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Home , Angola Basin, Congo Craton, Geology of Namibia, Kwango River, Lufilian Arc

Formation and Collapse of the Kalahari Duricrust [‘African Surface’] Across the , 10 with Implications for Changes in Rates of 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 System (EARS). Their relatively flat interior is covered by an exten- The Congo Basin (CB) of central lies at about 400 m sive Upper -Cenozoic succession of sand dunes, above mean (amsl), and is linked to the south, pan-lacustrine 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 is generally less than 100 m thick, representing one of the ’s most extensive, long- lived condensed stratigraphic sequences. The Kalahari Group directly overlies base- ment of the Kalahari and Central African Shields (Fig. 10.1b), late to mid- sequences of {Author was deceased at the time of publication. the including Lower Jurassic flood in southern Africa, dated at 178–183 Ma (the B. Linol (*) AEON-ESSRI (African Observatory Network – Earth ; Jourdan et al. 2007), and Cretaceous Stewardship Science Research Institute), Nelson Mandela Metropolitan volcanics and dykes in , 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 (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 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

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 Tsodilo Basalts Belt Samba Dekese 1 sea 0

Karoo Supergroup Cape Supergroup Lindian Supergroup Central African 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 (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 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 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, , , to the Kalahari duricrusts and accelerated erosion across the CB 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 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 ’) 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 (the Damara Supergroup; Haddon 1954), Zimbabwe (‘Pipe Sandstones’; Maufe 1936), Zambia and Roos 2001) and diamictites with black 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 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 paleo- climate and landscape evolution (e.g. Summerfield 1983; In cores (Figs. 10.4 and 10.5), the Nxau-Nxau Calcrete Holmgren and Shaw 1996; Ringrose et al. 1999; Thomas Formation comprises sandy and muddy carbonate rocks et al. 2003; Huntsman-Mapila et al. 2006; Kampunzu et al. that commonly have abundant micro-karst structures, root 2007; Burrough et al. 2009;Hu¨rkamp et al. 2011; Moore moulds and/or burrows, and cracks filled with coarser sands et al. 2012; Eckardt et al. 2013). However, significant and gravels. reworking, bioturbation and post-depositional modifications In thin sections (e.g. Fig. 10.5), the matrix is generally a of these sediments often hamper reliable age-dating and dark brown micritic groundmass supporting detrital . stratigraphic correlations. It has also been argued that a It is partially or totally replaced by sparite due to subsequent significant part of the Kalahari Group results from in-situ diagenesis. Fenestral fabric and circum-granular cracks weathering of its basement rocks, further hampering con- within the more muddy facies also indicate episodes of struction of a robust stratigraphy (McFarlane et al. 2010). immersion and desiccation (e.g. Wright and Tucker 1991; Only in the Etosha Pan of northern Namibia (Figs. 10.1a Tanner 2010). Although biogenic features, such as alveolar and 10.2 for location) recent borehole and paleontological structures, rhyzoliths and burrows are abundant, indicating investigations have described in detail a more complete intense biological activity (e.g. Goudie 1983), no fossils Kalahari succession (Miller 2008; Miller et al. 2010). Here, were observed. Thus, these facies have characteristics of the Kalahari Group includes basal gravels of the Beiseb both lacustrine and pedogenic deposits (c.f. Alonso-Zarza Formation (probably equivalent to the Botletle Beds 2003), and are interpreted to be deposited in a shallow lake described by Passarge 1904), and two thick sand-dominated wetland environment (Fig. 10.5). This suggests a humid and fan sequences, named the Olukonda and Andoni Formations, relatively hot climate during early deposition of the Kalahari maximum 150 m and 550 m in thicknesses, respectively. It Group. intercalates at the top with 50 m of saline clays that consti- tute the present-day pan bed, and which contain fossil verte- brate suites dated from the late to , ca. 10.3.2 Kalahari Lake Sediments 4–6 Ma (Miller et al. 2010). This relatively thick Kalahari succession in turn passes laterally into an 80 to 120 m thick, Previous sedimentological studies of Kalahari sediments massive ‘calcrete’ named the Etosha Calcrete Formation, have demonstrated wet conditions during the Pleistocene considered to represent a gigantic groundwater deposit that and suggested the occurrence of extensive paleo-lakes (e.g. developed <4 Ma under arid conditions (Miller 2008). Du Plessis and Le Roux 1995; Holmgren and Shaw 1996; These terrestrial carbonates, however, are complex because Thomas et al. 2003; Huntsman-Mapila et al. 2006; Eckardt of multiple phases of deposition and dissolution, and are thus et al. 2013). However, these studies often lack deep borehole difficult to interpret chronologically (e.g. Nash and McLaren data, particularly into the Kalahari lakes and wetlands (e.g. 2003; Wanke and Wanke 2007; Linol et al. 2009). Okavango Delta, Makgadikgadi Pan and Lake Ngami; 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 197

