2.1 Introduction
The geological history of the continental crust forming southern Africa dates back to the Eoarchaean (>3.6 Ga) with the oldest dated rock unit (part of the Ancient Gneiss Complex) having a U-Pb age of 3644±4 Ma (Compston and Kröner, Reference Compston and Kröner1988). This history is preserved as a series of events in both the Precambrian (Archaean and Proterozoic Eons of geological time) and the Phanerozoic (<545 Ma) that can be interpreted within the context of current plate tectonic theory. The rocks produced during this long and complex history are preserved as a mosaic of crustal fragments with each fragment providing evidence for both the various stages involved in the geological evolution of that fragment, and the geological events that characterise each of the stages recognised. In the sections that follow, this rich history will be summarised chronologically with reference to the geological map included as Figure 2.1. For a detailed description of the lithostratigraphy of the region, the reader is referred to the summary by McCarthy and Rubidge (Reference McCarthy and Rubidge2005), and individual chapters within Johnson et al. (Reference Johnson, Anhaeusser and Thomas2006a).
Fig. 2.1. Simplified geology of southern Africa, showing components named in the text
2.2 The Archaean Eon (3.85–2.65 Ga)
The Archaean Eon is the period of geological time between ~3850 and 2650 Ma and within South Africa as elsewhere, the early part of the Archaean (the Eoarchaean) is characterised by the formation of the first fragments of continental crust and the growth of these fragments into continents. During the succeeding Mesoarchean, these early continents acted as basement for the accumulation of sedimentary and volcanic rocks (e.g. the rocks of the Witwatersrand Supergroup) in some of the oldest sedimentary basins yet to be recognised. The processes by which these fragments of ancient continental crust formed are still a matter of debate but there is increasing evidence that processes similar to present-day plate tectonics have been in operation since the Mesoarchaean (3.2–2.8 Ga).
The earliest part of the geological history of South Africa is represented by the granitoid rocks and associated greenstone belts of the Kaapvaal Craton covering an area of 1.2 x 106 km2 within South Africa and adjacent parts of Botswana. Age data from single zircon U-Pb studies (e.g. Poujol et al., Reference Poujol, Robb, Anhaeusser and Gericke2003; Eglington and Armstrong, Reference Eglington and Armstrong2004) on the granitoids of the Kaapvaal Craton indicate multiple magmatic events over more than 1 Ga duration, with Na-rich tonalite–trondhjemite–granodiorite rocks dominant between 3.65 and 3.1 Ga and potassic granitoids thereafter. The greenstone belts are typically linear in plan with widths of 10–50 km and strike lengths of 100–300 km (Brandl et al., Reference Brandl, Cloete, Anhauesser, Johnson, Anhaeusser and Thomas2006), and are dominated by lavas of basaltic composition. Sedimentary rocks are predominantly clastic and typically dominate the upper part of the supracrustal sequence. Granitoid-greenstone terranes occur across the Kaapvaal Craton with the crust decreasing in age north and west from the Barberton terrane (Fig. 2.1). The Barberton granitoid-greenstone terrane comprises the supracrustal rocks of the Barberton Supergroup which defines the Barberton Greenstone Belt, together with granitoids that range in age from the Eoarchaean (>3.6 Ga) through to the early Mesoarchaean.
The Barberton Greenstone Belt comprises a 120 x 50 km ENE-trending remnant of strongly deformed Palaeoarchaean (3.6–3.2 Ga) supracrustal rocks (the Barberton Supergroup; SACS 1980) that define a lithotectonic complex subdivided into Northern and Southern Terranes separated by the strike-parallel Inyoka–Saddleback fault system (Brandl et al., Reference Brandl, Cloete, Anhauesser, Johnson, Anhaeusser and Thomas2006, and references therein). The oldest rocks of the Barberton Supergroup comprise a tectonic stack of ultramafic and mafic volcanic rocks some 12–15 km thick and termed the Onverwacht Group. The lower part of the Onverwacht Group comprises Mg-rich ultramafic volcanic rocks (komatiites) that may represent Archaean oceanic crust. The stratigraphy of the Onverwacht Group is a matter of some debate although there is general agreement that it represents a succession disrupted and duplicated by folding and faulting rather than an original sequence.
