single-grain geochronological methods. The fundamental principles of clay mineralogy and its dependence on climate and geology of source areas have long been established, and much work has been done on muds deposited in the deep sea (Biscaye, 1965, Chamley, 1989, Thiry, 2000). The composition of fine-grained sediments transported by large rivers worldwide has been studied extensively, but chiefly with geochemical methods (Martin and Meybeck, 1979, Canfield, 1997, Gaillardet et al., 1999, Bouchez et al., 2011, Lupker et al., 2012). Much remains to be learned about the relationships between mineralogy and chemistry of the suspended-load (Garzanti et al., 2011), specifically as regards the relative contribution of parent-rock lithology versus chemical weathering in different climatic conditions (Borges et al., 2008).
Tropical southern Africa is perfectly suited for such an investigation, being characterized by marked longitudinal and latitudinal climatic gradients, from sub-humid Mozambique to hyperarid Namibia, and from humid Angola to the arid Kalahari in Botswana (Fig. 1; McCarthy et al., 2000, Jury, 2010). In this vast area, straddling the Tropic of Capricorn between 30°S to 15°S, source rocks range from Archean cratonic basements and Proterozoic mobile belts to Mesozoic sediments and flood basalts (Fig. 1).
Information on the mineralogy and geochemistry of sediments carried by major African rivers is far from complete (Viers et al., 2009). We contribute to filling this gap by presenting a rich original dataset on silt-sized sediments. The different factors controlling sediment composition, and specifically how and to what extent the imprint of different parent-rock lithologies is modified by chemical weathering at the source, and subsequently by mixing along the routing system, will be analysed by diverse integrated techniques, including clay mineralogy, bulk geochemistry and isotope geochemistry. Another crucial problem that we will address is the distinction between weathered first-cycle and multicyclic sediments. This article, companion to a petrological analysis of river sands in the same region (Garzanti et al., 2014), follows studies carried out with the same rationale and methods on river muds in wet equatorial central Africa (Garzanti et al., 2013a) and dry tropical northern Africa (Padoan et al., 2011), with the ultimate goal to provide an overview of sediment-generation and weathering processes across the African continent.
Section snippets
Climate and fluvial systems in southern Africa
Dry climate in interior southern Africa is a consequence of quasi-stationary anticyclonic conditions. The continent divides the tropical high-pressure zone into the Indian Ocean and South Atlantic anticyclones, particularly in summer, when heating of the landmass is at its maximum. Moisture derived primarily from air masses moving inland from the warm Indian Ocean is reduced by orographic effects along the eastern escarpment, and declines progressively westward resulting in increasing aridity.
Sampling and analytical methods
Thirty-two samples of freshly deposited mud were collected in September–October 2011 along the banks of 25 rivers of southern Africa, including the Limpopo in South Africa, the Zambezi in Zimbabwe and Zambia, and the Okavango in Namibia and Botswana. Throughout the study area we have sampled also sands carried by numerous rivers including the Orange and the Kunene, and four vegetated Kalahari dunes in Botswana. We collected several replicate samples for the Okavango and Zambezi to assess
Rivers in Namibia
River muds in Namibia are mostly quartz-low and include K-feldspar, plagioclase and calcite (Table 1). Clay-mineral assemblages are rich in illite and subordinate chlorite in metamorphiclastic muds derived from the Damara Orogen, or dominated by smectite in volcaniclastic muds derived from Etendeka lavas. In all rivers draining into the Atlantic from the Orange to the Kunene, kaolinite represents less than 15% of clay minerals. Depletion in alkali and alkaline-earth elements is limited. The
Isotopic fingerprints
Rb–Sr and Sm–Nd isotopic systems, reflecting the weighted average of detrital components generated in different catchments, provide information on past events of crustal generation and thus represent a powerful means to investigate provenance of river sediments (Goldstein et al., 1984, Allègre et al., 1996). Sediments derived from older crust have relatively more radiogenic Sr and non-radiogenic Nd than those derived from younger crust, and a broad inverse relationship between 87Sr/86Sr and εNd
Provenance versus weathering control
In this section we discuss the areal distribution of mineralogical and geochemical parameters, and contrast the behaviour of diverse chemical elements in various catchments to investigate where and why weathering is most significant.
Conclusion
Integrating mineralogical and geochemical datasets is essential to critically appraise the different factors that control sediment composition, and specifically the effects of source-rock lithology, chemical weathering and recycling. 87Sr/86Sr ratios, 143Nd/144 Nd ratios and Sm–Nd tDM model ages of sediments are insensitive to weathering, and provide a faithful averaged signature of the geology of each drainage basin. Mesoarchean model ages characterize muds derived from the Kaapvaal and
Acknowledgments
The article benefited from the excellent constructive reviews provided by three unfortunately anonymous reviewers and Editor Carla Koretsky. We heartily thank Gaby Balzer for the skillful chemical separation, Paolo Censi for the geochemical advice, Luigi Marinoni and Lucia Galimberti for the assistance in the XRD and calcimetry analyses, and Pieter Vermeesch for the joyful company and thoughtful observations during the 2011 field campaign.
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