a Tsodilo Section b 1821C3 Approx. This study TsTsododililo C SfSmS cS Gr P ages HiHilllls 0 1821C10 B4 A15 TSODILO1 Sands > 50 ka 1821C16 A37 B6 KPH1 OkOkaavvanangogo Carbonates C15 21641A1 BM1 DeDeltlta and calcretes ca. 80 Ma Kalahari BM3 Nxau Nxau Red beds field

DrDrototsksky’y’s KIMBERLITE 100 m CaCaveve G3 G1 11471A JB7 11741A JB1 Maun t ultl Thamalakane faua Black shales rere JEB/1 a Karoo Supergroup Kunyere Fault umma GuG LaLakeke Ngagamimi NGAMI2 Diamictites 350 Ma NGAMI3 Damara 200 m ()

Fig. 10.3 (a) Location map of the Kalahari drilling project in the Ngamiland region of northwest Botswana, showing the studied boreholes (blue and red dots), and (b) simplified stratigraphic section (for more details see Linol 2013)

Figs. 10.1a and 10.2) that have great potential to preserve 10.3.2.2 Lake Ngami long Cenozoic sedimentary records. Below, we describe new Near the center of Lake Ngami (Fig. 10.6b), the Ngami- sequence analysis of the Kalahari Group from drilling up to 2 borehole (202903900S; 224304400E) is 125 m deep, without ca. 100–125 m depth at the edge of the Tsodilo Hills and into reaching the base of the lake sediments. The recovered Lake Ngami (Fig. 10.6). Details of the drilling technology Kalahari succession comprises calcareous fine sands with and core recovery are provided online: http://www.aeon.org. at depth, below À60 m, seven intervals of darker brown, za/content/pdf/kalahar_drilling_project_v7_10308.pdf. organic-rich muds that record successive episodes of lake expansion (i.e. flooding). These mud-rich horizons are fos- 10.3.2.1 Tsodilo Hills siliferous with ostracods, charophytes and charophyte gyrogonites (Fig. 10.7) that suggest an age younger than At the foot of the Tsodilo Hills, the Tsodilo-1 borehole   Miocene and confirm the existence of a deep paleo-lake. (18 46’58”S; 21 43’11”E) intercepted a ca. 42 m succession The ostracod shell fragments, from depth—106–107 m, of sands and calcretes of the Kalahari Group. This section were dated using AMS 14C technique (e.g. Rethemeyer overlies pre-Kalahari (e.g. Karoo-age) heterolithic et al. 2012) and resulted in an age greater than 50 ka, beyond sandstones and green mudstones that are increasingly the precise radiocarbon detection (Fig. 10.6c). Near the silicified upward to form a 4 m thick silcrete. This distinct surface, at a depth of À3.2 m, a horizon of organic-rich hardpan, likely formed by deep weathering and groundwater sediments was previously 14C dated at 18 ka (Huntsman- activity at the discontinuity that defines the base of the Mapila et al. 2006). Kalahari Group, possibly correlates with the Nxau-Nxau In summary, the Kalahari Group in the Ngamiland region (and Etosha) Calcrete Formation. The overlying Kalahari of northwest Botswana comprises basal clastic-carbonates succession at Tsodilo Hills comprises a lower, 32 m thick and calcretes (the Nxau-Nxau Calcrete Formation) that pass sand-dominated fluvial sequence, overlain by 8 m thick laterally and upward into a fluvial sequence of Kalahari sandy carbonates and a 2 m thick fossiliferous calcrete that sands, which in turn intercalates near the top with Pleistocene cap the flat present day topography (Fig. 10.6a). Previous pan-lacustrine and younger calcrete deposits (e.g. Thomas radiocarbon dates (Thomas et al. 2003), between 32 ka and et al. 2003; Huntsman-Mapila et al. 2006). Correlations of 36 ka, obtained from molluscs and shells collected at sur- these sequences (at the Tsodilo Hills and Lake Ngami; face, date this uppermost sequence of pan-lacustrine and Fig. 10.6c) with the Kalahari stratigraphy established at the calcrete deposits to the late Pleistocene (Fig. 10.6c). 198 B. Linol et al.

