Melville Koppies Nature Reserve
Melville Koppies Geology
Melville Koppies lies on the Kaapvaal craton, one of the earliest known pieces of the Earth's crust.
It took about a billion years for the molten planet to begin to form chunks of stable crust, which we call cratons, and which form the nuclei of later tectonic plates and continents.
At 3.5 billion years old the Kaapvaal Craton was one of the earliest. Roughly 1.2 million square kilometers in area, it underlies much of Southern Africa as we know it today, stretching from the Barberton mountain land in the East to beyond Gaberone in the West; from the Limpopo in the North to the Free State in the South. It is probably related to the Pilbara Craton, on the west coast of Australia, for the two areas were one at the time. The oldest rocks are exposed near Barberton, and are only rivalled by rocks of a similar age in Greenland.
Very ancient granitic rocks of the craton underlie northern Johannesburg, and are exposed as far as Midrand. Examples can be seen in the Montgomery Spruit and the Braamfontein Spruit. Here and there are remains of even older greenstone, visible in Linden, at Emma Park, for example.
By about 3.1 billion years ago the craton had sagged, forming a basin to the south of what we think of as the Witwatersrand. This basin filled with water. To the west, north, and east of the basin a range of mountains sent sediment in vast quantities into this inland sea.
The only form of life then on earth was prokatyotic bacteria, the rain was more like acid rain than the rain we know today, and so there was little to prevent the eroding mountain ranges from sending torrents of gravel, sand, and mud down to this sea. In a wide arc extending from today's Leander in the east, through the Witwatersrand, and south west to Welkom, the remains of the ancient sea shores are found. Over a period of three hundred million years or more, the mountains weathered into huge river deltas, and coastal sediments built up to a depth of about seven kilometres. These sediments were layered and differed depending on the circumstances of their deposition - layers of coarse pebbles deposited upstream, slightly less coarse sand forming beaches, and finally silt, deposited in deep and calm waters.
These sediments lithified (became stone) in the form of conglomerate, quartzite, shale, and siltstone. The Melville Koppies lies at the base of these sediments. In other words it represents the first sea shores and shallow beds of the ancient sea. It also forms part of the lowest level of one of the world's most well known geological features, the Witwatersrand Supergroup. Several fairly narrow layers of gravel, deposited quite late in the sequence, and bearing heavy elements, made the Witwatersrand Supergroup famous. These are the gold-bearing conglomerates of the main reefs.
A key to understanding the shape of these deposits today is that through tectonic action they no longer take the form of sprawling horizontal beds, but, as layers of rock, have been tipped southwards at a steep angle. They jut out of the present surface as long ridges, and plunge below the surface at an angle of about 70° to the horizontal. A dramatic view of this tipping effect can be seen when you drive in from OR Tambo airport, and first see the Linksfield Ridge. This tipping action was probably caused by the weight of water to the south, which caused the basin to sag and drag the edges downwards, as well as some other possible factors like the outpouring of the Ventersdorp lavas about 2.7 billion years ago (an event which ended the period of deposition), and the Vredefort asteroid impact, about 2.023 billion years ago.
The Melville Koppies Nature Reserve lies astride the geological "unconformity" - the place where the sediments of the Witwatersrand Supergroup lie upon the basement rocks. Thus the northern slopes have a thick reddish topsoil, the result of the decomposition of the mineral-rich granodiorite basement, while the ridges and southern part of the reserve consist of the quartzites and shales of the Supergroup. These rocks decompose to a thin, acidic soil, and this is reflected by the sparse grasses, proteas and other hardy shrubs which grow there.
In the quartzite there are many signs of its sedimentary origin - ripple marked rocks and cross bedding.
Ripple marks probably reflect the marks of the "last wave up the beach" which were then preserved by a sudden event like a fall of fine volcanic ash, or the dust of a sandstorm. This thin layer, later lost through erosion, nevertheless created a plane of weakness where the rock would later break, revealing the original marks in the sand.
Cross bedding is evidence of a long period of stable sedimentation followed by a violent event like a flash flood or a tsunami which caused large quantities of sediment to pour over the stable bed and settle at a different angle. In the picture on the right the (approximately) horizontal layers of the stable bed can be seen, while the cross layers go from top left to bottom right.
The shale and siltstone layers within the quartzite are softer, erode more easily, and form the shallow valleys between the quartzite ridges.
In one place on the Koppies you can see where the lowest level of the Orange Grove Quartzite lies upon the granitoid basement. This is the "unconformity". The basement rocks are far older than the quartzite - by about four hundred million years - and whatever geological processes happened in that length of time - presumably a long period of erosion - are irretrievably lost. The place where the two rock types meet shows that a period of violent tectonic action took place at some later time, thrusting the quartzite against the basement, shattering the granite. The result is a layer, about 10cm wide, of crushed granite, technically called schist. This phenomenon is only to be seen at two other sites in Johannesburg: others are unknown, lost below buildings.
Another feature of the Koppies is the abundance of quartz veins. Quartz was formed in the deep crust by hydrothermal action - the violent explosion of liquid trapped in deep layers, under conditions of great heat and pressure, leaching silicon dioxide from the silica rich quartzite. The veins vary from tiny seams only millimetres thick, to layers nearly a meter wide. Tectonic forces over the millions of years have largely shattered the quartz veins, meaning that the Koppies are covered with small milky white pebbles.
Finally, there is a completely different formation in the stream bed. Here, in the fault through which Beyers Naude Drive now runs, there is an intrusion of igneous rock - Gabbro or Dolerite (the difference is one of grain size, not chemical composition) which thrust up into the crust and cooled far below the surface about two billion years ago. It is probably associated with the huge volcanic activity which created the Bushveld Igneous Complex.
Thus the Melville Koppies represents in microcosm most of the features of the Witwatersrand Supergroup. What it does not have is gold-bearing rock. This occurs millions of years, and several kilometres, higher up in the sequence. The early owners, the Geldenhuys family, discovered this, to their regret.