Much of the bedrock geology of southwestern Africa is made up of a series of Pan-African orogenic belts that formed during the collision of Archaean to Mesoproterozoic continental fragments in the course of Gondwana amalgamation at the end of the Neoproterozoic Era. These are the West Congo, Mozambique and Lufilian/Zambezi belts to the west, east and south, respectively, of the Congo Craton. The latter continues southwestwards into the Damara Belt, located between the Kalahari and the Congo cratons. Along the South Atlantic coast are the Kaoko and Gariep belts located, with the latter extending into the Saldania Belt around the southern tip of Africa. The southern branch of the Saldania Belt represents an accretionary orogen at the margin of southwestern Gondwana but all the other belts are typical collisional orogens. While these orogenic belts share many similarities in lithostratigraphy and tectonic evolution, each belt has distinct characteristics that are briefly summarised in this chapter.
A new geodynamic model is presented for southwestern Gondwana amalgamation. Rifting of Rodinia was a protracted process, advancing from the boundaries of the supercontinent toward its core, represented by Laurentia. Whereas the Congo-Sao Francisco Craton was likely far away from Rodinia, it is proposed that the Rio de la Plata Craton was part of the supercontinent. We propose the name ‘ Arachania ’ for the block that comprises the Cuchilla Dionisio-Pelotas, Marmora, Tygerberg and correlative terranes, which likely represents a fragment of the Kalahari Craton that a later stage (650–570 l Ma) evolved into a magmatic arc. Available evidence points towards stepwise rifting of Rodinia, and the sequential closure of the thus formed oceanic basins from east to west (present coordinates). These basins were the Damara, Adamastor, Brazilides, Pampean and Iapetus oceans. Final amalgamation of Gondwana took place in the Cambrian (ca. 520 l Ma).
The Ediacaran-Lower Cambrian Pampean Orogen comprises a large area in northwest Argentina, which formed the western margin of Gondwana. This orogen evolved from an aulacogenic structure, which determined the outline of the large 'Puncoviscana Basin'. This basin was bounded by the Meso- to Neoproterozoic cratonic areas of the Rio de la Plata, Guaporé (West Amazonia), and the 'Arequipa Terrane'. A thick siliciclastic succession with subordinate carbonates and conglomerates was deposited. Body and trace fossils occurring in the siliciclastic sequence allow to assign it to the Ediacaran-Lower Cambrian. Carbon isotopes show that limestones represent different stratigraphic levels. Radiometric ages show that the deformational event F1 and the metamorphism M1 and M2 range from Lower to Middle Cambrian. Detrital zircon ages indicate a close relationship with the peripheral shield rocks. Current evidence supports an autochthonous origin for the Pampean Orogen, contrasting with other ideas supporting an assumed exotic block that collided with West Gondwana.
The Río de la Plata Craton (RPC) comprises part of southern Brazil, Paraguay, Uruguay and the central-eastern sector of Argentina. To the north, the RPC is bounded by the southern Amazonian craton, whereas to the east it is bounded by the allochthonous Cuchilla Dionisio Terrane. The RPC is limited in the west by the Pampia Terrane of Argentina. Three tectono-stratigraphic terranes compose the RPC: the Piedra Alta, Tandilia and Nico Pérez terranes, separated by the first-order Sarandí del Yí and Colonia shear zones. Basement rocks range in age from Archaean to Mesoproterozoic, with Palaeoproterozoic igneous–metamorphic complexes making up most of the Tandilia and Piedra Alta terranes. Neoproterozoic (mainly Ediacaran) sedimentary successions occur in the three terranes, and mainly represent passive margin deposits. Abundant granitic intrusions between 630 and 530 Ma in the Nico Pérez Terrane may represent rift-related granitoids or a transpressive regime with subduction towards the north-northwest during the Brasiliano Cycle.