KPH2 -30 KPH5 -13.5 C15 -24.5 PD25/2 -20.5

b a 6 cm d c

Fig. 10.4 Calcrete cores: (a) clastic-carbonates with large intraclasts (CCi facies); (b) and (c) clastic-carbonates with small pebbles and granules (CCg facies) and micro-karst structures; and (d) muddy-carbonates with cracks (CCm facies)

Etosha Pan in Namibia (Miller 2008) indicate increasingly DRC and northern Angola, to the Bate´ke´ region in the arid conditions during deposition of the uppermost Kalahari Republic of Congo and eastern (Fig. 10.2). Group regionally across southern Africa. Along the upper part of the KP at the southwestern margin of the CB, the Kalahari Group overlies a regional erosion surface across red sandstones of the Upper Jurassic to Upper Cretaceous Kwango Group (Linol 2013; Chap. 8, 10.4 Denudation Surfaces and Kalahari this Book). Here, the type section of the Kalahari Group Sediments of the Congo Basin ranges between about 100 m and 300 m thick and is gener- ally subdivided into two (Cahen and Lepersonne 1952): The KP extends northward into the CB, terminating at a 1. A lower, massive sequence comprises 60–80 m thick scarp (Fig. 10.8), between 400 m and 600 m high that extensively silicified ‘limestones’, calcareous sandstones extends from the Kwango and Kasai in southern and silcretes with chalcedony, known as the ‘Polymorph 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 199

SEDIMENTOLOGICAL LOGS FACIES DESCRIPTIONS MICRO-FACIES

Litho Grain-size C S fS mS cS Gr P a 7 m CCi: Sandy carbonates with large intra- clasts - Carbonated, fine- to coarse-grained sands with large intraclasts (1-5 cm). - Beds 1 to 5 m thick, bioturbated, with micro-karst infillings. 500 μm 0

Litho Grain-size b C S fS mS cS Gr P CCg: Sandy carbonates with granules 7 m - Carbonated fine- to coarse-grained sands with small pebbles and granules (0.5-3 cm). - Beds 1 to 10 m thick, root moulds and micro-karst infillings. 500 μm

0 c

Litho Grain-size CCm: Muddy carbonates C S fS mS cS Gr P - Carbonated muds with micrite agglomerates. 4 m - Beds 1 to 10 m thick with cracks infillings and recrystalizations.

500 μm

0

Litho Grain-size d C S fS mS cS Gr P CCl: Laminar carbonates 4 m - Carbonated muds with concentric and laminar structures (stromatolith?). - Beds 1 to 5 m thick with cracks infillings and recrystalizations.

0 500 μm

DEPOSITIONAL ENVIRONMENT INTERPRETATION Alternating Pedogenesis roots

<5 m

Microkarsts Clastic-carbonates Lime mud

Fig. 10.5 Facies descriptions of the Nxau-Nxau Calcrete Formation, (white) coated with sparite; (c) circum-granular crack; and (d) laminar and shallow lake wetland depositional model. Microfacies: (a) Alveo- micrite (dark brown) lar structure filled with sparite (white); (b) ‘floating’ quartz grains

Sandstones’ (c.f. Lepersonne 1945; see Chap. 14, this Erpetocypris sp., Gomphocythere? sp., Oncocypria sp. Book). These rocks are often cavernous, bioturbated and and Stenocypris bunzaensis; Leriche 1927; Polinard fossiliferous with shells that display early silicification 1932; Grekoff 1958; Colin 1994) and other lacustrine (De Ploey 1968). Ostracods (Cypris farnhami, C. lerichei, fossils (charophytes and gastropods) suggest an 200 B. Linol et al.

a b

TSODILO-1 NGAMI-2 C S fS mS cS Gr P C SfSmS cS Gr P

0 m 0 m 36 Ka PALUSTRINE CARBONATES 18 Ka Thomas 10 m AND CALCRETES Huntsmann- 10 m et al. (2003) Mapilaet al. (2006) 20 m 20 m

