Formation and Collapse of the Kalahari Duricrust [‘African Surface’] Across the Congo Basin, 10 with Implications for Changes in Rates of Cenozoic Off-Shore Sedimentation Bastien Linol, Maarten J. de Wit, Francois Guillocheau, Michiel C.J. de Wit, Zahie Anka, and Jean-Paul Colin{ 10.1 Introduction margins, and to the east by the East African Rift System (EARS). Their relatively flat interior is covered by an exten- The Congo Basin (CB) of central Africa lies at about 400 m sive Upper Cretaceous-Cenozoic succession of sand dunes, above mean sea level (amsl), and is linked to the south, pan-lacustrine sediments and alluviums with hard-caps across a central African drainage divide, to the high interior (duricrusts) of calcrete, silcrete and ferricrete, collectively Kalahari Plateau (KP) at ca. 1,100 m amsl (Fig. 10.1). The named the Kalahari Group (SACS, 1980). This succession CB and KP are flanked by distinct marginal escarpments reaches a maximum thickness of about 500 m, but across along the South Atlantic and southwest Indian Ocean southern and central Africa is generally less than 100 m thick, representing one of the world’s most extensive, long- lived condensed stratigraphic sequences. The Kalahari Group directly overlies Precambrian base- ment of the Kalahari and Central African Shields (Fig. 10.1b), late Paleozoic to mid-Mesozoic sequences of {Author was deceased at the time of publication. the Karoo Supergroup including Lower Jurassic flood basalts in southern Africa, dated at 178–183 Ma (the B. Linol (*) AEON-ESSRI (African Earth Observatory Network – Earth Stormberg Group; Jourdan et al. 2007), and Cretaceous Stewardship Science Research Institute), Nelson Mandela Metropolitan volcanics and dykes in Namibia, dated at 127–132 Ma (the University, Port Elizabeth, South Africa Etendeka Group; Miller 2008). By contrast, across the CB, Geological Sciences, Nelson Mandela Metropolitan University, Port the Kalahari Group overlies Upper Jurassic to Upper Creta- Elizabeth, South Africa ceous red sandstones (e.g. the Kwango Group) ranging in e-mail: [email protected] thickness between about 300 m and 1,000 m (Linol 2013; M.J. de Wit Chap. 8, this Book). The unconformity at the base of the AEON-ESSRI (African Earth Observatory Network – Earth Kalahari Group marks a subcontinental scale peneplanation Stewardship Science Research Institute), Nelson Mandela Metropolitan ‘ ’ University, Port Elizabeth, South Africa surface that is commonly referred to as the African Surface e-mail: [email protected] (e.g. King 1963; Partridge and Maud 2000; Haddon and F. Guillocheau McCarthy 2005; Decker et al. 2013), developed as a result Ge´osciences-Rennes, UMR 6118 Universite´ de Rennes 1 – CNRS, of extensive denudation and uplift following the break-out of OSUR, Universite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Africa from Gondwana during the opening of the Indian and cedex, France South Atlantic Oceans, and the onset of the Kalahari epeiro- e-mail: [email protected] geny (de Wit 2007). However, the precise age-range of this M.C.J. de Wit elevated, mega-denudation surface remains uncertain Delrand Resources Pty Ltd., Toronto, Ontario, Canada e-mail: [email protected] because the stratigraphy of the overlying Kalahari Group is poorly characterized, including many internal and local Z. Anka Helmholtz Centre Potsdam, GFZ German Research Centre for unconformities and abundant silcretes, calcretes and ferri- Geosciences, Telegrafenberg, 14473 Potsdam, Germany cretes that are difficult to date and correlate regionally. TOTAL Exploration/New Venture, 2 place Jean Millier, La Defence, Moreover, because large parts of this Kalahari succession 92078 Paris, France e-mail: [email protected] M.J. de Wit et al. (eds.), Geology and Resource Potential of the Congo Basin, Regional Geology Reviews, 193 DOI 10.1007/978-3-642-29482-2_10, # Springer-Verlag Berlin Heidelberg 2015 194 B. Linol et al. a East African RiV System Congo River Samba Kalahari Group Gilson Dekese Mbandaka Magkadigkadi Pan Tsodilo Okavango Delta Etosha Pan Orange River G reat Escarpment Congo Orange Basin Basin Outeniqua Fan b KP CB J-K sequences Kwango Kalahari Group EARS Cape NSnick-point extension Fold Oubanguides Belt Lufilian Arc Tsodilo Drakensberg Basalts Belt Samba Dekese 1 sea 0 Karoo Supergroup Cape Supergroup Lindian Supergroup Central African Shield Kalahari Shield -5 km 0 5000 km Fig. 10.1 (a) Digital elevation model of sub-Saharan Africa and (b) and with location of studied boreholes. Note that the transition between N–S cross-section of the CB and KP, highlighting the vast extension of the KP and CB is not related to the boundary between the Kalahari and Cenozoic sediments and duricrusts of the Kalahari Group (in yellow), Central African Shields are unconsolidated, there is relatively limited data available sedimentation in the Oligocene (e.g. Anka and Se´ranne from drill-cores (Haddon 2000; Miller 2008). 