The litho-, chemo-, biostratigraphic and radiometric data of the various Neoproterozoic to Early Palaeozoic syn- to late-orogenic sedimentary basins of southwestern Africa are discussed. The basins are located on the margin of the Kalahari palaeocontinent with the exception of the Owambo basin. The latter basin occurs on the western margin of the Congo palaeocontinent. An attempt has been made to unravel the evolution of each basin and to determine their interrelationship. At present the Nama Basin provides most data, whereas little is known of the Boland Basin. The greyish to greenish lower Nama Group contains a famous Ediacaran fauna and the upper Cambrian Nama contains red beds with trace fossils. Precise age constraints of the Nama Group are available in terms of radiometric data and palaeontological record.
The São Francisco and the Congo cratons represent sectors of a Neoproterozoic palaeocontinent preserved from the Brasiliano-Pan African orogeny, recorded in their marginal belts. The boundaries between the São Francisco Craton and the surrounding belts are marked by intensive folding and overthrusting. In the interior of the craton, Neoproterozoic sedimentary covers are horizontal or gently deformed, forming isolated basins. The term São Francisco Basin includes Palaeo- and Mesoproterozoic successions but in this chapter it is restricted to the Neoproterozoic sedimentary successions of the São Francisco Supergroup, characterised by a glaciogenic unit at the base and an argillaceous–carbonatic–arkosic unit at the top. The Neoproterozoic evolution of the São Francisco Craton, including its marginal belts and sedimentary cover, is reviewed. The lithostratigraphy, Neoproterozoic mineralisations (lead, zinc, fluorine, barium and phosphates) and geochronological and stable isotope studies are also presented. The geological record of glacial events and stable isotope data of related cap carbonates are also discussed.
The SW Gondwanan Parana Basin in Brazil is developed on a gneissic-granitic terrane differing from the other basement rocks. Its geologic and geophysical characteristics indicate the existence of a distinct continental lithosphere segment, the ‘Paranapanema Block’. Deep boreholes show a predominantly granitic composition. It acted as an upper plate during Neoproterozoic subduction processes. Zones of plate interaction mark the external boundaries of this portion of the basin, characterised by continental magmatic arcs and post-collision structures. Gravimetric data corroborate the geological observations. Well-defined gradients delineate its contour. Presence and location of its northern portion were confirmed by an MT survey. Small differences in seismic velocity suggested its cratonic nature. According to geochemistry, Sr and Pb isotopes, two major sources, divide the area into northern and southern segments. Recent studies point to the existence of two lithospheric blocks, one of which is the Paranapanema.
The Neoproterozoic Paraguay Belt comprises a thick sedimentary succession that was deposited and deformed along the southeastern margin of the Amazonian Craton. The stratigraphy consists of a lower succession of glaciogenic diamictites and turbidites related to the late Cryogenian glacial event, which is overlain by carbonates of the Corumbá and Araras groups. They are in turn overlain by younger glacial rocks deposited during the Ediacaran glacial age. The uppermost succession is composed of sandstones, siltstones and arkoses of the Alto Paraguay Group. This sedimentary sequence represents a passive margin that was deformed during late Neoproterozoic-Cambrian epoch. The cap dolomite of the Araras Group yielded a Pb/Pb isochron age of 633±25 Ma and the Tamengo Formation is constrained by a U-Pb zircon age of 543±25 Ma from an ash bed. Carbon, oxygen and strontium isotopic data are presented for different carbonate sections.
Syn- to post-tectonic intrusive rocks occur in the Kaoko Belt, but only west of the Purros Lineament and in the Southern Kaoko Zone. In the Damara Belt, granites occur almost exclusively in the Central Zone with some extending into the Northern Zone. The Donkerhuk Granite forms a major batholith along the northern edge of the Southern Zone and a few small plugs of alkali-feldspar granite intruded the eastern Southern Marginal Zone. Syn-tectonic granites are absent in the Gariep Belt, where only post-tectonic plutonic rocks occur along the Kuboos-Bremen line. In the Saldania Belt, however, a whole range from syn- to post-tectonic intrusive and extrusive rocks exists.
