Cape Fold Belt

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.
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.

The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa.[1] It was originally continuous with the Ventana Mountains near Bahía Blanca in Argentina and other fold and thrust belts in Antarctica and eastern Australia. The rocks involved are generally sandstones and shales, with the shales (Bokkeveld group) persisting in the valley floors and the erosion resistant sandstones forming the parallel ranges, the Cape Fold Mountains, which reach a maximum height of 2325 m at Seweweekspoortpiek (‘Seven Weeks Defile Peak’ in Afrikaans).

The Cape Fold Mountains form a series of parallel ranges that run along the south-western and southern coastlines of South Africa for 1000 km from the Cederberg 300 km to the north of the Cape Peninsula, and then along the south coast as far as Port Elizabeth, 700 km to the east (see the maps on the right and left).

Geological origin

Klein Swartberg Range, seen from the Laingsburg area
Folded rock formations of the Swartberg
Cape fold belt in the Keerom Koo area, near Robertson.
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,[2] 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.

The rocks were laid down as sediments in a rift valley that developed in southern Gondwana, just south of Southern Africa, during the Cambrian-Ordovician Periods (starting about 510 million years ago, and ending about 330-350 million years ago).[3][4][5] (See the Earth's geological timeline diagram at the end of the article, bottom right.) An 8 km thick layer of sediment, known as the Cape Supergroup (see below), accumulated on the floor of this rift valley.[4] 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 folded Cape Supergroup formed the northern foothills of this towering mountain range.

The weight of the Falkland-Cape Supergroup mountains caused the continental crust of Southern Africa to sag, forming a retroarc foreland system, into which the Karoo Supergroup was deposited.[1][4][6] Eventually much of the Cape Supergroup became buried under these Karoo deposits, only to re-emerge as mountains when upliftment of the subcontinent, about 180 million years ago, and again 20 million years ago, started an episode of continuous erosion that was to remove many kilometers of surface deposits from Southern Africa.[4] Although the tops of the original Cape Fold Mountains were eroded away, they eroded much slower than the considerably softer Karoo deposits to the north. Thus the Cape Fold Belt "erupted" from the eroding African landscape to form the parallel ranges of mountains that run for 800 km along the southern and south-western Cape coastline today. In fact, they form the coastline, either sloping steeply directly into the sea, or are separated from it by a relatively narrow coastal plain.

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 famous Table Mountain forms part of the Cape Fold Belt, being made up of the local lowest (oldest) strata of the Cape Supergroup, composed predominantly of quartzitic sandstone which forms the impressive, almost vertical cliffs which characterize the mountain and the rest of the range which constitutes the backbone of the Cape Peninsula.[3][7]

The degree to which the original Cape Fold mountains (formed during the Carboniferous and early Permian Periods) have been eroded is attested to by the fact that the 1 km high Table Mountain on the Cape Peninsula is a syncline mountain, meaning that it formed part of the bottom of a valley when the Cape Supergroup was initially folded. The anticline, or highest elevation of the fold between Table Mountain and the Hottentots-Holland Mountains (1.2 to 1.6 km elevation), on the opposite side of the isthmus connecting the Peninsula to the Mainland, has been eroded away. (See the diagram below, on the left.) The Malmesbury shale and granite basement on which this anticline mountain rested also formed an anticline; but being composed of much softer rocks, readily weathered into a 50 km wide sandy plain, called the "Cape Flats".[3]

The Cape Fold Belt extends from Cape Town in the southwest, to the Cederberg Mountains in the north, as well as to Port Elizabeth in the east.

The mountains, although only of moderate height, are majestic and dramatic. This is due in part to numerous geological factors; The ranges usually have few to no foothills and rise directly from valley floors. The bases of the mountains are usually at or near sea level.[8]

Cape Supergroup

The mountains of the Cape Fold Belt are composed of rocks belonging to the Cape Supergroup, which is more extensive than the Fold Belt. The Supergroup is divided into several distinct Groups.

