The Karoo Sea

This is swampland on a giant scale. Teaming, pushing, living, scrambling forest grows riotously along the muddy banks of lagoons and stinking back swamps. Trees, club mosses, cycads and ferns compete for space, water and light, while those that have seen their days out collapse back exhausted to the forest floor, to be rapidly colonised by fungi and bacteria. Beetles scramble through the leaf litter or fly ungainly through the canopy, while cockroaches scuttle and scratch in the leaf mould. In the creeks and backwaters amphibians populate the mud flats and dark pools, while out on the sunny waterways fish leap and squadrons of giant dragonflies keep station above the languid waters.

Times have clearly moved on from the icy grip of the Dwyka Ice Age. Gondwana has wandered northwards and South Africa now finds itself at approximately 30° south where temperatures are more conducive to life, which has seized the day, colonising the land to form vast subtropical forests along the northern rim of an ancient sea. But now we need to step back a little and look at the geological processes which have led to this change of circumstances. We have looked at the great ice age known as the Dwyka glaciation of 300 million years ago, but now we must take our focus off KwaZulu Natal for a short time and look at what was going on in South Africa as a whole. Many of us will have travelled through the passes of the Cape Mountains and wondered at the spectacular folding which characterise this mountain chain. These mountains began to form approximately 330 million years ago during the late stages of the Dwyka glaciation. The mountains formed the southern boundary of an ancient sea which covered the majority of Southern Africa – bearing in mind of course that we were part of a larger supercontinent. This sea had a low tidal range and is thought to resemble the modern Black Sea - open to the larger ocean, but without the dynamics of a full-blown ocean. To the north, stretching from the Northern Cape through Gauteng to Mpumalanga, were the Cargonian Highlands – a range of hills which defined the northern limits of this sea. Our Dwyka ice sheets have been bumping and grinding their way southward across these northern highlands before being cast adrift as icebergs on the turbulent waters. As the ice scoured the landscape, Gondwana continued to drift northwards, ultimately bringing us to warmer latitudes and creating conditions conducive for the burgeoning of life.

With decreasing latitude, the ice melted, to be replaced by southward flowing rivers which began to drain the Cargonian highlands, depositing vast quantities of sediment along the northern margins of the Karoo Sea to form large, meandering deltas. Like deltas everywhere, they supported the forests and swamp life where today’s narrative began. The trees that dominated these deltas were a Gondwana classic known as Glossopteris - the distribution of this species another nail in the coffin for the naysayers of continental drift and plate tectonics – it is found in South America, South Africa, Australia, India and Antarctica, strong evidence indeed for the supercontinent of Gondwana. Glossopteris, ferns, horsetails and club mosses thrived within these vast deltas, to the extent that thick accumulations of wood and organic matter were unable to rot and were buried and ultimately converted to coal. These deposits form our coal reserves of KwaZulu-Natal and the Highveld.

The southern margins of the Karoo Sea were characterised by deep water and a narrow littoral, set against the towering peaks of the Cape Mountains. Small, fast flowing rivers typical of mountainous regions deposited their load along a narrow littoral on the southern margin of the Karoo Sea. Every so often some of this accumulation of sediments would slump into the depths to form large, fan like deposits on the sea floor called turbidites, and these may still be seen fossilised in the hills of the Eastern Cape. But this is treatise on the local geology, so we will stay focussed on what is in our backyard. The accumulation of sediment in the Karoo Sea continued for in excess of 150 million years, eventually filling the basin. Karoo rocks have the widest distribution across the subcontinent and reflect the changing latitude and depositional environments as the basin began to fill. It is an ongoing tail of evolution and change and we will continue with the Karoo story in the next article. However any journey inland from Pietermaritzburg will traverse sediments once laid down either on an ancient sea floor or on the swampy margins thereof, where verdant forests thrived in a magnificent outburst of life. Karoo rocks extend to the crest of van Reenen’s Pass and beyond – a truly magnificent heap of sediment which reflects in part the geological history of a subcontinent.