Review articleProvenance of Kalahari Sand: Paleoweathering and recycling in a linked fluvial-aeolian system
Introduction
The intracratonic Kalahari sag basin hosts several dunefields that, largely inactive at present, represent the largest sand sea on Earth (Fig. 1). The compositional signatures of such a vast expanse of aeolian sand and their provenance have not been systematically studied so far, and yet encrypted in them lies a bounty of information on the geological, geomorphological, and environmental history of the region. Formed as a consequence of the multistep break-up of Gondwana, the Kalahari Basin presently occupies the core of southern Africa, which experienced dynamic uplift in the Cenozoic and is currently cut across by the southwestward-propagating East African rift system (Haddon and McCarthy, 2005; De Wit, 2007). Complex landscape evolution during the Pleistocene and Holocene was punctuated by a high-frequency alternation of arid and humid climatic stages and consequent repeated changes in hydrology, drainage patterns, and interaction of fluvial and aeolian processes (Burrough et al., 2009a; Hürkamp et al., 2011; Moore et al., 2012; Matmon et al., 2015). Decrypting the Kalahari sedimentary archive is an essential step to improve our understanding not only of the evolution of tropical southern Africa but also of the interplay between tectonic and climatic forces that mould the Earth's surface. Clarifying the control exerted by key climate variables on arid landscapes can, in turn, help test the robustness of numerical models simulating dunefield dynamics and improve model simulations that are used to predict the impact of future climate change on aeolian-dune remobilisation (e.g., Thomas et al., 2005; Mayaud et al., 2017; Vainer et al., 2021a).
This article considers what is known about the Kalahari Basin and its hydrological systems and dunefields (Fig. 2), including an overview of their Quaternary history. The geology of the region is first outlined in the wider context of southern Africa (Fig. 3), before reviewing what was currently known about the provenance of the dunefields and potential fluvial feeder systems. To date, much of the information on provenance has been inferred from likely palaeowind directions (e.g., Thomas, 1987) or by applying a limited number of techniques (mostly sediment textures and heavy minerals) to some parts of this vast basin. Several sedimentological issues have thus remained controversial, including the relative role played by fluvial processes versus aeolian reworking and the origin of weathering. Quantitative petrographic data were obtained only on a few aeolian-dune sands in the north and west, and detrital-zircon ages only on fluvial sands in the north (Gärtner et al., 2014; Garzanti et al., 2014a).
For these reasons, we present new results from bulk-petrography, heavy-mineral, and detrital-zircon U–Pb geochronology analyses on 100 aeolian-dune and river-bar sands collected across 17 degrees of latitude from Zambia to South Africa and over 13 degrees of longitude from Angola to Zimbabwe. A set of statistical techniques was applied to this multi-proxy dataset to adequately illustrate the compositional variability of aeolian sand across the Kalahari Basin, reveal meaningful mineralogical patterns, identify the original sediment sources, and gain insight into sand dispersal pathways. In particular, this paper investigates inheritance from past climatic conditions, buffering of environmental signals through linked fluvial and aeolian systems, and progressive compositional homogenization and concentration of most durable minerals acquired through multiple cycles of erosion, transport, deposition, and diagenesis. The new provenance data are integrated and reviewed in terms of what is known about fluvial-aeolian interactions, chemical weathering, and drainage evolution in the Kalahari. Understanding the complexities of sediment transport systems, and particularly how sediment-routing connectivity regulates the transmission of environmental signals from source areas to depositional sinks over spatial and temporal scales, is essential for a realistic interpretation of the stratigraphic record (Romans et al., 2016; Allen, 2017; Caracciolo, 2020).
Section snippets
Geology of southern Africa
Southern Africa was amalgamated through multiple tectono-magmatic events dating back to the Archean and culminated with the Neoproterozoic Pan-African orogeny (Fig. 3; Hanson, 2003). The Archean core consists of the Kaapvaal and Zimbabwe Cratons, welded by the Limpopo Belt. The Kaapvaal Craton, progressively amalgamated between 3.7 and 2.7 Ga, was stabilized by 2.6 Ga, and eventually intruded by the Bushveld Complex at 2.06 Ga (Eglington and Armstrong, 2004). The Zimbabwe Craton, comprising
The Kalahari Basin
The intracratonic Kalahari sag basin comprises the largest continuous sand sea on Earth, which extends for over 2.5 × 106 km2 (Fig. 1). The interior of the Kalahari is an elevated plateau with flat topography (average altitude 1200 m), delimited by relatively steep escarpments down to the Atlantic Ocean in the west and to the Indian Ocean in the east. Kalahari Group sediments — including basal gravel and conglomerate, sandstone with calcrete, and unconsolidated sand —stretch north from the
Overview of previous work on the provenance of Kalahari sands