30 m 30 m FLUVIAL 40 m SANDS 40 m hardpan 50 m 50 m

60 m 60 m 1

70 m 70 m 2 3 DEBRIS-FLOW 80 m 80 m 4 5 90 m 90 m (EastAfrican Rift extension) 6 100 m LAKE EPISODES c >50 Ka 7 This study 110 m

120 m

Kunyere Fault

Fig. 10.6 AEON drill sites (a) at the Tsodilo Hills and (b) at Lake sequence of palustrine carbonates and calcretes, in total 10 m thick. In Ngami, and (c) sequence correlations of the Kalahari Group between the Ngami-2 section, the equivalent lacustrine succession is more than these two sites (Figs. 10.2 and 10.3a for locations), with location of 14C 125 m thick, including at depth, below À60 m, seven horizons of dated samples (in red). In the Tsodilo-1 borehole, the Kalahari Group fossiliferous, dark organic-rich mud. These correlations indicate wetter comprises a lower, 30 m thick fluvial sequence overlain by an upper conditions during deposition of the lower Kalahari Group

age for this lower Kalahari sequence. Lithostratigra- 2. An upper sequence of unconsolidated sands, up to 120 m phically, this sequence of duricrusts correlates well with thick, overlies another weathering surface, marked by the similar formations of calcrete and silcrete described at ferricrete deposits at the top of the ‘Polymorph the base of the Kalahari Group in northern Namibia and Sandstones’ (Cahen and Lepersonne 1952). This Botswana (the Nxau Nxau and Etosha Calcrete sequence comprises fine ochreous sands, silts and kaolin- Formations). Differences in lithology between these itic muds devoid of stratification, possibly due to intense deposits (silcretes versus calcretes) likely reflect region- bioturbation, and which are interpreted as sheet wash ally variable weathering conditions related to more humid deposits (De Ploey 1968; see also Chap. 14, this Book). and hotter climate in central Africa. 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 201

Fig. 10.7 Sieved-residues of sample—106–107 m from the Ngami-2 borehole (Fig. 10.6c). (a) Ostracode shell fragments dated at ca. >50 ka by 14C method, and (b) charophyte stems

Hardpan ‘AFRICAN SURFACE’

Poorly consolidated red sandstones

Fig. 10.8 Northernmost extension of the KP (ca. 1,100 amsl) in the [the’African Surface’] across red sandstones of the Kwango Group. southwest CB (ca. 400 amsl). A hardpan of silcrete and calcrete (‘Poly- This distinct pedogenic unit at the base of the Kalahari Group can be morph Sandstones’) overlies a regional peneplanation surface correlated with similar calcretes in Botswana and Namibia

10.4.1 Silcrete and Calcrete Boulders on Flats the lower sequence of the Kalahari Group (the ‘Polymorph and River Terraces of the Kwango Valley Sandstones’), and are displaced lower down along the slopes (Fig. 10.9a). In the lower-lying Kwango Valley, at The Kwango Valley is deeply incised into the Kwango elevations between 400 m and 600 m amsl, remnant outcrops Group red sandstones along the northern flank of the KP in of red sandstones form flat erosional surfaces devoid of the southwest CB (Fig. 10.2 for location). Along the scarp at silcrete or calcrete caps (Fig. 10.9b). These are commonly the upper part of the KP, large blocks of silcretized covered with residual blocks of silcrete and calcrete sandstones (5–15 m in diameter) have broken-away from (0.5–5 m in diameter), and often best preserved within the 202

c b a

WEST EAST Kalahari Plateau 1000 Hardpan

900 A

800 B Wamba River

700

Elevation (m) 600 Kwango River C Kwango Group 500

Precambrian Basement 400

‘ ’

Fig. 10.9 Simplified E–W cross-section across the Kwango Valley in southwestern DRC. (a) Large blocks of silcrete and calcrete ( Polymorph Sandstones ) fragmented from the upper part of al. et Linol B. the adjacent KP, (b) preserved on slopes, and (c) re-deposited within the Kwango river terraces 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 203