The sedimentary rocks of the Barberton Greenstone Belt are subdivided into the older Fig Tree Group which unconformably overlies the Onverwacht Group and comprises deep- to shallow-water shale, greywacke, jaspilitic banded iron formation and carbonaceous chert (Hofmann, Reference Hofmann2005); and the younger Moodies Group of conglomerate, coarse, cross-bedded, quartz-rich sandstones and shales that were deposited in tidally influenced basins (Eriksson, Reference Eriksson1980). The rocks of the Moodies Group are unconformable with respect to the Fig Tree Group and represent a molasse-type sequence deposited in synorogenic basins (Heubeck et al., Reference Heubeck, Engelhardt, Byerly, Zeh, Sell, Luber and Lowe2013).
The oldest granitoid rocks of the Barberton terrane (e.g. the Ancient Gneiss Complex, the 3.51 Ga Steynsdorp Pluton, and the 3.44 Ga Theespruit Pluton) were emplaced as diapirs which contributed to the deformation seen in the supracrustal rocks. The younger K-granitoids are sheet-like intrusions and include the 3.10 Ga Nelspruit and Mpuluzi batholiths (Robb et al., Reference Robb, Brandl, Anhaeusser, Poujol, Johnson, Anhaeusser and Thomas2006, and references therein). An important result of this magmatism was the progressive stabilisation of the developing continental crust, such that by 3.1 Ga a continental fragment of definable size had formed. de Wit et al. (Reference de Wit, de Ronde, Tredoux, Roering, Hart, Armstrong, Green, Peberdy and Hart1992) refer to this fragment as the Kaapvaal Shield and identify it as the micro-continent on which the Mesoarchaean cover sequences were deposited, and which are now preserved as the Dominion Group and Witwatersrand Supergroup in the central part of the Kaapvaal Craton, and the Pongola Supergroup in the southeast of the craton.
The rocks of the Dominion Group comprise quartzite with minor conglomerate overlain by volcanic rocks that include both basalt and rhyolite, a sequence that suggests the Dominion Group accumulated in a rift basin. A minimum age for this rifting event is 3.07 Ga, the age of the rhyolite lava in the upper part of the Dominion Group (Armstrong et al., Reference Armstrong, Compston, Retief, Williams and Welke1991). The Nsuze Group forms the lower part of the Pongola Supergroup and, like the Dominion Group, is characterized by a ~3.0 Ga rift-related volcano-sedimentary sequence, thereby allowing for correlation between units of the Nsuze Group and the Dominion Group (Gold, Reference Gold, Johnson, Anhaeusser and Thomas2006). In the central part of the craton, the Dominion Group is overlain by the sedimentary rocks of the Witwatersrand Supergroup, whereas to the southeast the Nsuze Group is overlain by the largely sedimentary Mozaan Group which defines the upper part of the Pongola Supergroup. The rocks of the Witwatersrand Supergroup are subdivided into the lower West Rand Group and the upper Central Rand Group. Deposition of the West Rand Group started around 2.97 Ga (Robb and Meyer, Reference Robb and Meyer1995) and the Crown Lava towards the top of the Group was erupted at 2.91 Ga. The rocks of the West Rand Group are dominated by quartzite and shale in roughly equal proportions, and are subdivided into three subgroups (Hospital Hill, Government and Jeppestown) and sixteen formations (McCarthy, Reference McCarthy, Johnson, Anhaeusser and Thomas2006). The Central Rand Group consists mainly of quartzite and conglomerate including the renowned auriferous units (gold reefs), and was deposited between 2.91 Ga and the eruption of the Klipriviersberg Group (see below) at 2.714 Ga. The rocks of the Central Rand Group are subdivided into two subgroups (Johannesburg and Turffontein) and nine formations (McCarthy, Reference McCarthy, Johnson, Anhaeusser and Thomas2006). Correlation between the cover rocks of the central and southeastern parts of the Kaapvaal Craton has been extended into the Witwatersrand Supergroup, with the Mozaan Group (Pongola Supergroup) regarded as a correlative of the West Rand Group (Beukes and Cairncross, Reference Beukes and Cairncross1991).
