Reading about Reg Sprigg keeps leading me back to one word I had never heard before this trip started taking shape: superbasin. Sprigg himself never used it. He and Douglas Mawson called the thing the Adelaide Geosyncline, and geologists later called it the Adelaide Rift Complex. Only in the last few years has it been renamed the Adelaide Superbasin, after a thorough re-sorting of the rock layers.
Video explaining the geology of South Australia (21 minutes)
The name matters less than the size. This is a stack of sedimentary rock covering much of eastern South Australia and reaching into western New South Wales and Victoria — laid down over roughly 340 million years, from about 830 to 490 million years ago. It is one of the largest and best-preserved records anywhere on Earth of a supercontinent tearing itself apart. And for four days in the Flinders Ranges we will be driving straight through the middle of it, on roads that cut across the layers like a knife through a very old, very crumpled cake.
Here is what it is, how it got there, and why exploration companies have been poking at it for seventy years.
Act one: Rodinia comes apart
Long before Gondwana, there was a supercontinent called Rodinia. Around 830 million years ago it began to split, and the crack ran right through what is now South Australia.
Rodinia comes apart, and the tear runs through our route. Panel 1 is one plausible reconstruction among several — the local evidence in panels 2 and 3 is much firmer.
The superbasin from above: a long, narrow gash running north–south, and the road we take down the length of it.
It helps enormously to see the shape of the thing first. Basins are usually drawn as bowls, and this one is nothing of the sort: it is a long, narrow trough running roughly north–south for something like 800 kilometres, pinned hard against the eastern edge of the Gawler Craton. That shape is the single best clue to its origin. Bowls form where crust sags. Gashes form where crust is pulled apart.
A caveat is worth making here, because it is the kind of thing that gets stated too confidently. Rodinia itself is a reconstruction, not an observation, and geologists still argue about which continent sat where. The evidence in South Australia, though, is not in doubt: the belt of rift-filled sediment is there, the basalt dykes are dated to about 827 million years, and no explanation other than a continent pulling apart accounts for them.
The rift never quite finished the job. In the south it matured into a proper passive continental margin, an ocean edge like the east coast of North America today; in the north it stalled as a failed rift. Either way, the trough kept sinking and kept filling, for hundreds of millions of years.
Four acts: how the Adelaide Superbasin was made, and what erosion left behind.
Act two: a sea, twice frozen
What accumulated in that basin is an extraordinary archive, and it is the reason geologists from all over the world come to the Flinders. Working up from the bottom, the pile records:
the copper-bearing dolomites that later made the fortunes of Burra and Kapunda — the mines that kept the young colony of South Australia solvent;
two Snowball Earth glaciations. The Sturtian, starting around 717 million years ago, was the longest ice age our planet has ever endured; the Marinoan followed. Both are recorded here as tillite — frozen mud studded with boulders dropped from floating ice — and the Sturtian is named for the Sturt River south of Adelaide;
the Acraman impact, a large meteorite strike west of Lake Gairdner whose shattered debris was blown across the basin and settled as a distinct layer you can trace for hundreds of kilometres;
a great rise in atmospheric oxygen, and then, in the shallow sandy shelf sediments at the top, the Ediacara biota — the soft, quilted, frond-like creatures Sprigg found in 1946, the oldest complex animals known.
Two consequences of this stack are worth knowing before we set off. First, the golden spike that defines the base of the Ediacaran Period for the entire planet is driven into a rock face at Enorama Creek in Brachina Gorge — Australia's only such global reference point. Second, the Rawnsley Quartzite, high in the sequence and tough as iron, is the rock that holds up the walls of Wilpena Pound.
The pile from bottom to top, and where the road crosses each part of it.
Act three: the squeeze
Around 514 million years ago the filling stopped and the crushing began. As Gondwana assembled, the whole basin was shoved sideways in the Delamerian Orogeny, folding the layers into a mountain belt and shutting the basin down for good by about 490 million years ago.
And the salt came back. Buried salt is less dense than the rock above it and behaves, over geological time, like a very slow liquid. Squeezed, it flowed upward and punched through the overlying layers as diapirs — salt plugs that dome the strata around them. Several of these break the surface in the Flinders, most famously near Blinman, and they are a big part of why the geology of the ranges is so wonderfully contorted.
What we will actually drive through is the eroded stump of those mountains. Something like fifteen kilometres of rock has been stripped off the top. The present-day relief is much younger — the ranges have been re-lifted in the last few million years — but the folds themselves are Cambrian.
Why the explorers care, part one: uranium
Sprigg's biography is subtitled with four words: oil, uranium, geology, conservation. The uranium came first — as a young man he worked at Radium Hill and in the Mount Painter country, hunting material for the first atomic bombs. Mount Painter is at the northern end of the Flinders, and Arkaroola, which Sprigg later bought and turned into a sanctuary, sits squarely on it.
