Cape Fold Belt
The Cape Fold Belt is a long fold-and-thrust mountain belt along the western and southern coastlines of Western Cape, South Africa. The Cape Fold Belt formed during the Permian period in the late Paleozoic age, affecting the sequence of sedimentary rock layers of the siliciclastic Cape Supergroup with folding and faulted rocks, which were deposited in the Cape Basin in the southwestern corner of South Africa.
The Cape Fold Belt was once part of a larger orogenic belt with other mountain ranges that formed as part of the same tectonic event that originally extended from Argentina, across southern Africa, and into Antarctica. It included the Ventana Mountains near Bahía Blanca in Argentina, the Pensacola Mountains in East Antarctica, the Ellsworth Mountains in West Antarctica, and the Hunter–Bowen orogeny in eastern Australia. The rocks involved in this fold system are primarily sandstones and shales, with shales from the Bokkeveld Group persisting in valley floors, while the more erosion-resistant sandstones of the Peninsula Formation form the parallel ranges of the Cape Fold Mountains. The highest peak in the range is Seweweekspoortpiek, which reaches 2,325meters.
The Cape Fold Mountains form a series of parallel ranges that run along the south-western and southern coastlines of South Africa for 850 km from the Cederberg 200 km to the north of the Cape Peninsula, and then along the south coast as far as Port Elizabeth, 650 km to the east.
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. 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, 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. 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 making them very resistant to weathering. The famous Table Mountain forms part of the Cape Fold Belt, being made up of the local lowest 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 degree to which the original Cape Fold mountains 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, on the opposite side of the isthmus connecting the Peninsula to the Mainland, has been eroded away. The Malmesbury shale and granite basement on which this anticline mountain rested also formed an anticline; but being composed of much softer rocks, readily eroded into a 50 km wide flat plain, called the "Cape Flats".
The Cape Fold Belt extends from about Clanwilliam, to about Port Elizabeth. The Cape Supergroup sediments beyond these points are not folded into mountain ranges, but do, in places, form steep cliffs or gorges, where the surrounding sediments have been eroded away.
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.
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. 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.
The Cape Supergroup extends eastwards beyond the Fold Belt into the Eastern Cape and northern KwaZulu-Natal, where no folding took place.
Klipheuwel and Natal Groups
The initial sedimentation into the rift valley which developed in southern Gondwana 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. 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.The Natal Group, which is similar to the Table Mountain Group, 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. Most of the group has been deeply eroded by numerous rivers creating the ragged plateaus and scarps of the Durban-Pietermaritzburg region.
Table Mountain Group
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, leading to a seaway across the southern and eastern parts of the country, called the Agulhas Sea. The sea floor consisted of the Klipheuwel Group in the west, the Natal Group in the east, and eroded pre-Cambrian rocks in between.File:Ou Kaapse weg.jpg|thumb|upright=1.35|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 an area of inland drainage, possibly below sea level. 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.
File:Cederberg geology.jpg|thumb|right|upright=1.35|Schematic diagram of an approximate 100 km west-east geological cross-section through the Cederberg portion of the Cape Fold Belt. The rocky layers belong to the Cape Supergroup. Not to scale. The green layer is the Pakhuis diamictite formation which divides the Peninsula Formation Sandstone into a lower and upper portion. It is the lower 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. 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. 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.
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. 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. This layer, known as the Graafwater Formation, reaches a maximum thickness of 400 m, but on the Cape Peninsula it is only 60–70 m thick. No fossils have been found in the Graafwater rocks, but shallow-water animal tracks have been found. 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 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 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, 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. A deposit, known as the Peninsula Formation, 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.
The Peninsula formation can be traced from 300 km north of Cape Town, southwards to Cape Town and then eastwards to northern KwaZulu-Natal, a total distance of approximately 1800 km, roughly along the South African coastline. Only the section between Clanwilliam and Port Elizabeth is folded into the Cape Fold Mountains.
During the laying down of the Peninsula Formation sediments, the western portion of the region was covered for a geologically brief period by glaciers. The diamictite sediments that were laid down by these glaciers, or in glacial lakes, are known as the Pakhuis and Cederberg Formations. A small patch of Pakhuis tillite occurs on the top of Table Mountain at Maclear's Beacon, but most of the Pakhuis and Cederberg Formations are found as a thin layer in the Peninsula Formation rocks of the more inland mountains to the west of a line between Swellendam and Calvinia. 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. In several locations the quartzites below the glacial horizon have been rucked into a series of folds. This is believed to have been caused by the movement of ice ploughing into the underlying unconsolidated sands. 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, and especially on the Pakhuis Pass near Clanwilliam, from which the formation derives its name.
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.
The Upper Peninsula Formation, above the Pakhuis and Cederberg Formations, consists of much softer sandstone than the Lower Peninsula Formation, and is often referred to as the Nardouw Formation. 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.