The Witwatersrand (, ; ; locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which account for the name Witwatersrand, meaning 'white water ridge' in Afrikaans.Truswell, J.F. (1977). The Geological Evolution of South Africa. pp. 21, 27–28, 33–36. Cape Town: Purnell. This east-west-running scarp can be traced with only one short gap, from Bedfordview (about west of O.R. Tambo International Airport) in the east, through Johannesburg and Roodepoort, to Krugersdorp in the west (see the diagram at left below).Norman, N.; Whitfield, G. (2006) Geological Journeys. pp. 38–49, 60–61. Cape Town: Struik Publishers.
The scarp forms the northern edge of a plateau (or ridge) which rises about above the surrounding plains of the Highveld. A number of picturesque Johannesburg suburbs, including Observatory, Linksfield Ridge and Upper Houghton are located along the scarp, overlooking the rest of northern Johannesburg with views up to the Magaliesberg (although locals refer to segments of the scarp using area-specific names, such as Linksfield Ridge, Parktown Ridge or Observatory Ridge). The entire plateau-like structure is also often called the Witwatersrand. The plateau's elevation above sea-level is between .
The Witwatersrand plateau forms a continental divide, with the run-off to the north draining into the Indian Ocean via the Crocodile and Limpopo rivers, while the run-off to the south drains via the Vaal into the Orange River and ultimately into the Atlantic Ocean.Encyclopædia Britannica (1975). Micropaedia, Vol. X. p. 720. Helen Hemingway Benton, Chicago.
Because of the extraordinary quantities of gold that have been extracted from the Witwatersrand rocks, the South African currency was named the rand in 1961 upon the declaration of the republic.
Witwatersrand and the Rand are names for the conurbation that developed along the range, although the terms are falling into disuse and Witwatersrand was the "W" in PWV (Pretoria-Witwatersrand-Vereeniging), the initial name of Gauteng province. In this context, it has lent its name to institutions including the University of the Witwatersrand (Wits University) and the defunct Rand Afrikaans University (RAU, now part of the University of Johannesburg), and to towns and regions such as the East Rand, West Rand and Randburg.
Geology
The Witwatersrand plateau consists of a layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2.97 billion years ago. The entire series of rocks, known as the "Witwatersrand Supergroup", consists of very hard erosion resistant quartzites, banded ironstones and some marine lava deposits, interspersed with softer, more easily eroded tillites, mudstones and conglomerates. The oldest rocks (laid down 2.97 billion years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2.71 billion years ago) are those that form the southern edge of the plateau.
Gold is found in the conglomerate strata of the younger members of the Supergroup, locally referred to as banket. The abundance of this gold is without a natural equal anywhere else in the world. Over have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 22% of all the gold that is accounted for today.
Not all the conglomerates contain gold, and of those that do (known as "reefs" by the miners), the gold is not uniformly distributed throughout the layer, but tends to occur in streaks, where the pebbles that make up the conglomerate are larger than elsewhere. Here the gold is associated with other minerals, especially iron pyrite and uraninite, as well as carbon rich materials such as kerogen, or bitumen, which occurs in small balls less than in size, called "flyspeck carbon", or as continuous layers about thick. The gold-bearing conglomerates occur chiefly in the upper, younger layers of the Witwatersrand Supergroup of rocks, on the southern side of the Witwatersrand plateau.
The Witwatersrand Supergroup strata which reach the surface in Johannesburg dip downwards to the south at an angle of about 30°. From there on they are almost everywhere, with very few exceptions (see below), covered by younger rocks.Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa. Gold mining in these buried portions of the Witwatersrand Supergroup is sometimes carried out at depths of below the surface.
Witwatersrand Basin
The Witwatersrand Basin is a largely underground geological formation which surfaces in the Witwatersrand. It holds the world's largest known gold reserves and has produced over , which represents about 22% of all the gold accounted for above the surface. The basin straddles the old provinces of Transvaal and the Orange Free State, and consists of a 5000–7000 m thick layer of Archean, mainly sedimentary rocks laid down over a period of about 260 million years, starting about 3 billion years ago. The entire series of rocks, known as the "Witwatersrand Supergroup" consists of quartzites, banded ironstones, mudstones, tillites, conglomerates and some marine lava deposits. Most of the basin is deeply buried under younger rocks, but outcrops occur in Gauteng, the Free State, as well as in some of the surrounding Provinces. The outcrop in Gauteng forms the Witwatersrand ridge, from which the basin and its rocks derive their name. It was on the southern portion of this ridge that gold was first discovered on the farm Langlaagte in 1886, 5 km west of Johannesburg. Since this gold was embedded in a conglomerate, it was first assumed that this was alluvial gold in an old riverbed, that had been tilted as a result of earth movements. However, when it was found that, traced downdip, the conglomerate was not merely developed for the narrow width of a river, but continued in depth, there came the realisation that this conglomeratic zone was part of a sedimentary succession. The conglomerate was quickly traced east and westward for a total continuous distance of to define what became known as the "Central Rand Gold Field".