Fig. 10.10 Lupemban stone tools typically made out of silcrete rock fragments (‘Polymorph Sandstones’), found at artisanal diggings within the Kwango river terrace deposits erosional gullies and ravines, indicating fragmentation and Old seismic refraction surveys (Evrard 1960) mapped the erosion of an overlying unit of Kalahari duricrusts, like the sediment thicknesses at 111 stations across the entire central ‘Polymorph Sandstones’. CB (Fig. 10.11a). On this refraction data the Kalahari Lower down, along the large meandering Kwango River, sequence is imaged as a thin distinctive upper unit with a river terraces comprise basal conglomerates (1 to 5 m thick) low velocity range of ca. 1,800–2,200 m/s (Linol 2013; see supporting blocks and boulders of silcrete and calcrete, also Chap. 7, this Book). The reconstructed surface elevation ranging between 1 to 10 m in length (Fig. 10.9c). This type of this uppermost main reflector shows the thickest accumu- of deposit also appears to have originated from the fragmen- lations (ca. 180–200 m) to be located beneath the Middle tation of the Kalahari ‘polymorph’ duricrusts and re- Congo River and Lake Ndombe, in the western part of the deposited by groundwater along the river terraces (e.g. the central CB (Fig. 10.2). It is bounded to the south by an E-W ‘detrital model’ of Goudie 1983). These giant boulders of unconformity that coincides at surface with the Lukenie silcrete and calcrete form good trap for alluvial River (Fig. 10.11a). (Chap. 16, this Book), and also contain some human arte- In the four deep boreholes drilled in the center of the CB facts (e.g. Fig. 10.10). The latter are stone tools mainly with (Fig. 10.11b), this Kalahari succession corresponds to elongated bifacial points constructed out of ‘Polymorph unconsolidated sandstones with pebbles, mainly of quartz, Sandstones’, and characteristic of the Lupemban industry flint (e.g. silcretes) and sandstones, and varicolored clay- in central Africa (e.g. Clark and Brown 2001), which is as stones (Cahen et al. 1959, 1960; Esso-Zaire 1981a, b). Its old as middle Pleistocene in age (300 Ka). These indicate thickness ranges from 37 m at Dekese to 242 m thick in the that deposition of river sands (and underlying calcrete Gilson-1 well. In the Samba section, only the uppermost boulders?) onto the Kwango terraces is a relatively recent 80–110 m (‘Couche’ [Bed] 1 of Cahen et al. 1959)was event. originally attributed to the Kalahari Group. However, new logging of the cores (Linol 2013) has identified a marked erosional surface etched across red mudstones at a depth of -192 m (Fig. 10.11c), which more likely corresponds to the 10.4.2 The Kalahari Group Across the Central base of the Kalahari Group, consistent with the seismic data Congo Basin and the depth of the first Cretaceous fossils found in this section, at -195 m (Cahen et al. 1959). Biostratigraphically, Based on seismic, litho- and bio-stratigraphy, the Kalahari the Kalahari Group is dated from the Eocene to the Oligo- Group has also been identified across the CB (Fig. 10.11). Miocene in the lower parts (Units G1 and M1) of the 204 B. Linol et al.

a

Luk enie Ka Riv sa er er i o Riv Riv ang er Kw Con go River

Dekese

Gilson Samba Mbandaka

Isosurface 2400 m/s

Isosurface 3600 m/s Isosurface 4200 m/s NORTH

b

North 337 km 360 km 185 km South Samba (core) Mbandaka (well log) Gilson (well log) Dekese (core)

C SfSmS cS Gr P C S fS mS cS Gr P C S fS mS cS Gr P C SfSmS cS Gr P 0 m 0 m 0 m 0 m D1 S1 M1 Kalahari Group 100 m 100 m 100 m 100 m G1 S2 D2 200 m 200 m M2 200 m 200 m S3 Cretaceous red-beds G2

Fig. 10.11 (continued)

Gilson-1 and Mbandaka-1 wells (Fig. 10.11b), based on have been derived also from the calcretized Kalahari cara- freshwater ostracods (Potamocypris? sp., Eucypris sp., pace (‘Polymorph Sandstones’). Cyprinotus sp. and Cypridopsis sp.) and charophytes (Grambastichara) that can be tentatively correlated to the ‘Polymorph Sandstones’ (Colin 1981; Colin and Jan du 10.5 Regional Correlation and Synthesis Cheˆne 1981). In summary, seismic and borehole correlations identify a Across the central Kalahari (in Botswana and Namibia), a vast extent of fluvial sediments of the Kalahari Group in the distinct, condensed sequence (10–100 m thick) of calcretes center of the CB. This succession contains relatively abun- (the Nxau-Nxau and Etosha Calcrete Formations) cap a dant pebbles of silcrete and calcrete, similar to those regional unconformity across Precambrian basement, the observed in the Kwango Valley, and which we interpret to (Carboniferous-) Karoo Supergroup, and Upper 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 205