That is not a coincidence. The basement rocks beneath and beside the superbasin include granites unusually rich in uranium, thorium and potassium. They are radioactive enough to be measurably warm — this is the "hot rocks" country that also drew geothermal drillers, and it is why the Paralana hot springs near Arkaroola are hot.
But the uranium being mined today is not in the ancient rocks at all. It is in the young sands next door, and the process is beautifully simple:
Rain falls on the uranium-rich granites of the ranges and leaches uranium into the groundwater.
The groundwater flows east, down into buried river channels in the Frome Embayment — the flat, featureless country between the ranges and Lake Frome, filled with sand and clay only tens of millions of years old.
Those channels are sealed above and below by clay, so the water is funnelled along them.
Somewhere along the way the water meets sand full of buried plant matter, which has stripped all the oxygen out of it. Uranium is soluble in oxygenated water and insoluble without it, so at that boundary it simply drops out of solution — concentrating into a crescent-shaped ore body called a roll front, which creeps downstream over millions of years.
This is the recipe that produced Beverley (found in 1969, on the eastern doorstep of the northern Flinders), Honeymoon, Four Mile and Goulds Dam. Because the ore sits in permeable sand between two clay seals, it can be mined by in-situ recovery: no pit and no shaft, just wells that dissolve the uranium back out of the ground and pump the solution to the surface. South Australia has been one of the world's significant uranium producers on the strength of it.
So the superbasin's contribution is indirect but decisive. It built the mountains, exposed the radioactive basement, and provided the erosional plumbing that has been quietly concentrating uranium ever since.
The roll front: how the ranges, the groundwater and a buried swamp combine to make an ore body.
Why the explorers care, part two: oil and gas
Here the story is one of stubborn, seventy-year, not-quite-yet optimism — and it needs one clarification first. Sprigg's great petroleum success, the Cooper Basin gas fields that made Santos, is in a much younger basin, a few hundred million years younger, sitting well to the north-east. That is not this.
The Adelaide Superbasin's own petroleum story is more tantalising. The ingredients are genuinely there:
Source rock. The Tapley Hill Formation, laid down on a black, airless sea floor after the first Snowball Earth, is rich in organic carbon.
Reservoir and seal. Porous sandstones and carbonates, with those thick salt beds making superb seals — and the diapirs making natural traps.
Evidence of hydrocarbons. Oil shows were logged in Cambrian rocks in the 1950s. When Blinman 1 and 2 were drilled in 1990–91 beside one of the salt diapirs, traces of gas came out of fractures in the Tapley Hill Formation. Solid bitumen — dead oil — turns up in the basin too.
The problem is timing and heat. These rocks are so old, and were cooked so thoroughly during the Delamerian squeeze, that most of the oil generated has long since escaped or been destroyed. Modelling suggests the Neoproterozoic source rocks are mature to overmature for oil, and better prospects for gas.
Yet the play refuses to die, for a good reason: rocks of exactly this age are commercially productive elsewhere. Oman, Siberia, India, China and North Africa all produce from late-Precambrian source rocks. The nearby Officer Basin, of similar age, has recorded oil shows from four distinct oil families without a commercial discovery yet. As one government summary drily puts it, it remains one of the last onshore frontiers where something large might still be found. More recently the same salt-and-old-basement geology has drawn interest for helium and naturally occurring hydrogen, which is a very 2020s twist on a very old idea.
Sprigg would have enjoyed that. He spent his life betting that ancient rocks had more to give.
What to look for from the bus window
Brachina Gorge. Set up as a geological trail that walks you forward through time as you drive. The signposted golden spike is the base of the Ediacaran — the point that defines the period worldwide.
Wilpena Pound. The rim is Rawnsley Quartzite, the same unit that carries the Ediacara fossils.
Tillite. Snowball Earth in the hand: mudstone with random boulders in it, dropped from melting icebergs.
Blinman and the diapirs. Broad domes with jumbled cores — salt that flowed upward for a hundred million years.
Arkaroola. Radioactive granites, hot springs and, in the Ridgetop Tour, the most spectacular section through the basement anyone offers.
The plains east of the ranges. They look like nothing at all. They are the uranium.
Sources and further reading
Adelaide Superbasin — Wikipedia overview, with references to the recent stratigraphic revision.
Arrowie Basin and Adelaide Rift Complex — South Australian Department for Energy and Mining, including the Blinman wells.
Uranium in South Australia — the same department, on deposit types and the Frome Embayment mines.
Officer Basin — Geoscience Australia, on the neighbouring Neoproterozoic petroleum frontier.
Kristin Weidenbach, Rock Star: The story of Reg Sprigg — still the best way in.
Diagrams above are schematic and drawn for this blog: vertical scales are wildly exaggerated and horizontal distances compressed. Corrections from anyone who actually knows their Adelaidean stratigraphy are very welcome in the comments.
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