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). However, the entire suite in this region slopes downwards towards the north and east, so that the oldest rocks are exposed in the south and west, while the youngest members of the Supergroup are exposed in the north, where the entire Cape Supergroup dives beneath the Karoo rocks. Drilling in the Karoo has established that Cape Supergroup rocks are found below the surface up to approximately 150 km north of their northernmost exposure on the surface.[5]

The Cape Supergroup extends eastwards beyond the Fold Belt into the northern Eastern Cape and KwaZulu-Natal, where no folding took place.[5]

Klipheuwel and Natal Groups

The initial sedimentation into the rift valley which developed in southern Gondwana (see diagram at top left) was confined to the western and eastern ends of the rift. Rivers diverted into these early rifts deposited sand and gravel to form the Klipheuwel Group in the west, and the Natal Group in the east.[4][5] These formations contain no fossils. Today the Klipheuwel Group is exposed in several small patches near Lamberts Bay, Piketberg, and to the south-west of Paarl Rock.[9]

The Natal Group, which is similar to the Table Mountain Group (see below), is found in several elongated patches near the coast of the northern Eastern Cape and KwaZulu-Natal. 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.[4] Most of the group has been deeply eroded by numerous rivers creating the ragged plateaus and scarps of the Durban-Pietermaritzburg region.[5][10]

Table Mountain Group

An west-east (left to right) geological cross section through Table Mountain on the Cape Peninsula, the Cape Flats (the isthmus connecting the Peninsula to the African mainland) and the Hottentots-Holland Mountains on the mainland, indicating how much of the Cape Fold Mountains has been eroded away.[3]

With the widening and deepening of the rift valley, the entire southern portion of what was to become South Africa, as far as a line extending from Calvinia in the west to East London in the east, plus the KwaZulu-Natal coastline, became flooded,[5] leading to a seaway across the southern and eastern parts of the country, called the Agulhas Sea.[4] The sea floor consisted of the Klipheuwel Group in the west, the Natal Group in the east, and eroded pre-Cambrian rocks in between.[4]

The initial sediments into the still shallow sea were alternating layers of maroon colored mudstones and buff colored sandstones, each mostly between 10 and 30 cm thick.[3] 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.[3] This layer, known as the Graafwater Formation, reaches a maximum thickness of 400 m,[10] but on the Cape Peninsula it is only 60–70 m thick.[3] No fossils have been found in the Graafwater rocks, but shallow-water animal tracks have been found.[4][10] 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.[5]

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 photograph was taken at an elevation of 250 m.
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.

The cutting for Chapman’s Peak Drive, on the Cape Peninsula, is carved into the Graafwater Formation which overlays the Cape Granite basement rock below the road. 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. In the cutting one can also see the abrupt and obvious transition into the Peninsula Formation above it. Looking up the slope from below to the first hairpin bend, the granite basement on which the Graafwater formation rests is visible. And in the cutting at the first hairpin bend, the ocher-colored, gritty clay into which the granite weathers is clearly displayed.

With further subsidence of the rift valley floor, the sediments became more sandy. A deposit, known as the Peninsula Formation (also often referred to as Table Mountain Sandstone), consisting of thickly layered quartzitic sandstone, with a maximum thickness of 2000 m, was laid down. These sandstones are very hard, and erosion resistant. They therefore form the bulk of mountains and steep cliffs and rugged crags of the Cape Fold Belt, including the upper 600 m of the 1 km high Table Mountain, below which Cape Town is situated. It contains no fossils.

During the laying down of the Peninsula Formation sediments, the western portion of the region was covered for a brief period of time by glaciers. The diamictite sediments that were laid down by these glaciers, or in glacial lakes, are known as the Pakhuis and Cederberg Formations.[4] A small patch of Pakhuis tillite occurs on the top of Table Mountain at Maclear’s beacon,[3] but most of the Pakhuis and Cederberg Formations are found as a thin layer (on average only about 60 m thick[5]) in the Peninsula Formation rocks of the more inland mountains to the west of a line between Swellendam and Calvinia.[5] These diamictite rocks are composed of finely ground mud, containing a jumble of faceted pebbles. 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.[5] In several locations the quartzites below the glacial horizon have been rucked into a series of folds. This believed to have been caused by the movement of ice ploughing into the underlying unconsolidated sands.[5] A good example of this can be seen on a ridge of rocks near Maclear’s Beacon on Table Mountain, close to the edge of the plateau overlooking the Cape Town City Bowl and Table Bay.

