Article from the May 2003 issue of
Johannesburg Geology
By Ann Cripps
Recent OU course material has led me to look with new eyes at the geology of my hometown.
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Figure 1. Lonehill Koppie |
The story starts about 3.5 Ga in the basement with Archaean greenstones, basaltic komatiitic lavas, some with pillow structures, exposed round the edges of the Johannesburg Dome, a granitic intrusion lying between Johannesburg and Pretoria. This intrusion metamorphosed the pyroxenes, peridotites, dunites and harzburgites to amphibolites and serpentinites. The granodiorite and granite magmas were intruded in two phases at about 3.2 and 3 Ga respectively, and erosion has today revealed characteristic koppies, where differential weathering has created small hills of rounded, exfoliate rocks. I grew up near one, called Lonehill, and often climbed it. It is now a nature reserve surrounded by houses and shops.
The greenstone-granite basement stabilised by about 2.7 Ga to form the Kaapvaal Craton, one of the earliest continents, and basins evolved on it by mechanisms not yet clearly understood (Tinker et al, 2002). The largest (about 300 km x 150 km) seems to have been the Witwatersrand Basin, a NE to SW trending sea, sometimes open to the ocean in the south-east, with high granitic mountains to the north and west. Sediment washed down from these mountains in fastflowing braided-channel rivers, accumulated in an arc on the shoreline, the heaviest pebbles being dropped first, and the finest shales being carried out furthest. Sandy deltas built out into the sea and the sediments were reworked by currents during successive transgressions. Aeolian structures indicate that the area was probably near the equator at 2.7 Ga, though these overlie some of the earliest known glacially produced rocks, debris-flow diamictites, suggesting rapid climatic change (Tyson, 1986). There was also some interlayering of volcanic rocks indicating crustal instability (MacCrae, 1999).
The oldest sedimentary facies, known as the Witwatersrand Supergroup, is about 8 km thick, with gold-bearing pebble conglomerates in a sandy matrix in the upper units. Extensive research has of course been done by mining companies, and the highland areas and alluvial fans clearly identified. This concentration of gold particles by sedimentary processes appears to be unique. Most of the world’s gold deposits occur in quartz and carbonate veins in fault zones, i.e. lode deposits, possibly as a result of metamorphic hydrothermal activity (Gibson & Reimold, 2001). The ultimate origin of the gold is still debatable. Recent Re/Os isotope dating has shown it to be older than the surrounding sedimentary rocks, so confirming that it is detrital, and of mantle origin. (Kirk et al., 2002)
Another point of interest about the gold-bearing reefs is that a number of high-yielding seams occur in seams of kerogen or carbon, associated with periods of non-deposition during shoreline transgressions. Some researchers linked this to the high uranium values found, or to the reducing effect of the carbon on gold-bearing fluids, but carbon isotopic analysis indicated a biological origin for the carbon. Detailed investigations have suggested that the original organisms were tough and leathery, unlike algae, and probably lichenlike. Studies on modern lichens showed they are capable of accumulating inorganic materials, particularly radioactive and heavy metals which are deadly to higher plants. It is suggested that lichen-like structures, with spherical spores, grew in mats just below or above the water level, and when dislodged and degraded by bacteria produced amorphous carbon which, when buried, compacted, and geothermally heated, became black kerogen. The age of this material – 2.9 to 2.7 Ga - suggests far more complicated and differentiated early forms of life than has been believed possible, and that biological organisms played a decisive role in concentrating the gold and uranium particles (MacCrae, 1999).
The Witwatersrand Supergroup facies is capped by the Ventersdorp Supergroup, an outpouring of lava 1.6 km thick at 2.3 Ga which led to extensive faulting and folding on the northern edge of the Witwatersrand Basin. There was then a marine incursion when carbonate rocks were laid down starting about 2.2 Ga – one of the earliest carbonate deposits on Earth, contributing to the evolution of the carbon cycle – and these became dolomitised to the west of Johannesburg. The caves in this area now contain some of the earliest hominid fossils and have been proclaimed a World Heritage Site called the Cradle of Mankind.
Geological activity in southern Africa centred on different areas after this, though iron- and magnesium-rich dykes belonging to the 1.1 – 1.4 Ga dyke swarm from Pilanesberg north-west of Johannesburg did reach the city and can be traced for about 20 km in the 3.2 Ga granitic rocks of the Johannesburg Dome. There are also tillites from Gondwana’s Great Ice Age about 320 to 270 Ma ago.
The Witwatersrand itself is a prominent highly-resistant quartzite ridge running east to west. The gold mines lie to the south of it, and have produced something like 40% of all the world’s gold in recorded history. The seams are thin, but reliable, and dip at between 30º and 45º to the south. Older works (Mendelsohn, 1986) attribute this dip to the weight of sediments in the Witwatersrand Basin to the south, but since 1996 it has emerged that at 2.02 Ga a meteorite about 10 km in diameter slammed into the Basin at Vredefort, about 120 km south-west of Johannesburg, and that the Witwatersrand is part of the northern rim of the impact crater, causing all the strata to dip inwards. This is believed to have preserved the gold-bearing layers from erosion to provide present-day South Africa with its economic foundation.
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Figure 2. Layered shale and quartzite in the Witwatersrand
Botanic Gardens |
The Ridge itself is a continental watershed. Streams flowing down the quartzite northern slopes, which are steeper, are clear and fast-flowing, and gave rise to its name – the Ridge of White Waters. They flow into the Crocodile River, and ultimately the Limpopo and the Indian Ocean. Those flowing down the southern slopes, which are the remains of the Ventersdorp Lavas, are sluggish and muddy, and flow into the Vaal (meaning dun or grey) River, and ultimately the Orange River and the Atlantic Ocean.
One of the curiosities of South African geology is that strata are about 500 m higher than their equivalents elsewhere – known as the Southern African Superswell. Seismic tomography has discovered an underlying large, hot, seismically slow region located just above the core-mantle boundary. One result is that, although not far south of the Tropic of Capricorn, at 1 500 m Johannesburg is more equable climatically than one would expect. The Ridge also claims one of the highest incidences of lightning strikes in the world, though I have never heard a geological explanation of that.
Johannesburg is the largest city in the world not on a major river or the sea, which leads to a certain lack of focus in layout. However, that layout still follows the structure dictated by the distribution of the gold deposits along the shoreline of the ancient Witwatersrand Basin.
By Ann Cripps
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Last modified on Monday, May 09, 2005 at 15:24
