Cape Fold Belt

[6] Closure of the rift valley, starting 330 million years ago, resulted from the development of a subduction zone along the southern margin of Gondwana, and the consequent drift of the Falkland Plateau back towards Africa, during the Carboniferous and early Permian periods.

After closure of the rift valley, and rucking of the Cape Supergroup into a series of parallel folds, running mainly east-west (with a short section running north-south in the west, due to collision with eastward moving Patagonia), the continued subduction of the paleo-Pacific Plate beneath the Falkland Plateau and the resulting collision of the latter with Southern Africa, raised a mountain range of immense proportions to the south of the former rift valley.

The Falkland Mountain range had probably eroded into relative insignificance by the mid-Jurassic Period, and started drifting to the south-west soon after Gondwana began to break up 150 million years ago, leaving the Cape Fold Belt to edge the southern portion of the newly formed African continent.

Even though the mountains are very old by Andean and Alpine standards, they remain steep and rugged due to their quartzitic sandstone geology (see below) making them very resistant to weathering.

The western and southern extents of the Supergroup have been folded into a series of longitudinal mountain ranges, by the collision of the Falkland Plateau into what would later become South Africa (see diagrams on the left).

Today the Klipheuwel Group is exposed in several small patches near Lamberts Bay, Piketberg, and to the south-west of Paarl Rock.

It forms the impressive cliffs of Oribi Gorge, and can also be seen in a road cutting between Durban and Pietermaritzburg, particularly at the Marian Hill Toll Plaza.

[6] The first sediments into the initially still shallow, possibly inland, sea were alternating layers of maroon-colored mudstones and buff-colored sandstones, each mostly between 10 and 30 cm thick.

[5] The mudstone units commonly display ripple marks from the ebb and flow of tidal currents, as well as polygonal sand-filled mud cracks that indicate occasional exposure to desiccation.

[6][12] A particularly good example of these tracks can be viewed in the foyer of Geology Department of the University of Stellenbosch, where a slab of Graafwater rock from the Cederberg mountains has been built into the wall.

The Graafwater Formation can also be clearly seen in the cutting on the second hairpin bend as the Ou Kaapse Weg (road) goes up the slope from Westlake on to the Silvermine plateau.

With further subsidence of the rift valley floor, and possibly breaking through to the ocean, the sediments abruptly become more sandy, indicative of a sudden increase in the depth of the Agulhas Sea (see photograph on the right).

They can easily be recognized at a distance as this formation readily erodes into fertile, gently sloping, green swaths in a landscape where this contrasts starkly with the bare rocky surfaces of the quartzites above and below.

[7] The glaciers which formed the Pakhuis and Cederberg formations came from the north west, in the direction of the South Pole which was located in the neighborhood of Cameroon at the time.

[7][14] In the Cederberg this formation has been eroded by the wind into a wide variety of "sculptures", caves, and other fascinating structures for which these mountains have become well-known.

Here they form the fertile soils on which the vineyards and fruit orchards of the Western Cape flourish with the help of irrigation from the rivers that have their sources in the surrounding mountains.

[citation needed] The Bokkeveld Group extends eastwards to Port Alfred (near Grahamstown), approximately 120 km beyond the eastern extent of the Cape Fold Belt.

These rocks were laid down 370 – 330 million years ago in the silted up, and therefore shallow marine conditions of what remained of the Agulhas Sea.

[11] The Witteberg sediments were laid down in what remained of the Agulhas Sea – a shallow, much reduced expanse of water compared to its size during the Bokkeveld period.

They were created when the Falkland Plateau collided with Southern Africa, when Pangaea, the supercontinent formed during the Cambrian-Ordovician periods (from 510 to about 330–350 million years ago),.

A remarkable feature of these ranges is that the 1500 m high mountains (from base to crest) are cut through by very narrow, almost vertically walled defiles, not much more than 50–70 m wide at the bottom, through which rivers flow from the inland Great Escarpment to the sea.

It is from within these narrow defiles, many of which can be traveled by road, that one is afforded a cross sectional view of the mountains, and can appreciate their intense folding and distortion (see the photographs on the top right above).