C Samba Section

C SfSmS cS Gr P

0 m

10 m

20 m

30 m

40 m Unconsolidated 50 m sands S1 60 m

70 m

80 m

90 m Kaolinite

100 m Silcrete pebbles 110 m

120 m

130 m

140 m bioturbation (roots) 150 m S2 Kaolinite 160 m

170 m

180 m

190 m Cretaceous 200 m S3 (ostracods and phyllopods)

Fig. 10.11 (a) 3-D seismic block diagram showing a relatively flat column of Samba cores (photos on the right) archived at the RMCA uppermost main reflector at 2,400 m/s (surface in yellow), and (b) bore- museum in Tervuren, Belgium. The Kalahari succession comprises two hole stratigraphic correlations of the Kalahari Group across the central CB sequences separated by a horizon with pebbles of silcrete, similar to those (see also Chaps. 7 and 8, this Book). (c) New Kalahari stratigraphic observed in the Kwango Valley along the southwest margin of the CB

Cretaceous (83 Ma) kimberlites that corresponds to the (Fig. 10.8), where it merges with the characteristic calcrete ‘African Surface’ (e.g. du Toit 1954;King1963; and silcrete deposits of the ‘Polymorph Sandstones’ (Cahen Summerfield 1983; Partridge and Maud 2000; Decker et al. and Lepersonne 1952). Below the KP, in the Kwango Valley 2013; Linol 2013). This basal sequence of the Kalahari and across the central CB, abundant boulders and pebbles of Group extends continuously to the northern flank of the calcrete and silcrete appear to be derived from the disinte- KP, overlapping the southwestern margin of the CB gration of this polymorph unit of duricrusts. We thus 206 B. Linol et al.

JURASSIC - CRETACEOUS propose the following simple model of regional excavation Aeolian sand dunes deposition (desert) (subterranean erosion) and collapse of this Kalahari cara- wind pace, remnants of which are now found in many lower-lying Cenozoic to Recent deposits of the CB (Fig. 10.12): 1. Jurassic-Cretaceous northeast-derived aeolian red sandstones of the Kwango Group were deposited across central Africa (and eastern ), forming a giant

Kimberlites Gondwana desert under arid conditions (Myers et al. 2011; Linol 2013; Chap. 13, this Book). The sandstones Congo Fan 3) remained largely unconsolidated (Fig. 10.12a). (200 000 km a 2. With the onset of more humid conditions at the end of the Cretaceous and during the early Cenozoic, Kalahari END CRETACEOUS Peneplanation and weathering (hot, humid) silcretes and calcretes formed a thick layer of hard-cap rocks across most of the basement of the KP and the red ~1000 km sandstones of the CB, and is here named the ‘Kalahari Surface’ (Fig. 10.12b).

~500 m 3. Increasingly wet conditions, following the Paleocene-

~1000 km Eocene thermal maximum (e.g. Zachos et al. 2001), resulted in fluvial activity across the Kalahari Surface, excavating and eroding preferentially underlying poorly consolidated (soft red) sandstones of the Kwango Group, Kwango Group creating subsurface karsts topography (Fig. 10.12c). unconsolidated red-beds b ca. 500 000 km3 4. Accelerated erosion in the CB removed an estimated volume of 0.5 x 106 km3 of red sandstones that ultimately OLIGO-MIOCENE led to the demise of the Kalahari Surface during sustained Increased surface and subterranean fluvial activity (wet) riverine erosion and to re-deposition of giant boulders of silcrete and calcrete (Fig. 10.12d). The most recent deposits trap Stone Age artefacts and alluvial diamonds concentrates. In summary, a prolonged period of peneplanation and weathering at the end of the Cretaceous formed a thick

Caves carapace (the Kalahari Surface) across the KP and the CB. This was followed by rapid subterranean erosion and ‘flushing’ of the CB during wetter times that must have Congo Fan 3) (ca. 700 000 km generated large volumes of sediment to an off-shore sink after the opening of the South . c RECENT Silcrete and calcrete boulders re-deposition (wet) 10.6 Off-Shore Sedimentation History of the Congo Fan