Acritarchs are key for unravelling Neoproterozoic biodiversity changes because they represent the only fossil group occurring throughout the era. Their Neoproterozoic evolution is characterised by drastic diversity changes, which, coupled to the occurrence of taxa with restricted stratigraphic range, represents a powerful biostratigraphic tool for the subdivision of the era. Diverse acritarch assemblages characterise the Tonian-early Cryogenian, middle Ediacaran (Ediacaran Complex Acanthomorph Palynoflora, ECAP) and the Cambrian. Impoverished palynofloras include two Bavlinella-dominated assemblages in the mid-Cryogenian (770–740 Ma) and late Cryogenian (ca. 670–635 Ma), the Early Ediacaran Leiosphere Palynoflora (EELP) and the Late Ediacaran Leiosphere Palynoflora (LELP). Criteria are proposed to distinguish between the latter two assemblages. The LELP is characterised by smaller leiosphaerids (<150 μm), small acanthomorphs (Asteridium), co-occurrence with Cloudina and other shelly fossils, and higher 87Sr/86Sr of coeval carbonates. Instead of a bipartite sudivision of the Ediacaran, a tripartite subdivision characterised by the EELP, ECAP and LELP becomes feasible.
The Luís Alves Microplate (LAM) is an allochthonous block accreted to the eastern border of the Rio de La Plata and Paranapanema cratons, located between the Neoproterozoic Ribeira Belt and Dom Feliciano belts. It comprises an Archaean-Palaeoproterozoic basement (orthogneisses), a Neoproterozoic volcano-sedimentary cover and anorogenic alkaline–peralkaline granitoids. The geological history of the basement rocks is complex, including two high-grade metamorphic events (2,350 and 2,180 Ma). K-Ar cooling ages on biotite and amphibole of ca. 1,800 Ma indicate that the LAM has been stable since the Palaeoproterozoic, Brasiliano deformation being restricted to shear zones at its margins. An ophiolite and deformed Neoproterozoic calc-alkaline granitoids rocks at its north–northwestern margin record oceanic crust consumption followed by continental collision. In contrast, the southern margin of the LAM is covered by the Neoproterozoic Itajaí Group, a foreland basin that was deformed and overthrust by the Brusque Group in the Cambrian (ca. 535 Ma).
The metamorphic zonation of the Damara and Kaoko belts is very similar. Both have paired medium-pressure, Barrovian-type and low-pressure–high-temperature, Buchan-type belts with abundant to scattered granites in the latter and large-scale nappe structures in the former. In each, minerals of the early, highest pressure assemblages are enclosed in decompression coronas that record as much a 3 kbar of decompression within 20–30 myr. Unique to the western Kaoko Belt is a granulite-facies metamorphic event with associated calc-alkine magmatism at approximately 650 Ma, thus predating the peak of metamorphism and deformation in the Damara and Gariep belts at about 542 Ma. During both the M1 and M2 phases of metamorphism in the Damara Belt, the southern Central Zone was the leading edge of the high-temperature–low pressure active continental margin, whereas the Southern Zone and Southern Marginal Zone of that belt formed the low-temperature–medium pressure regions of the accretionary wedge riding atop the subducting Kalahari plate. In contrast to the above, only low-grade metamorphism is recorded further south in the Gariep and Saldania belts. There the main structural imprint was caused by sinistral transpression with top-to-northeast transport, similar as in the Kaoko Belt.
The Ediacaran-Early Cambrian transition in the southwestern margin of Gondwana is represented in the Andean Margin of South America by the thick sequence of the Puncoviscana Basin. In southern Africa, the Nama and Vanrhynsdorp groups display partially equivalent successions. Trace fossil assemblages are well known from both regions, with relatively simple, low diversity forms in the lower stratigraphic levels. An increase in complexity is recorded in the upper sector of the Andean and African sequences, with a variety of Treptichnus-type traces that denotes a more gradual diversity increase towards the Cambrian than generally accepted. Trace fossil associations indicate that the fossiliferous levels of the Puncoviscana Basin may predate the uppermost lower Cambrian (pre-trilobite) successions of the Nama and Vanrhynsdorp groups, and lasted into the 'trilobitic' Cambrian. Intergradations between Treptichnus and different ichnogenera cast doubts on using related ichnospecies in Ediacaran-Early Cambrian biozonation.