Sedimentation to produce the Witwatersrand Supergroup was replaced by a volcanic event that produced the thick pile of lava preserved as the Ventersdorp Supergroup. This volcanic event started with the outpouring of the basaltic Klipriviersberg Group followed unconformably by the, initially sedimentary, Platberg Group in which felsic volcanics of the Makwassie Formation have been dated to 2.708 Ga (Armstrong et al., Reference Armstrong, Compston, Retief, Williams and Welke1991). The Platberg Group is overlain with a pronounced unconformity by the Bothaville Formation, and the sequence completed by mafic to intermediate lava of the ~2.68 Ga Allanridge Formation.
The Limpopo Orogeny was the event that produced the Limpopo Belt, a zone of granulite facies gneisses between the Kaapvaal and Zimbabwe cratons. The geology of the Limpopo Belt is very complex, being the product of high-grade metamorphism, extreme ductile deformation and widespread crustal melting (van Reenen et al., Reference van Reenen, Kramers, McCourt and Perchuk2011). The details of the processes involved in its formation are still a matter of debate, but there is general agreement that the high-grade gneisses represent the root zone of an ancient mountain belt formed by plate tectonic processes similar to those responsible for the Himalaya (see Kramers et al., Reference Kramers, McCourt, Roering, Smit, van Reenen, van Reenen, Kramers, McCourt and Perchuk2011, for discussion). The collision of the Kaapvaal and Zimbabwe cratons to produce the Limpopo Belt occurred between ~2.7 and 2.65 Ga and the resultant crustal fragment, the core of the continental crust of southern Africa, has been referred to as the Azanian Craton (McCourt et al., Reference McCourt, Kampunzu, Bagai and Armstrong2004).
2.3 The Neoarchaean and Palaeoproterozoic Eras
Following the Limpopo Orogeny, or perhaps in response to it, a new rift developed in the Kaapvaal section of the Azanian Craton in which the rocks of the 2.66 Ga Wolkberg Group and associated sequences accumulated. This rifting was followed by thermal subsidence of the majority of the Kaapvaal Craton to form a large shallow continental shelf basin on which the sedimentary rocks of the Transvaal Supergroup were deposited. The remnants of this sequence are preserved in three structural basins with the same basic succession recognised in each, viz: a basal clastic unit overlain by a thick sequence of chemical sedimentary rocks which are in turn separated by an unconformity from an upper clastic unit. The hiatus represented by this unconformity is poorly constrained but in the case of the Transvaal Basin it may be as much as 150 Ma (Moore et al., Reference Moore, Polteau, Armstrong, Corfu and Tsikos2012). Age constraints indicate that the basal unit was deposited around 2.64 Ga, the carbonate rocks around 2.58 Ga and the overlying sequence between 2.5 and 2.43 Ga. Within the Transvaal Basin, the minimum age of the upper clastic sequence (the Pretoria Group) is provided by the emplacement age of the world’s largest mafic layered intrusion, the Bushveld Complex.
The Bushveld Complex comprises three different groups of igneous rocks. The oldest component of the Complex is the 2.061 Ga Rooiberg Group, a succession of predominantly rhyolitic lavas that erupted along an unconformity surface at the top of the Pretoria Group. The second component is the 2.054 Ga Rustenburg Layered Suite (RLS), a sill-like intrusion of mafic and ultramafic magma into the rocks of the Pretoria Group and host to enormous reserves of platinum-group elements, chromite and vanadium-bearing magnetite. The youngest component of the Bushveld Complex is the Lebowa Granite Suite produced by partial melting of the lower crust during the emplacement of the RLS.
