Cape Fold Belt

Paleozoic fold and thrust belt in South Africa

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, the Pensacola Mountains (East Antarctica), the Ellsworth Mountains (West Antarctica) and the Hunter Bowen orogeny in eastern Australia. The rocks involved are generally sandstones and shales, with the shales (Bokkeveld Group) persisting in the valley floors while the erosion resistant sandstones (of the Peninsula Formation) form 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 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 (see the two maps one above the other on the right).

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). (See the yellow block labeled C on the Earth's geological timeline diagram on the right.) An 8 km thick layer of sediment, known as the Cape Supergroup (see below), accumulated on the floor of this rift valley. 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. 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 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, (now covered with dune sands) called the "Cape Flats".

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. 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.

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, 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. 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. Although the Dwyka and Ecca sediments adjoining the Cape Fold Mountains were subjected to the same compression forces that gave rise to the Cape Mountains, they do not form the same mountain ranges as do the Cape Fold Mountains. This is because they are composed of much softer rocks than the Peninsula Formation Sandstone, and were thus soon eroded into the flat plains of the "Lower Karoo", except where they were protected by hard, erosion resistant dolerite or turbidite caps, to form isolated mountains that stand out from the plain. Sediments eroded from these mountains provided the bulk of the 6 km thick Beaufort sediments in the Karoo Basin,