Cape Fold Belt
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. 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.
Geological origin
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). An 8 km thick layer of sediment, known as the Cape Supergroup, accumulated on the floor of this rift valley. 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.
Image 1: 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,but compass bearings are nevertheless given as if Africa was in its present position.
"Southern Gondwana" by Oggmus - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Southern_Gondwana.png#mediaviewer/File:Southern_Gondwana.png
Image 2: 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.
"Formation of Cape Fold Mountains" by Oggmus - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Formation_of_Cape_Fold_Mountains.jpg#mediaviewer/File:Formation_of_Cape_Fold_Mountains.jpg
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. 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. 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.
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 following is a list of the 24 largest individual ranges within the Cape Fold Belt include from west to east.
- Cederberg Mountains
- Olifants River Mountains
- Piketberg
- Winterhoek Mountains
- Skurweberge
- Hex River Mountains
- Cape Peninsula and Table Mountain
- Du Toitskloof Mountains
- Drakenstein Mountains
- Simonsberg
- Hottentots-Holland Mountains
- Kogelberg
- Stettynsberge
- Langeberg
- Riviersonderend Mountains
- Kleinrivier Mountains
- Witteberge
- Swartberge
- Outeniqua Mountains
- Langkloof Mountains
- Kouga Mountains
- Tsitsikamma Mountains
- Baviaanskloof Mountains
- Zuurberge
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.
Geological Fold Terms
Image3: Taken from: Geological Journeys by Nick Normand and Gavin Whitfield.
Swartberg Mountains
The Swartbergmountains are a mountain range in the Western Cape province of South Africa. It is composed of two main mountain chains running roughly east-west along the northern edge of the semi-arid Little Karoo. To the north of the range lies the other large semi-arid area in South Africa, the Great Karoo. Most of the Swartberg Mountains are above 2000 m high, making them the tallest mountains in the Western Cape. It is also one of the longest, spanning some 230 km from south of Laingsburg in the west to between Willowmore and Uniondale in the east. Geologically, these mountains are part of the Cape Fold Belt .Much of the Swartberg is part of a UNESCO World Heritage Site.
The two ranges
The Swartberg consists of two officially named ranges. The Smaller and the Greater Swartberg Mountains.
Klein Swartberge
The Smaller Swartberg are the westernmost of the two. Ironically, this range is the higher one, including the province's highest peak, Seweweekspoortpiek at 2325 m. The famous Towerkop towers over the Klein Karoo town of Ladismith at a height of 2189 m. The peak is so named for its cleft peak, that, according to legend, was split by a spell and subsequent bolt of lightning.
Groot Swartberge
The Greater Swartberg is located to the east, with the dividing line between the two ranges being the Gouritz River, which cuts a gorge directly through the range. This section, almost of a similar height, is slightly lower in elevation, with the Tierberg at 2132 m being the highest. These mountains are home to the Cango Caves, the most famous subterranean system in South Africa, located just north of Oudtshoorn.
Swartberg Pass
The spectacular Swartberg Pass on the R328 run through the Swartberg mountain range which runs roughly east-west along the northern edge of the semi-arid area called the Little Karoo in the Western Cape province of South Africa.
The Swartberg is amongst the best exposed fold mountain chains in the world, and the pass slices through magnificently scenic geological formations. To the north of the range lies the other large semi-arid area in South Africa, the Great Karoo. Much of the Swartberg is part of a UNESCO World Heritage Site.
It was built using convict labour by Thomas Bain and opened on 10 January 1888. The drystone work supporting some of its picturesque hairpin bends is particularly noteworthy.
The pass runs between Oudtshoorn in the south and Prince Albert in the north. The pass is not tarred and can be a little treacherous after rain, but offers spectacular views over the Little Karoo to the south and the Great Karoo to the north. The plant life along the pass is very interesting, many hundreds of species being found on the Swartberg.
The Swartberg pass was built between 1881 and 1888 by Thomas Bain, son of the famous Andrew Geddes Bain who built Bain's Kloof Pass and many more. The dry-stone retaining walls are still in place and almost 120 years old.
The pass is especially famous due to the spectacular geology that is exposed at its Northern end. The contortions in the rock display astonishing anticlines and synclines, and the vivid coloration of the surrounding Quartzite is remarkable. At the Northern end of the pass seven hundred metre high quartzite cliffs of the upper Table Mountain Group can be seen, and these are often tilted through 90 degrees (sometimes even more). Arguably the most famous of all these cliff faces is the spectacular 'Wall of Fire'.
Please answer the follwoing questions to qualify your log:
1. Why, do you think, does many of these ranges within the Cape Fold Belt, not have any foothills?
2. The Cape Fold Belt is made up of rocks belonging to the Cape Supergroup. Are these rocks metamorphic or sedimentary rock? Qualify your answer?
3. There are several good quality folds visible in the Swartberg pass. Choose one of them and describe the scene by mentioning the estimated size angles of the folds and the colouring of the feature.
4. OPTIONAL: Upload as many photographs of the features in the pass?
Scources:
- Text and Imaged 1 and 2 obtained from Wikipedia.
- Image 3 were taken from Geological Journeys by Nick Normand and Gavin Whitfield.