At the end of the Karoo Period about 180 million years ago, the subcontinent was covered by a thick layer of Drakensberg lavas, an event that was accompanied by upliftment or bulging of Southern Africa, ushering in an almost uninterrupted period, continuing to the present, of erosion removing many kilometers of surface rocks from the entire subcontinent.

[6] Rivers running off this bulging interior into the seas that were forming around South Africa as Gondwana was breaking up 150 million years ago, eventually encountered rocky ridges as the protective layer over the Cape Fold Belt eroded away, exposing their mountain tops.

The rivers breached these ridges, after possibly being dammed back for a short period, creating a narrow passage through the low rocky barrier.

The Swartberg Mountains were uplifted along this fault, to such an extent that in the Oudtshoorn region the rocks that form the base of the Cape Supergroup are exposed.

Map of the Western Cape , showing the main Cape Fold Mountain ranges. The Cape Fold belt is not part of the Great Escarpment shown in blue: the Roggeveld, Nuweveld and Sneeuberg "mountains". They are geographically and geologically distinct from the Cape Fold Mountains. The remaining named mountain ranges, to the south and southwest of the Escarpment, are all part of the Cape Fold Belt, which extends to the east as far as Port Elizabeth , about 150 km beyond the right-hand edge of the map.
A map of South Africa showing the central plateau edged by the Great Escarpment and its relationship to the Cape Fold Mountains in the south. The portion of the Great Escarpment shown in red is known as the Drakensberg .
A timeline of the earth's geological history, with an emphasis on events in Southern Africa. The yellow block labeled C indicates when the Cape Supergroup was deposited, in relation to the Karoo Supergroup , K , immediately after it. The W indicates when the Witwatersrand Supergroup was laid down, very much further in the past. The graph also indicates the period during which banded ironstone formations were formed on Earth, indicative of an oxygen-free atmosphere . The Earth's crust was wholly or partially molten during the Hadean Eon; the oldest rocks on Earth are therefore less than 4000 million years old. One of the first microcontinents to form was the Kaapvaal Craton , which forms the foundation of the north-eastern part of the country. The assembly and break-up of Gondwana are, in terms of the Earth's and South Africa's geological history, relatively recent events.
Klein Swartberg Range, seen from the Laingsburg area
Folded rock formations of the Swartberg
Southern Gondwana during the Cambrian - Ordovician Periods. Today's continents into which this Supercontinent eventually broke up, are indicated in brown. A rift developed about 510 million years ago, separating Southern Africa from the Falkland Plateau. Flooding of the rift formed the Agulhas Sea. The sediments which accumulated in this shallow sea consolidated to form the Cape Supergroup of rocks, which form the Cape Fold Belt today. This portion of Gondwana was probably located on the opposite side of the South Pole from Africa's present position, [ 4 ] but compass bearings are nevertheless given as if Africa was in its present position.
A north-south cross-section through the Agulhas Sea (see above). The brown structures are continental plates, the thick black layer on the left is paleo-Pacific Oceanic plate, red indicates the upper mantle , and blue indicates flooded areas or ocean. The top illustration depicts the geology about 510 million years ago, with the sediments which would eventually form the Cape Supergroup settling in the Agulhas Sea. The middle illustration depicts the Falkland Plateau drifting northwards once again to close the Agulhas Sea, causing the Cape Supergroup to be rucked into a series of folds, running predominantly east-west. The lowest illustration shows how subduction of the paleo-Pacific Oceanic plate under the Falkland Plateau, during the Early Permian period, raised a massive range of mountains. These eventually eroded into the Karoo Sea, forming the Karoo Supergroup . Ultimately, the Falkland Mountains eroded almost completely away, but the Cape Fold Mountains had, by this time, become buried under the Karoo sediments. Being composed largely of quartzitic sandstone , they resisted subsequent erosion, when continental uplifting caused several kilometers of Southern Africa's surface to be planed away, and thus persist to this day as the Cape Fold Belt. The remnant of the Falkland Plateau broke away from Africa, and drifted south-westwards to its present position in the western South Atlantic Ocean, following the breakup of Gondwana about 150 million years ago.