At around 2.15 Ga, rifting occurred along the northwest margin of the Azanian Craton (now northwest Zimbabwe) to accommodate the development of the Magondi Basin and the deposition of the rocks of the Magondi Supergroup. By about 1.95 Ga, this rift system had produced a continental shelf along the western margin of the former Kaapvaal Craton on which shallow-water marine sediments began to accumulate. Today this shelf sequence is represented by the quartzite, shale and local carbonate of the Olifantshoek Supergroup, the age of which is constrained by the 1.93 Ga lava of the Hartley Formation towards the base of the sequence (Cornell et al., Reference Cornell, Armstrong and Walraven1998). Palaeoproterozoic extension also affected the interior of the Kaapvaal Craton and produced basins in which the rocks of the Waterberg Group and Soutpansberg Group were deposited. The clastic rocks in both these sequences are characterised by a strong red colour, indicating deposition in an atmosphere that contained free oxygen and, as such, record an important change in the Earth’s environment. These ‘Red Bed’ successions were deposited between 2.05 and 1.87 Ga.
2.4 Rodinia and Gondwana (1.6–0.5 Ga)
The geological history of southern Africa during the Mesoproterozoic Era (1.6–1.0 Ga) records the formation of island arcs to the south and west of the Kaapvaal Craton followed by accretion of these arc complexes to the craton margin to form the Namaqua–Natal Metamorphic Province. The rocks defining the Namaqua–Natal Metamorphic Province can be traced from southern Namibia, southeast through the Northern Cape Province, to the coastline of KwaZulu–Natal (Fig. 2.1). Within each sector of the orogen a number of terranes have been recognised (McCourt et al., Reference McCourt, Armstrong, Grantham and Thomas2006, and references therein). The orogenic event responsible for the Namaqua–Natal Metamorphic Province is linked to the assembly of the supercontinent Rodinia and, within southern Africa, led to the growth of the Azanian Craton into the Kalahari Craton.
The geology formed during the Neoproterozoic Era (1000–524 Ma) is represented by lithostratigraphic units that characterise the Gariep Belt to the north of 32°S (Richtersveld Suite, Gariep Supergroup, Nama Group and Vanrhynsdorp Group) and the Saldania Belt to the south (Gresse et al., Reference Gresse, von Veh, Frimmel, Johnson, Anhaeusser and Thomas2006). The sedimentary rocks of the Gariep Supergroup were deposited in a shallow-water marine environment and then deformed and metamorphosed to produce the Gariep Belt. The rocks of the Nama and Vanrhynsdorp Groups represent foreland basin deposits inboard of the belt. Within the Saldania Belt, the main occurrence of supracrustal rocks lies to the north and northeast of Cape Town (Malmesbury Group) with smaller occurrences farther east in the Oudtshoorn (Cango Caves and Kansa Groups), George (Kaaimans Group) and Port Elizabeth (Gamtoos Group) areas (Gresse et al., Reference Gresse, von Veh, Frimmel, Johnson, Anhaeusser and Thomas2006). The metamorphosed rocks of the Malmesbury and Kaaimans Groups are intruded by the 550–507 Ma granitoids of the Cape Granite Suite and are well exposed at the Sea Point locality in Cape Town. Globally, the Gariep and Saldania belts are related to the assembly of the southwest part of Gondwana and, together with the Kaoko and Damara belts in Namibia and the Mozambique Belt, define the southern African sector of the Pan-African orogenic system.
2.5 The Phanerozoic Eon
Subsequent to the Saldanian Orogeny and the intrusion of the Cape Granite Suite, southern Africa lay close to the southern margin of Gondwana, and it is within this context that the rocks which record the final chapter in the geological history of continental southern Africa formed. Today these rocks of Gondwana (McCarthy and Rubidge, Reference McCarthy and Rubidge2005) are preserved as the Cape Supergroup, the Natal Group, and the Karoo Supergroup. Collectively these Palaeozoic to Mesozoic rocks cover more than 60% of the surface of South Africa, are integral to the landscape of the region, and preserve a unique record of the early evolution of fishes, amphibians, reptiles and mammals (McCarthy and Rubidge, Reference McCarthy and Rubidge2005). The Cape Supergroup was deposited between ~500 and 330 Ma in a basin that developed along the southern margin of South Africa from present-day Clanwilliam to Port Alfred. The rocks of the Cape Supergroup are Palaeozoic in age and subdivided into three lithologically distinctive units called the Table Mountain Group (Ordovician to early Devonian sandstones), the Bokkeveld Group (early to middle Devonian shale and sandstone containing a rich and diverse marine invertebrate fossil record), and the Witteberg Group (late Devonian to early Carboniferous sandstone and mudrock with a rich invertebrate, fish and plant fossil record). The Natal Group (Marshall, Reference Marshall, Johnson, Anhaeusser and Thomas2006) is preserved in a northeast-trending, coast-parallel basin that has been interpreted as part of the system that accommodated the deposition of the Cape Supergroup (McCarthy and Rubidge, Reference McCarthy and Rubidge2005), and as a foreland basin to the Pan African Mozambique Belt (Marshall, Reference Marshall, Johnson, Anhaeusser and Thomas2006). Deposition of the reddish-grey sandstone-dominated Natal Group started in the early Ordovician (~490 Ma; Thomas et al., Reference Thomas, Marshall, Watkeys, Fitch and Miller1992), but although similar in age, there is no evidence to support a correlation, on lithological grounds, with the Table Mountain Group.