2004), we propose a new model of rapid disintegration of Here, we present new sedimentological and stratigraphic the Kalahari duricrusts carapace and preferential erosion data from drilling into the Kalahari Group on top of the KP, (‘flushing out’) of the soft underlying red-beds across the in the Ngamiland region of northwest Botswana, and from CB, driven by increased fluvial activity in response to global field investigations in the Kwango Valley of the southwest cooling in the mid- to late Cenozoic (e.g. Zachos et al. 2001). CB, flanking the transition to the KP in the southern Demo- cratic Republic of Congo (DRC). On the basis of well/core and seismic data, we also extend this Kalahari sequence to 10.2 The Kalahari Group the center of the CB (Fig. 10.2). Here, very little of the Kalahari duricrusts cover is preserved, leaving a denuded The Kalahari Group covers most of southern and central landscape (‘Bad-Lands’) of Cretaceous red-beds sometimes Africa (albeit poorly exposed), extending continuously covered by residual blocks and large boulders of silcrete and from the Orange River in South Africa, through Namibia, calcrete that suggest relatively recent collapse of the Botswana, western Zimbabwe, Zambia, Angola, to the Kalahari duricrusts and accelerated erosion across the CB Congo River in DRC and the Republic of Congo, covering of its underlying poorly consolidated red-beds. Because the some 2.7 million km2 (Figs. 10.1a and 10.2). off-shore sedimentation history of the Congo Fan along the A thickness map of the Kalahari Group in southern Atlantic margin reveals a sudden episode of rapid Africa, compiled mainly from borehole data of water wells 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 195 Fig. 10.2 Isopach map of the Kalahari Group, extended from Haddon (1999) by Linol (2013). Three study areas referred to in the text are shown by red rectangles Mbandaka Samba 1 L. Victoria Gilson Dekese L. Tanganyika N Kalahari thickness 420-450 m 390-420 m 360-390 m 330-360 m 300-330 m 270-300 m L. Kariba 240-270 m 210-240 m 4 Tsodilo 180-210 m 2 150-180 m 3 120-150 m Ngami 90-120 m 60-90 m 30-60 m 0-30 m Active wetlands 1: West Congo Swamps 2: Okavango Delta 3: Makgadikgadi Pan 4: Etosha Pan Major rivers Congo-Kalahari watershed 0250 500 1,000 km by Haddon (1999), has now been extended to include central emphasizes that the thickest sediment accumulations are Africa (Fig. 10.2), based on new field observations in the preserved along the western margin of the KP: in northern Kwango Valley along the northern flank of the KP, and Namibia (ca. 200–450 m thick), in central Angola (ca. re-examination of the seismic and well/core data from the 200–300 m thick), in northern Botswana (ca. 150–250 m 1950s and 1970s in the center of the CB (Linol 2013; see thick), and in the western part of the central CB (ca. also Chaps. 7 and 8, this Book). This isopach map 100–200 m thick). The distinct decrease in thicknesses 196 B. Linol et al. eastward across south-central Africa likely reflects the influ- ence of Cenozoic uplift related to the formation of the 10.3 Drill-Cores from the Central Kalahari EARS, between 20–40 Ma (e.g. Chorowicz 2005; Pik et al. Plateau, Northwest Botswana 2008; Roberts et al. 2012). The classical ‘Kalahari type-section’, as first described in In the Ngamiland region of northwest Botswana (Fig. 10.3), Botswana by Passarge (1904), consists of basal gravels (the new borehole stratigraphy has also characterized widespread Botletle Beds), calcretized or silcretized sandstones and calcretes in subsurface, between 10 m and 60 m thick, and marls (‘Kalahari Limestones’) and aeolian sands (‘Kalahari named the Nxau-Nxau Calcrete Formation (Linol 2013). Sands’). Lithostratigraphic equivalents were later also These carbonate rocks overly a regional unconformity across recognized in South Africa (the Kalahari Beds; du Toit Precambrian basement (the Damara Supergroup; Haddon 1954), Zimbabwe (‘Pipe Sandstones’; Maufe 1936), Zambia and Roos 2001) and diamictites with black shales and red- (the Barotse Formation; Money 1972) and DRC (‘Poly- beds of the Karoo Supergroup that are deformed and locally morph Sandstones’; Cahen and Lepersonne 1952). Much of intruded by kimberlites in the Nxau-Nxau area, dated at this historic mapping and research has been summarized 83 Ma (Batumike et al. 2007; de Wit 2013a). Calcretization since in regional reviews (e.g. Thomas and Shaw 1990, also frequently penetrates into the Karoo Supergroup and 1993; Haddon 2000; Giresse 2005), but very little new Precambrian basement, and xenoliths of calcrete occur in the field data have been generated to provide detailed strati- kimberlites (de Wit 2013b). graphic sections and test these regional correlations. Most recent studies across the Kalahari have focused on near-surface pan sediments, silcretes/calcretes and aeolian 10.3.1 The Nxau-Nxau Calcrete Formation sand dunes to provide information about Quaternary paleo- climate and landscape evolution (e.g.