The Congo deep-sea fan is one of the largest submarine fan systems in the world (ca. 300,000 km2; Fig. 10.13), formed on the continental margin of west-equatorial Africa Alluvial diamonds

d the Kwango red-beds into the off-shore Congo Fan. A calculated volume of 0.5 million km3 sediments removed on-shore closely Fig. 10.12 Regional model of formation and collapse of the Kalahari matches the estimated off-shore accumulation of 0.7 million km3 of hard-cap (the ‘Kalahari Surface’) across the CB, and re-deposition of Oligocene to Recent sediments 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 207

5°E 10°E 15°E 5°E 10°E 15°E

0° Cretaceous sediments accumulation 0° 0° Cenozoic sediments accumulation 0°

1000 1000

2000 2000

3000 3000

4000 4000

C C 5000 A 5000 A 5°S B 5°S 5°S B 5°S D D Thickness (m) 4000 Congo Congo Thickness (m) canyon canyon 2600 3000

1800 2000 1000

200 a 1000 b

5°E 10°E 15°E 5°E 10°E 15°E

SW NE 850 km D C B A 0 R: Miocene - Pliocene boundary 1 bMM: base mid-Miocene BO: base Oligocene 2 TC: top 3 Syn-rift 4 R bMM 5 Oligo-Miocene fan 6 salt BO TC 7 Cretaceous fan Aptian 8 50km c 9 10 km

Fig. 10.13 Off-shore isopach maps of (a) Cretaceous and (b) Cenozoic sediment accumulations of the Congo Fan, and (c) interpreted reflection seismic profiles (from Anka and Se´ranne 2004). Note the relative thickness between the Cretaceous and the Oligo-Miocene deep-sea fans

following the Early Cretaceous rifting of the South Atlantic These Cretaceous deposits, together with their proximal (e.g. Moulin et al. 2010; Heine et al. 2013). It is currently age-equivalent sequences on the shelf, contain a minimum sourced by the Congo River (the world’s second largest river of 0.2 x106 km3 sediments, representing almost one-third of by drainage area; Runge 2007), and extends over 1,000 km the volume of the Cenozoic fan (Anka and Se´ranne 2004; offshore from the shelf as far as to the (Savoye Anka et al. 2009). These observations suggest that the Congo et al. 2000; Droz et al. 2003). The existence of a direct River has been one of the major off-shore sediment suppliers connection between the Congo River outlet and the subma- since the opening of the South Atlantic, and that the location rine fan through an impressive submarine canyon is one of of its outlet has remained relatively stable ever since. Thus, the most important characteristics of this system the observed variations in the off-shore sedimentation his- (Barbonneau et al. 2002), linking directly the marine strati- tory of the Congo Fan (Fig. 10.14) are most likely primarily graphic record of the Congo Fan to the on-shore erosion and controlled by changes in the on-shore CB. sediment transport dynamics of the Congo River (Anka et al. The two episodes of high-sediment supply, represented 2009). by the deposition of both (Cretaceous and Oligo-Miocene) Detailed analysis of deep-offshore seismic reflection data submarine fan systems, are separated by a long period (Anka and Se´ranne 2004) has revealed that the volume of the (Coniacian-Eocene; 89–34 Ma) of condensed sedimentation Cenozoic fan (Oligocene–Recent) is at least 0.7 Â 106 km3. and basin starvation (Fig. 10.14). On-shore, however, apatite In addition, an Albian-Turonian (mid-Cretaceous) wedge fission track results generally suggest accelerated erosion that extends across the -Ocean boundary beneath and denudation at that time (e.g. Brown et al. 1990; Spiegel the Cenozoic fan (Fig. 10.13c), and whose depocenter is et al. 2007; Tinker et al 2008; Turner et al. 2008). We centered near the present-day Congo canyon, indicates that propose that off-shore resumption of high sedimentation an older Cretaceous fan was also sourced by a paleo-Congo following this event was driven by enhanced continental located nearby the present-day river (Anka et al. 2010). erosion of the CB due to onset of karst topography beneath 208 B. Linol et al.

