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The Ironstone heart of the Karoo

The Ironstone heart of the Karoo

What defines the Karoo, that charismatic semi-desert sprawled across South Africa’s midriff? First, its clay soils are evenly sprinkled with dwarf bossies and surprisingly high biodiversity. Second, it is huge, sprawling over 400 000km² (slightly larger than Germany) and covering one-third of South Africa.

Third, there are way more sheep than humans (by a ratio of 7:1). Fourth, many of its hills are flat-topped. Fifth, there are so many windpumps they’ve become a visual shorthand for this semi-desert. Sixth (which you’d only know if you owned a kettle or a horse in the Karoo) the soils are very high in minerals, especially calcium.

120 million years

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Much of this has to do with a catastrophe that happened 182.5 million years ago, the traces of which are irrevocably embedded in the Karoo’s rock formations. Professor Bruce Rubidge from the University of the Witwatersrand Centre of Excellence in Palaeosciences explains that the region’s geology is a uniquely intact library safeguarding a story told nowhere else in the world. “Rocks of the Karoo Basin preserve a world-class assemblage of fossils that document the early evolution of tortoises, dinosaurs and mammals.

“These rocks, deposited from the Late Carboniferous (300 million years ago) to the Early Jurassic (180 million years ago), witnessed environmental change from glaciers, through aeolian desert dunes, and the vast outpouring of volcanic lava.” The last part of his description explains one of this semi-desert’s geological names – Karoo Large Igneous Province, or KLIP for short.

For 120 million years, the Karoo’s rock formations were laid down in sedate sedimentary layers, eroded from high mountains by water and wind, occasionally encapsulating the now fossilised remains of ancient life.

Then a geological drama began, with molten magma rising from deep within the Earth 182.5 million years ago, oozing steadily upwards from 100 kilometres or more below the surface through all the Karoo’s layered sediments (Dwyka, Ecca, Beaufort, Stormberg). Like melted chocolate rising through piles of layercake, it filled every tiny and large crack, crevice, pocket and fault, and still kept coming.

All kinds of hell broke loose in comparative slow motion. Some parts of South Africa were covered in lava to a depth of three kilometres; the jagged basalt peaks of the Drakensberg mountain range are the most visible remnant of that. Lava pressed down and ‘baked’ sand dunes into the striking formations now visible around Clarens in the Eastern Free State.

Characteristic mountains

Ironstone

Julian ‘Goonie’ Marsh, Professor Emeritus of geology at Rhodes University in the Eastern Cape.

South Africa’s leading authority on Karoo dolerite (nicknamed ironstone, for very good reason) is Julian ‘Goonie’ Marsh, Professor Emeritus of geology at Rhodes University in the Eastern Cape. “In geological terms, the igneous event was brief, but intense. The volumes were incredible, amounting to at least 1.5 million km³ of molten rock.”

As the magma rose closer to the surface, it would have come into contact with wet sediments or groundwater. The hydrothermal explosions that resulted punched upwards, creating thousands of holes in the Earth’s crust, through which hydrocarbon gases poured.

Over thousands of years, the molten rock cooled, trapped in the basement of Karoo formations. Underground it set in layered sills atop pancaked sediments, in saucer-shaped rings and vertical dykes, an irregularly patterned subterranean jungle gym of iron-hard rock sheets, pillars and random shapes, veined throughout the Karoo bedrock like the roots of a tree.

The characteristic flat-topped mountains and hills of the Karoo are the most widespread visible remnant of the ironstone dolerite’s tough, level sills. They protected the softer underlying sandstone from erosion. The thrusting dolerite crest of the Compassberg near Nieu-Bethesda, and the jointed stone pillars at the Valley of Desolation outside Graaff-Reinet, are also among the most iconic exposed remains of that magma upwelling today. Near Williston and Loxton they appear as pop-up koppies made from piled blockish boulders, burnt black by the sun.

Less visible are the underground cracks left in the adjoining sediment as the superheated dolerite rose.

Stone age tools

Ironstone
Groundwater collects in the crevices around sills and dykes. Anyone looking to drill for water in the Karoo will do so in the vicinity of ironstone dolerite. Windpumps generally mark the spot of these priceless caches of underground water.

The Karoo’s clay soils are full of minerals thanks to the dolerite. Wander around an ironstone koppie and between the stones you will find unique plants thriving in the sheltered, comparatively fertile clay. The western side of the semi-desert, called Namaqualand or the Succulent Karoo, is one of only two arid-area biodiversity hotspots in the world. (The other is the Horn of Africa.)

Ironstone Dolerite’s other role has been its importance to human evolution and South Africa’s culture. This takes at least two forms. Firstly, all across the Karoo you can find Stone Age artefacts and tools. They were made out of hornfels, a hard, fine-grained surface shale that was heated (metamorphosed) by dolerite.

Hornfels is very hard and, like glass or obsidian, it breaks with a conchoidal fracture. With it, Stone Age people created tools for cutting, scraping and stabbing.

