WA's amazing geology

When coming from the east and crossing the imaginary north-south meridian of 129°E to enter Western Australia, it seems like you are entering another land. And if it is true that it is the underlying geology which determines the surface of the landscape, this is certainly true when you enter WA from the east.

The traveller must first cross the remarkable Nullarbor Plain equipped with ‘Beware of Camels’ signs and its famed treeless expanse which at first seems a great mystery but is easily explained by an almost complete lack of available water for trees - or travellers for that matter. If you take a short walk almost anywhere on its flat surface, you are bound to discover that you are walking over a limestone landscape which was submerged beneath the ocean until relatively recently geologically speaking. This is demonstrated by an abundance of marine fossil shells which are readily recognisable as almost identical to those found on modern seashores. Beneath the waterless surface, the plain is host to numerous limestone caves that penetrate deep into the interior of the plain. Most are above current sea-level and so are never flooded with salty water. In these there are reserves of fresh water that has accumulated over many years and which supply fresh water to the scattered grazing properties and infrequent roadhouses providing motorists with necessities and an occasional change of scenery. Travelling further into the state via the Eyre Highway it takes quite some time to encounter landscape features that are distinctive to Western Australia, but once you approach the Greenstone Belt it is clear that you are in a very ancient land. This is a part of WA where huge granite ‘domes’ project upwards through the surface of the plain interrupting the flat

By Peter Bindon

The treeless expanse that is the Nullarbor Plain.

landscape. The granites provide sheltered and varied environments for various shrubs and trees that take advantage of the water that drains off the rock surface during rainstorms or condenses on the rock when conditions allow. The rocks are often the focus of Aboriginal people’s occupation of the plains as the impermeable rocks contain hollows and crevices that are known as ‘gnammas’ and hold precious water close to the surface. These water resources have proved invaluable not only to the Indigenous owners of the landscape but also to European explorers and prospectors seeking mineral resources to exploit. In the style of the modern tendency to label everything using shortened language, the ages of the rocks in the granite-greenstone region are marked with the abbreviation ‘Ga’ short for “giga annum”, meaning ‘billions of years ago’. Perhaps there were too many other ‘BAs’ around to use that abbreviation for a rock label. And the remarkable thing about this truly ancient time in Western Australia’s story, or the world’s story for that matter, is that it is still possible to view remnants of life from that time far in the past when you explore Western Australia today. Readers will probably be familiar with Stromatolites, the layered stony structures whose fame brought attention to Western Australian fossil occurrences when it was shown that their age extends back to almost threequarters of the age of the entire universe – not a concept that is easily digested. Not only are structures of this age visible in certain rock formations in the Pilbara, but they have relatives which are still living. Fancy living from 3.5 Ga to the present (I was dying to use that abbreviation) and being able to relate the history that passed before your eyes – except that the tiny cyanobacteria which created the layers in a stromatolite are eyeless. These stony structures are built by colonies of tiny photosynthesising critters called ‘cyanobacteria’. They gradually grew over thin layers of fine sand and sediment binding the grains together until millimetre by millimetre they formed rounded structures visible today as mounds.

Hamelin Pool in WA is one of only two places on earth where living marine stromatolites exist. The location contains by far the biggest colony on earth.

Remarkably, as I said some of these belong way back in time, but some colonies are still alive today. So, they certainly are survivors. I think that when I first encountered stromatolites, we referred to all mounded structures found in a few of the coastal lakes and inlets along the west coast as stromatolites. However, it turns out that there are related structures that instead of being layered like the stromatolites, retain the mounded shape but are created with various ‘clotted’ agglomerations of similar tiny organisms that instead of binding the tiny rock particles together in layers, do so in clotted groups that together form mounds similar to stromatolites except that these are called ‘thrombolites.’ It turns out that the celebrated Professor Brian Cox had an encounter with thrombolites in Western Australia and brought these unassuming structures into the international spotlight. The Greenstone Belt was enormously important to the economy of Western Australia because it was and is the location of many of the discoveries of gold that contributed to the exploration, settlement and prosperity of the state. But we are going further north through the goldfields and mid-west until we reach the Pilbara. To my mind it is the latter region that holds the most spectacular scenery to be found in the state. I love the rolling spinifex-clad hills dissected by deep gorges containing lush green vegetation that is in such contrast to the yellow spikey spinifex plants that live on the tops. There is more than one kind of spinifex, but that is not obvious to the traveller in an air-conditioned vehicle passing swiftly through the landscape. In fact, one variety is called ‘Soft Spinifex’, but I can tell you that this is a relative term! But

Pilbara landscape dotted with spinifex.

spinifex plants proved very useful to Aboriginal people even if the plants feel less than friendly to those who insist on wearing thongs or sandals. Clumps were collected after softening rain had fallen or were turned over with a cautious foot and weighted down in a convenient waterhole until the leaves were soft. These were then pounded in small bunches to release the fibres which could be spun into strings that were plied together to form strong cords. With these cords, nets were made with meshes about 10 centimetres square and these were used in hunting large game, by being propped up in chosen locations and having the game driven into them from the surrounds. A further use by Aboriginal people of another type of spinifex product is quite amazing. Goodness knows how many ages ago they discovered that the sticky resin could be extracted from the leaf bases of spinifex during the cold times of the year. This resinous material is quite solid when it is cold, but when heated becomes plastic and can be carefully moulded on artefacts to form a very strong bond, say between a stone point and a wooden spear shaft. Now it was not until the discovery of synthetic resin glues in the 20th century that western craftspeople could make a stone adhere directly on to a stick, but the technique was discovered in Australia more than ten thousand years ago. There are many more stories like that which could be told but having looked quickly at the surface features of the Pilbara, let us descend into some of the spectacular gorges that dissect the plains. Many of the gorges have streams flowing along their shaded bottoms or else are the location of pools of water remaining from the summer storms which are the main source of surface water in the region. The sides of the gorges are invariably cut through BIF which is a short-hand term used by some locals for ‘banded iron formation.’ These rocks produce the current fortunes of the Pilbara mines, and remarkably, their occurrence is also due to tiny oceanic bacteria like those we discussed earlier. Banded iron formations are the source of most of the world’s deposits of iron ore and all of them belong to what geologists call the ’Precambrian Age,’ which means that they are more than about 540 million years old and remarkably they occur on all the world’s continents. The world’s oceans, although not in exactly the same configuration that we see now, became the ‘sink’ into which were deposited dissolved elements from rocks exposed in the landmasses of the time and carried along by rivers to the ocean. These elements included silica, aluminium and

“It is quite astounding iron. Oxygen either in combination with one of these elements or as a dissolved gas was also to think that a present, but a more important microscopic organism source of oxygen was a could be responsible cyanobacteria that about for such huge quantities 2500 million years ago developed the capacity to use photosynthesis for growth just of ore” like ordinary garden plants, and in the process also released oxygen. Seasonal ‘blooms’ just like the algal blooms we sometimes see today, released huge amounts of oxygen into the oceans that reacted with the iron compounds brought into the ocean by rivers to form insoluble iron oxides that fell out of the seawater to form a layer on the sea floor. In turn, this layer was covered by silica rich sediments to be followed once more by an iron-rich bloom and this sequence of sediment followed by iron oxide precipitation was repeated over and over, building up the thick deposits of iron rich rocks mined today as haematite and magnetite in the Pilbara mines. It is quite astounding to think that a microscopic organism could be responsible for such huge quantities of ore that have to be moved to the coast in staggeringly long trains to be shipped away in huge ore-carriers before returning to Australia as a myriad of manufactured goods in daily use.