Congo Fan volume (x104 km3) δ18Ο (‰) 87Sr/86Sr (Gradstein et al., 1994) R 0 10 20 30 40 50 60 70 4 3 2 1 0 0.7075 0.7080 0.7085 0.7090 0.7095 0 . GELASIAN PIACENZIAN PLIO. ZANCLEAN 0.732 0.746 MESSINIAN TORTONIAN SERRAVALLIAN LANGHIAN

BURDIGALIAN 3 MIOCENE NEOGENE 0.7 million km AQUITANIAN CHATTIAN

CENE RUPPELIAN

OLIGO Onset of accelerated off-shore sedimentation PRIABONIAN BARTONIAN

LUTETIAN

50 EOCENE

PALEOGENE YPRESIAN

Age (Ma) THANETIAN SELANDIAN

CENE DANIAN Basin starvation PALEO MAASTRICHTIAN

CAMPANIAN

SANTONIAN UPPER CONIACIAN TURONIAN

CENOMANIAN 100 3 0.2 million km o

ALBIAN Temperature ( C) CRETACEOUS

LOWER 08124 APTIAN

Fig. 10.14 Comparison between the off-shore sedimentation history ice conditions inferred from oxygen isotope records (from Zachos et al. of the Congo Fan (from Anka and Se´ranne 2004) and global proxies of 2001) and the rapid increase in the 87Sr/86Sr ratio of sea-water (from paleo-climate. The onset of Cenozoic accelerated off-shore sedimenta- McCauley and DePaolo 1997) at the Eocene-Oligocene boundary. R ¼ tion (dotted blue line) correlates with the transition from greenhouse to Range in 87Sr/86Sr of the on-shore CB red sandstones the Kalahari duricrusts in response to wetter climatic been re-deposited regionally within the lower-lying, Ceno- conditions following major global cooling during the zoic to Recent alluviums of the central CB. Rapid prefer- Eocene-Oligocene transition (e.g. Zachos et al. 2001), and ential erosion of the red-beds of the CB, underneath the cap doing so ‘flushing-out’ the unconsolidated red-beds rock Kalahari Surface, led to the rapid disintegration (cav- (Fig. 10.12). This may also have been further enhanced by ing) and collapse of this carapace across an area >1 Â 106 uplifts of the Angolan Highland (Walford and White 2005; km2 (Fig. 10.12). Initiation of the erosion of the CB can be Pritchard et al. 2009; Roberts and White 2010) and the matched to the marine records of accelerated sediments EARS between 20 Ma and 40 Ma (Lavier et al. 2001; Pik accumulation associated to the onset of the Congo Fan et al. 2008; Roberts et al. 2012). during the Eocene-Oligocene transition (Fig. 10.14). This sudden regional ‘transfer’ of sediment from on-shore to Conclusion off-shore may be linked to increasingly humid conditions due to rapid global cooling at that time; since our initial The Kalahari Group overlies a subcontinental scale results of 87Sr/86Sr of the red sandstones reveal very high ‘ ’ peneplanation surface [the African Surface ]acrossthe initial ratio and is consistent with the onset of rapid KP and the CB that resulted from significant exhumation increase in the 87Sr/86Sr ratio of sea-water (e.g. McCauley and erosion-weathering following the separation of Africa and DePaolo 1997), which may not be solely related there- from , during the opening of the South fore to the onset of the Alpine-Himalaya uplift/erosion as Atlantic Ocean. On the top of the KP this major denudation previously suggested (e.g. Raymo et al. 1988). surface is capped (protected) regionally by a thin sequence of terrestrial carbonates and calcretes (e.g. the Etosha and Nxau-Nxau Calcrete Formations) covering Paleozoic and Precambrian basement rocks. This lowermost Kalahari Acknowledgments We acknowledge funding through the Inkaba carapace unit extends northward across the CB where it yeAfrica and !Khure Africa programs, supported by the DST/NRF of merges with the ‘Polymorph Sandstones’ that, in contrast, South Africa. B. Linol is particularly grateful to James Bruchs from his overlies a thick sequence of poorly consolidated Jurassic- invitations to the core shed of Tsodilo Resources Ltd. in Maun. Frank Cretaceous red-beds. At the scarp marking the northern- Eckardt and Tyrel Flugel are thanks for their help with Figures 10.1A and 10.2. We also thank A. Tankard and G.A. Botha for valuable most extent of the KP, the Kalahari carapace is fragmented reviews that improved the chapter. This is AEON contribution number and large boulders and pebbles of silcrete and calcrete have 129 and Inkaba yeAfrica number 99. 10 Formation and Collapse of the Kalahari Duricrust [‘African Surface’]... 209

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