The provenance of Kalahari sands has not been investigated thoroughly by a multi-technique approach so far. Previous surveys recognized the highly quartzose composition of aeolian sands, but their compositional variability has been only broadly evaluated, and the origin of Kalahari dunefields was mostly ascribed to either reworking of older sediments (e.g., Du Toit, 1954; Baillieul, 1975; Thomas, 1987) or dominant fluvial processes (e.g., De Ploey et al., 1968; Verboom, 1974; Moore and Dingle,
Methods
In this provenance study, we have analysed 57 samples of aeolian dunes collected across the vast Kalahari sand sea in the frame of diverse research projects (Stone and Thomas, 2008; Matmon et al., 2018; Burrough et al., 2019; Stone et al., 2019; Wittmann et al., 2020; Vainer et al., 2021a). Another 43 sand samples collected from exposed sandbars or dry riverbeds in Angola, Botswana, Zambia, Zimbabwe, Namibia and South Africa, and previously studied with similar and complementary methodological
Mineralogy of river sands
The main rivers that drain into the Kalahari Basin are sourced in the humid regions of Angola and Zambia in the north. The Caculuvar and Mucope tributaries of the Cunene River, draining the western edge of the Kalahari Erg in Angola (Fig. 1A), carry pure quartzose sand with a few K-feldspar grains. The extremely poor, zircon-rich tHM suite includes epidote, tourmaline, and minor andalusite, staurolite and rutile (Table 1). The Okavango and Cuando Rivers, which are also sourced in Angola and
Mineralogy of aeolian-dune sands
Dune sand is quartz-rich over most the vast Kalahari Basin: out of the 57 studied samples, 31 are pure quartzose, 12 quartzose, and 10 quartz-rich feldspatho-quartzose (Fig. 5). Throughout the northern Kalahari, in Angola, northeastern Namibia, Caprivi Strip, northern Botswana and western Zambia, sand consists virtually exclusively of monocrystalline quartz commonly showing rounded to subrounded outline and abraded overgrowths (Fig. 4L). Aeolian sand in the Caprivi Strip contains abundant red
Ages of detrital zircons
Five age ranges recur among the analysed samples (Fig. 6), corresponding to main orogenic episodes in southern Africa (Hanson, 2003; Dirks et al., 2009; Andersen et al., 2016, Andersen et al., 2018): I) Limpopo (2.5–2.8 Ga, Neoarchean; peak at 2720 Ma); II) Eburnean (1.8–2.05 Ga, Orosirian; peak at 1893 Ma); III) Sinclair-Kibaran (1.2–1.4 Ga, Ectasian; peak at 1312 Ma); IV) Namaqua-Irumide (1.0–1.1 Ga, Stenian; peak at 1056 Ma); V) Damara-Lufilian (0.45–0.65 Ga, Ordovician-Cryogenian; peak at
Provenance of Kalahari aeolian-dune sand
In most of the Kalahari Basin, including the NWK and NEK dunefields in Angola, Namibia and Zambia, much of Botswana, and part of the EK dunefield in Zimbabwe, dune sand is dominated by monocrystalline quartz associated with very poor tHM suites including durable ZTR minerals, staurolite, and kyanite (Fig. 8, Fig. 9, Fig. 10). Such a homogenized mineralogical signature reveals multiple recycling of older quartzose sandstones through geological time.
A local exception is the quartz-rich
Fluvial-aeolian interactions and multistep recycling
The mineralogical composition of aeolian dunes and its variability across a sand sea reflect the relative importance of fluvial and aeolian processes and the degree of their interplay. Sand seas largely fed by river systems are typically characterized by partly first-cycle detritus including various amounts of diverse types of rock fragments, feldspars and heavy minerals, generally allowing identification of a single dominant source, as for the Namib Erg (Garzanti et al., 2012). The opposite
Paleoweathering in the Kalahari
The extent to which a sediment has been subjected to chemical weathering, integrated over a series of sedimentary cycles, can be evaluated by combining evidence from petrographic, heavy-mineral, clay-mineral, and geochemical data.
Distilled multicyclic sand of northern Kalahari aeolian dunes and of the Okavango, Cuando, and upper Zambezi Rivers is composed dominantly of quartz with strongly depleted tHM suites including zircon, tourmaline, rutile, staurolite, and kyanite but virtually no garnet
Drainage integration as a driver of provenance change
Sand seas such as the Kalahari or the Great Nafud and Rub'Al Khali in Arabia are huge reservoirs of quartz-rich polycyclic sand trapped in the continental interiors. In the hyperarid climate of the tropics, precipitation and runoff can be so scarce that ephemeral rivers are unable to carry sediment across and beyond these vast rift-related sags, where sand is dumped and multiply reworked while remaining largely untapped for tens or even hundreds of million years. An analogous ancient case is
Conclusions
Aeolian dunes of the Kalahari are homogeneously pure quartzose in Angola, Zambia, and over much of Botswana and parts of Zimbabwe and Namibia, where they also contain a few K-feldspar grains and strongly depleted heavy-mineral assemblages dominated by ZTR minerals and including common staurolite in Botswana and kyanite in Zambia, but virtually no garnet. Composition varies markedly only at the western and eastern edges of the erg, ranging from feldspar-rich feldspatho-quartzose and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Several samples from Kalahari aeolian dunes were obtained thanks to the generosity of Sallie Burrough, Tom Kemp, and Matt Telfer; some fluvial samples were provided by Hella Wittmann, Hanna Haedke, Armanda Trindade, Dario Fornara, Lindani Ncube, and Helena Johanna Van Niekerk. Most petrographic and heavy-mineral analyses were made by Giuditta Radeff, Marta Padoan, and Mara Limonta. Marta Padoan helped substantially with the compilation of geochronological ages and Martin Rittner with
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