Dr Marsh says, “I have found some places along dolerite dykes intruding shales, where the ground is littered with hundreds of worked hornfels fragments. One can imagine such sites to literally have been stone-tool ‘factories’. Without dolerite to make hornfels, one can wonder whether this Stone Age culture could have thrived as it did across the Karoo Basin.”

Like bells

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Professor Bruce Rubidge from the University of the Witwatersrand Centre of Excellence in Palaeosciences.

Secondly, ironstone has qualities that lent themselves to early human creativity. As this rock ages, the exposed area darkens to reddish-brown and then pitch-black, as if becoming sunburnt. This is called a rock varnish, patina or oxidation. But if that is pecked or scraped away, the pale reddish-brown rock underneath is exposed.

Some of the planet’s earliest human artistic expression is etched on dolerite, at sites all over the Karoo. In just one rock-etching site near Nelspoort (north of Beaufort West), there are literally hundreds of etchings and engravings of animals and geometric symbols, thought by many to date back thousands of years, and made by South Africa’s Bushmen (San) and Khoi.

These ‘First People’ engraved eland, black rhinoceros, giraffe, hartebeest, zebra, bird-human figures, spirit beings, jackal, blesbok and elephant. Dark patina has reclaimed many of them again, and the oldest ones can only be seen at certain angles, in certain light.

There is also an acoustic quality to ironstone. Many large dolerite boulders balanced near the top of flat-topped hills give off a bell-like sound when struck. Ancient strike-marks show that Bushmen and Khoi people used them for drumming.

Archaeology professor John Parkington of the University of Cape Town writes in his co-authored book Karoo Rock Engravings, ‘Karoo rock gongs are almost without exception located among clusters of engravings and are recognisable as sets of hammered or pecked patches usually on dolerite boulders, nearly always on the lips of rocky ridges’.

He explains that most sedimentary rocks, like sandstone, produce no ringing sound, but that igneous rocks are ‘homogenous, dense, metallic in structure and will allow the force of a blow to resonate… to ring in effect’.

When or why were they used? No one knows any more, but Parkington and co-authors David Morris and Neil Rusch mention rain-making rituals as a possibility.

Gas long gone?

Ironstone
Back to present day, and the threat of hydraulic fracturing for shale gas that lies, potentially, in the deep carbon-rich Ecca formations. From late 2010, Karoo shale attracted the attention of companies like Royal Dutch Shell, Falcon Oil & Gas, Bundu Gas & Oil and, more recently in the Free State and KwaZulu-Natal, Rhino Oil and Gas.

Like the Marcellus shale fields in the US, the Karoo Basin seems to have had exactly the right mix of events, depth and pressure to create gassy hydrocarbons. On paper at least. In 2015, the US Energy Information Administration calculated the technically recoverable shale gas resource at 390tcf (trillion cubic feet) or more, which would have made it the eighth-richest shale basin in the world. But it is also among very few such basins in the world that is veined with dolerite.

So what did the ‘igneous event’ do to the methane gas trapped in the shale (Ecca) layers? More and more scientists think the gas that the frackers seek is long gone. Or at least gone in commercially viable quantities.

Dr Billy de Klerk is a Grahamstown-based geologist, with a PhD in magmatic processes. He points out that the doleritic magma was at about 1 200°C when it intruded. “It’s entirely likely that the enormous heat of this dolerite – hot-boxed for millennia by the insulated, layered sediments – would have driven off most of the volatile hydrocarbon gases like methane.”

Some of the best scientists in South Africa worked to produce the Strategic Environmental Assessment for Shale Gas Development in the Central Karoo, finalised in June 2017. In the summary, they note, ‘The total quantity of shale gas that occurs within the study area is uncertain, as is where exactly it may be concentrated. There may be no economically extractable gas. Geological upheavals hundreds of millions of years ago, specifically the intrusion of hot lava (dolerite) and the pushing-up of the Cape Fold Mountains, are believed to have reduced the volume of gas originally in place.’

The physics of drilling

r Billy de Klerk is a Grahamstown-based geologist, with a PhD in magmatic processes.

In September 2017, the South African Journal of Sciences published a study by eight geologists (three from the University of Johannesburg), called Deflating the Shale Gas Potential of the Main Karoo Basin.

They write, ‘Contact metamorphism by dolerite sills has resulted in catastrophic and explosive degassing and alteration of shale’, and conclude that, if there is gas, it would be at the bottom end of a range of estimates – perhaps 13tcf. ‘To be economically viable, the resource would be required to be confined to a small, well-delineated ‘sweet spot’ area in the vast southern area of the basin.’

Companies like Shell that have been seeking to exploit this shale pushed hard for exploration rights, and ANC mineral resource ministers (most recently, Mosebenzi Zwane and Gwede Mantashe) have expressed great keenness for shale gas exploitation.

But ironstone may have put paid to their dreams. Even had it not cooked off the gas, the sheer physics of drilling through this extremely hard and faulted rock is enough to daunt the keenest fracker. So the next time you pass
a Karoo ironstone koppie, doff your hat. Those are our fortresses against frackers, our secret water stashes, our ancient art canvases and our singing rocks.

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