It has since been established that the rocks that make up the Witwatersrand Ridge dip downwards and southwards to form the largely underground "Witwatersrand Basin" which covers an elliptical area with a long major axis from Evander in the north-east to Theunissen in the south-west, and wide stretching from Steynsrus in the south-east to Coligny in the north-west, with a small subsidiary basin at Kinross. Gold occurs only along the northern and western margins of this basin, but not in a continuous band. The gold-bearing rocks are limited to six sites where Archean rivers from the north and west formed fan deltas, with many braided channels, before flowing into the "Witwatersrand Sea" to the south, where the earlier sediments that form the older rocks of the Witwatersrand Supergroup had been deposited. Some of these gold bearing fan deltas are now at depths of below the surface. Although many of the older mines, around Johannesburg, are now nearly exhausted, the Witwatersrand Basin still produces most of South Africa's gold and much of the total world output. Silver, uranium, and iridium are recovered as gold-refining by-products.
Geological origin The Witwatersrand basin was created during the Archean Eon, and is therefore amongst the oldest geological structures on Earth. It was laid down in two stages, over the course of 260 million years starting just short of 3 billion years ago. The first phase, lasting 60 million years, consisted of sedimentary deposits in a shallow sea, conveniently named the "Witwatersrand Sea". The resulting 2500–4500 m thick layer of sediments is termed the "West Rand Group" of Witwatersrand rocks. The second phase, which lasted for 200 million years, followed on from the first phase, with on-land deposits, resulting from the retreat of the Witwatersrand Sea, leaving a wide almost flat coastal plain over which rivers from the north formed wide braided river deltas, into some of which rich deposits of gold were deposited. The resulting 2500 m thick layer of rock is termed the "Central Rand Group". The "West Rand Group" and "Central Rand Group" of rocks together form the "Witwatersrand Supergroup", the full horizontal extent of which is termed the Witwatersrand Basin.
There were no continents during the early stages of the Archean Eon,Stanley, S.M. (1999). Earth System History. pp. 297–301. W.H. Freeman and Company, New York. . but island arcs did form. It was the coalescence of several of these island arcs that led to the formation of the Kaapvaal craton, one of the first microcontinents to form on Earth about 3.9 billion years ago. Its size and position relative to Southern Africa today are indicated in the diagram on the left. About 3 billion years ago local cooling of the underlying asthenosphere caused subsidence of the south eastern portion of this microcontinent below sea level. The floor of this newly formed "Witwatersrand sea" consisted of smoothly eroded granites. Sandy sediments brought in by rivers from the north started being deposited on the granite about 2.97 billion years ago. This sandy layer eventually became compressed to form the Orange Grove Quartzite, the lowermost layer of the Witwatersrand Supergroup. This quartzite layer can be seen lying on its granite base in Johannesburg, where it forms a 56 km long east–west ridge over which several rivers running to the north form waterfalls, giving rise to the name Witwatersrand, which in Afrikaans means "Ridge of White Waters".
There was no free oxygen in Earth's atmosphere until about 2 billion years ago, before the appearance and sufficient development of photosynthesizing cyanobacteria. The oxygen produced by these microorganisms rapidly reacted with, amongst others, Fe2+ ions dissolved in water, precipitating insoluble red iron oxide (hematite). For various reasons, the sedimentation of the fine grain mud was affected by cyclic episodes with more or less precipitation of iron oxides. The result was alternating red and beige layers of mud which, when consolidated, became banded ironstones.
As the sea deepened, finer grained and muddy sediments accumulated. Changing geographical conditions resulted in the accumulation of a wide variety of sediments, ranging from mud, to sand, to gravel, and banded ironstones. Tillite deposits, dating from 2.95 billion years ago, are indicative of the first glaciation episodes on earth.Tankard, A.J., Jackson, M.P.A, Erikson, K.A., Hobday, D.K., Hunter, D.R., Minter, W.E.L. (1982). Crustal Evolution of Southern Africa. 3.8 Billion Years of Earth History.pp. 118–139. Springer-Verlag, New York. Within 60 million years, up to 4500 m of sediment had accumulated on the granite base, to become the "West Rand Group" of rocks that contribute over 60% of the total thickness of the Witwatersrand Supergroup.