Geological Formations

Originally, some 4 500 million years ago, the earth consisted of one continent (Pangaea), which subsequently broke up into two supercontinents, viz. Gondwanaland in the south and Laurasia in the north. Gondwanaland split about 200 million years ago to form the continents of South America, Africa, India, Australia, Antarctica, and New Zealand. Before the breakup of Gondwanaland, the geological formations were formed by volcanic actions, sedimentation, and metamorphic processes.

The following are important geological formations in South Africa.

Basement Complex

These rocks, consisting of granites and metamorphic gneiss and schists, were formed about 1 000 million years ago. They form the foundation on which sediments and lava were laid.

Table Mountain Series (Natal and Cape Groups)

This is the first sedimentary sequence deposited on the Basement complex (490million years ago). It is sandy in composition.

Karoo System

Great basin-shaped depression was formed 300 million years ago, which covers more than half the area of South Africa. For the next 120 million years, layers of sediments were deposited in the Karoo basin and topped with volcanic lava. The layers consist of the Dwyka, Ecca, Beaufort, Stormberg, Drakensberg and Lebombo groups.

Dwyka Group

The rocks overlying the Table Mountain Series are tillites, deposited in a glacial environment by retreating ice sheets 300 million years ago. Africa, part of Gondwanaland, was situated near the South Pole and covered in ice. The rocks embedded in the slow-moving ice sheets consisted of granite, gneiss, quartzite and sandstone. The Dwyka Group forms the lowest and oldest deposit in the Karoo Basin.

Ecca Group

As Gondwanaland moved north towards the Equator, thick clay beds were laid down in a large sea that occupied the Karoo basin in a cool temperate environment. The Ecca shales and sandstones lie above the Dwyka Group. The shales form part of the Pietermaritzburg formation. Overlying this is a sequence consisting of light grey sandstone called the Vryheid formation, where vegetation in swamps were buried to form coal seams. Silt and sandstone, under drier conditions, were laid above this sequence to form the Volksrust formation.

Beaufort Group

The red, green and purple mudstones that characterise this sedimentary sequence were deposited in a drying swampland 250 million years ago. During this period, 95% of the known species became extinct.

Stormberg Group

This group is subdivided into three formations that record gradual desertification. The Molteno formation is the lowest deposit, consisting of sandstone, exposed as small cliffs in the lower Drakensberg, which sparkle due to quartz binding the sand grains. The Red Beds of the Elliot formation overlie the Molteno formation and contain early dinosaur fossils. The Clarens formation (Cave Sandstone) forms the white cliffs of the middle Drakensberg. These sandstones were deposited as large dunes in a desert environment. Dinosaur footprints can be found in these rocks.

Drakensberg and Lebombo Groups

Some 180 million years ago, a huge lava flow occurred, which spread across Gondwanaland. This indicated the breakup of the continent. The 1 500m thick basalt lava deposit can be seen along the Drakensberg Escarpment. The magma that reached the surface solidified rapidly into fine-grained crystal basalt. The lava that did not reach the surface formed sills and dykes of dolerite. This lava cooled slowly to form medium-grained rocks. The final volcanic event produced Rhyolite lava, which forms the Lebombo Mountains. Rhyolite has a similar composition to granite but because it cooled rapidly, it has fine-grained crystals.

These volcanic events were followed by upliftment and faulting that eventually separated Africa and Antarctica. The first deposits formed in the newly-opened Indian Ocean were silt and sandstone of the Cretaceous age. Fluctuations in sea level, due to changing climate, have shaped the coastline. During the last glacial period (18 000 years ago), the sea level was 100m below what it is at present and the coastline would have been 5km out to sea.