Rocks of the Karoo Supergroup range in age from late Carboniferous to middle Jurassic (310–180 Ma) and represent the last major supracrustal sequence to be deposited on the continental crust of southern Africa. The bulk of the dominantly sedimentary succession occupies the erosional remnant of the main basin which covers some 700,000 km2 of south-central South Africa (Johnson et al., Reference Johnson, van Vuuren, Visser, Cole, de V. Wickens, Christie, Roberts, Brandl, Johnson, Anhaeusser and Thomas2006b) with smaller but significant deposits present in the Springbok Flats, Ellisras, Tshipise and Tuli Basins farther north. The succession in the Main Karoo Basin is strongly asymmetric in cross section, being significantly thicker in the south against the mountains of the Cape Fold Belt and thinning northwards across the Kaapvaal Craton. Based on age and lithological character, Johnston et al. (Reference Johnson, van Vuuren, Visser, Cole, de V. Wickens, Christie, Roberts, Brandl, Johnson, Anhaeusser and Thomas2006b) subdivide the sedimentary rocks in the Main Basin into the basal Dwyka Group (glacial deposits), the Ecca Group (deltaic deposits including coal), Beaufort Group (floodplain deposits), Molteno Formation, Elliot Formation and Clarens Formation (desert conditions), reflecting a significant change in climate due to the northward drift of Gondwana from polar to warmer latitudes. The youngest unit of the Karoo Supergroup is the Drakensberg Group comprising the basaltic lava and associated dolerite intrusions that identify the start of Gondwana breakup at some 180 Ma (Duncan and Marsh, Reference Duncan, Marsh, Johnson, Anhaeusser and Thomas2006; Watkeys, Reference Watkeys, Johnson, Anhaeusser and Thomas2006), an event that was to have an important effect on the present-day landscape of South Africa.
The Mesozoic (Jurassic and Cretaceous) clastic sedimentary rocks that are preserved in isolated fault-bound onshore basins from northern KwaZulu–Natal to Worcester in the Western Cape accumulated in basins related to extension of the newly-formed continental margin during the break-up of Gondwana (Shone, Reference Shone, Johnson, Anhaeusser and Thomas2006). The Algoa Basin to the north of Port Elizabeth is the largest of the basins along the southern margin of South Africa, and accommodated the deposition of the late Jurassic to early Cretaceous Uitenhage Group, comprising the Enon, Kirkwood and Sundays River Formations. The Cretaceous deposits of the Zululand Group in northern KwaZulu–Natal are subdivided into the Makatini, Mzinene and St Lucia Formations. Exposure is confined to a narrow belt from Mthubathuba north to the border with Mozambique, but drilling on the coastal plain has confirmed up to 2000 m of Cretaceous rocks under the Cenozoic cover (Shone, Reference Shone, Johnson, Anhaeusser and Thomas2006).