The Pakhuis Formation is also well exposed on the road along Michell’s Pass just below the Tolhuis,[3] and especially on the Pakhuis Pass near Clanwilliam, from which the formation derives its name.[5]

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.[10]

Bokkeveld Group

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.

About 400 million years ago (in the early Devonian Period) there was further subsidence of the rift valley floor. This brought about the deposition of deeper-water, fine grained sediments of the Bokkeveld Group. This is in marked contrast to the predominantly sandy sediments of the Table Mountain Group. The Bokkeveld Group consists predominantly mudstones.[4]

After the Cape Supergroup had been folded into the Cape Fold Mountains, these soft mudstones readily washed away from the mountain tops, and only remain in the valleys. Here they form the fertile soils on which the vines and fruit orchards of the Western Cape flourish with the help of irrigation from the rivers that have their sources in the surrounding mountains.[4]

The Bokkeveld Group does not extend on to the Cape Peninsula or its isthmus (the Cape Flats). Here the Stellenbosch, Franschhoek, Paarl, Durbanville, Tulbagh and Constantia vineyards have been planted on the weathered Cape Granite and Malmesbury shale soils, which form the basement rocks on which the Cape Supergroup rocks in this region rest.

The Bokkeveld Group extends eastwards to Port Alfred (near Grahamstown), approximately 120 km beyond the eastern extent of the Cape Fold Belt.[9]

The bulk of the fossils found in the Cape Supergroup occur in the Bokkeveld mudstones. They include a variety of brachiopods, as well as trilobites, molluscs, echinoderms (including starfish, crinoids, and the extinct blastoids and cystoids), foraminifera and fish with jaws (placoderms).[4][5][10]

Witteberg Group

The upper layers of the Bokkeveld Group become increasingly more sandy, grading into the sandstone of the Witteberg Group, named for the range of mountains to the south of Matjiesfontein and Laingsburg in the southern Karoo. 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.[4] The group contains fewer fossils than the Bokkeveld Group, but the assemblage that is preserved includes primitive fish, an extinct species of shark, brachiopods, bivalves, and a meter long sea scorpion. There are also plant fossils and numerous animal tracks.[4][5] The Witteberg Group is truncated by the overlying Dwyka sediments of glacial origin. The latter forms part of the Karoo Supergroup. Therefore, the Witteberg Group forms the uppermost layer of the Cape Supergroup. It tends to form the most inland outcrops of the Cape Supergroup, and can be traced eastwards as far as the Bokkeveld Group can be traced (i.e. to Port Alfred), about 120 km beyond the Cape Fold Belt.[5][9]

The Bokkeveld and Witteberg groups do not occur in the northern Eastern Cape and KwaZulu-Natal, where the Cape Supergroup is represent only by the Natal Group and a trace of the Peninsula Formation (without the intervening Graafwater Formation).[9]

Appearance

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 upliftment along this fault line. The subsurface structures are not to scale.

The mountains are not particularly ancient, despite their old-looking appearance. They are considered middle-aged in geologic terms. 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),.[3][4][5] Their stature, with heights varying from 1000m to 2300m, is mostly due to the weather-resistant rocks of quartzitic sandstone of the Table Mountain Group (see above).

They occur in long parallel ranges each not much more than 10 km wide, separated by equally long valleys with a maximum width of about 50 km (mostly only 15–30 km wide). Almost all of these ranges consist of Table Mountain Group rocks. The valleys tend to be floored by Bokkeveld Group mudstones. 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 right above). Their origin is as follows:

After the fold mountains had been formed they became buried under sediments derived from the massive Himalaya-sized Falkland Mountains to the south of the Cape Fold Belt.[4][11] Sediments eroded from these mountains provided the bulk of the 6 km thick Beaufort sediments in the Karoo Basin,[4] but they also covered the Cape Fold Belt, thereby protecting them from erosion. 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.[4] 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. Continued erosion exposed more and more of these quartzitic mountain ranges, but the rivers, now confined to narrow, fast flowing gorges, continued breaking through each barrier as the surrounding landscape eroded to lower and lower levels, particularly during the past 20 million years.[4]