The half a billion of years claimed by the Neoproterozoic Era and Cambrian Period marks a great turning point in the history of Earth, beginning with a low diversity and low-PO2 ocean and atmosphere and ending with a more familiar, oxygen-rich atmosphere–ocean system, populated by diverse animal life. This time period encapsulates many other extraordinary events that helped shape the Earth's surface environment, including the break-up and amalgamation of supercontinents, snowball glaciations, true polar wander, and enormous perturbations to the global carbon cycle. The wealth of data emerging from abundant Neoproterozoic-Cambrian sedimentary successions increasingly points to an intimate connection between tectonic, geochemical, climatic, and biospheric change during this pivotal time, highlighting the complexity of the Earth system. Here we briefly review the tectonic, geochemical, and palaeontological records spanning the Neoproterozoic-Cambrian transition as a template for reconstructing the biogeochemical evolution of the surface environment as a habitable Earth emerged.
In this review we provide a correlation scheme for Neoproterozoic palaeoclimatic events in southwestern Gondwana based on combined carbon and strontium isotope variations informed by emerging radiometric constraints on glacial deposits. Strontium isotope ratios in particular hold promise as chronostratigraphic tools if well-preserved samples can be identified, and if long-term trends through the Neoproterozoic show a regular progression to more radiogenic values. Based on analyses of high Sr limestone samples from cap carbonate lithofacies, we recognise four discrete glacial events in southwestern Gondwana, including two Sturtian, one Marinoan and one Gaskiers ice age. Notably, the Namibian Maieberg cap carbonate – considered by many to be a Marinoan archetype – sits stratigraphically below a profound positive carbon isotope excursion (called the Hüttenberg anomaly) and is correlated using Sr isotope ratios with an identical post-glacial cap in Brazil dated at ca. 740 Ma by Pb-Pb carbonate techniques. If correct, the Pb-Pb age constraint places the Ghaub ice age and its equivalents into the Sturtian epoch.
The Neoproterozoic stands out as a period of innovations and upheavals. The extreme palaeoclimatic, palaeoceanographic and biotic events that characterize the Neoproterozoic Era are reviewed, and may ultimately be a result of its unusual tectonic history. The final accretion and subsequent break-up of Rodinia, followed by the amalgamation of Gondwana were probably paramount in influencing Earth's surface environments, although the precise mechanisms remain controversial. Deep-Earth processes, such as mantle avalanches and superplumes, may have been the unheralded engines of a dynamic Neoproterozoic tectonic regime. Rapidly evolving palaeogeography may have in turn contributed to biogeochemical and climatic oscillations, which themselves were likely inextricably linked to biospheric evolution and ultimately the Cambrian explosion. Thus, Neoproterozoic-Cambrian Earth history is a case in point of the complexity and intrigue of the interactions between the deep earth, the lithosphere, the oceans, the atmosphere and the biosphere, helping to improve our current understanding of the Earth system.
Neoproterozoic shelly fossils include calcareous (Cloudina, Namacalathus, Namapoikia, Sinotubulites), siliceous (vase-shaped microfossils, sponge spicules, Tindir scales), phosphatic (Waltheria, Vendoconularia) and agglutinated skeletons (Titanotheca). Vase-shaped microfossils are the oldest, potentially useful skeletal fossils for Neoproterozoic biostratigraphy, especially for Cryogenian subdivision. The Cloudina Range Zone is erected, which could be used to characterise the youngest, still unnamed series of the Ediacaran. Titanotheca is another potential index fossil of the Late Ediacaran, because of its unusually high preservation potential, wide palaeogeographic distribution (three palaeocontinents), relatively short stratigraphic range, abundance and occurrence in different facies. Predation was the most likely trigger of skeletogenesis in early protists and metazoans. However, it was not the only driver, and an ecological ‘inhibitor’ prevented significant diversification before the Cambrian. Oxygen deficiency and, more importantly, repeated ice ages were the most important inhibiting factors. The Cambrian explosion could only ‘detonate’ once oxygen and a stable, warm climate were available.
