Cape Fold Belt

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Map of the Western Cape, showing the main ranges mountains. The Roggeveld, Nuweveld and Sneeuberg "mountains", are part of the Great Escarpment which edges South Africa's central plateau. They do not form part of 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, not shown on the map.
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 450 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,[1] but compass bearings are nevertheless referred to as if Africa was in its present position.
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.
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 450 mya (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.
Klein Swartberg Range, seen from the Laingsburg area
Folded rock formations of the Swartberg
Cape fold belt in the Keerom Koo area, near Robertson.


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.[2] 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 rocks were laid down as sediments in a rift valley that developed in southern Gondwana, just south of Southern Africa, upon the Malmesbury unconformity during the Cambrian-Ordovician Periods (starting about 450 million years ago, and ending about 330-350 million years ago).[3][4] An 8 km thick layer of sediment, known as the Cape Supergroup, accumulated on the floor of this rift valley.[3] 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, 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.[2][3][5] Eventually much of the Cape Supergroup became buried under these Karoo deposits, only to re-emerge as mountains when uplifing 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.[3] 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 making them very resistant to weathering. The famous Table Mountain forms part of the Cape Fold Belt, being made up of the lower (oldest) strata of the Cape Supergroup, composed predominantly of quartzitic sandstone which form the impressive, almost vertical cliffs which characterize the mountain and the rest of the range which constitutes the backbone of the Cape Peninsula.[6][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. The Malmesbury and granitic 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".[6]

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 mountain's base is usually at or near sea level.[8]

Orogeny[edit]

The mountains are not particularly ancient, despite their old-looking appearance. They are considered middle-aged in geologic terms. They were created when the Falklands Plateau collided with Southern Africa, when Pangaea, the supercontinent formed from the late Carboniferous to early Permian periods. Their stature is mostly due to their weather-resistant rocks of quartzitic sandstone, with the softer shale formations (the Bokkeveld group), which originally overlay the lower sandstones, surviving only in the valleys.

The mountains were folded during this period when they were still buried beneath the Earth's surface. Due to the extreme temperatures and lithic pressure at these depths, folding was not a brittle process, as the rocks retained a "plastic" constituency, making the Cape Fold Mountains spectacular in their exposed folded, faulted and twisted sedimentary strata. A number of parallel faults still runs roughly parallel with the coast, having formed during the Gondwana rifting when South America 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. This faulting has caused the Table Mountain Sandstone to be exposed closer to and above sea level close to the coast, but gradually becoming deeper embedded in the overlying strata further inland, except in areas where folding has caused anticlines to expose the rocks higher up (e.g. Swartberg Mountains north of the Oudtshoorn Fault.

The "conservation" of the Cape Fold belt resulted from the Cape Supergroup of rocks being buried under the deposition of the Karoo Sequence of sedimentary deposits, including the Dwyka group, which is compacted glacial till deposited during the late Carboniferous southern ice age, when Gondwana had drifted over the South Pole.[9]

The ranges[edit]

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[edit]

References[edit]

  1. ^ Jackson, A.A., Stone, P. (2008). "Bedrock Geology UK South". p. 6-7. Keyworth, Nottingham: British Geological Survey.
  2. ^ a b 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. 
  3. ^ a b c d McCarthy, T., Rubridge, B. (2005). The Story of Earth and Life. pp. 161, 187-195. Struik Publishers, Cape Town
  4. ^ Truswell, J.F. (1977). The Geological Evolution of South Africa. pp. 131-159. Purnell, Cape Town.
  5. ^ Catuneanu O. (2004). "Retroarc foreland systems––evolution through time". Journal of African Earth Sciences 38 (3): 225–242. doi:10.1016/j.jafrearsci.2004.01.004. 
  6. ^ a b Compton, J.S. (2004).The Rocks and Mountains of Cape Town. p. 24-26. Double Storey Books, Cape Town.
  7. ^ Geology of the Cape Peninsula - Cape Fold Belt
  8. ^ The Cape Fold Belt
  9. ^ Norman, N., Whitfield, G. (2006). Geological Journeys. pp. 28, 122. Struik Publishers, Cape Town.

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