These 150‑million-year-old rivers therefore cut the defile, starting by flowing over, and then through the gradually erupting Cape Fold Mountains, to form the spectacular “poorte” and “klowe” (plural of “poort” and “kloof”, the Afrikaans for defile or chasm) that characterize these mountains today. The best known defiles are: Meiringspoort, Seweweekspoort, the Tradouw pass, Kogmanskloof, Garcia’s pass, the Gourits River gorge (which does not have a road running through it), and Michell's Pass, which is, however, more V-shaped than the others, but impressive nevertheless. In addition several roads and highways crest the mountains over cols, or go through them via tunnels.

A number of parallel faults still run roughly parallel with the coast, having formed during the Gondwana rifting when South America and the Falkland Plateau separated from Africa. Patagonia was to the west of Cape Town and the Falkland Islands were to the south during the Jurassic Period prior to separation. Most of these faults are inactive today, but in 1969 the towns of Ceres and Tulbagh, about 160 km north-east of Cape Town, were severely damaged by an earthquake, originating from movement of the northern end of the Worcester Fault.[4][5]

Another major (inactive) fault line runs for 300 km along the southern edge of the Swartberg Mountains. 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. These are locally known as the "Cango Group", but are probably continuous with the "Malmesbury Group" that forms the base of Table Mountain on the Cape Peninsula, and similar outcrops in the Western Cape.[4] In the Little Karoo the outcrop is composed of limestone, into which an underground stream has carved the impressively extensive Cango Caves.[9][11][12][13]

The ranges

The following is a list of the largest individual ranges within the Cape Fold Belt include from west to east. (Translations in brackets; berg is Afrikaans for mountain; its plural is berge.)

See also

References

  1. 1 2 Shone R.W. & Booth P.W.K. (2005). "The Cape Basin, South Africa: A review". Journal of African Earth Sciences 43 (1-3): 196–210. doi:10.1016/j.jafrearsci.2005.07.013.
  2. ↑ Jackson, A.A., Stone, P. (2008). "Bedrock Geology UK South". p. 6-7. Keyworth, Nottingham: British Geological Survey.
  3. 1 2 3 4 5 6 7 8 9 10 Compton, J.S. (2004).The Rocks and Mountains of Cape Town. p. 24-26, 44-70. Double Storey Books, Cape Town.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 McCarthy, T., Rubridge, B. (2005). The Story of Earth and Life. pp. 159-161, 182, 187-195, 202-207, 267-269, 302. Struik Publishers, Cape Town
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Truswell, J.F. (1977). The Geological Evolution of South Africa. pp. 93-96, 114-159. Purnell, Cape Town.
  6. ↑ Catuneanu O. (2004). "Retroarc foreland systems––evolution through time" (PDF). Journal of African Earth Sciences 38 (3): 225–242. doi:10.1016/j.jafrearsci.2004.01.004.
  7. ↑ Geology of the Cape Peninsula - Cape Fold Belt
  8. ↑ The Cape Fold Belt
  9. 1 2 3 4 5 Geological Map of South Africa, Lesotho and Swaziland. (1970). Council for Geoscience, Geological Survey of South Africa.
  10. 1 2 3 4 5 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. 333-363. Springer-Verlag. New York.
  11. 1 2 Norman, N., Whitfield, G. (2006). Geological Journeys. pp. 300-311. Struik Publishers, Cape Town.
  12. ↑ Reader’s Digest Illustrated Guide to Southern Africa. (5th Ed. 1993). pp. 78-89. Reader’s Digest Association of South Africa Pty. Ltd., Cape Town.
  13. ↑ Bulpin, T.V. (1992). Discovering Southern Africa. pp. 271-274, 301-314. Discovering Southern Africa Productions, Muizenberg.

Coordinates: 33°24′S 22°00′E / 33.4°S 22°E / -33.4; 22

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