Mesozoic offshore basins are located along the western, southern and eastern margins of continental South Africa and are presently important exploration targets for oil and gas. The basins developed in response to the separation of East and West Gondwana (e.g. Watkeys, Reference Watkeys, Johnson, Anhaeusser and Thomas2006). Rifting along the western margin led to the development of the Orange Basin, the fill of which comprises two synrift phases and five drift intervals (Broad et al., Reference Broad, Jungslager, McLachlan, Roux, Johnson, Anhaeusser and Thomas2006). The sedimentary rocks are siliclastic and predominantly deltaic with the oldest being early Aptian (112 Ma) rocks ranging from continental red beds to marine sandstones and shales (Broad et al., Reference Broad, Jungslager, McLachlan, Roux, Johnson, Anhaeusser and Thomas2006). The Outeniqua Basin is situated off the south coast of South Africa and comprises a series of southeast-trending sub-basins named, from west to east, the Bredasdorp, Pletmos, Gamtoos and Algoa sub-basins (Broad et al., Reference Broad, Jungslager, McLachlan, Roux, Johnson, Anhaeusser and Thomas2006). The Bredasdorp sub-basin is approximately 200 km long and 80 km wide and the site of the majority of oil and gas exploration activities, thus the geology is well documented (Broad et al., Reference Broad, Jungslager, McLachlan, Roux, Johnson, Anhaeusser and Thomas2006). The east coast of South Africa has two Mesozoic offshore basins, namely the Durban and Zululand Basins located between Port Shepstone and the Mozambique border, with the Zululand Basin being the southern component of the Mozambique Basin (Broad et al., Reference Broad, Jungslager, McLachlan, Roux, Johnson, Anhaeusser and Thomas2006). Exploration activity is limited, thus the geology of these basins is poorly understood. In the Durban Basin, synrift rocks have been correlated with continental red beds at the base of the Zululand Group onshore (Broad et al., Reference Broad, Jungslager, McLachlan, Roux, Johnson, Anhaeusser and Thomas2006).
The formation and stratigraphy of the Cenozoic deposits of southern Africa can be directly linked to the tectonic and geomorphic events that followed the breakup of Gondwana (Partridge et al., Reference Partridge, Botha, Haddon, Johnson, Anhaeusser and Thomas2006). At this time the average elevation of southern Africa was around 2000 m (Partridge and Maud, Reference Partridge and Maud1987). Rifting of this elevated region created a narrow coastal plain flanked by a high escarpment. The climate was warm and humid which, together with the high elevation, promoted rapid and effective erosion such that by the end of the Cretaceous (66 Ma) when the Earth’s climate started to cool, the escarpment had been eroded back to a position 120 km inland from the southeastern coast and 50 km from the west coast (Partridge et al., Reference Partridge, Botha, Haddon, Johnson, Anhaeusser and Thomas2006). The result of this post-rifting period of erosion and denudation was the formation of an interior plateau separated from surrounding lower-lying regions by the Great Escarpment. The coeval erosion surfaces above and below the Great Escarpment define the African Surface (Partridge and Maud, Reference Partridge and Maud1987, and references therein) on which pedogenic duricrusts formed above deeply kaolinised weathering profiles (Botha, Reference Botha, Partridge and Maud2000) and Cenozoic sediments were deposited in both interior (Kalahari Group; Partridge et al., Reference Partridge, Botha, Haddon, Johnson, Anhaeusser and Thomas2006) and coastal (Maputland, Algoa and Bredasdorp Groups; Roberts et al., Reference Roberts, Botha, Maud, Pether, Johnson, Anhaeusser and Thomas2006) basins. At ~30 Ma, the African plate stopped its northward drift and began to bulge upwards (Burke, Reference Burke1996, and references therein) with important periods of uplift documented in the early Miocene (~20 Ma) and particularly the Pliocene (~5 Ma). These periods of uplift would have increased erosion and facilitated the processes responsible for shaping southern Africa’s current topography. This recent geologic history is discussed by Knight and Grab (this volume).
2.6 Summary
The long geological history of southern Africa is testimony to the interplay between tectonic, sedimentary and climatic processes, which have driven periods of uplift, rifting and volcanism; metamorphism and formation of ore deposits; the formation and closure of sedimentary basins; and landscape denudation. The geological relationship to the present-day landscape is shown in patterns of landforms, soils and ecosystems, and provides the context for later human occupation of the landscape and exploitation of its resources.