A view of the eastern crags (or cliffs) of Table Mountain and Devils Peak as seen from Newlands Forest, above the University of Cape Town. The two layers of the Table Mountain Group which are represented on the Cape Peninsula, namely the Graafwater Formation and the Table Mountain Sandstone or Peninsula Formation, can readily be distinguished. The remaining layer, the Pakhuis Formation, occurs as a tiny remnant on the top of Table Mountain, not visible in the photograph. The granite basement on which the Table Mountain Group rests on the Cape Peninsula is obscured by the forest in the foreground.
A view of the cutting at the second hairpin bend on Ou Kaapse Weg , as it winds on to the Silvermine Plateau. The transition between the lowermost layer of the Table Mountain Group on the Cape Peninsula – the Graafwater Formation – and the layer above it, the Table Mountain Sandstone or Peninsula Formation, can be clearly seen. The very abrupt transition between them suggests that the Aghulas Sea was initially (during the Graafwater period) an area of inland drainage, possibly below sea level (comparable to the Dead Sea in the Middle East today). When the rift extended into the ocean, the rift valley suddenly flooded to become a deep water passage similar to the Red Sea today. The photograph was taken at an elevation of 250 m.
Schematic diagram of an approximate 100 km west-east (left to right) geological cross-section through the Cederberg portion of the Cape Fold Belt. The rocky layers (in different colors) belong to the Cape Supergroup. Not to scale. The green layer is the Pakhuis diamictite formation which divides the Peninsula Formation Sandstone (or Table Mountain Sandstone ) into a lower and upper portion. It is the lower (older) portion that is particularly hard and erosion resistant, and, therefore, forms most of the highest and most conspicuous peaks in the Western Cape, as well as the steepest cliffs of the Cape Fold Mountains (including Table Mountain ). [ 13 ] The Upper Peninsula Formation, above the Pakhuis tillite layer, is considerably softer and more easily eroded than the lower Formation. In the Cederberg Mountains This formation has been sculpted by wind erosion into many fantastic shapes and caverns, for which these mountains have become famous. [ 14 ] The Graafwater Formation forms the lowermost layer of the Cape Supergroup in this region, but is, for simplicity, incorporated into the Table Mountain Sandstone Formation in this diagram.
Wolfberg Arch in the relatively easily eroded Nardouw Formation (or Upper Peninsula Formation) rocks of the Cederberg.
The Agulhas Bank is the broad, shallow part of the southern African continental shelf which extends up to 250 km (160 mi) south of Cape Agulhas before falling steeply to the abyssal plain . It represents the remains of the Falkland Plateau still attached to Southern Africa. The Agulhas Ridge (or Agulhas-Falkland Fracture Zone) which runs from the south-eastern edge of the Bank towards the south west for 1,200 kilometres (750 mi) across the South Atlantic, is the transform fault along which the remainder of the Falkland Plateau moved to the southern tip of South America, a short while after Gondwana broke up, 110 million years ago. [ 6 ]
An approximate SW-NE geological cross section through South Africa, with the Cape Peninsula (with Table Mountain ) on the far left, and north-eastern KwaZulu-Natal on the right. Diagrammatic and only roughly to scale to scale. The difference in both composition and structure of the Cape Fold Mountains and the Central Plateau surrounded by the Great Escarpment , in particular the Drakensberg , can clearly be seen.
A diagrammatic 400 km north-south cross-section through the southern portion of the country at approximately 21° 30' E (i.e. close to Calitzdorp in the Little Karoo), showing the relationship between the Cape Fold Mountains (and their geological structure) and the geology of the Little and Great Karoo , as well as the position of the Great Escarpment . The colour code for the geological layers is the same as those used in the diagram above. The heavy black line flanked by opposing arrows is the fault that runs for nearly 300 km along the southern edge of the Swartberg Mountains. The Swartberg Mountain range owes some of its great height to uplift along this fault. The subsurface structures are not to scale.